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ArticleNovember 13, 2018

Molecular Engineering and Structure-Related Properties of Squaraine Dyes Based on the Core and Wings Concept
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G. Hanumantha Rao
G. Hanumantha Rao
Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
Academy of Scientific and Innovative Research (AcSIR), CSIR-Human Resource Development Centre, (CSIR-HRDC) Campus, Kamla Nehru Nagar, Ghaziabad, Uttar Pradesh 201002, India
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Prem Jyoti Singh Rana
Prem Jyoti Singh Rana
Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
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http://orcid.org/0000-0003-4819-947X
Ramesh Kumar Chitumalla
Ramesh Kumar Chitumalla
Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea
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http://orcid.org/0000-0002-9523-7056
Joonkyung Jang*
Joonkyung Jang
Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea
*E-mail: [email protected]. Phone: +8251-510-7348 (J.J.).
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http://orcid.org/0000-0001-9028-0605
Surya Prakash Singh*
Surya Prakash Singh
Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India
*E-mail: [email protected]. Phone: +9140-2719-1700 (S.P.S).
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http://orcid.org/0000-0001-5670-7329

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ACS Omega

Cite this: ACS Omega 2018, 3, 11, 15416–15425

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https://doi.org/10.1021/acsomega.8b01809

Published November 13, 2018

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Abstract

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Three new squaraine-based functional π-conjugated molecules were synthesized considering the core and wings concept. The molecules, SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH, were end-capped with three different wings, such as malononitrile, 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile, and 3-ethyl-2-thioxothiazolidin-4-one. Among the three dyes, SQ-DICN-RH showed the highest molar extinction coefficient. The photoluminescence of all the dyes showed an opposite trend to that of the absorption maximum. The electrochemical results showed that the lowest unoccupied molecular orbital level of all the dyes ranged from −3.72 to −3.82 eV, whereas the highest occupied molecular orbital ranged from −4.89 to −4.94 eV. Solvatochromism was carried out to observe the effects of the solvent containing the dyes. The electronic structure of the dyes was examined using ab initio simulations. The dyes were characterized theoretically, and the red-shifted absorption of SQ-DICN-RH was explained and correlated with its biradicaloid character and singlet–triplet energy gap.

Subjects

Introduction

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The absorption of photons in the near-infrared (NIR) region by fluorescent dyes has an increasing number of applications in biomedical science, (1) materials science, and nanotechnology. (2) A series of NIR dyes, such as BODIPY, DPP, phthalocyanine, and squaraine, have been reported. (3) Among them, squaraine dyes have shown promising applications in fluorescent probes, (4,5) photosensitizers [in dye-sensitized solar cells (DSSCs)], (6) electron donors [in organic photovoltaic (OPV)], (7) optical storage media, (8−10) sensitizers for photodynamic cancer therapy, (11−13) and two-photon absorbing materials. (14,15) Because OPV devices are being fabricated with a blend of fullerene and polymer/small molecule (SM) derivatives, which play a key role in enhancing the power conversion efficiency (PCE). (16) Fullerene derivatives such as phenyl-C61-butyric acid methyl ester and phenyl-C71-butyric acid methyl ester have been widely accepted as electron-accepting materials, whereas polymers are used as donor materials. Fullerenes have high dominance in OPV because of their advantageous properties such as (i) electron-withdrawing capability, (ii) high electron mobility, and (iii) three-dimensional transportability of electrons due to their shape. (17)

Notwithstanding advantages, fullerene has some limitations, that is, it does not cover the entire visible region of the incident solar spectrum because of weak absorption, cost-effective synthesis, and effects in the aggregation and morphology of thin films. In the past few years, a considerable concentration has been waged on the development of nonfullerene acceptors (NFAs) (18,19) in OPVs instead of fullerene derivatives. Novel NFAs show number of superior properties over the fullerene, such as ease of synthesis, high absorption in the visible to NIR region of the solar spectrum, and structural plasticity, which allows tuning the energy levels of donors and acceptors. By considering all these superior properties, NFAs have been designed and applied in OPVs. In this regard, Beaujuge et al. reported (compound 1, 2, and 3 shown in Figure 1) small molecular acceptors having malononitrile as the end group with a PCE of 5.3%. (20) Lim et al. introduced fluorene/carbazole-containing rhodanine-based acceptors (compound 4 and 5 shown in Figure 1) for OPV and resulted in a PCE of 3.08%. (21) Recently, Zhan et al. reported indacenodithiophene as a core and ethyl rhodamine/malononitrile as an end group (22,23)(6a, 6b) and achieved a PCE of 5.12 and 4.81%, respectively. Malononitrile end group-based organic molecules have also been used in organic field-effect transistors (OFETs) and have achieved the mobility (24,25) up to 0.64 cm² V^–1 s^–1.

Figure 1. Molecular structures of NFAs based on the core and wings concept.

NIR squaraine dyes, showing absorption beyond 700 nm, helped in constructing transparent and semitransparent photovoltaic devices. (26,27) The squaraine dyes were classified into two categories: symmetrical and unsymmetrical. The symmetrical squaraine molecules followed a simple synthetic pathway and could be synthesized via the condensation of electron-rich molecules (such as indolium salts) with squaric acid. The electronic and optical properties of such dyes depend on their energy gap, that is, the energy difference between the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO). The absorption and emission properties of fluorescent dyes that undergo wide changes in charge dispersal and photoexcitation processes involving intramolecular electron transfer are quite sensitive to the nature of the solvent used.

Because squaraine dyes possess reasonable photo- and thermal stability and exhibit an intense absorption peak, they have been used widely as an electron-donating material in the bulk-heterojunction (BHJ) solar cells. (28−30) Although considerable attention has been paid toward the molecular engineering of new squaraine dyes, there have been few studies on the structural–property relationship of these promising optoelectronic materials. (31) In general, the molecular architecture of these squaraine dyes is in a donor (D)–acceptor (A)–donor fashion. Their photophysical and electrochemical properties can be tuned finely further by altering the basic architecture through the design of a newer concept, that is, donor−π–acceptor–donor (D−π–A–D), donor−π–acceptor−π–donor (D−π–A−π–D), and acceptor−π–donor–acceptor−π–donor (A−π–D–A−π–D). The most successful design of photonic materials was reported by the Bazan research group. (32) They introduced a new class of small molecule donor materials for solution-processed OPVs. Before this work, SMs appeared to be intrinsically inferior to polymers in their ability to generate useful BHJ solar cells. They developed a new framework that can generally be explained as consisting of an electron-rich central core flanked by a poor electron-withdrawing unit and finished with π-conjugated end caps. This scaffold recommended the suitable synthetic entry to a wide class of materials with tailored electronic, optical, and physical properties.

This paper reports three symmetrical novel SQ dyes with the acceptor−π–donor–acceptor–donor−π–acceptor (A−π–D–A–D−π–A) architecture. The three SQ dyes were decorated with three different acceptors as end groups (wings) [malononitrile, 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile, and 3-ethyl-2-thioxothiazolidin-4-one], respectively, and a central squaraine unit as the core. This study examined the photochemical properties in different solvents regarding the solubility and potential applications. The geometries (cis- and trans-isomers) and biradical character of the dyes were analyzed by density functional theory (DFT). The calculated electrochemical and photophysical properties were compared with the experimental data. The reasons behind the red-shifted absorption for SQ-DICN-RH were investigated using theoretical simulations.

Results and Discussion

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The core and wings concept is useful for symmetrical systems when compared with another multichromophoric system for absorption of sunlight in the visible to NIR regions of the solar spectrum. Using the core and wings concept, we can attach different kinds of end groups (wings) to know the electronic properties of respective dyes by keeping the core as constant. Bazan et al. proposed a core and an end-capping group concept in SM BHJ solar cells as donors, and they reported the DTS(PTTh₂)₂ structure based on a core donor–acceptor framework with end-capping units with a PCE of 6.7%. (32) We recently reported four SMs based on the core and wings concept and dithienosilole (DTS) used as a core for four molecules at the center of the molecule as an electron donor (Figure 2). (33)

Figure 2. Molecular structures of SMs based on the core and wings concept.

Squaraine dyes having a multichromophoric system, (34) explored in OPVs, DSSCs, and OFET; however, the core and end-capped (wing) concept are untouched. In this article, we report that three electron-acceptor units [malononitrile or 3-ethyl rhodanine or 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile] are connected in conjugation with the central squaraine core moiety, which as a result enhanced the absorption in the solar spectrum and electron affinities. Complete representation of core and wings is shown in Figure 3.

Figure 3. Squaraine structure having the core and wings design.

Absorption and Photoluminescence Spectroscopy

The optical absorption spectra of SQ dyes were collected in dichloromethane (DCM) and are shown in Figure 4; the related data are summarized in Table 1. SQ-DIEt-RH, SQ-DICN, and SQ-DICN-RH dyes show a prominent absorption maximum at 710, 716, and 720 nm with vibration progression at 650, 656, and 661 nm, respectively. The absorption at longer wavelengths was attributed to a charge transfer π–π* transition. The new less-intense broad absorption band in the region from 365 to 450 nm was assigned to the prolonged conjugation occurring from the dicyanovinyl unit. SQ-DCN-RH, showing a greater λ_max with respect to the other two SQ dyes, was attributed to the presence of extended conjugation and the dicyanovinyl unit. The emission spectra of the three SQ dyes were measured in a DCM solution and are displayed in Figure 4. The fluorescence spectrum of SQ-DIEt-RH, SQ-DICN, and SQ-DICN-RH dyes showed the emission maximum at 738, 745, and 757 nm with a Stokes shift of 27, 29, and 28 nm, respectively, which is in contrast to the absorption maxima trend. (35) These smaller Stokes shifts arise because of the conformational rigidity, that is, the presence of an analogous configuration of the dye molecules in the ground state as well as in the excited states.

Figure 4. Normalized absorption (solid lines) and photoluminescence (dotted lines) spectra of the SQ dyes in a DCM solution.

Table 1. Absorption and Photoluminescence Data of SQ Dyes in a DCM Solution at Ambient Temperature

sample code	λ_abs (nm)^a	λ_em (nm)^b	ε (dm³mol^–1 cm^–1) × 10⁵	Stokes shift (nm)^c
SQ-DIEt-RH	710	738	1.18	28
SQ-DICN	716	745	1.20	29
SQ-DICN-RH	720	757	1.33	27
SQ-2Br	643	653	1.17	10

^{^a}

Optical absorption maxima.

^{^b}

Emission maxima of SQ dyes in the DCM solution.

^{^c}

Difference of absorption and emission maximum.

Electrochemical Properties

The redox propensity of the dyes was analyzed by cyclic voltammetry (CV) and differential pulse voltammetry in DCM solution shown in Figure 5. Table 2 lists the relevant data of the dyes. All dyes showed two quasi-reversible oxidation couples attributed to simultaneous electron elimination from indolium salts. For the first anodic peak, SQ-DCN showed difficult oxidation because of the strong electron-withdrawing nature of the dicyanovinyl unit on both sides of the squaraine building block. The order of the oxidation pattern is as follows: SQ-DIEt-RH = SQ-DCN-RH (0.53) < SQ-DCN (0.57). On the other hand, the second oxidation was similar for all dyes due to the same electron richness of the cation dyes. Therefore, the band gap of SQ-DCN is lower than that of the other dyes. The HOMO of the dyes was calculated from the oxidation of SQ dyes with respect to the ferrocene/ferrocenium cation. The HOMO and LUMO of SQ-DCN were high compared to the other dyes and it can act as a better donor to construct the organic solar cell devices by blending the PCBM electron acceptor. The remaining rhodanine and dicyanorhodanine dyes showed similar HOMO and LUMO values.

Figure 5. Cyclic voltammograms of the squaraine dyes [1 mM solution of SQ dyes in DCM using 0.1 M of the supporting electrolyte (Bu)₄NPF₆, reference electrode (Ag/AgCl), and internal standard ([Fc]⁺/[Fc]⁰) with a scan rate of 100 mV s^–1].

Table 2. Electrochemical Data of SQ Dyes in DCM under an Inert Condition at RT^a

sample code	E_ox¹	E_ox²	E_ox^*	E_0–0 (eV)	HOMO (eV)	LUMO (eV)	ΔG_inj (V)
SQ-DCN-Et-RH	0.532	0.94	1.15	1.68	–4.89	–3.72	0.66
SQ-DCN	0.576	0.96	1.12	1.69	–4.94	–3.82	0.62
SQ-DCN-RH	0.531	0.95	1.16	1.69	–4.89	–3.72	0.65
SQ-2Br	0.651	1.11	1.24	1.90	–4.99	–3.09

^{^a}

E_ox^* (excited-state oxidation potential) of the SQ dyes was evaluated by the ground-state potential of the SQ dye in the Rehm–Weller equation (E_ox^* = E_ox – E_0–0). E_ox was calculated from CV; it is the ground-state oxidation potential. The thermodynamic driving force of the electron injection is as follows: (ΔG_inj) = E_ox^* – E_CB.

Solvatochromism

To examine excited-state properties of SQ dyes, the absorption and the emission profile of all the SQ dyes were scrutinized with different solvents, that is, toluene (TOL), chloroform (CHL), DCM, tetrahydrofuran (THF), ethyl acetate (EA), and dimethylformamide (DMF) as shown in Figure 6. Generally, all the absorption profiles of the SQ dyes are insensitive to the solvent polarity, which indicates the presence of the less polar ground state. The absorption spectra of SQ-DICN-RH, SQ-DICN, and SQ-DIEt-RH in different solvents did not show a significant shift in the λ_max value, that is, 10, 4, and 13 nm, respectively, in the ground state. On the other hand, in emission spectra, SQ-DICN-RH showed an increase in wavelength from 738 nm (nonpolar solvent) to 761 nm (polar solvent); SQ-DICN emission spectra showed an increase in wavelength from 744 to 768 nm, and similarly, the SQ-DIEt-RH emission spectra ranged from 736 to 760 nm. In SQ-DICN-RH, 740 nm emissions were observed in DCM and were red-shifted 20 nm in DMF because of the increase in polarity.

Figure 6. Absorption (a–c) and emission (d–f) spectra of the SQ dyes in different solvents.

In contrast, the dye SQ-DICN in DCM showed an emission maximum at 745 nm, whereas it was 760 nm in DMF. SQ-DIET-RH in EA showed an emission at 730 nm, showing a 30 nm shift compared to that in DMF. Therefore, the emission spectra showed a reasonable positive response toward the solvatochromism with increasing solvent polarity. This suggests that the excited state of all the SQ dyes is more polar than the ground state because they may have a planar type structure in the excited state. In general, the difference in the Stokes shift between the polar and nonpolar solvents is due to charge transfer from the organic luminogens in the excited state. (36)

Spectroelectrochemical Properties

The spectroelectrochemistry study was carried out to better understand the spectroscopic and electrochemical behavior of the dyes. This study focused mainly on the UV–visible region because electron transfer in the electrochemical system occurs in the same region because of the high molar absorption coefficient of the dyes. CV of all three dyes was performed in DCM and showed two quasi-reversible oxidation waves in the range from 0.53 to 0.57 and 0.94 to 0.96 V in the anodic region versus calomel electrode. Upon applying the controlled oxidation potential to all the SQ dyes, the intense absorption peak in the visible region decreased gradually with the formation of isosbestic points nearly at 582 and 757 nm, which indicates the oxidation outcome in a single product. This might be due to the squaraine cation radical formation. The potential of 0.94–0.96 V was also applied for all the dyes, but no significant change in the absorption spectra was observed. SQ-DICN-RH shows a fast gradual decrease because of the presence of an electron acceptor and extended conjugation. When applied oxidation potential was removed, the absorption spectra returned almost to its original absorption. Figure 7 presents the UV–visible spectroelectrochemical changes with the SQ-DICN-RH, SQ-DICN, and SQ-DIEt-RH dyes recorded in DCM at the respective applied potentials.

Figure 7. Absorption spectroelectrochemical changes of (a) **SQ-DICN-RH**, (b) **SQ-DICN**, and (c) **SQ-DIEt-RH** dyes recorded in DCM at the respective applied potentials.

Theoretical Calculations

To gain deeper insights into the structural (geometrical isomerism), electrochemical, and optical behavior (bathochromic shift for SQ-DICN-RH), a detailed theoretical investigation was carried out using first-principles ab initio simulations. The cis- and trans-isomers of the synthesized SQ dyes were modeled; Figure 8 shows the optimized geometries and the deviations in the dihedral angles of their wings. The corresponding geometries of the cis-isomers are given in Figure S18 in the Supporting Information. The trans-isomer of the SQ dye is relatively more stable than the corresponding cis-isomer and the relative energies are given in Figure S18. The trans-isomers of SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH were 0.95, 0.93, and 0.96 kcal/mol more stable, respectively, than the corresponding cis-isomers. The low stability of the cis-isomers can be attributed to the steric hindrance caused by the one-sided alignment of the wings with respect to the squaraine moiety. The ground-state geometries of the cis-isomers possess higher dipole moments than the corresponding trans-isomers. Both the cis- and trans-isomers were planar and differed only in the orientation of the wings attached to the squaraine moiety.

Figure 8. Ground-state optimized structures of the SQ dyes (*trans*-isomers) and the deviations from the planarity of their wings. Hydrogen atoms were omitted for clarity.

The frontier molecular orbitals (FMOs) of each dye were calculated at the B3LYP/6-31G(d) level of theory. The electron cloud distribution in the FMOs of the dyes is shown in Figure 9, whereas Figure S19 in the Supporting Information shows the corresponding electron density distribution of cis-isomers. For both the cis- and trans-isomers, the electron density was localized mainly over the squaraine core in the HOMO and was dispersed toward the attached wings in the LUMO. The calculated HOMOs and LUMOs of the dyes were well matched with those obtained from the CV measurements. The theoretical HOMO eigenvalues of the dyes were −5.92, −5.82, and −5.86 eV, respectively, for SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH. The LUMO energies were calculated by adding the time-dependent DFT (TDDFT) transition energy (E_S₀→S₁) to the corresponding HOMO energy, rather than considering the unreliable Kohn–Sham LUMO eigenvalue. (37) The LUMO energies for SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH obtained from this method were −4.26, −4.12, and −4.21 eV, respectively. The calculated LUMO eigenvalues were in good agreement with those obtained experimentally.

