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Novel Heteroleptic Ruthenium(II) Complexes with 2,2′- Bipyridines Containing a Series of Electron-Donor and Electron-Acceptor Substituents in 4,4′-Positions: Syntheses, Characterization, and Application as Sensitizers for ZnO Nanowire-Based Solar Cells
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Novel Heteroleptic Ruthenium(II) Complexes with 2,2′- Bipyridines Containing a Series of Electron-Donor and Electron-Acceptor Substituents in 4,4′-Positions: Syntheses, Characterization, and Application as Sensitizers for ZnO Nanowire-Based Solar Cells
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  • Fernando F. Salomón
    