Figure 9. Isodensity plots (isosurface = 0.02 e Å^–3) of the FMOs of **SQ-DICN** (top), **SQ-DIEt-RH** (middle), and **SQ-DICN-RH** (bottom).

The optical properties of the dyes were evaluated using the TDDFT formalism; Figure 10 shows the simulated UV–visible absorption spectra. The simulated absorption spectra reproduced the main bands observed in the experimental spectra. The calculated absorption maxima (λ_max) for SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH in the low energy region were 748, 728, and 752 nm, respectively. These intense transitions were attributed mainly to HOMO → LUMO excitation. The other two absorption peaks with less intensity were observed at ca. 550 nm and ca. 450 nm. The red-shifted absorption (NIR absorption) of the squaraine dyes can be attributed to their biradicaloid character (BRC) rather than the charge transfer. (38,39) The method suggested by Nakano et al. based on orbital occupation numbers was used to estimate the BRC percentage of the three dyes. (40)

where S_i

is the orbital overlap.

Figure 10. Simulated UV–visible absorption spectra of the SQ dyes obtained at the TD-B3LYP/6-31G(d) level of theory in a CHL solution.

The calculated BRC percentage of the dyes was 12.9 and 13.3% for SQ-DICN and SQ-DICN-RH, respectively, whereas the SQ-DIEt-RH dye did not show any BRC. The red-shifted absorption (λ_cal = 752 nm) of SQ-DICN-RH can be attributed to the high BRC (13.3%) among the three dyes. Although the calculated BRC of the dyes was low, the order of the BRC is in accordance with that of the absorption maxima of the dyes. As per the earlier reports, the BRC is underestimated by DFT functionals; (41,42) hence, we used the UHF/6-31G(d) level of theory to evaluate the BRC of the dyes. The adiabatic singlet–triplet energy gap (ΔE_S–T) was taken as the difference in the energies of the lowest singlet ground state (S₀) and the corresponding lowest triplet state (T₁). The S₀ geometries were considered to be the initial geometries to optimize T₁ at the UB3LYP/6-31g(d) level of theory. The calculated ΔE_S–T of the dyes was 21.51, 21.68, and 21.41 kcal/mol for SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH, respectively. The relatively high ΔE_S–T values exhibited by the dyes are responsible for their lower BRC (Table 3).

Table 3. Simulated Electrochemical and Optical Properties of the Dyes Collected at the B3LYP/6-31G(d) Level of Theory

dye	HOMO (eV)	E_S₀→S₁^a (eV)	LUMO^b (eV)	λ_cal (nm)	f^c	μ_ge^d (D)	predominant contribution	BRC (%)	ΔE_S–T^e
SQ-DICN	–5.92	1.66	–4.26	748	2.0980	7.19	H → L (99%)	12.9	21.51
				551	0.8164	3.85	H → L + 2 (93%)
				423	0.8621	3.46	H – 3 → L (54%)
SQ-DIEt-RH	–5.82	1.70	–4.12	728	2.4760	7.70	H → L (98%)	0	21.68
				562	0.7298	3.67	H → L + 2 (92%)
				455	0.8288	3.52	H – 1 → L + 1 (85%)
SQ-DICN-RH	–5.86	1.65	–4.21	752	2.3913	7.69	H → L (98%)	13.3	21.41
				568	0.9545	4.23	H → L + 2 (91%)
				462	0.9620	3.83	H – 1 → L + 1 (77%)

^{^a}

TDDFT transition energy (S₀ → S₁).

^{^b}

LUMO = HOMO + E_0–0.

^{^c}

Oscillator strength.

^{^d}

Ground- to excited-state transition dipole moment.

^{^e}

ΔE_S–T in kcal/mol.

Conclusions

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Three new squaraine dyes were synthesized and characterized systematically. The core and wings concept was adopted to design these dyes, which is the most successful design for developing high-performing photonic materials. The absorption and emission spectra of these squaraine dyes were recorded in DCM (10^–5 M). Among these three dyes, SQ-DICN-RH showed a greater λ_max with respect to the other two SQ dyes because of the presence of extended conjugation and a dicyanovinyl unit. The absorption spectra of these dyes were not sensitive toward the solvent polarity, which indicates the less polar ground state but the emission spectra showed a reasonable positive response toward the solvatochromism with increasing solvent polarity. This suggests that the excited state of all the SQ dyes is more polar than the ground state. The results agreed well with the computational study. This study paves the way to the design and synthesis of other derivatives for high-performing optoelectronic materials.

Experimental Section

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General Methods and Materials

All precursor materials, 5-formyl-2-thienylboronic acid, malononitrile, 3-ethyl rhodamine, and squaric acid, were purchased directly from commercial sources and used as received. The ¹H and ¹³C NMR spectra were measured using AVANCE ACP-300, AVANCE-400, and AMX2-500 spectrometers at 300, 400, and 500 MHz, respectively, using tetramethylsilane (TMS) as an internal standard. High resolution mass spectrometry (HRMS) and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) were performed on a Shimadzu LCMS-2010 EV model with an ESI probe. CV was performed on CH Instruments with a three-electrode system consisting of a glassy carbon working electrode, an Ag/AgCl reference electrode, and a platinum wire counter electrode. A UV–visible spectrophotometer was used to record the absorption spectra of the SQ dyes in DCM solution. The redox potentials of the dyes were measured in DCM containing 0.1 M Bu₄NClO₄ at a scan rate of 100 mV s^–1.

Synthesis

Scheme 1 presents the synthetic pathway used to prepare the dyes, SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH. 5-Bromo-2,3,3-trimethyl-3H-indole and (43) 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile (44) were synthesized using the methodologies reported in the literature. (43,44) The designed intermediate symmetrical squaraine dye, SQ-2CHO, was synthesized via Suzuki cross-coupling between SQ-2Br and 5-formyl-2-thienylboronic acid. A condensation reaction between SQ-2CHO and the end groups, malononitrile, 3-ethyl rhodamine, and 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile, in CHL with base afforded the target molecules. All compounds were characterized thoroughly by nuclear magnetic resonance (NMR, ¹H and ¹³C) spectroscopy, Fourier transform infrared spectroscopy, MALDI, and HRMS.

Scheme 1. Synthetic Procedure of the Novel SQ Dyes

5-Bromo-1-butyl-2,3,3-trimethyl-3H-indol-1-ium Iodide

5-Bromo-2,3,3-trimethyl-3H-indole (2 g, 8.4 mmol), iodobutane (3.09 g, 16.8 mmol), and acetonitrile (15 mL) were taken in a round bottom flask. The reaction mixture was heated under reflux up to 12 h. The solvent was removed using vacuum and washed with diethyl ether and n-pentane several times to obtain compound (2) as a brown color solid. Yield: 1.8 g (76%). mp 162–164 °C. ¹H NMR (400 MHz, CDCl₃, TMS): δ (ppm) 7.70–7.74 (m, 2H), 7.61 (d, J = 8 Hz, 1H), 4.67 (t, J = 8 Hz, 2H), 3.08 (s, 3H), 1.88–1.96 (m, 2H), 1.68 (s, 6H), 1.47–1.55 (m, 2H), 1.02 (t, J = 8 Hz, 3H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 195.5, 143.3, 139.8, 132.8, 126.8, 124.7, 117.1, 54.8, 50.3, 29.9, 23.2, 20.2, 17.0, 13.7 ppm.

(Z)-4-((5-Bromo-1-butyl-2,3,3-trimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(5-bromo-1-butyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-2Br)

Squaric acid (0.25 g, 2.19 mmol) and 5-bromo-1-butyl-2,3,3-trimethyl-3H-indol-1-ium iodide (1.29 g, 4.38 mmol) were dissolved in a mixture of 10 mL TOL/n-butanol (1:1 v/v, 25 mL) in a two-necked round bottom flask, which was followed by the addition of one drop of pyridine. The reaction mixture was refluxed for 12 h using a Dean–Stark apparatus. After cooling the reaction mixture, the solvents were removed using vacuum. To the crude product, DCM was added and washed with water, dried over Na₂SO₄, and purified by column chromatography on silica gel using DCM/methanol (97:3, v/v) as the eluent to obtain SQ-2Br. mp 258–260 °C, blue solid. Yield: 1.12 g (77%). IR (KBr) ν̃: 3446, 2926, 1610 (ν_C═O) cm^–1. ¹H NMR (400 MHz, CDCl₃, TMS): δ (ppm) 7.44–7.45 (m, 2H), 7.424–7.429 (m, 1H), 7.404–7.408 (m, 1H), 6.85 (s, 1H), 6.83 (s, 1H), 5.95 (s, 2H), 3.95 (br s, 4H), 1.78 (s, 16H), 1.42–1.47 (m, 4H), 0.98 (t, J = 8 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 182.0, 180.2, 169.3, 144.1, 141.4, 130.5, 125.6, 116.4, 110.6, 87.0, 49.2, 43.5, 28.9, 26.9, 20.2, 13.7.

(Z)-4-((1-Butyl-5-(5-formylthiphen-2-yl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(1-butyl-5-(5-formylthiphen-2-yl)-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-2CHO)

SQ-2Br (0.1 g, 0.15 mmol) and 5-formyl-2-thienylboronic acid (0.07 g, 0.4 mmol) were dissolved in a round bottom flask containing 10 mL of dry THF in a nitrogen atmosphere. Subsequently, 1 mL of 2 M Na₂CO₃ solution and Pd(PPh₃)₄ (0.05 equiv) were added. The reaction mixture was degassed with nitrogen for approximately 15 min and refluxed for 18 h under nitrogen conditions. After cooling the reaction mixture, the solvent was removed using vacuum. DCM was then added and the mixture was washed twice with water, dried over Na₂SO₄, and purified by column chromatography using DCM/methanol (95:5, v/v) to obtain SQ-2CHO. mp 242–244 °C, green solid. Yield: 70 mg (64%). IR (KBr) ν̃: 3421, 2925, 1658 (ν_C═O), 1597 (ν_C═O) cm^–1. ¹H NMR (500 MHz, CDCl₃, TMS): δ (ppm) 9.88 (s, 2H), 7.74 (d, J = 5 Hz, 2H), 7.44–7.48 (m, 4H), 7.40 (d, J = 5 Hz, 2H), 7.03 (d, J = 10 Hz, 2H), 6.04 (s, 2H), 4.02 (br s, 4H), 1.84 (s, 12H), 1.45–1.53 (m, 4H), 1.24–1.34 (m, 4H), 1.01 (t, J = 10 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 182.5, 180.3, 169.6, 153.8, 143.4, 141.9, 137.5, 132.0, 131.9, 128.8, 128.5, 128.3, 126.5, 123.6, 120.2, 109.9, 87.7, 76.9, 49.1, 43.7, 29.1, 27.1, 20.3, 13.8.

General Synthetic Procedure for SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH

In a two-necked round bottom flask, the corresponding SQ-2CHO and malononitrile or 3-ethyl rhodanine or 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile were placed in 6 ml of dry CHCl₃. To this, one drop of piperidine was added and the resulting mixture was refluxed. The reaction mixture was observed by TLC. Upon completion of the reaction, it was allowed to cool to room temperature followed by work-up with water, drying over Na₂SO₄, and concentration under vacuum. The final product was separated by the column chromatography technique and DCM/methanol (99:1, v/v) as the eluent to obtain the pure compounds.

(Z)-4-((1-Butyl-5-(5-(2,2-dicyanovinyl)thiophen-2-yl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(1-butyl-5-(5-(2,2-dicyanovinyl)thiophen-2-yl))-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-DICN)

SQ-2CHO (50 mg, 0.06 mmol) and malononitrile (13 mg, 0.20 mmol) were used to obtain SQ-DICN. Yield: 28 mg, (50%) was obtained as a solid. mp 277–279 °C. IR (KBr) ν̃: 3448, 2926, 2217 (ν_C≡N), 1593 (ν_C═O) cm^–1. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 7.79 (s, 2H), 7.71 (d, J = 5 Hz, 2H), 7.67 (dd, J₁ = 5 Hz, J₂ = 1.15 Hz, 2H), 7.61 (d, J = 5 Hz, 2H), 7.44 (d, J = 5 Hz, 2H), 7.04 (d, J = 10 Hz, 2H), 6.08 (s, 2H), 4.03 (br s, 4H), 1.84 (s, 12H), 1.81–1.84 (m, 4H), 1.61 (s, moisture), 1.44–1.52 (m, 4H), 1.01 (t, J = 10 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 159.5, 159.0, 158.6, 158.2, 150.5, 143.9, 140.3, 127.3, 116.0, 113.1, 111.1, 110.3, 49.7, 44.2, 33.2, 29.6, 29.3, 26.7, 26.6, 20.1, 13.7. ESI m/z calcd for [M]⁺ (C₅₀H₄₄N₆O₂S₂), 825.05; found, 825.

(Z)-4-((1-Butyl-5-(5-((Z)-(3-ethyl-4-oxo-2-thioxothiazolidin-5-ylidene)methyl)thiophen-2-yl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(1-butyl-5-(5-(-((Z)-(3-ethyl-4-oxo-2-thioxothiazolidin-5-ylidene)methyl)thiophen-2-yl)-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-DIEt-RH)

SQ-2CHO (50 mg, 0.06 mmol) and 3-ethyl rhodanine (22 mg, 0.13 mmol) were used to produce SQ-DIEt-RH. Yield: 28 mg, (40%) was obtained as solid. mp 280–282 °C. IR (KBr) ν̃: 3418, 2925, 1699 (ν_C═O), 1581 (ν_C═O) cm^–1. ¹H NMR (500 MHz, CDCl₃): δ (ppm) 7.87 (s, 2H), 7.61–7.63 (m, 2H), 7.58–7.59 (m, 2H), 7.38–7.41 (m, 4H), 7.02 (d, J = 8 Hz, 2H), 6.05 (s, 2H), 4.21 (q, 4H), 4.03 (br s, 4H), 1.79–1.84 (m, 14H), 1.60 (s, moisture), 1.46–1.53 (m, 6H), 1.30 (t, J = 8 Hz, 6H), 1.01 (t, J = 8 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 159.6, 159.2, 158.8, 158.4, 116.0, 113.1, 40.0, 38.8, 33.2, 29.6, 29.5, 30.1, 26.7, 22.6, 20.1, 14.1, 13.7, 12.2. MALDI-TOF (ESI, m/z) calcd for [M]⁺ (C₅₄H₅₄N₄O₄S₆), 1015.42; found, 1018.55.

(Z)-4-((1-Butyl-5-(5-((Z)-(2-(dicyanomethylene)-3-hexyl-4-oxothiazolidin-5-ylidene)methyl)thiophen-2-yl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(1-butyl-5-(5-(-((Z)-(2-(dicyanomethylene)-3-hexyl-4-oxothiazolidin-5-ylidene)methyl)thiophen-2-yl)-)-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-DICN-RH)

SQ-2CHO (50 mg, 0.06 mmol) and 2-(3-hexyl-4-oxothiazolidin-2-ylidene)malononitrile (34 mg, 0.13 mmol) were used to synthesize SQ-DICN-RH. Yield: 15 mg, (18%) was obtained as solid. mp 282–284 °C. IR (KBr) ν̃: 2926, 2213 (ν_C≡N), 1719 (ν_C═O), 1611 (ν_C═O), cm^–1. ¹H NMR (400 MHz, CDCl₃): δ (ppm)8.07 (s, 2H), 7.67 (d, J = 4 Hz, 1H), 7.65 (d, J = 4 Hz, 1H), 7.61 (m, 2H), 7.48 (d, J = 4 Hz, 2H), 7.43 (d, J = 4 Hz, 2H), 7.05 (s, 1H), 7.03 (s, 1H), 6.05 (s, 2H), 4.22 (t, J = 8 Hz, 4H), 4.04 (br s, 4H), 1.86 (s, 12H), 1.71–1.83 (m, 8H), 1.6 (s, moisture), 1.40–1.52 (m, 9H), 1.33–1.36 (m, 7H), 1.01 (t, J = 8 Hz, 6H), 0.95 (t, J = 8 Hz, 6H). ¹³C NMR (100 MHz, CDCl₃): δ (ppm) 169.5, 166.0, 165.5, 153.3, 143.6, 136.8, 135.5, 128.8, 128.4, 126.7, 124.6, 119.9, 113.3, 112.2, 110.0, 88.0, 55.7, 49.2, 45.3, 43.7, 31.2, 29.1, 28.7, 27.2, 25.6, 22.4, 20.3, 13.9, 13.8. MALDI-TOF (ESI, m/z) calcd for [M]⁺ (C₆₈H₇₀N₈O₄S₄), 1191.59; found, 1191.54.

Computational Details

The Gaussian 09 quantum chemical program was used for DFT and TDDFT calculations of the three SQ dyes. (45) The DFT method was used to examine the structural and electrochemical properties, and the TDDFT method was used for the excited-state properties. Each of the three SQ dyes was modeled with two isomers, that is, cis and trans. The geometries were optimized with RB3LYP hybrid exchange–correlation functional in combination with the 6-31G(d) basis set. (46) The method employed in the present study has been successfully used to deal with the other biradical systems such as croconate dyes. (47,48) The optimized geometries of the dyes were subjected to vibrational frequency analysis to ensure the real minima on the potential energy surface. All positive harmonic vibrational frequencies confirmed the real minima. Based on the ground state optimized geometries, TDDFT simulations were performed to evaluate the excitation energies and corresponding oscillator strengths. The TDDFT simulations were also performed at the same level of theory, and the solvent effect was included, as used in the experiment. The experimental CHL solution was mimicked using a polarizable continuum model, as implemented in Gaussian 09. (49,50) The excitation energies and oscillator strengths for the first 25 singlet–singlet excitations were evaluated under the TDDFT framework. The input files were prepared, and the results were visualized using the GaussView graphical interface. (51)

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Structural characterization by ¹H, ¹³C NMR, ESI MS spectra, MALDI-TOF MS spectra, and DFT calculations (PDF)

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Author Information

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Corresponding Authors
- Joonkyung Jang - Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea; http://orcid.org/0000-0001-9028-0605; Email: [email protected]
- Surya Prakash Singh - Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India; Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India; ; http://orcid.org/0000-0001-5670-7329; Email: [email protected]
Authors
- G. Hanumantha Rao - Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India; Academy of Scientific and Innovative Research (AcSIR), CSIR-Human Resource Development Centre, (CSIR-HRDC) Campus, Kamla Nehru Nagar, Ghaziabad, Uttar Pradesh 201002, India
- Prem Jyoti Singh Rana - Polymers and Functional Materials Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500007, India; http://orcid.org/0000-0003-4819-947X
- Ramesh Kumar Chitumalla - Department of Nanoenergy Engineering, Pusan National University, Busan 46241, Republic of Korea; http://orcid.org/0000-0002-9523-7056
Notes
The authors declare no competing financial interest.

Acknowledgments

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G.H.R. wishes to thank the AcSIR for PhD enrolment and CSIR for the Senior Research Fellowship. S.P.S. thanks the DST Indo-Poland project DST/INT/POL/P-26/2016. This work was supported by the Korea Research Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2016H1D3A1936765).

References

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Chen, Guo; Sasabe, Hisahiro; Sasaki, Yusuke; Katagiri, Hiroshi; Wang, Xiao-Feng; Sano, Takeshi; Hong, Ziruo; Yang, Yang; Kido, Junji
Chemistry of Materials (2014), 26 (3), 1356-1364CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Squaraine dyes are considered an important group of photoactive materials in the field of org. photovoltaic devices. The authors purposely tuned the side chains and no. of hydroxyl (OH) groups in squaraine (SQ) dyes, i.e., SQ1-4, to study the effect of structural variations on the material properties as well as the performance of these dyes as donor materials in bulk heterojunction (BHJ) photovoltaic cells. The material structure and properties of these SQs were systematically characterized using various tools. Soln.-processed BHJ photovoltaic cells based on these SQ compds. combined with [6,6]-phenyl-C71-butyric acid Me ester (PC71BM) as an acceptor gave efficient power conversion efficiencies > 4.0% under AM 1.5 G solar illumination at 100 mW/cm2. The authors' observations show that the OH groups deepened the HOMO of the donor and thus enhanced the open-circuit voltage, whereas the linear chain improved the charge transport properties in the BHJ films. Both the side chain and the no. of OH groups play important roles in detg. the aggregation behavior of these SQs in solid-state films: SQ1, which contains four OH groups and branched side chains, exhibits J-aggregation because of the steric hindrance of its side chains; SQ2, which contains four OH groups and linear side chains, exhibits both H-aggregation and J-aggregation; SQ3, which contains two OH groups and linear side chains, exhibits preferential H-aggregation; SQ4, which contains linear side chains without OH groups, exhibits J-aggregation, this is most likely because of its strong intermol. coupling and intermol. H-bonding interactions to form a head-to-tail packing mode, i.e., J-aggregation. The absorption of J-aggregates in BHJ cells contributes to the cells' photoresponse at long wavelengths, and thus results in higher photocurrent. The authors' results demonstrate a clear relation between the mol. structures of SQ dyes and their phys. properties that govern their photovoltaic performance.
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Corredor, C. C.; Huang, Z.-L.; Belfield, K. D. Two-Photon 3D Optical Data Storage via Fluorescence Modulation of an Efficient Fluorene Dye by a Photochromic Diarylethene. Adv. Mater. 2006, 18, 2910– 2914, DOI: 10.1002/adma.200600826
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Two-photon 3D optical data storage via fluorescence modulation of an efficient fluorene dye by a photochromic diarylethene
Corredor, Claudia C.; Huang, Zhen-Li; Belfield, Kevin D.
Advanced Materials (Weinheim, Germany) (2006), 18 (21), 2910-2914CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
A two-photon 3-dimensional optical storage system based on a photochromic diarylethene (1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene) and a highly efficient two-photon absorbing fluorescent dye (2,7-bis[4-(9,9-didecylfluoren-2-yl)vinyl]phenylbenzothiazole) is demonstrated. The figure shows two-photon fluorescent readout of data recorded by single-photon excitation. The system is suitable for recording data in thick storage media and providing a nondestructive readout method.
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Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Materials for Optical Data Storage. Angew. Chem., Int. Ed. 1989, 28, 1445– 1471, DOI: 10.1002/anie.198914453
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Jipson, V. B.; Jones, C. R. Infrared dyes for optical storage. J. Vac. Sci. Technol. 1981, 18, 105– 109, DOI: 10.1116/1.570684
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Infrared dyes for optical storage
Jipson, V. B.; Jones, C. R.
Journal of Vacuum Science and Technology (1981), 18 (1), 105-9CODEN: JVSTAL; ISSN:0022-5355.
There is current interest in developing optical storage materials that can be written with GaAlAs lasers. Dyes which absorb strongly at those wavelengths are potential candidates for this application due to their attractive thermal properties. Through optical and thermal modeling, the properties that are necessary if they are to be writeable at energies of ≤1 nJ were examd. A specific class of IR absorbing dyes, squarylium, is discussed, and preliminary data on optical characteristics, writing energy, and stability are presented.
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Karpenko, I. A.; Klymchenko, A. S.; Gioria, S.; Kreder, R.; Shulov, I.; Villa, P.; Mély, Y.; Hibert, M.; Bonnet, D. Squaraine as a Bright, Stable and Environment-Sensitive Far-Red Label for Receptor-Specific Cellular Imaging. Chem. Commun. 2015, 51, 2960– 2963, DOI: 10.1039/c4cc09113b
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Luo, C.; Zhou, Q.; Jiang, G.; He, L.; Zhang, B.; Wang, X. The synthesis and ¹O₂ photosensitization of halogenated asymmetric aniline-based squaraines. New J. Chem. 2011, 35, 1128– 1132, DOI: 10.1039/c0nj00949k
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Kurhuzenkau, S. A.; Woodward, A. W.; Yao, S.; Belfield, K. D.; Shaydyuk, Y. O.; Sissa, C.; Bondar, M. V.; Painelli, A. Ultrafast Spectroscopy, Superluminescence and Theoretical Modeling of a Two-Photon Absorbing Fluorene Derivative. Phys. Chem. Chem. Phys. 2016, 18, 12839– 12846, DOI: 10.1039/c6cp01393g
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Ultrafast spectroscopy, superluminescence and theoretical modeling of a two-photon absorbing fluorene derivative
Kurhuzenkau, S. A.; Woodward, A. W.; Yao, S.; Belfield, K. D.; Shaydyuk, Y. O.; Sissa, C.; Bondar, M. V.; Painelli, A.
Physical Chemistry Chemical Physics (2016), 18 (18), 12839-12846CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
A comprehensive study of photophys. and photochem. properties of an unsym. fluorene deriv. is presented, including linear absorption, fluorescence excitation anisotropy, photochem. stability, steady-state fluorescence, and fluorescence lifetimes in org. solvents of different polarities. Nonlinear optical properties were investigated using Z-scan measurements of degenerate two-photon absorption and femtosecond pump-probe spectroscopy. The strongly fluorescent compd. exhibited good photostability, positioning it for use in a no. of applications. A dramatic increase in fluorescence intensity along with spectral narrowing was obsd. under femtosecond pumping, demonstrating amplified spontaneous emission. An extensive set of exptl. data is rationalized based on essential state models.
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Zhang, Y.; Kim, B.; Yao, S.; Bondar, M. V.; Belfield, K. D. Controlled Aggregation and Enhanced Two-Photon Absorption of a Water-Soluble Squaraine Dye with a Poly(acrylic acid) Template. Langmuir 2013, 29, 11005– 11012, DOI: 10.1021/la4023129
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Controlled aggregation and enhanced two-photon absorption of a water-soluble squaraine dye with a poly(acrylic acid) template
Zhang Yuanwei; Kim Bosung; Yao Sheng; Bondar Mykhailo V; Belfield Kevin D
Langmuir : the ACS journal of surfaces and colloids (2013), 29 (35), 11005-12 ISSN:.
Controlling the aggregation behavior of organic dyes is important for understanding and exploring supramolecular assembly utilizing the specific characteristics of aggregation. Regulating J-aggregation by electrostatic interactions between anionic polyelectrolytes and cationic dyes has gained growing interest. Here, we report the formation of J-aggregates of a water-soluble cationic squaraine dye, 4-(pyridinium-1-yl)butylbenzothiazolium squaraine (SQ), using poly(acrylic acid) sodium salt (PAA-Na) as a template. Electrostatic interactions between the PAA-Na polyelectrolyte and the cationic SQ dye enhanced J-aggregation; the absorbance of the resulting J-band with the polyelectrolyte template was much sharper than the absorbance of the J-aggregate formed using a high concentration of NaCl. Significantly, removal of the polyelectrolyte PPA-Na template by the introduction of calcium ions, which can form stronger ionic binding with carboxylate groups, dissociated J-aggregates, freeing the SQ molecules back to unaggregated or lower aggregate forms. To demonstrate the reversibility of the J-aggregate formation cycle, an in situ experiment was conducted that showed 60% reversibility of the second cycle. In addition, an enhancement by ca. 23 times per repeat unit of the two-photon absorption (2PA) cross section was observed at 920 nm for the polyelectrolyte template-SQ J-aggregate compared to unaggregated or lower aggregate SQ. These results suggest a prominent role of polyelectrolyte templated SQ J-aggregation in the enhancement of 2PA efficiency and provide a means of modulating supramolecular assembly.
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Sun, C.-L.; Liao, Q.; Li, T.; Li, J.; Jiang, J.-Q.; Xu, Z.-Z.; Wang, X.-D.; Shen, R.; Bai, D.-C.; Wang, Q.; Zhang, S.-X.; Fu, H.-B.; Zhang, H.-L. Rational Design of Small Indolic Squaraine Dyes with Large Two-Photon Absorption Cross Section. Chem. Sci. 2015, 6, 761– 769, DOI: 10.1039/c4sc02165g
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Rational design of small indolic squaraine dyes with large two-photon absorption cross section
Sun, Chun-Lin; Liao, Qing; Li, Ting; Li, Jun; Jiang, Jian-Qiao; Xu, Zhen-Zhen; Wang, Xue-Dong; Shen, Rong; Bai, De-Cheng; Wang, Qiang; Zhang, Sheng-Xiang; Fu, Hong-Bing; Zhang, Hao-Li
Chemical Science (2015), 6 (1), 761-769CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
Small org. dyes with large two-photon absorption (TPA) cross sections (δ) were more desirable in many applications compared with large mols. Herein, we proposed a facile theor. method for the fast screening of small org. mols. as potential TPA dyes. This method is based on a theor. anal. to the natural transition orbitals (NTOs) directly assocd. with the TPA transition. Exptl. results on the small indolic squaraine dyes (ISD) confirmed that their TPA cross sections was strongly correlated to the delocalization degree of the NTOs of the S2 excited states. Aided by this simple and intuitive method, we have successfully designed and synthesized a small indolic squaraine dye (ISD) with a remarkable δ value above 8000 GM at 780 nm. The ISD dye also exhibited a high singlet oxygen generation quantum yield about 0.90. The rationally designed TPA dye was successfully applied in both two-photon excited fluorescence cell imaging and in vivo cerebrovascular blood fluid tracing.
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Zhang, G.; Zhao, J.; Chow, P. C. Y.; Jiang, K.; Zhang, J.; Zhu, Z.; Zhang, J.; Huang, F.; Yan, H. Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells. Chem. Rev. 2018, 118, 3447– 3507, DOI: 10.1021/acs.chemrev.7b00535
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Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells
Zhang, Guangye; Zhao, Jingbo; Chow, Philip C. Y.; Jiang, Kui; Zhang, Jianquan; Zhu, Zonglong; Zhang, Jie; Huang, Fei; Yan, He
Chemical Reviews (Washington, DC, United States) (2018), 118 (7), 3447-3507CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a soln.-processed org. photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene deriv. have been the most commonly used electron donor and electron acceptor, resp. While most advances of the device performance come from the design of new polymer donors, fullerene derivs. have almost been exclusively used as electron acceptors in org. photovoltaics. Recently, nonfullerene acceptor materials, particularly small mols. and oligomers, have emerged as a promising alternative to replace fullerene derivs. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chem., thermal, and photostability. The facile functionalization of these mols. affords excellent tunability to their optoelectronic and electrochem. properties. Within the past five years, there have been over 100 nonfullerene acceptor mols. synthesized, and the power conversion efficiency of nonfullerene org. solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the mol. design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphol., and device physics are discussed.
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Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.; McCulloch, I. Non-Fullerene Electron Acceptors for Use in Organic Solar Cells. Acc. Chem. Res. 2015, 48, 2803– 2812, DOI: 10.1021/acs.accounts.5b00199
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Non-Fullerene Electron Acceptors for Use in Organic Solar Cells
Nielsen, Christian B.; Holliday, Sarah; Chen, Hung-Yang; Cryer, Samuel J.; McCulloch, Iain
Accounts of Chemical Research (2015), 48 (11), 2803-2812CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. The active layer in a soln. processed org. photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing mol. design considerations and the structure-property relationships that are emerging. The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystn., resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coeffs., thus contributing to device photocurrent. The ability to vary and optimize the LUMO (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient arom. mols. can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers with a large intramol. twist, which suppresses both nucleation and crystal growth. The generic design concept of rotationally sym. arom. small mols. with extended π orbital delocalization, including polyarom. hydrocarbons, phthalocyanines, etc., has also provided some excellent small mol. acceptors. In most cases, addnl. electron withdrawing functionality, such as imide or ester groups, can be incorporated to stabilize the LUMO and improve properties. New calamitic acceptors have been developed, where MO hybridization of electron rich and poor segments can be judiciously employed to precisely control energy levels. Conformation and intermol. assocns. can be controlled by peripheral functionalization leading to optimization of crystn. length scales. In particular, the use of rhodanine end groups, coupled electronically through short bridged arom. chains, has been a successful strategy, with promising device efficiencies attributed to high lying LUMO energy levels and subsequently large open circuit voltages.
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Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K.-Y.; Marder, S. R.; Zhan, X. Non-fullerene Acceptors for Organic Solar Cells. Nat. Rev. Mater. 2018, 3, 18003, DOI: 10.1038/natrevmats.2018.3
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Non-fullerene acceptors for organic solar cells
Yan, Cenqi; Barlow, Stephen; Wang, Zhaohui; Yan, He; Jen, Alex K.-Y.; Marder, Seth R.; Zhan, Xiaowei
Nature Reviews Materials (2018), 3 (3), 18003CODEN: NRMADL; ISSN:2058-8437. (Nature Research)
A review. Non-fullerene acceptors (NFAs) are currently a major focus of research in the development of bulk-heterojunction org. solar cells (OSCs). In contrast to the widely used fullerene acceptors (FAs), the optical properties and electronic energy levels of NFAs can be readily tuned. NFA-based OSCs can also achieve greater thermal stability and photochem. stability, as well as longer device lifetimes, than their FA-based counterparts. Historically, the performance of NFA OSCs has lagged behind that of fullerene devices. However, recent developments have led to a rapid increase in power conversion efficiencies for NFA OSCs, with values now exceeding 13%, demonstrating the viability of using NFAs to replace FAs in next-generation high-performance OSCs. This Review discusses the important work that has led to this remarkable progress, focusing on the two most promising NFA classes to date: rylene diimide-based materials and materials based on fused arom. cores with strong electron-accepting end groups. The key structure-property relationships, donor-acceptor matching criteria and aspects of device physics are discussed. Finally, we consider the remaining challenges and promising future directions for the NFA OSCs field.
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Cheng, P.; Li, G.; Zhan, X.; Yang, Y. Next-generation Organic Photovoltaics based on Non-fullerene Acceptors. Nat. Photonics 2018, 12, 131– 142, DOI: 10.1038/s41566-018-0104-9
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Next-generation organic photovoltaics based on non-fullerene acceptors
Cheng, Pei; Li, Gang; Zhan, Xiaowei; Yang, Yang
Nature Photonics (2018), 12 (3), 131-142CODEN: NPAHBY; ISSN:1749-4885. (Nature Research)
Over the past three years, a particularly exciting and active area of research within the field of org. photovoltaics has been the use of non-fullerene acceptors (NFAs). Compared with fullerene acceptors, NFAs possess significant advantages including tunability of bandgaps, energy levels, planarity and crystallinity. To date, NFA solar cells have not only achieved impressive power conversion efficiencies of ∼13-14%, but have also shown excellent stability compared with traditional fullerene acceptor solar cells. This Review highlights recent progress on single-junction and tandem NFA solar cells and research directions to achieve even higher efficiencies of 15-20% using NFA-based org. photovoltaics are also proposed.
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Wang, K.; Firdaus, Y.; Babics, M.; Cruciani, F.; Saleem, Q.; El Labban, A.; Alamoudi, M. A.; Marszalek, T.; Pisula, W.; Laquai, F.; Beaujuge, P. M. π-Bridge-Independent 2-(Benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile-Substituted Nonfullerene Acceptors for Efficient Bulk Heterojunction Solar Cells. Chem. Mater. 2016, 28, 2200– 2208, DOI: 10.1021/acs.chemmater.6b00131
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π-Bridge-Independent 2-(Benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile-Substituted Nonfullerene Acceptors for Efficient Bulk Heterojunction Solar Cells
Wang, Kai; Firdaus, Yuliar; Babics, Maxime; Cruciani, Federico; Saleem, Qasim; El Labban, Abdulrahman; Alamoudi, Maha A.; Marszalek, Tomasz; Pisula, Wojciech; Laquai, Frederic; Beaujuge, Pierre M.
Chemistry of Materials (2016), 28 (7), 2200-2208CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Mol. acceptors are promising alternatives to fullerenes (e.g., PC61/71BM) in the fabrication of high-efficiency bulk-heterojunction (BHJ) solar cells. While soln.-processed polymer-fullerene BHJ devices have recently met the 10% efficiency threshold, mol. acceptors have yet to prove comparably efficient with polymer donors. At this point in time, it is important to forge a better understanding of the design parameters that directly impact small-mol. (SM) acceptor performance in BHJ solar cells. In this report, we show that 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM)-terminated SM acceptors can achieve efficiencies as high as 5.3% in BHJ solar cells with the polymer donor PCE10. Through systematic device optimization and characterization studies, we find that the nonfullerene analogs (FBM, CBM, and CDTBM) all perform comparably well, independent of the mol. structure and electronics of the π-bridge that links the two electron-deficient BM end groups. With estd. electron affinities within range of those of common fullerenes (4.0-4.3 eV), and a wider range of ionization potentials (6.2-5.6 eV), the SM acceptors absorb in the visible spectrum and effectively contribute to the BHJ device photocurrent. BM-substituted SM acceptors are promising alternatives to fullerenes in soln.-processed BHJ solar cells.
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Kim, Y.; Song, C. E.; Moon, S.-J.; Lim, E. Rhodanine dye-based small molecule acceptors for organic photovoltaic cells. Chem. Commun. 2014, 50, 8235– 8238, DOI: 10.1039/c4cc01695e
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Rhodanine dye-based small molecule acceptors for organic photovoltaic cells
Kim, Yujeong; Song, Chang Eun; Moon, Sang-Jin; Lim, Eunhee
Chemical Communications (Cambridge, United Kingdom) (2014), 50 (60), 8235-8238CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
The soln.-processable small mols. based on carbazole or fluorene contg. rhodanine dyes at both ends were synthesized and introduced as acceptors in org. photovoltaic cells. The high energy levels of their lowest unoccupied MOs resulted in a power conversion efficiency of 3.08% and an open circuit voltage of up to 1.03 V.
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Wu, Y.; Bai, H.; Wang, Z.; Cheng, P.; Zhu, S.; Wang, Y.; Ma, W.; Zhan, X. A Planar Electron Acceptor for Efficient Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 3215– 3221, DOI: 10.1039/c5ee02477c
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A planar electron acceptor for efficient polymer solar cells
Wu, Yao; Bai, Huitao; Wang, Zaiyu; Cheng, Pei; Zhu, Siya; Wang, Yifan; Ma, Wei; Zhan, Xiaowei
Energy & Environmental Science (2015), 8 (11), 3215-3221CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
A novel planar acceptor IDT-2BR was designed and synthesized. Polymer solar cells (PSCs) based on P3HT:IDT-2BR blended films gave power conversion efficiencies of up to 5.12%, which are much higher than that of PC61BM-based control devices (3.71%) and the highest values reported for P3HT-based fullerene-free PSCs.
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Bai, H.; Wu, Y.; Wang, Y.; Wu, Y.; Li, R.; Cheng, P.; Zhang, M.; Wang, J.; Ma, W.; Zhan, X. Nonfullerene acceptors based on extended fused rings flanked with benzothiadiazolylmethylenemalononitrile for polymer solar cells. J. Mater. Chem. A 2015, 3, 20758– 20766, DOI: 10.1039/c5ta05901a
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Nonfullerene acceptors based on extended fused rings flanked with benzothiadiazolylmethylenemalononitrile for polymer solar cells
Bai, Huitao; Wu, Yao; Wang, Yifan; Wu, Yang; Li, Rong; Cheng, Pei; Zhang, Mingyu; Wang, Jiayu; Ma, Wei; Zhan, Xiaowei
Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (41), 20758-20766CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
Two novel A-D-A type mols. IDT-2BM and IDTT-2BM with extended fused-ring indacenodithiophene (IDT) or indacenodithienothiophene (IDTT) units as cores and strong electron-withdrawing unit 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM) as the end-capping group were synthesized and studied as electron acceptors in soln.-processed polymer solar cells (PSCs). IDT-2BM and IDTT-2BM exhibited strong and broad absorption from 300 to 800 nm, and appropriate LUMO (-3.8 eV) and HOMO (-5.5 to -5.6 eV) levels matching with the classical polymer donor PBDTTT-C-T. IDT-2BM and IDTT-2BM films exhibited intrinsic electron mobilities of about 3.7 × 10-6 and 1.0 × 10-5 cm2 V-1 s-1, resp. Fullerene-free PSCs employing PBDTTT-C-T as the donor and IDT-2BM or IDTT-2BM as the acceptor afforded power conversion efficiencies of 4.26% and 4.81%, resp.
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Lv, A.; Stolte, M.; Würthner, F. Head-to-Tail Zig-Zag Packing of Dipolar Merocyanine Dyes Affords High-Performance Organic Thin-Film Transistors. Angew. Chem., Int. Ed. 2015, 54, 10512– 10515, DOI: 10.1002/anie.201504190
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Liess, A.; Huang, L.; Arjona-Esteban, A.; Lv, A.; Gsänger, M.; Stepanenko, V.; Stolte, M.; Würthner, F. Organic Thin Film Transistors Based on Highly Dipolar Donor-Acceptor Polymethine Dyes. Adv. Funct. Mater. 2015, 25, 44– 57, DOI: 10.1002/adfm.201402678
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Organic Thin Film Transistors Based on Highly Dipolar Donor-Acceptor Polymethine Dyes
Liess, Andreas; Huang, Lizhen; Arjona-Esteban, Alhama; Lv, Aifeng; Gsaenger, Marcel; Stepanenko, Vladimir; Stolte, Matthias; Wuerthner, Frank
Advanced Functional Materials (2015), 25 (1), 44-57CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
Org. thin film transistors (OTFTs) of twenty dipolar donor-acceptor-substituted polymethine dyes (D-A dyes, dipole moments from 3-15 D) were studied. The employed merocyanine dyes contain a dimethine bridge that is substituted with 1-alkyl-3,3-dimethylindolin-2-ylidene (Fischer base), 3-alkyl-2,3-dihydrobenzothiazol-2-ylidene or 1,3-benzodithiole-2-ylidene, resp., as electron-donating unit and various acceptor heterocycles. Thin films formed by these D-A dyes upon deposition in high vacuum are all composed of antiparallel π-stacked dimers. However, they are either amorphous, discontinuous or highly cryst. due to the interplay between mol.-substrate and dimer-dimer interactions. With the help of single crystal x-ray anal., out-of-plane X-ray studies (XRD), selected area electron diffraction (SAED), and at. force microscopy (AFM), a correlation between the mol. structure, film ordering, and hole charge transport ability can be established. The mobility values are compared to Baessler's disorder charge transport theory and a film growth mechanism is proposed based on DFT calcns. and single crystal structures. With carefully adjusted bulky substituents and high dipolarity an intimate centrosym. packing with a slipped, but tight π-stacking arrangement could be realized. This provides two-dimensional percolation pathways for holes and ultimately results in charge carrier mobilities up to 0.18 cm2 V-1 s-1.
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Heiniger, L.-P.; O’Brien, P. G.; Soheilnia, N.; Yang, Y.; Kherani, N. P.; Grätzel, M.; Ozin, G. A.; Tétreault, N. See-Through Dye-Sensitized Solar Cells: Photonic Reflectors for Tandem and Building Integrated Photovoltaics. Adv. Mater. 2013, 25, 5734– 5741, DOI: 10.1002/adma.201302113
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See through dye sensitized solar cells: Photonic reflectors for tandem and building integrated photovoltaics
Heiniger, Leo-Philipp; O'Brien, Paul G.; Soheilnia, Navid; Yang, Yang; Kherani, Nazir P.; Graetzel, Michael; Ozin, Geoffrey A.; Tetreault, Nicolas
Advanced Materials (Weinheim, Germany) (2013), 25 (40), 5734-5741CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
Dye sensitized solar cells are made with abundant and low purity materials are a promising alternative to the prevailing solid state p-n junction photovoltaic devices because of their low fabrication and environmental cost as well as their spectral tunability. This work relates to a conducting Bragg reflector composed os alternative layers of SiO2 nanoparticles and sputtered tin-doped indium oxide employed as the counter electrode for DSCs. These conducting 1D photonic crystals can be utilized as intermediate reflectors in micromorph Si solar cells.
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Tai, Q.; Yan, F. Emerging Semitransparent Solar Cells: Materials and Device Design. Adv. Mater. 2017, 29, 1700192, DOI: 10.1002/adma.201700192
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Keil, D.; Hartmann, H.; Moschny, T. Synthesis and characterization of 1,3-bis-(2-dialkylamino-5-thienyl)-substituted squaraines—a novel class of intensively coloured panchromatic dyes. Dyes Pigm. 1991, 17, 19– 27, DOI: 10.1016/0143-7208(91)85025-4
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Synthesis and characterization of 1,3-bis(2-dialkylamino-5-thienyl)-substituted squaraines - a novel class of intensively colored panchromatic dyes
Keil, Dietmar; Hartmann, Horst; Moschny, Torsten
Dyes and Pigments (1991), 17 (1), 19-27CODEN: DYPIDX; ISSN:0143-7208.
By refluxing N,N-disubstituted 2-aminothiophenes with squaric acid in BuOH-PhMe, a novel heterocyclic type of deeply colored squaraines (I; NR2 = morpholino, pyrrolidino, piperidino; R1 = H, Ph, 3-methoxyphenyl; R2 = H, Ph, 4-bromophenyl, 2-chlorophenyl) is obtained in generally moderate yields. These sparingly sol. dyes exhibit narrow absorption bands at ∼700 nm in org. solvents and, in a dispersed form, wide absorption bands ranging into the IR region at ∼900 nm.
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Sreejith, S.; Carol, P.; Chithra, P.; Ajayaghosh, A. Squaraine Dyes: A Mine of Molecular Materials. J. Mater. Chem. 2008, 18, 264– 274, DOI: 10.1039/b707734c
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Squaraine dyes: a mine of molecular materials
Sreejith, Sivaramapanicker; Carol, Priya; Chithra, Parayalil; Ajayaghosh, Ayyappanpillai
Journal of Materials Chemistry (2008), 18 (3), 264-274CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
A review. This feature article highlights the recent developments in the field of squaraine chem. Attempts were made to address the relevance of squaraine dyes as a class of functional org. materials useful for electronic and photonic applications. Due to the synthetic access of a variety of squaraine dyes with structural variations and due to the strong absorption and emission properties which respond to the surrounding medium, these dyes were receiving significant attention. Therefore, squaraine dyes were extensively studied in recent years, from both fundamental and technol. viewpoints.
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Mayerhöffer, U.; Deing, K.; Gruß, K.; Braunschweig, H.; Meerholz, K.; Würthner, F. Outstanding Short-Circuit Currents in BHJ Solar Cells Based on NIR-Absorbing Acceptor-Substituted Squaraines. Angew. Chem., Int. Ed. 2009, 48, 8776– 8779, DOI: 10.1002/anie.200903125
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Outstanding short-circuit currents in BHJ solar cells based on NIR-absorbing acceptor-substituted squaraines
Mayerhoffer Ulrich; Deing Kaja; Gruss Katrin; Braunschweig Holger; Meerholz Klaus; Wurthner Frank
Angewandte Chemie (International ed. in English) (2009), 48 (46), 8776-9 ISSN:.
There is no expanded citation for this reference.
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Wang, S.; Hall, L.; Diev, V. V.; Haiges, R.; Wei, G.; Xiao, X.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. N,N-Diarylanilinosquaraines and Their Application to Organic Photovoltaics. Chem. Mater. 2011, 23, 4789– 4798, DOI: 10.1021/cm2020803
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N,N-diarylanilinosquaraines and their application to organic photovoltaics
Wang, Siyi; Hall, Lincoln; Diev, Vyacheslav V.; Haiges, Ralf; Wei, Guodan; Xiao, Xin; Djurovich, Peter I.; Forrest, Stephen R.; Thompson, Mark E.
Chemistry of Materials (2011), 23 (21), 4789-4798CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
We report new derivs. of sym. squaraine dyes with N,N-diarylanilino substituents that have high soly. and high absorptivity (ε = 0.71-4.1 × 105 M-1 cm-1) in the red solar spectral region (λmax = 645-694 nm) making them promising candidates for application in org. photovoltaics (OPVs). Unsym. N,N-diisobutylanilino- and N,N-diphenylanilino(diphenylamino)squaraines have also been prepd. that give blue-shifted absorption spectra (λmax = 529-535 nm) relative to their sym. counterparts. Compared to bis(N,N-diisobutylanilino)squaraine, both sym. and unsym. N,N-diarylanilino squaraines show markedly broader absorption bands in soln. than their N,N-dialkylanilino squaraine counterparts: the full width at half-max. (fwhm) for N,N-diarylanilino squaraines range from 1280-1980 cm-1, while the fwhm value for the N,N-diisobutylanilino squarine is only 630 cm-1. The absorption bands for thin films of N,N-diarylanilino squaraines broaden further to 2500-3300 cm-1. N,N-Diarylanilino squaraines are fluorescent, albeit with lower quantum yields than bis(N,N-diisobutylanilino)squaraine (.vphi.PL = 0.02-0.66 and 0.80, resp.). OPVs were prepd. with soln. processed squaraine layers using the following structure: ITO/squaraine (66-85 Å)/C60 (400 Å)/BCP (100 Å)/Al (1000 Å), BCP = bathocuproine. Devices using thin films of the bis(N,N-diarylanilino)squaraines as donor layers show improved performance relative to OPVs prepd. with bis(N,N-dialkylanilino)squaraines, i.e. bis(N,N-diisobutylanilino)squaraine: open-circuit voltage Voc = 0.59 ± 0.05 V, short-circuit current Jsc = 5.58 ± 0.16 mA/cm2, fill factor FF = 0.56 ± 0.03, and power conversion efficiency η = 1.8 ± 0.2% under 1 sun, AM1.5G simulated illumination, compared with bis(N,N-diphenylanilino)squaraine: Voc = 0.82 ± 0.02 V, Jsc = 6.71 ± 0.10 mA/cm2, FF = 0.59 ± 0.01, and η = 3.2 ± 0.1%. Morphol. studies of thin films suggest that the soly. of bis(N,N-diarylanilino)squaraines plays an important role in controlling the optoelectronic properties of the OPVs.
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Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency. Nat. Mater. 2012, 11, 44– 48, DOI: 10.1038/nmat3160
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Solution-processed small-molecule solar cells with 6.7% efficiency
Sun, Yanming; Welch, Gregory C.; Leong, Wei Lin; Takacs, Christopher J.; Bazan, Guillermo C.; Heeger, Alan J.
Nature Materials (2012), 11 (1), 44-48CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
Org. photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial labs. because of their potential to enable mass prodn. of flexible and cost-effective devices. Most of the attention has been focused on soln.-processed polymer bulk-heterojunction solar cells. A combination of polymer design, morphol. control, structural insight and device engineering has led to power conversion efficiencies reaching the 6-8% range for conjugated polymer/fullerene blends. Soln.-processed small-mol. bulk-heterojunction solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. Here, we report efficient soln.-processed small-mol. bulk-heterojunction solar cells based on a new mol. donor, DTS(PTTh2)2. A record power conversion efficiency of 6.7% under air-mass 1.5G irradn. (100 mW/cm2) is achieved for small-mol. bulk-heterojunction devices from DTS(PTTh2)2:PC70BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25 vol.% of 1,8-diiodooctane) during the film-forming process, which leads to reduced domain sizes in the bulk-heterojunction layer. These results provide important progress for soln.-processed org. photovoltaics and demonstrate that solar cells fabricated from small donor mols. can compete with their polymeric counterparts.
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McDowell, C.; Narayanaswamy, K.; Yadagiri, B.; Gayathri, T.; Seifrid, M.; Datt, R.; Ryno, S. M.; Heifner, M. C.; Gupta, V.; Risko, C.; Singh, S. P.; Bazan, G. C. Impact of Rotamer Diversity on the Self-Assembly of Nearly Isostructural Molecular Semiconductors. J. Mater. Chem. A 2018, 6, 383– 394, DOI: 10.1039/c7ta09972j
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Impact of rotamer diversity on the self-assembly of nearly isostructural molecular semiconductors
McDowell, Caitlin; Narayanaswamy, Kamatham; Yadagiri, Bommaramoni; Gayathri, Thumuganti; Seifrid, Martin; Datt, Ram; Ryno, Sean M.; Heifner, Michael C.; Gupta, Vinay; Risko, Chad; Singh, Surya Prakash; Bazan, Guillermo C.
Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (2), 383-394CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
Conformational diversity due to different orientations of structural subunits has a complex impact on morphol. disorder of org. semiconductors. Here, we isolate the impact of a specific structural change: replacing bithiophene (biTh) units with thieno[3,2-b]thiophene (TT). We compare four mols. with an alternating donor-acceptor structure (D'-A-D-A-D') composed of a central, electron-rich dithienosilole (DTS) unit flanked by pyridyl-[2,1,3]thiadiazole (PT) or fluorinated benzo[c][1,2,5]thiadiazole (FBT) and end-capped with bithiophene biTh or TT groups. We find that using TT instead of biTh results in an increased degree of order within films cast directly from soln. by influencing the self-assembly tendencies of the different mols. Unlike switching the acceptor subunit, such as FBT for PT, the TT for biTh structural change has little impact on the electronic structure of these mol. semiconductors. Instead, these morphol. effects can be understood within the context of the predicted conformational diversity. TT units limit the no. of rotational conformations (rotamers) available within this mol. architecture; low rotamer dispersity facilitates self-assembly into ordered domains. As a practical illustration of this greater drive toward self-assembly, we use the TT-contg. mols. as donors in bulk heterojunction solar cells with PC70BM. Devices with TT-contg. mols. show improved photovoltaic performance compared to their previously characterized biTh analogs (d-DTS(PTTh2)2 and p-DTS(FBTTh2)2) in both as-cast and optimized conditions, with efficiencies up to 6.4% and 8.8% for PT-TT and FBT-TT, resp. The TT subunit and, more broadly, the strategy of limiting conformational diversity can be readily applied toward the design of soln.-processable org. semiconductors with increased as-cast order.
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Rao, G. H.; Rana, P. J. S.; Islam, A.; Singh, S. P. Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via C-H Arylation for See-through Photovoltaic. ACS Appl. Energy Mater. 2018, 1, 4786– 4793, DOI: 10.1021/acsaem.8b00862
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Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via C-H Arylation for See-through Photovoltaic
Rao, G. Hanumantha; Rana, Prem Jyoti Singh; Islam, Ashraful; Singh, Surya Prakash
ACS Applied Energy Materials (2018), 1 (9), 4786-4793CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
Developing org. mols. as light harvesters is the key for transparent solar cells technologies. In this paper, a novel metal-free near-IR (NIR) panchromatic asym. squaraine sensitizers (SQ-SPS) are designed and synthesized with the thieno[3,2-b]thiophene (TT) unit as π-spacer, dicyanovinyl and cyanoacrylic acid unit as acceptor and successfully used this sensitizer for dye-sensitized solar cells (DSSCs) application. The C-H arylation protocol is adapted to achieve the final target mol. which allows to avoid conventional and multistep coupling reactions at low-cost. The time-dependent d. functional theory (TDDFT) gives an overview of transitions involved and is in good agreement with the exptl. UV-vis absorption data of SQ-SPS. The av. lifetime of SQ-SPS is 2.45 ns, which is decent value for squaraine dye. SQ-SPS dye alone shows the power conversion efficiency (PCE) of 5.86%, Voc 0.564 mV, Jsc15.00 mA cm-2, and after cosensitization with N3 dye there is a tremendous increase in PCE to 8.84%, Voc to 0.650 mV, and Jsc to 15.00 mA cm-2. The remarkable increase in efficiency is mainly due to entire spectral coverage of IPCE spectra after cosensitization. The dye casted on TiO2 film shows the transparent nature, which revealed that SQ-SPS dye has potential for transparent solar cell applications.
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Marchena, M. J.; de Miguel, G.; Cohen, B.; Organero, J. A.; Pandey, S.; Hayase, S.; Douhal, A. Real-Time Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells with Iodide and Cobalt Electrolytes. J. Phys. Chem. C 2013, 117, 11906– 11919, DOI: 10.1021/jp401399j
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Real-Time Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells with Iodide and Cobalt Electrolytes
Marchena, Maria Jose; de Miguel, Gustavo; Cohen, Boiko; Organero, Juan Angel; Pandey, Shyam; Hayase, Shuzi; Douhal, Abderrazzak
Journal of Physical Chemistry C (2013), 117 (23), 11906-11919CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
A series of dye-sensitized solar cells (DSSCs) has been prepd. by using indole-based or quinoline-based squaraines (SQs) as the sensitizer and contg. the commonly used I3-/I- redox pair or the lately employed cobalt complexes, [Co(dimethylbipyridine)3]3+/2+, [Co(bipyridine)3]3+/2+, and [Co(phenanthroline)3]3+/2+ redox electrolytes. The photodynamics of the different electron transfer reactions were investigated by means of the femto- to millisecond pump-probe techniques. In the femtosecond transient absorption expts., the electron injection rate consts. and efficiencies, kei and φei, were detd. for each cell. Larger values of kei and φei for the indole-based (SQ 8) compared to the quinoline-based (SQ 12) squaraines were obtained (13.2 × 1010 s-1 and 0.95 × 1010 vs. 6.9 × 1010 s-1 and 0.81 for SQ 8 or SQ 12 with the I3-/I- pair, resp.), despite the similar values of the electron injection driving forces (-ΔG0ei = 0.75 vs. 0.76 eV). This is due to the lower electron d. in the LUMO at the anchoring group (-COOH) in SQ 12 compared to SQ 8. However, the type of electrolyte did not affect the kinetics of the electron injection processes. In the flash photolysis expts., the kinetic parameters of the electron recombination via dye or electrolyte and the cation regeneration were calcd. from the decays of the transient absorption signals of the electrons (1550 nm) or the SQ cation (570 nm). It was found that the electron recombination with the oxidized redox species is faster with the Co-based compared to the I3-/I- electrolytes for both SQs, τrec = 3 vs. ∼0.5-1 ms. This proves that the steric hindrance in these SQs is not sufficient to avoid the approach of the Co3+ species to the surface of the TiO2 nanoparticle. Moreover, the regeneration rate consts. and efficiencies, kreg and φreg, are considerably smaller for the cells with the different Co-based electrolytes compared to those with the I3-/I- pair (i.e., kreg = 30 × 104 vs. 8 × 104 M-1s-1 and φreg = 0.96 vs. 0.75 with the [Co(dmb)3]3+/2+ for SQ 8). This is explained by the lower regeneration driving force, -ΔGreg, in the Co-based electrolytes (0.3-0.1 eV). Thus, the use of Co-based electrolytes in these two SQs is detrimental to the overall efficiency of the cell, since -ΔGreg values below 0.4 eV do not give complete regeneration efficiency. Finally, the measured photocurrent is compared with the calcd. electron injection and regeneration efficiencies, a good correlation between both parameters is found.
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Sung, J.; Kim, P.; Lee, Y. O.; Kim, J. S.; Kim, D. Characterization of Ultrafast Intramolecular Charge Transfer Dynamics in Pyrenyl Derivatives: Systematic Change of the Number of Peripheral N,N-Dimethyaniline Substituents. J. Phys. Chem. Lett. 2011, 2, 818– 823, DOI: 10.1021/jz200042s
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Characterization of ultrafast intramolecular charge transfer dynamics in pyrenyl derivatives: systematic change of the number of peripheral N,N-dimethyaniline substituents
Sung, Jooyoung; Kim, Pyosang; Lee, Yoen Ok; Kim, Jong Seung; Kim, Dongho
Journal of Physical Chemistry Letters (2011), 2 (7), 818-823CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
We have comparatively investigated the charge transfer (CT) dynamics of a series of covalently linked N,N-dimethylaniline (DMA) and pyrenyl derivs. I, II, III, IV and V. Their CT characters have been estd. in the order of I > II ≃ III > IV > V by an increase of Stokes shift with increasing the solvent polarity. The femtosecond time-resolved fluorescence spectra show very early excited-state dynamics, a transition from the initially populated locally excited (LE) state to the CT state. The CT characters of studied mol. systems are strongly influenced by the no. and position of donor (DMA) groups onto the acceptor (pyrene) moiety.
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Zhang, G.; Musgrave, C. B. Comparison of DFT Methods for Molecular Orbital Eigenvalue Calculations. J. Phys. Chem. A 2007, 111, 1554– 1561, DOI: 10.1021/jp061633o
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Comparison of DFT Methods for Molecular Orbital Eigenvalue Calculations
Zhang, Gang; Musgrave, Charles B.
Journal of Physical Chemistry A (2007), 111 (8), 1554-1561CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
We report how closely the Kohn-Sham HOMO and LUMO eigenvalues of 11 d. functional theory (DFT) functionals, resp., correspond to the neg. ionization potentials (-IPs) and electron affinities (EAs) of a test set of mols. We also report how accurately the HOMO-LUMO gaps of these methods predict the lowest excitation energies using both time-independent and time-dependent DFT (TD-DFT). The 11 DFT functionals include the local spin d. approxn. (LSDA), five generalized gradient approxn. (GGA) functionals, three hybrid GGA functionals, one hybrid functional, and one hybrid meta GGA functional. We find that the HOMO eigenvalues predicted by KMLYP, BH and HLYP, B3LYP, PW91, PBE, and BLYP predict the -IPs with av. abs. errors of 0.73, 1.48, 3.10, 4.27, 4.33, and 4.41 eV, resp. The LUMOs of all functionals fail to accurately predict the EAs. Although the GGA functionals inaccurately predict both the HOMO and LUMO eigenvalues, they predict the HOMO-LUMO gap relatively accurately (∼0.73 eV). On the other hand, the LUMO eigenvalues of the hybrid functionals fail to predict the EA to the extent that they include HF exchange, although increasing HF exchange improves the correspondence between the HOMO eigenvalue and -IP so that the HOMO-LUMO gaps are inaccurately predicted by hybrid DFT functionals. We find that TD-DFT with all functionals accurately predicts the HOMO-LUMO gaps. A linear correlation between the calcd. HOMO eigenvalue and the exptl. -IP and calcd. HOMO-LUMO gap and exptl. lowest excitation energy enables us to derive a simple correction formula.
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Prabhakar, C.; Chaitanya, G. K.; Sitha, S.; Bhanuprakash, K.; Rao, V. J. Role of the Oxyallyl Substructure in the Near Infrared (NIR) Absorption in Symmetrical Dye Derivatives: A Computational Study. J. Phys. Chem. A 2005, 109, 2614– 2622, DOI: 10.1021/jp044954d
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Role of the oxyallyl substructure in the near infrared (NIR) absorption in symmetrical dye derivatives: A computational study
Prabhakar, Ch.; Chaitanya, G. Krishna; Sitha, Sanyasi; Bhanuprakash, K.; Rao, V. Jayathirtha
Journal of Physical Chemistry A (2005), 109 (11), 2614-2622CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
It is well-known from exptl. studies that the oxyallyl-substructure-based squarylium and croconium dyes absorb in the NIR region of the spectrum. Recently, another dye has been reported (M. Tian et al., 2003) which contains the same basic chromophore, but the absorption is red-shifted by at least 300 nm compared to former dyes and is obsd. near 1100 nm. To analyze the reasons behind the large red shift, in this work we have carried out symmetry-adapted cluster-CI (SAC-CI) studies on some of these NIR dyes which contain the oxyallyl substructure. From this study, contrary to the earlier reports, it is seen that the donor groups do not seem to play a major role in the red shift of the absorption. On the other hand, on the basis of the results of the high-level calcns. carried out here and using qual. MO theory, it is obsd. that the orbital interactions play a key role in the red shift. Finally, design principles for the oxyallyl-substructure-based NIR dyes are suggested.
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Prabhakar, C.; Yesudas, K.; Krishna Chaitanya, G.; Sitha, S.; Bhanuprakash, K.; Rao, V. J. Near-Infrared Absorption in Symmetric Squarylium and Croconate Dyes: A Comparative Study Using Symmetry-Adapted Cluster-Configuration Interaction Methods. J. Phys. Chem. A 2005, 109, 8604– 8616, DOI: 10.1021/jp0516184
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Near-infrared absorption in symmetric squarylium and croconate dyes: A comparative study using symmetry-adapted cluster-configuration interaction methods
Prabhakar, Ch.; Yesudas, K.; Chaitanya, G. Krishna; Sitha, Sanyasi; Bhanuprakash, K.; Rao, V. Jayathirtha
Journal of Physical Chemistry A (2005), 109 (38), 8604-8616CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
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Isabelle Toubia, Yann Bernhard, Valentin Diez Cabanes, Stéphania Abdallah, Rana Mhanna, Tioga Gulon, Stéphane Parant, Jean-Pierre Malval, Jean-Bernard Regnouf-de-Vains, Antonio Monari, Mariachiara Pastore, Andreea Pasc. Enhancing Photothermal Energy Transduction through Inter- and Intramolecular Interactions of Multiple Two-Photon Dyes Appended onto Calix[4]arene. The Journal of Physical Chemistry B 2024, 128 (41) , 10086-10102. https://doi.org/10.1021/acs.jpcb.4c04820
Alicja Balcerak, Janina Kabatc. Recent progress in the development of highly active dyeing photoinitiators based on 1,3-bis( p -substituted phenylamino)squaraines for radical polymerization of acrylates. Polymer Chemistry 2022, 13 (13) , 1787-1812. https://doi.org/10.1039/D1PY01519B

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Abstract
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Figure 1
Figure 1. Molecular structures of NFAs based on the core and wings concept.
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Figure 2
Figure 2. Molecular structures of SMs based on the core and wings concept.
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Figure 3
Figure 3. Squaraine structure having the core and wings design.
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Figure 4
Figure 4. Normalized absorption (solid lines) and photoluminescence (dotted lines) spectra of the SQ dyes in a DCM solution.
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Figure 5
Figure 5. Cyclic voltammograms of the squaraine dyes [1 mM solution of SQ dyes in DCM using 0.1 M of the supporting electrolyte (Bu)₄NPF₆, reference electrode (Ag/AgCl), and internal standard ([Fc]⁺/[Fc]⁰) with a scan rate of 100 mV s^–1].
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Figure 6
Figure 6. Absorption (a–c) and emission (d–f) spectra of the SQ dyes in different solvents.
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Figure 7
Figure 7. Absorption spectroelectrochemical changes of (a) SQ-DICN-RH, (b) SQ-DICN, and (c) SQ-DIEt-RH dyes recorded in DCM at the respective applied potentials.
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Figure 8
Figure 8. Ground-state optimized structures of the SQ dyes (trans-isomers) and the deviations from the planarity of their wings. Hydrogen atoms were omitted for clarity.
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Figure 9
Figure 9. Isodensity plots (isosurface = 0.02 e Å^–3) of the FMOs of SQ-DICN (top), SQ-DIEt-RH (middle), and SQ-DICN-RH (bottom).
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Figure 10
Figure 10. Simulated UV–visible absorption spectra of the SQ dyes obtained at the TD-B3LYP/6-31G(d) level of theory in a CHL solution.
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Scheme 1
Scheme 1. Synthetic Procedure of the Novel SQ Dyes
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References
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  Chen, Guo; Sasabe, Hisahiro; Sasaki, Yusuke; Katagiri, Hiroshi; Wang, Xiao-Feng; Sano, Takeshi; Hong, Ziruo; Yang, Yang; Kido, Junji
  Chemistry of Materials (2014), 26 (3), 1356-1364CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Squaraine dyes are considered an important group of photoactive materials in the field of org. photovoltaic devices. The authors purposely tuned the side chains and no. of hydroxyl (OH) groups in squaraine (SQ) dyes, i.e., SQ1-4, to study the effect of structural variations on the material properties as well as the performance of these dyes as donor materials in bulk heterojunction (BHJ) photovoltaic cells. The material structure and properties of these SQs were systematically characterized using various tools. Soln.-processed BHJ photovoltaic cells based on these SQ compds. combined with [6,6]-phenyl-C71-butyric acid Me ester (PC71BM) as an acceptor gave efficient power conversion efficiencies > 4.0% under AM 1.5 G solar illumination at 100 mW/cm2. The authors' observations show that the OH groups deepened the HOMO of the donor and thus enhanced the open-circuit voltage, whereas the linear chain improved the charge transport properties in the BHJ films. Both the side chain and the no. of OH groups play important roles in detg. the aggregation behavior of these SQs in solid-state films: SQ1, which contains four OH groups and branched side chains, exhibits J-aggregation because of the steric hindrance of its side chains; SQ2, which contains four OH groups and linear side chains, exhibits both H-aggregation and J-aggregation; SQ3, which contains two OH groups and linear side chains, exhibits preferential H-aggregation; SQ4, which contains linear side chains without OH groups, exhibits J-aggregation, this is most likely because of its strong intermol. coupling and intermol. H-bonding interactions to form a head-to-tail packing mode, i.e., J-aggregation. The absorption of J-aggregates in BHJ cells contributes to the cells' photoresponse at long wavelengths, and thus results in higher photocurrent. The authors' results demonstrate a clear relation between the mol. structures of SQ dyes and their phys. properties that govern their photovoltaic performance.
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8. 8
  Corredor, C. C.; Huang, Z.-L.; Belfield, K. D. Two-Photon 3D Optical Data Storage via Fluorescence Modulation of an Efficient Fluorene Dye by a Photochromic Diarylethene. Adv. Mater. 2006, 18, 2910– 2914, DOI: 10.1002/adma.200600826
  8
  Two-photon 3D optical data storage via fluorescence modulation of an efficient fluorene dye by a photochromic diarylethene
  Corredor, Claudia C.; Huang, Zhen-Li; Belfield, Kevin D.
  Advanced Materials (Weinheim, Germany) (2006), 18 (21), 2910-2914CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A two-photon 3-dimensional optical storage system based on a photochromic diarylethene (1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene) and a highly efficient two-photon absorbing fluorescent dye (2,7-bis[4-(9,9-didecylfluoren-2-yl)vinyl]phenylbenzothiazole) is demonstrated. The figure shows two-photon fluorescent readout of data recorded by single-photon excitation. The system is suitable for recording data in thick storage media and providing a nondestructive readout method.
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9. 9
  Emmelius, M.; Pawlowski, G.; Vollmann, H. W. Materials for Optical Data Storage. Angew. Chem., Int. Ed. 1989, 28, 1445– 1471, DOI: 10.1002/anie.198914453
  There is no corresponding record for this reference.
10. 10
  Jipson, V. B.; Jones, C. R. Infrared dyes for optical storage. J. Vac. Sci. Technol. 1981, 18, 105– 109, DOI: 10.1116/1.570684
  10
  Infrared dyes for optical storage
  Jipson, V. B.; Jones, C. R.
  Journal of Vacuum Science and Technology (1981), 18 (1), 105-9CODEN: JVSTAL; ISSN:0022-5355.
  There is current interest in developing optical storage materials that can be written with GaAlAs lasers. Dyes which absorb strongly at those wavelengths are potential candidates for this application due to their attractive thermal properties. Through optical and thermal modeling, the properties that are necessary if they are to be writeable at energies of ≤1 nJ were examd. A specific class of IR absorbing dyes, squarylium, is discussed, and preliminary data on optical characteristics, writing energy, and stability are presented.
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11. 11
  Karpenko, I. A.; Klymchenko, A. S.; Gioria, S.; Kreder, R.; Shulov, I.; Villa, P.; Mély, Y.; Hibert, M.; Bonnet, D. Squaraine as a Bright, Stable and Environment-Sensitive Far-Red Label for Receptor-Specific Cellular Imaging. Chem. Commun. 2015, 51, 2960– 2963, DOI: 10.1039/c4cc09113b
  There is no corresponding record for this reference.
12. 12
  Luo, C.; Zhou, Q.; Jiang, G.; He, L.; Zhang, B.; Wang, X. The synthesis and ¹O₂ photosensitization of halogenated asymmetric aniline-based squaraines. New J. Chem. 2011, 35, 1128– 1132, DOI: 10.1039/c0nj00949k
  There is no corresponding record for this reference.
13. 13
  Kurhuzenkau, S. A.; Woodward, A. W.; Yao, S.; Belfield, K. D.; Shaydyuk, Y. O.; Sissa, C.; Bondar, M. V.; Painelli, A. Ultrafast Spectroscopy, Superluminescence and Theoretical Modeling of a Two-Photon Absorbing Fluorene Derivative. Phys. Chem. Chem. Phys. 2016, 18, 12839– 12846, DOI: 10.1039/c6cp01393g
  13
  Ultrafast spectroscopy, superluminescence and theoretical modeling of a two-photon absorbing fluorene derivative
  Kurhuzenkau, S. A.; Woodward, A. W.; Yao, S.; Belfield, K. D.; Shaydyuk, Y. O.; Sissa, C.; Bondar, M. V.; Painelli, A.
  Physical Chemistry Chemical Physics (2016), 18 (18), 12839-12846CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
  A comprehensive study of photophys. and photochem. properties of an unsym. fluorene deriv. is presented, including linear absorption, fluorescence excitation anisotropy, photochem. stability, steady-state fluorescence, and fluorescence lifetimes in org. solvents of different polarities. Nonlinear optical properties were investigated using Z-scan measurements of degenerate two-photon absorption and femtosecond pump-probe spectroscopy. The strongly fluorescent compd. exhibited good photostability, positioning it for use in a no. of applications. A dramatic increase in fluorescence intensity along with spectral narrowing was obsd. under femtosecond pumping, demonstrating amplified spontaneous emission. An extensive set of exptl. data is rationalized based on essential state models.
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14. 14
  Zhang, Y.; Kim, B.; Yao, S.; Bondar, M. V.; Belfield, K. D. Controlled Aggregation and Enhanced Two-Photon Absorption of a Water-Soluble Squaraine Dye with a Poly(acrylic acid) Template. Langmuir 2013, 29, 11005– 11012, DOI: 10.1021/la4023129
  14
  Controlled aggregation and enhanced two-photon absorption of a water-soluble squaraine dye with a poly(acrylic acid) template
  Zhang Yuanwei; Kim Bosung; Yao Sheng; Bondar Mykhailo V; Belfield Kevin D
  Langmuir : the ACS journal of surfaces and colloids (2013), 29 (35), 11005-12 ISSN:.
  Controlling the aggregation behavior of organic dyes is important for understanding and exploring supramolecular assembly utilizing the specific characteristics of aggregation. Regulating J-aggregation by electrostatic interactions between anionic polyelectrolytes and cationic dyes has gained growing interest. Here, we report the formation of J-aggregates of a water-soluble cationic squaraine dye, 4-(pyridinium-1-yl)butylbenzothiazolium squaraine (SQ), using poly(acrylic acid) sodium salt (PAA-Na) as a template. Electrostatic interactions between the PAA-Na polyelectrolyte and the cationic SQ dye enhanced J-aggregation; the absorbance of the resulting J-band with the polyelectrolyte template was much sharper than the absorbance of the J-aggregate formed using a high concentration of NaCl. Significantly, removal of the polyelectrolyte PPA-Na template by the introduction of calcium ions, which can form stronger ionic binding with carboxylate groups, dissociated J-aggregates, freeing the SQ molecules back to unaggregated or lower aggregate forms. To demonstrate the reversibility of the J-aggregate formation cycle, an in situ experiment was conducted that showed 60% reversibility of the second cycle. In addition, an enhancement by ca. 23 times per repeat unit of the two-photon absorption (2PA) cross section was observed at 920 nm for the polyelectrolyte template-SQ J-aggregate compared to unaggregated or lower aggregate SQ. These results suggest a prominent role of polyelectrolyte templated SQ J-aggregation in the enhancement of 2PA efficiency and provide a means of modulating supramolecular assembly.
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15. 15
  Sun, C.-L.; Liao, Q.; Li, T.; Li, J.; Jiang, J.-Q.; Xu, Z.-Z.; Wang, X.-D.; Shen, R.; Bai, D.-C.; Wang, Q.; Zhang, S.-X.; Fu, H.-B.; Zhang, H.-L. Rational Design of Small Indolic Squaraine Dyes with Large Two-Photon Absorption Cross Section. Chem. Sci. 2015, 6, 761– 769, DOI: 10.1039/c4sc02165g
  15
  Rational design of small indolic squaraine dyes with large two-photon absorption cross section
  Sun, Chun-Lin; Liao, Qing; Li, Ting; Li, Jun; Jiang, Jian-Qiao; Xu, Zhen-Zhen; Wang, Xue-Dong; Shen, Rong; Bai, De-Cheng; Wang, Qiang; Zhang, Sheng-Xiang; Fu, Hong-Bing; Zhang, Hao-Li
  Chemical Science (2015), 6 (1), 761-769CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  Small org. dyes with large two-photon absorption (TPA) cross sections (δ) were more desirable in many applications compared with large mols. Herein, we proposed a facile theor. method for the fast screening of small org. mols. as potential TPA dyes. This method is based on a theor. anal. to the natural transition orbitals (NTOs) directly assocd. with the TPA transition. Exptl. results on the small indolic squaraine dyes (ISD) confirmed that their TPA cross sections was strongly correlated to the delocalization degree of the NTOs of the S2 excited states. Aided by this simple and intuitive method, we have successfully designed and synthesized a small indolic squaraine dye (ISD) with a remarkable δ value above 8000 GM at 780 nm. The ISD dye also exhibited a high singlet oxygen generation quantum yield about 0.90. The rationally designed TPA dye was successfully applied in both two-photon excited fluorescence cell imaging and in vivo cerebrovascular blood fluid tracing.
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16. 16
  Zhang, G.; Zhao, J.; Chow, P. C. Y.; Jiang, K.; Zhang, J.; Zhu, Z.; Zhang, J.; Huang, F.; Yan, H. Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells. Chem. Rev. 2018, 118, 3447– 3507, DOI: 10.1021/acs.chemrev.7b00535
  16
  Nonfullerene Acceptor Molecules for Bulk Heterojunction Organic Solar Cells
  Zhang, Guangye; Zhao, Jingbo; Chow, Philip C. Y.; Jiang, Kui; Zhang, Jianquan; Zhu, Zonglong; Zhang, Jie; Huang, Fei; Yan, He
  Chemical Reviews (Washington, DC, United States) (2018), 118 (7), 3447-3507CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  The bulk-heterojunction blend of an electron donor and an electron acceptor material is the key component in a soln.-processed org. photovoltaic device. In the past decades, a p-type conjugated polymer and an n-type fullerene deriv. have been the most commonly used electron donor and electron acceptor, resp. While most advances of the device performance come from the design of new polymer donors, fullerene derivs. have almost been exclusively used as electron acceptors in org. photovoltaics. Recently, nonfullerene acceptor materials, particularly small mols. and oligomers, have emerged as a promising alternative to replace fullerene derivs. Compared to fullerenes, these new acceptors are generally synthesized from diversified, low-cost routes based on building block materials with extraordinary chem., thermal, and photostability. The facile functionalization of these mols. affords excellent tunability to their optoelectronic and electrochem. properties. Within the past five years, there have been over 100 nonfullerene acceptor mols. synthesized, and the power conversion efficiency of nonfullerene org. solar cells has increased dramatically, from ∼2% in 2012 to >13% in 2017. This review summarizes this progress, aiming to describe the mol. design strategy, to provide insight into the structure-property relationship, and to highlight the challenges the field is facing, with emphasis placed on most recent nonfullerene acceptors that demonstrated top-of-the-line photovoltaic performances. We also provide perspectives from a device point of view, wherein topics including ternary blend device, multijunction device, device stability, active layer morphol., and device physics are discussed.
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17. 17
  Nielsen, C. B.; Holliday, S.; Chen, H.-Y.; Cryer, S. J.; McCulloch, I. Non-Fullerene Electron Acceptors for Use in Organic Solar Cells. Acc. Chem. Res. 2015, 48, 2803– 2812, DOI: 10.1021/acs.accounts.5b00199
  17
  Non-Fullerene Electron Acceptors for Use in Organic Solar Cells
  Nielsen, Christian B.; Holliday, Sarah; Chen, Hung-Yang; Cryer, Samuel J.; McCulloch, Iain
  Accounts of Chemical Research (2015), 48 (11), 2803-2812CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. The active layer in a soln. processed org. photovoltaic device comprises a light absorbing electron donor semiconductor, typically a polymer, and an electron accepting fullerene acceptor. Although there has been huge effort targeted to optimize the absorbing, energetic, and transport properties of the donor material, fullerenes remain as the exclusive electron acceptor in all high performance devices. Very recently, some new non-fullerene acceptors have been demonstrated to outperform fullerenes in comparative devices. This Account describes this progress, discussing mol. design considerations and the structure-property relationships that are emerging. The motivation to replace fullerene acceptors stems from their synthetic inflexibility, leading to constraints in manipulating frontier energy levels, as well as poor absorption in the solar spectrum range, and an inherent tendency to undergo postfabrication crystn., resulting in device instability. New acceptors have to address these limitations, providing tunable absorption with high extinction coeffs., thus contributing to device photocurrent. The ability to vary and optimize the LUMO (LUMO) energy level for a specific donor polymer is also an important requirement, ensuring minimal energy loss on electron transfer and as high an internal voltage as possible. Initially perylene diimide acceptors were evaluated as promising acceptor materials. These electron deficient arom. mols. can exhibit good electron transport, facilitated by close packed herringbone crystal motifs, and their energy levels can be synthetically tuned. The principal drawback of this class of materials, their tendency to crystallize on too large a length scale for an optimal heterojunction nanostructure, has been shown to be overcome through introduction of conformation twisting through steric effects. This has been primarily achieved by coupling two units together, forming dimers with a large intramol. twist, which suppresses both nucleation and crystal growth. The generic design concept of rotationally sym. arom. small mols. with extended π orbital delocalization, including polyarom. hydrocarbons, phthalocyanines, etc., has also provided some excellent small mol. acceptors. In most cases, addnl. electron withdrawing functionality, such as imide or ester groups, can be incorporated to stabilize the LUMO and improve properties. New calamitic acceptors have been developed, where MO hybridization of electron rich and poor segments can be judiciously employed to precisely control energy levels. Conformation and intermol. assocns. can be controlled by peripheral functionalization leading to optimization of crystn. length scales. In particular, the use of rhodanine end groups, coupled electronically through short bridged arom. chains, has been a successful strategy, with promising device efficiencies attributed to high lying LUMO energy levels and subsequently large open circuit voltages.
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18. 18
  Yan, C.; Barlow, S.; Wang, Z.; Yan, H.; Jen, A. K.-Y.; Marder, S. R.; Zhan, X. Non-fullerene Acceptors for Organic Solar Cells. Nat. Rev. Mater. 2018, 3, 18003, DOI: 10.1038/natrevmats.2018.3
  18
  Non-fullerene acceptors for organic solar cells
  Yan, Cenqi; Barlow, Stephen; Wang, Zhaohui; Yan, He; Jen, Alex K.-Y.; Marder, Seth R.; Zhan, Xiaowei
  Nature Reviews Materials (2018), 3 (3), 18003CODEN: NRMADL; ISSN:2058-8437. (Nature Research)
  A review. Non-fullerene acceptors (NFAs) are currently a major focus of research in the development of bulk-heterojunction org. solar cells (OSCs). In contrast to the widely used fullerene acceptors (FAs), the optical properties and electronic energy levels of NFAs can be readily tuned. NFA-based OSCs can also achieve greater thermal stability and photochem. stability, as well as longer device lifetimes, than their FA-based counterparts. Historically, the performance of NFA OSCs has lagged behind that of fullerene devices. However, recent developments have led to a rapid increase in power conversion efficiencies for NFA OSCs, with values now exceeding 13%, demonstrating the viability of using NFAs to replace FAs in next-generation high-performance OSCs. This Review discusses the important work that has led to this remarkable progress, focusing on the two most promising NFA classes to date: rylene diimide-based materials and materials based on fused arom. cores with strong electron-accepting end groups. The key structure-property relationships, donor-acceptor matching criteria and aspects of device physics are discussed. Finally, we consider the remaining challenges and promising future directions for the NFA OSCs field.
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19. 19
  Cheng, P.; Li, G.; Zhan, X.; Yang, Y. Next-generation Organic Photovoltaics based on Non-fullerene Acceptors. Nat. Photonics 2018, 12, 131– 142, DOI: 10.1038/s41566-018-0104-9
  19
  Next-generation organic photovoltaics based on non-fullerene acceptors
  Cheng, Pei; Li, Gang; Zhan, Xiaowei; Yang, Yang
  Nature Photonics (2018), 12 (3), 131-142CODEN: NPAHBY; ISSN:1749-4885. (Nature Research)
  Over the past three years, a particularly exciting and active area of research within the field of org. photovoltaics has been the use of non-fullerene acceptors (NFAs). Compared with fullerene acceptors, NFAs possess significant advantages including tunability of bandgaps, energy levels, planarity and crystallinity. To date, NFA solar cells have not only achieved impressive power conversion efficiencies of ∼13-14%, but have also shown excellent stability compared with traditional fullerene acceptor solar cells. This Review highlights recent progress on single-junction and tandem NFA solar cells and research directions to achieve even higher efficiencies of 15-20% using NFA-based org. photovoltaics are also proposed.
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20. 20
  Wang, K.; Firdaus, Y.; Babics, M.; Cruciani, F.; Saleem, Q.; El Labban, A.; Alamoudi, M. A.; Marszalek, T.; Pisula, W.; Laquai, F.; Beaujuge, P. M. π-Bridge-Independent 2-(Benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile-Substituted Nonfullerene Acceptors for Efficient Bulk Heterojunction Solar Cells. Chem. Mater. 2016, 28, 2200– 2208, DOI: 10.1021/acs.chemmater.6b00131
  20
  π-Bridge-Independent 2-(Benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile-Substituted Nonfullerene Acceptors for Efficient Bulk Heterojunction Solar Cells
  Wang, Kai; Firdaus, Yuliar; Babics, Maxime; Cruciani, Federico; Saleem, Qasim; El Labban, Abdulrahman; Alamoudi, Maha A.; Marszalek, Tomasz; Pisula, Wojciech; Laquai, Frederic; Beaujuge, Pierre M.
  Chemistry of Materials (2016), 28 (7), 2200-2208CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Mol. acceptors are promising alternatives to fullerenes (e.g., PC61/71BM) in the fabrication of high-efficiency bulk-heterojunction (BHJ) solar cells. While soln.-processed polymer-fullerene BHJ devices have recently met the 10% efficiency threshold, mol. acceptors have yet to prove comparably efficient with polymer donors. At this point in time, it is important to forge a better understanding of the design parameters that directly impact small-mol. (SM) acceptor performance in BHJ solar cells. In this report, we show that 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM)-terminated SM acceptors can achieve efficiencies as high as 5.3% in BHJ solar cells with the polymer donor PCE10. Through systematic device optimization and characterization studies, we find that the nonfullerene analogs (FBM, CBM, and CDTBM) all perform comparably well, independent of the mol. structure and electronics of the π-bridge that links the two electron-deficient BM end groups. With estd. electron affinities within range of those of common fullerenes (4.0-4.3 eV), and a wider range of ionization potentials (6.2-5.6 eV), the SM acceptors absorb in the visible spectrum and effectively contribute to the BHJ device photocurrent. BM-substituted SM acceptors are promising alternatives to fullerenes in soln.-processed BHJ solar cells.
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21. 21
  Kim, Y.; Song, C. E.; Moon, S.-J.; Lim, E. Rhodanine dye-based small molecule acceptors for organic photovoltaic cells. Chem. Commun. 2014, 50, 8235– 8238, DOI: 10.1039/c4cc01695e
  21
  Rhodanine dye-based small molecule acceptors for organic photovoltaic cells
  Kim, Yujeong; Song, Chang Eun; Moon, Sang-Jin; Lim, Eunhee
  Chemical Communications (Cambridge, United Kingdom) (2014), 50 (60), 8235-8238CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  The soln.-processable small mols. based on carbazole or fluorene contg. rhodanine dyes at both ends were synthesized and introduced as acceptors in org. photovoltaic cells. The high energy levels of their lowest unoccupied MOs resulted in a power conversion efficiency of 3.08% and an open circuit voltage of up to 1.03 V.
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22. 22
  Wu, Y.; Bai, H.; Wang, Z.; Cheng, P.; Zhu, S.; Wang, Y.; Ma, W.; Zhan, X. A Planar Electron Acceptor for Efficient Polymer Solar Cells. Energy Environ. Sci. 2015, 8, 3215– 3221, DOI: 10.1039/c5ee02477c
  22
  A planar electron acceptor for efficient polymer solar cells
  Wu, Yao; Bai, Huitao; Wang, Zaiyu; Cheng, Pei; Zhu, Siya; Wang, Yifan; Ma, Wei; Zhan, Xiaowei
  Energy & Environmental Science (2015), 8 (11), 3215-3221CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  A novel planar acceptor IDT-2BR was designed and synthesized. Polymer solar cells (PSCs) based on P3HT:IDT-2BR blended films gave power conversion efficiencies of up to 5.12%, which are much higher than that of PC61BM-based control devices (3.71%) and the highest values reported for P3HT-based fullerene-free PSCs.
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23. 23
  Bai, H.; Wu, Y.; Wang, Y.; Wu, Y.; Li, R.; Cheng, P.; Zhang, M.; Wang, J.; Ma, W.; Zhan, X. Nonfullerene acceptors based on extended fused rings flanked with benzothiadiazolylmethylenemalononitrile for polymer solar cells. J. Mater. Chem. A 2015, 3, 20758– 20766, DOI: 10.1039/c5ta05901a
  23
  Nonfullerene acceptors based on extended fused rings flanked with benzothiadiazolylmethylenemalononitrile for polymer solar cells
  Bai, Huitao; Wu, Yao; Wang, Yifan; Wu, Yang; Li, Rong; Cheng, Pei; Zhang, Mingyu; Wang, Jiayu; Ma, Wei; Zhan, Xiaowei
  Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015), 3 (41), 20758-20766CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
  Two novel A-D-A type mols. IDT-2BM and IDTT-2BM with extended fused-ring indacenodithiophene (IDT) or indacenodithienothiophene (IDTT) units as cores and strong electron-withdrawing unit 2-(benzo[c][1,2,5]thiadiazol-4-ylmethylene)malononitrile (BM) as the end-capping group were synthesized and studied as electron acceptors in soln.-processed polymer solar cells (PSCs). IDT-2BM and IDTT-2BM exhibited strong and broad absorption from 300 to 800 nm, and appropriate LUMO (-3.8 eV) and HOMO (-5.5 to -5.6 eV) levels matching with the classical polymer donor PBDTTT-C-T. IDT-2BM and IDTT-2BM films exhibited intrinsic electron mobilities of about 3.7 × 10-6 and 1.0 × 10-5 cm2 V-1 s-1, resp. Fullerene-free PSCs employing PBDTTT-C-T as the donor and IDT-2BM or IDTT-2BM as the acceptor afforded power conversion efficiencies of 4.26% and 4.81%, resp.
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24. 24
  Lv, A.; Stolte, M.; Würthner, F. Head-to-Tail Zig-Zag Packing of Dipolar Merocyanine Dyes Affords High-Performance Organic Thin-Film Transistors. Angew. Chem., Int. Ed. 2015, 54, 10512– 10515, DOI: 10.1002/anie.201504190
  There is no corresponding record for this reference.
25. 25
  Liess, A.; Huang, L.; Arjona-Esteban, A.; Lv, A.; Gsänger, M.; Stepanenko, V.; Stolte, M.; Würthner, F. Organic Thin Film Transistors Based on Highly Dipolar Donor-Acceptor Polymethine Dyes. Adv. Funct. Mater. 2015, 25, 44– 57, DOI: 10.1002/adfm.201402678
  25
  Organic Thin Film Transistors Based on Highly Dipolar Donor-Acceptor Polymethine Dyes
  Liess, Andreas; Huang, Lizhen; Arjona-Esteban, Alhama; Lv, Aifeng; Gsaenger, Marcel; Stepanenko, Vladimir; Stolte, Matthias; Wuerthner, Frank
  Advanced Functional Materials (2015), 25 (1), 44-57CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Org. thin film transistors (OTFTs) of twenty dipolar donor-acceptor-substituted polymethine dyes (D-A dyes, dipole moments from 3-15 D) were studied. The employed merocyanine dyes contain a dimethine bridge that is substituted with 1-alkyl-3,3-dimethylindolin-2-ylidene (Fischer base), 3-alkyl-2,3-dihydrobenzothiazol-2-ylidene or 1,3-benzodithiole-2-ylidene, resp., as electron-donating unit and various acceptor heterocycles. Thin films formed by these D-A dyes upon deposition in high vacuum are all composed of antiparallel π-stacked dimers. However, they are either amorphous, discontinuous or highly cryst. due to the interplay between mol.-substrate and dimer-dimer interactions. With the help of single crystal x-ray anal., out-of-plane X-ray studies (XRD), selected area electron diffraction (SAED), and at. force microscopy (AFM), a correlation between the mol. structure, film ordering, and hole charge transport ability can be established. The mobility values are compared to Baessler's disorder charge transport theory and a film growth mechanism is proposed based on DFT calcns. and single crystal structures. With carefully adjusted bulky substituents and high dipolarity an intimate centrosym. packing with a slipped, but tight π-stacking arrangement could be realized. This provides two-dimensional percolation pathways for holes and ultimately results in charge carrier mobilities up to 0.18 cm2 V-1 s-1.
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26. 26
  Heiniger, L.-P.; O’Brien, P. G.; Soheilnia, N.; Yang, Y.; Kherani, N. P.; Grätzel, M.; Ozin, G. A.; Tétreault, N. See-Through Dye-Sensitized Solar Cells: Photonic Reflectors for Tandem and Building Integrated Photovoltaics. Adv. Mater. 2013, 25, 5734– 5741, DOI: 10.1002/adma.201302113
  26
  See through dye sensitized solar cells: Photonic reflectors for tandem and building integrated photovoltaics
  Heiniger, Leo-Philipp; O'Brien, Paul G.; Soheilnia, Navid; Yang, Yang; Kherani, Nazir P.; Graetzel, Michael; Ozin, Geoffrey A.; Tetreault, Nicolas
  Advanced Materials (Weinheim, Germany) (2013), 25 (40), 5734-5741CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Dye sensitized solar cells are made with abundant and low purity materials are a promising alternative to the prevailing solid state p-n junction photovoltaic devices because of their low fabrication and environmental cost as well as their spectral tunability. This work relates to a conducting Bragg reflector composed os alternative layers of SiO2 nanoparticles and sputtered tin-doped indium oxide employed as the counter electrode for DSCs. These conducting 1D photonic crystals can be utilized as intermediate reflectors in micromorph Si solar cells.
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27. 27
  Tai, Q.; Yan, F. Emerging Semitransparent Solar Cells: Materials and Device Design. Adv. Mater. 2017, 29, 1700192, DOI: 10.1002/adma.201700192
  There is no corresponding record for this reference.
28. 28
  Keil, D.; Hartmann, H.; Moschny, T. Synthesis and characterization of 1,3-bis-(2-dialkylamino-5-thienyl)-substituted squaraines—a novel class of intensively coloured panchromatic dyes. Dyes Pigm. 1991, 17, 19– 27, DOI: 10.1016/0143-7208(91)85025-4
  28
  Synthesis and characterization of 1,3-bis(2-dialkylamino-5-thienyl)-substituted squaraines - a novel class of intensively colored panchromatic dyes
  Keil, Dietmar; Hartmann, Horst; Moschny, Torsten
  Dyes and Pigments (1991), 17 (1), 19-27CODEN: DYPIDX; ISSN:0143-7208.
  By refluxing N,N-disubstituted 2-aminothiophenes with squaric acid in BuOH-PhMe, a novel heterocyclic type of deeply colored squaraines (I; NR2 = morpholino, pyrrolidino, piperidino; R1 = H, Ph, 3-methoxyphenyl; R2 = H, Ph, 4-bromophenyl, 2-chlorophenyl) is obtained in generally moderate yields. These sparingly sol. dyes exhibit narrow absorption bands at ∼700 nm in org. solvents and, in a dispersed form, wide absorption bands ranging into the IR region at ∼900 nm.
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29. 29
  Sreejith, S.; Carol, P.; Chithra, P.; Ajayaghosh, A. Squaraine Dyes: A Mine of Molecular Materials. J. Mater. Chem. 2008, 18, 264– 274, DOI: 10.1039/b707734c
  29
  Squaraine dyes: a mine of molecular materials
  Sreejith, Sivaramapanicker; Carol, Priya; Chithra, Parayalil; Ajayaghosh, Ayyappanpillai
  Journal of Materials Chemistry (2008), 18 (3), 264-274CODEN: JMACEP; ISSN:0959-9428. (Royal Society of Chemistry)
  A review. This feature article highlights the recent developments in the field of squaraine chem. Attempts were made to address the relevance of squaraine dyes as a class of functional org. materials useful for electronic and photonic applications. Due to the synthetic access of a variety of squaraine dyes with structural variations and due to the strong absorption and emission properties which respond to the surrounding medium, these dyes were receiving significant attention. Therefore, squaraine dyes were extensively studied in recent years, from both fundamental and technol. viewpoints.
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30. 30
  Mayerhöffer, U.; Deing, K.; Gruß, K.; Braunschweig, H.; Meerholz, K.; Würthner, F. Outstanding Short-Circuit Currents in BHJ Solar Cells Based on NIR-Absorbing Acceptor-Substituted Squaraines. Angew. Chem., Int. Ed. 2009, 48, 8776– 8779, DOI: 10.1002/anie.200903125
  30
  Outstanding short-circuit currents in BHJ solar cells based on NIR-absorbing acceptor-substituted squaraines
  Mayerhoffer Ulrich; Deing Kaja; Gruss Katrin; Braunschweig Holger; Meerholz Klaus; Wurthner Frank
  Angewandte Chemie (International ed. in English) (2009), 48 (46), 8776-9 ISSN:.
  There is no expanded citation for this reference.
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31. 31
  Wang, S.; Hall, L.; Diev, V. V.; Haiges, R.; Wei, G.; Xiao, X.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. N,N-Diarylanilinosquaraines and Their Application to Organic Photovoltaics. Chem. Mater. 2011, 23, 4789– 4798, DOI: 10.1021/cm2020803
  31
  N,N-diarylanilinosquaraines and their application to organic photovoltaics
  Wang, Siyi; Hall, Lincoln; Diev, Vyacheslav V.; Haiges, Ralf; Wei, Guodan; Xiao, Xin; Djurovich, Peter I.; Forrest, Stephen R.; Thompson, Mark E.
  Chemistry of Materials (2011), 23 (21), 4789-4798CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  We report new derivs. of sym. squaraine dyes with N,N-diarylanilino substituents that have high soly. and high absorptivity (ε = 0.71-4.1 × 105 M-1 cm-1) in the red solar spectral region (λmax = 645-694 nm) making them promising candidates for application in org. photovoltaics (OPVs). Unsym. N,N-diisobutylanilino- and N,N-diphenylanilino(diphenylamino)squaraines have also been prepd. that give blue-shifted absorption spectra (λmax = 529-535 nm) relative to their sym. counterparts. Compared to bis(N,N-diisobutylanilino)squaraine, both sym. and unsym. N,N-diarylanilino squaraines show markedly broader absorption bands in soln. than their N,N-dialkylanilino squaraine counterparts: the full width at half-max. (fwhm) for N,N-diarylanilino squaraines range from 1280-1980 cm-1, while the fwhm value for the N,N-diisobutylanilino squarine is only 630 cm-1. The absorption bands for thin films of N,N-diarylanilino squaraines broaden further to 2500-3300 cm-1. N,N-Diarylanilino squaraines are fluorescent, albeit with lower quantum yields than bis(N,N-diisobutylanilino)squaraine (.vphi.PL = 0.02-0.66 and 0.80, resp.). OPVs were prepd. with soln. processed squaraine layers using the following structure: ITO/squaraine (66-85 Å)/C60 (400 Å)/BCP (100 Å)/Al (1000 Å), BCP = bathocuproine. Devices using thin films of the bis(N,N-diarylanilino)squaraines as donor layers show improved performance relative to OPVs prepd. with bis(N,N-dialkylanilino)squaraines, i.e. bis(N,N-diisobutylanilino)squaraine: open-circuit voltage Voc = 0.59 ± 0.05 V, short-circuit current Jsc = 5.58 ± 0.16 mA/cm2, fill factor FF = 0.56 ± 0.03, and power conversion efficiency η = 1.8 ± 0.2% under 1 sun, AM1.5G simulated illumination, compared with bis(N,N-diphenylanilino)squaraine: Voc = 0.82 ± 0.02 V, Jsc = 6.71 ± 0.10 mA/cm2, FF = 0.59 ± 0.01, and η = 3.2 ± 0.1%. Morphol. studies of thin films suggest that the soly. of bis(N,N-diarylanilino)squaraines plays an important role in controlling the optoelectronic properties of the OPVs.
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32. 32
  Sun, Y.; Welch, G. C.; Leong, W. L.; Takacs, C. J.; Bazan, G. C.; Heeger, A. J. Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency. Nat. Mater. 2012, 11, 44– 48, DOI: 10.1038/nmat3160
  32
  Solution-processed small-molecule solar cells with 6.7% efficiency
  Sun, Yanming; Welch, Gregory C.; Leong, Wei Lin; Takacs, Christopher J.; Bazan, Guillermo C.; Heeger, Alan J.
  Nature Materials (2012), 11 (1), 44-48CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
  Org. photovoltaic devices that can be fabricated by simple processing techniques are under intense investigation in academic and industrial labs. because of their potential to enable mass prodn. of flexible and cost-effective devices. Most of the attention has been focused on soln.-processed polymer bulk-heterojunction solar cells. A combination of polymer design, morphol. control, structural insight and device engineering has led to power conversion efficiencies reaching the 6-8% range for conjugated polymer/fullerene blends. Soln.-processed small-mol. bulk-heterojunction solar cells have received less attention, and their efficiencies have remained below those of their polymeric counterparts. Here, we report efficient soln.-processed small-mol. bulk-heterojunction solar cells based on a new mol. donor, DTS(PTTh2)2. A record power conversion efficiency of 6.7% under air-mass 1.5G irradn. (100 mW/cm2) is achieved for small-mol. bulk-heterojunction devices from DTS(PTTh2)2:PC70BM (donor to acceptor ratio of 7:3). This high efficiency was obtained by using remarkably small percentages of solvent additive (0.25 vol.% of 1,8-diiodooctane) during the film-forming process, which leads to reduced domain sizes in the bulk-heterojunction layer. These results provide important progress for soln.-processed org. photovoltaics and demonstrate that solar cells fabricated from small donor mols. can compete with their polymeric counterparts.
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33. 33
  McDowell, C.; Narayanaswamy, K.; Yadagiri, B.; Gayathri, T.; Seifrid, M.; Datt, R.; Ryno, S. M.; Heifner, M. C.; Gupta, V.; Risko, C.; Singh, S. P.; Bazan, G. C. Impact of Rotamer Diversity on the Self-Assembly of Nearly Isostructural Molecular Semiconductors. J. Mater. Chem. A 2018, 6, 383– 394, DOI: 10.1039/c7ta09972j
  33
  Impact of rotamer diversity on the self-assembly of nearly isostructural molecular semiconductors
  McDowell, Caitlin; Narayanaswamy, Kamatham; Yadagiri, Bommaramoni; Gayathri, Thumuganti; Seifrid, Martin; Datt, Ram; Ryno, Sean M.; Heifner, Michael C.; Gupta, Vinay; Risko, Chad; Singh, Surya Prakash; Bazan, Guillermo C.
  Journal of Materials Chemistry A: Materials for Energy and Sustainability (2018), 6 (2), 383-394CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
  Conformational diversity due to different orientations of structural subunits has a complex impact on morphol. disorder of org. semiconductors. Here, we isolate the impact of a specific structural change: replacing bithiophene (biTh) units with thieno[3,2-b]thiophene (TT). We compare four mols. with an alternating donor-acceptor structure (D'-A-D-A-D') composed of a central, electron-rich dithienosilole (DTS) unit flanked by pyridyl-[2,1,3]thiadiazole (PT) or fluorinated benzo[c][1,2,5]thiadiazole (FBT) and end-capped with bithiophene biTh or TT groups. We find that using TT instead of biTh results in an increased degree of order within films cast directly from soln. by influencing the self-assembly tendencies of the different mols. Unlike switching the acceptor subunit, such as FBT for PT, the TT for biTh structural change has little impact on the electronic structure of these mol. semiconductors. Instead, these morphol. effects can be understood within the context of the predicted conformational diversity. TT units limit the no. of rotational conformations (rotamers) available within this mol. architecture; low rotamer dispersity facilitates self-assembly into ordered domains. As a practical illustration of this greater drive toward self-assembly, we use the TT-contg. mols. as donors in bulk heterojunction solar cells with PC70BM. Devices with TT-contg. mols. show improved photovoltaic performance compared to their previously characterized biTh analogs (d-DTS(PTTh2)2 and p-DTS(FBTTh2)2) in both as-cast and optimized conditions, with efficiencies up to 6.4% and 8.8% for PT-TT and FBT-TT, resp. The TT subunit and, more broadly, the strategy of limiting conformational diversity can be readily applied toward the design of soln.-processable org. semiconductors with increased as-cast order.
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34. 34
  Rao, G. H.; Rana, P. J. S.; Islam, A.; Singh, S. P. Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via C-H Arylation for See-through Photovoltaic. ACS Appl. Energy Mater. 2018, 1, 4786– 4793, DOI: 10.1021/acsaem.8b00862
  34
  Synthesis of Multichromophoric Asymmetrical Squaraine Sensitizer via C-H Arylation for See-through Photovoltaic
  Rao, G. Hanumantha; Rana, Prem Jyoti Singh; Islam, Ashraful; Singh, Surya Prakash
  ACS Applied Energy Materials (2018), 1 (9), 4786-4793CODEN: AAEMCQ; ISSN:2574-0962. (American Chemical Society)
  Developing org. mols. as light harvesters is the key for transparent solar cells technologies. In this paper, a novel metal-free near-IR (NIR) panchromatic asym. squaraine sensitizers (SQ-SPS) are designed and synthesized with the thieno[3,2-b]thiophene (TT) unit as π-spacer, dicyanovinyl and cyanoacrylic acid unit as acceptor and successfully used this sensitizer for dye-sensitized solar cells (DSSCs) application. The C-H arylation protocol is adapted to achieve the final target mol. which allows to avoid conventional and multistep coupling reactions at low-cost. The time-dependent d. functional theory (TDDFT) gives an overview of transitions involved and is in good agreement with the exptl. UV-vis absorption data of SQ-SPS. The av. lifetime of SQ-SPS is 2.45 ns, which is decent value for squaraine dye. SQ-SPS dye alone shows the power conversion efficiency (PCE) of 5.86%, Voc 0.564 mV, Jsc15.00 mA cm-2, and after cosensitization with N3 dye there is a tremendous increase in PCE to 8.84%, Voc to 0.650 mV, and Jsc to 15.00 mA cm-2. The remarkable increase in efficiency is mainly due to entire spectral coverage of IPCE spectra after cosensitization. The dye casted on TiO2 film shows the transparent nature, which revealed that SQ-SPS dye has potential for transparent solar cell applications.
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35. 35
  Marchena, M. J.; de Miguel, G.; Cohen, B.; Organero, J. A.; Pandey, S.; Hayase, S.; Douhal, A. Real-Time Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells with Iodide and Cobalt Electrolytes. J. Phys. Chem. C 2013, 117, 11906– 11919, DOI: 10.1021/jp401399j
  35
  Real-Time Photodynamics of Squaraine-Based Dye-Sensitized Solar Cells with Iodide and Cobalt Electrolytes
  Marchena, Maria Jose; de Miguel, Gustavo; Cohen, Boiko; Organero, Juan Angel; Pandey, Shyam; Hayase, Shuzi; Douhal, Abderrazzak
  Journal of Physical Chemistry C (2013), 117 (23), 11906-11919CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  A series of dye-sensitized solar cells (DSSCs) has been prepd. by using indole-based or quinoline-based squaraines (SQs) as the sensitizer and contg. the commonly used I3-/I- redox pair or the lately employed cobalt complexes, [Co(dimethylbipyridine)3]3+/2+, [Co(bipyridine)3]3+/2+, and [Co(phenanthroline)3]3+/2+ redox electrolytes. The photodynamics of the different electron transfer reactions were investigated by means of the femto- to millisecond pump-probe techniques. In the femtosecond transient absorption expts., the electron injection rate consts. and efficiencies, kei and φei, were detd. for each cell. Larger values of kei and φei for the indole-based (SQ 8) compared to the quinoline-based (SQ 12) squaraines were obtained (13.2 × 1010 s-1 and 0.95 × 1010 vs. 6.9 × 1010 s-1 and 0.81 for SQ 8 or SQ 12 with the I3-/I- pair, resp.), despite the similar values of the electron injection driving forces (-ΔG0ei = 0.75 vs. 0.76 eV). This is due to the lower electron d. in the LUMO at the anchoring group (-COOH) in SQ 12 compared to SQ 8. However, the type of electrolyte did not affect the kinetics of the electron injection processes. In the flash photolysis expts., the kinetic parameters of the electron recombination via dye or electrolyte and the cation regeneration were calcd. from the decays of the transient absorption signals of the electrons (1550 nm) or the SQ cation (570 nm). It was found that the electron recombination with the oxidized redox species is faster with the Co-based compared to the I3-/I- electrolytes for both SQs, τrec = 3 vs. ∼0.5-1 ms. This proves that the steric hindrance in these SQs is not sufficient to avoid the approach of the Co3+ species to the surface of the TiO2 nanoparticle. Moreover, the regeneration rate consts. and efficiencies, kreg and φreg, are considerably smaller for the cells with the different Co-based electrolytes compared to those with the I3-/I- pair (i.e., kreg = 30 × 104 vs. 8 × 104 M-1s-1 and φreg = 0.96 vs. 0.75 with the [Co(dmb)3]3+/2+ for SQ 8). This is explained by the lower regeneration driving force, -ΔGreg, in the Co-based electrolytes (0.3-0.1 eV). Thus, the use of Co-based electrolytes in these two SQs is detrimental to the overall efficiency of the cell, since -ΔGreg values below 0.4 eV do not give complete regeneration efficiency. Finally, the measured photocurrent is compared with the calcd. electron injection and regeneration efficiencies, a good correlation between both parameters is found.
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36. 36
  Sung, J.; Kim, P.; Lee, Y. O.; Kim, J. S.; Kim, D. Characterization of Ultrafast Intramolecular Charge Transfer Dynamics in Pyrenyl Derivatives: Systematic Change of the Number of Peripheral N,N-Dimethyaniline Substituents. J. Phys. Chem. Lett. 2011, 2, 818– 823, DOI: 10.1021/jz200042s
  36
  Characterization of ultrafast intramolecular charge transfer dynamics in pyrenyl derivatives: systematic change of the number of peripheral N,N-dimethyaniline substituents
  Sung, Jooyoung; Kim, Pyosang; Lee, Yoen Ok; Kim, Jong Seung; Kim, Dongho
  Journal of Physical Chemistry Letters (2011), 2 (7), 818-823CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  We have comparatively investigated the charge transfer (CT) dynamics of a series of covalently linked N,N-dimethylaniline (DMA) and pyrenyl derivs. I, II, III, IV and V. Their CT characters have been estd. in the order of I > II ≃ III > IV > V by an increase of Stokes shift with increasing the solvent polarity. The femtosecond time-resolved fluorescence spectra show very early excited-state dynamics, a transition from the initially populated locally excited (LE) state to the CT state. The CT characters of studied mol. systems are strongly influenced by the no. and position of donor (DMA) groups onto the acceptor (pyrene) moiety.
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37. 37
  Zhang, G.; Musgrave, C. B. Comparison of DFT Methods for Molecular Orbital Eigenvalue Calculations. J. Phys. Chem. A 2007, 111, 1554– 1561, DOI: 10.1021/jp061633o
  37
  Comparison of DFT Methods for Molecular Orbital Eigenvalue Calculations
  Zhang, Gang; Musgrave, Charles B.
  Journal of Physical Chemistry A (2007), 111 (8), 1554-1561CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  We report how closely the Kohn-Sham HOMO and LUMO eigenvalues of 11 d. functional theory (DFT) functionals, resp., correspond to the neg. ionization potentials (-IPs) and electron affinities (EAs) of a test set of mols. We also report how accurately the HOMO-LUMO gaps of these methods predict the lowest excitation energies using both time-independent and time-dependent DFT (TD-DFT). The 11 DFT functionals include the local spin d. approxn. (LSDA), five generalized gradient approxn. (GGA) functionals, three hybrid GGA functionals, one hybrid functional, and one hybrid meta GGA functional. We find that the HOMO eigenvalues predicted by KMLYP, BH and HLYP, B3LYP, PW91, PBE, and BLYP predict the -IPs with av. abs. errors of 0.73, 1.48, 3.10, 4.27, 4.33, and 4.41 eV, resp. The LUMOs of all functionals fail to accurately predict the EAs. Although the GGA functionals inaccurately predict both the HOMO and LUMO eigenvalues, they predict the HOMO-LUMO gap relatively accurately (∼0.73 eV). On the other hand, the LUMO eigenvalues of the hybrid functionals fail to predict the EA to the extent that they include HF exchange, although increasing HF exchange improves the correspondence between the HOMO eigenvalue and -IP so that the HOMO-LUMO gaps are inaccurately predicted by hybrid DFT functionals. We find that TD-DFT with all functionals accurately predicts the HOMO-LUMO gaps. A linear correlation between the calcd. HOMO eigenvalue and the exptl. -IP and calcd. HOMO-LUMO gap and exptl. lowest excitation energy enables us to derive a simple correction formula.
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38. 38
  Prabhakar, C.; Chaitanya, G. K.; Sitha, S.; Bhanuprakash, K.; Rao, V. J. Role of the Oxyallyl Substructure in the Near Infrared (NIR) Absorption in Symmetrical Dye Derivatives: A Computational Study. J. Phys. Chem. A 2005, 109, 2614– 2622, DOI: 10.1021/jp044954d
  38
  Role of the oxyallyl substructure in the near infrared (NIR) absorption in symmetrical dye derivatives: A computational study
  Prabhakar, Ch.; Chaitanya, G. Krishna; Sitha, Sanyasi; Bhanuprakash, K.; Rao, V. Jayathirtha
  Journal of Physical Chemistry A (2005), 109 (11), 2614-2622CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  It is well-known from exptl. studies that the oxyallyl-substructure-based squarylium and croconium dyes absorb in the NIR region of the spectrum. Recently, another dye has been reported (M. Tian et al., 2003) which contains the same basic chromophore, but the absorption is red-shifted by at least 300 nm compared to former dyes and is obsd. near 1100 nm. To analyze the reasons behind the large red shift, in this work we have carried out symmetry-adapted cluster-CI (SAC-CI) studies on some of these NIR dyes which contain the oxyallyl substructure. From this study, contrary to the earlier reports, it is seen that the donor groups do not seem to play a major role in the red shift of the absorption. On the other hand, on the basis of the results of the high-level calcns. carried out here and using qual. MO theory, it is obsd. that the orbital interactions play a key role in the red shift. Finally, design principles for the oxyallyl-substructure-based NIR dyes are suggested.
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39. 39
  Prabhakar, C.; Yesudas, K.; Krishna Chaitanya, G.; Sitha, S.; Bhanuprakash, K.; Rao, V. J. Near-Infrared Absorption in Symmetric Squarylium and Croconate Dyes: A Comparative Study Using Symmetry-Adapted Cluster-Configuration Interaction Methods. J. Phys. Chem. A 2005, 109, 8604– 8616, DOI: 10.1021/jp0516184
  39
  Near-infrared absorption in symmetric squarylium and croconate dyes: A comparative study using symmetry-adapted cluster-configuration interaction methods
  Prabhakar, Ch.; Yesudas, K.; Chaitanya, G. Krishna; Sitha, Sanyasi; Bhanuprakash, K.; Rao, V. Jayathirtha
  Journal of Physical Chemistry A (2005), 109 (38), 8604-8616CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  Sym. croconate (CR) and squarylium dyes (SQ) are well-known near-IR (NIR) dyes and, in general, are considered to be donor-acceptor-donor type mols. It is established in the literature that CR dyes absorb in a longer wavelength region than the corresponding SQ dyes. This has been attributed to the CR ring being a better acceptor than the SQ ring. Thus increasing the donor capacity should lead to a bathochromic shift in both SQ and CR. On the other hand, some expts. reported in the literature have revealed that increasing the conjugation in the donor part of the SQ mol. leads first to a red shift, which upon further increase of the conjugation changes to a blue shift. Hence, to understand the role of the central ring and the substitutions in the absorption of these dyes, we carried out high-level symmetry-adapted cluster-CI (SAC-CI) calcns. of some substituted SQ and CR dyes and compare the absorption energy with existing exptl. data. We found that there is very good agreement. We also carried out SAC-CI calcns. of some smaller model mols., which contain the main oxyallyl substructure. We varied the geometry (angle) of the oxyallyl subgroup and the substitution in these model mols. to establish a correlation with the bathochromic shift. We found that the charge transfer is very small and does not play the key role in the red shift, but on the other hand, the perturbation of the HOMO-LUMO gap (HLG) from both the geometry and substitution seems to be responsible for this shift. We suggest as a design principle that increasing the donor capacity of the groups may not help in the red shift, but introducing groups which perturb the HLG and decrease it without changing the MO character should lead to a larger bathochromic shift.
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  Yamanaka, S.; Okumura, M.; Nakano, M.; Yamaguchi, K. EHF theory of chemical reactions Part 4. UNO CASSCF, UNO CASPT2 and R(U)HF coupled-cluster (CC) wavefunctions. J. Mol. Struct. 1994, 310, 205– 218, DOI: 10.1016/s0022-2860(10)80071-7
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  Nakano, M. Second Hyperpolarizabilities (γ) of Bisimidazole and Bistriazole Benzenes: Diradical Character, Charged State, and Spin State Dependences. J. Phys. Chem. A 2006, 110, 4238– 4243, DOI: 10.1021/jp056672z
  41
  Second Hyperpolarizabilities (γ) of Bisimidazole and Bistriazole Benzenes: Diradical Character, Charged State, and Spin State Dependences
  Nakano, Masayoshi; Kishi, Ryohei; Nakagawa, Nozomi; Ohta, Suguru; Takahashi, Hideaki; Furukawa, Shin-Ichi; Kamada, Kenji; Ohta, Koji; Champagne, Benoit; Botek, Edith; Yamada, Satoru; Yamaguchi, Kizashi
  Journal of Physical Chemistry A (2006), 110 (12), 4238-4243CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  The second hyperpolarizabilities of bisimidazole- and bistriazole-benzene compds. were calcd. at different levels of approxn. to unravel the effects of diradical character and of charge and spin multiplicity. The largest second hyperpolarizabilities are assocd. with intermediate diradical character, provided pos. charging does not compensate for this effect. For the neutral diradical bisimidazole compd., the singlet diradical species possesses a second hyperpolarizability two to three times larger than the corresponding triplet, demonstrating the possibility of spin state control of the third-order NLO responses for diradical species.
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  Nakano, M. Origin of the Enhancement of the Second Hyperpolarizability of Singlet Diradical Systems with Intermediate Diradical Character. J. Chem. Phys. 2006, 125, 074113, DOI: 10.1063/1.2213974
  42
  Origin of the enhancement of the second hyperpolarizability of singlet diradical systems with intermediate diradical character
  Nakano, Masayoshi; Kishi, Ryohei; Ohta, Suguru; Takebe, Akihito; Takahashi, Hideaki; Furukawa, Shin-Ichi; Kubo, Takashi; Morita, Yasushi; Nakasuji, Kazuhiro; Yamaguchi, Kizashi; Kamada, Kenji; Ohta, Koji; Champagne, Benoit; Botek, Edith
  Journal of Chemical Physics (2006), 125 (7), 074113/1-074113/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  The origin of the diradical character dependence of the second hyperpolarizability (γ) of neutral singlet diradical systems is clarified based on the perturbation formula of γ using the simplest diradical mol. model with different diradical characters, i.e., H2 under bond dissocn. The enhancement of γ in the intermediate diradical character region turns out to originate from the increasing magnitude of the transition moment between the first and second excited states and the decrease of that between the ground and first excited states, resp., with the increase in diradical character. This feature confirms that open-shell singlet conjugated mols. with intermediate diradical characters constitute a new class of third-order nonlinear optical systems, whose γ values can be controlled by the diradical character in addn. to the conjugation length.
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  Völker, S. F.; Renz, M.; Kaupp, M.; Lambert, C. Squaraine Dyes as Efficient Coupling Bridges between Triarylamine Redox Centres. Chem.—Eur. J. 2011, 17, 14147– 14163, DOI: 10.1002/chem.201102227
  There is no corresponding record for this reference.
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  Mao, J.; He, N.; Ning, Z.; Zhang, Q.; Guo, F.; Chen, L.; Wu, W.; Hua, J.; Tian, H. Stable Dyes Containing Double Acceptors without COOH as Anchors for Highly Efficient Dye-Sensitized Solar Cells. Angew. Chem. 2012, 124, 10011– 10014, DOI: 10.1002/ange.201204948
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45. 45
  Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2013.
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  Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648– 5652, DOI: 10.1063/1.464913
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  Density-functional thermochemistry. III. The role of exact exchange
  Becke, Axel D.
  Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
  Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
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47. 47
  Thomas, A.; Srinivas, K.; Prabhakar, C.; Bhanuprakash, K.; Rao, V. J. Estimation of the First Excitation Energy in Diradicaloid Croconate Dyes Having Absorption in the near Infra Red (NIR): A DFT and SF-TDDFT Study. Chem. Phys. Lett. 2008, 454, 36– 41, DOI: 10.1016/j.cplett.2008.01.074
  There is no corresponding record for this reference.
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  Chitumalla, R. K.; Lim, M.; Gao, X.; Jang, J. Substituent Effects on the Croconate Dyes in Dye Sensitized Solar Cell Applications: A Density Functional Theory Study. J. Mol. Model. 2015, 21, 297, DOI: 10.1007/s00894-015-2845-4
  48
  Substituent effects on the croconate dyes in dye sensitized solar cell applications: a density functional theory study
  Chitumalla Ramesh Kumar; Jang Joonkyung; Lim Manho; Gao Xingfa
  Journal of molecular modeling (2015), 21 (11), 297 ISSN:.
  Using the density functional theory (DFT), we studied two model croconate dyes, one with an electron-donating substituent (CR1) and the other with an electron-withdrawing group (CR2). The geometric, electronic, and optical properties of these dyes were compared. Upon switching from CR1 to CR2, a considerable bathochromic shift was observed in the electronic absorption spectrum. We also investigated the adsorption behavior of the two dyes on a TiO2 (101) anatase surface by employing periodic DFT simulations. The periodic electronic-structure calculations revealed that the diketo group of CR1 bound more strongly to the TiO2 surface than that of CR2, with a binding strength comparable to that of a typical organic D-π-A dye. In this work we evaluate in particular the effect of the electron withdrawing/donating nature of the substituent on the electronic, optical, and adsorption properties of the croconate dyes. Finally, we hope that the present study will help in the design of highly efficient dyes for dye sensitized solar cells by considering substituent effects. Graphical abstract Effect of substituent on binding energy and charge transfer.
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  Miertuš, S.; Scrocco, E.; Tomasi, J. Electrostatic Interaction of a Solute with a Continuum. A Direct Utilizaion of AB Initio Molecular Potentials for the Prevision of Solvent Effects. Chem. Phys. 1981, 55, 117– 129, DOI: 10.1016/0301-0104(81)85090-2
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  Electrostatic interaction of a solute with a continuum. A direct utilization of ab initio molecular potentials for the prevision of solvent effects
  Miertus, S.; Scrocco, E.; Tomasi, J.
  Chemical Physics (1981), 55 (1), 117-29CODEN: CMPHC2; ISSN:0301-0104.
  A method is presented which utilizes the calcn. of the mol. electrostatic potential or the elec. field at a discrete no. of preselected points to evaluate the environmental effects of a solvent on the properties of a mol. system. No limitations are imposed on the compn. and dimension of the solute, on the value of the corresponding wavefunction, or on the shape of the cavity in the dielec. Several levels of approxn., which evidence the effect of self-polarization of the system of surface charges, the influence of the tails of the solute charge distribution beyond the limits of the cavity, and the effect of the polarization of the solute are discussed.
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  Cossi, M.; Barone, V.; Cammi, R.; Tomasi, J. Ab initio study of solvated molecules: a new implementation of the polarizable continuum model. Chem. Phys. Lett. 1996, 255, 327– 335, DOI: 10.1016/0009-2614(96)00349-1
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  Ab initio study of solvated molecules: a new implementation of the polarizable continuum model
  Cossi, Maurizio; Barone, Vincenzo; Cammi, Roberto; Tomasi, Jacopo
  Chemical Physics Letters (1996), 255 (4,5,6), 327-335CODEN: CHPLBC; ISSN:0009-2614. (Elsevier)
  The authors have implemented an efficient version of the polarizable continuum solvation model in the GAUSSIAN94 package. This version exploits a new definition of surface elements area, and a direct formulation of the electrostatic self-consistent problem. Non-electrostatic contributions to the mol. free-energy in soln. are calcd. in the same framework. Several possible definitions of the mol. cavity are examd., and the results compared to those of other continuum solvation methods already available in GAUSSIAN94.
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Supporting Information
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b01809.
- Structural characterization by ¹H, ¹³C NMR, ESI MS spectra, MALDI-TOF MS spectra, and DFT calculations (PDF)
- ao8b01809_si_001.pdf (877.32 kb)
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Molecular Engineering and Structure-Related Properties of Squaraine Dyes Based on the Core and Wings ConceptClick to copy article linkArticle link copied!

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Abstract

Introduction

Figure 1

Results and Discussion

Figure 2

Figure 3

Absorption and Photoluminescence Spectroscopy

Figure 4

Electrochemical Properties

Figure 5

Solvatochromism

Figure 6

Spectroelectrochemical Properties

Figure 7

Theoretical Calculations

Figure 8

Figure 9

Figure 10

Conclusions

Experimental Section

General Methods and Materials

Synthesis

Scheme 1

5-Bromo-1-butyl-2,3,3-trimethyl-3H-indol-1-ium Iodide

(Z)-4-((5-Bromo-1-butyl-2,3,3-trimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(5-bromo-1-butyl-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-2Br)

(Z)-4-((1-Butyl-5-(5-formylthiphen-2-yl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(1-butyl-5-(5-formylthiphen-2-yl)-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-2CHO)

General Synthetic Procedure for SQ-DICN, SQ-DIEt-RH, and SQ-DICN-RH

(Z)-4-((1-Butyl-5-(5-(2,2-dicyanovinyl)thiophen-2-yl)-3,3-dimethyl-3H-indol-1-ium-2-yl)methylene)-2-((E)-(1-butyl-5-(5-(2,2-dicyanovinyl)thiophen-2-yl))-3,3-dimethylindolin-2-ylidene)methyl)-3-oxocyclobut-1-enolate (SQ-DICN)

Computational Details

Supporting Information

Terms & Conditions

Author Information

Acknowledgments

References

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Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Scheme 1

References

Supporting Information

Supporting Information

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Molecular Engineering and Structure-Related Properties of Squaraine Dyes Based on the Core and Wings Concept
Click to copy article linkArticle link copied!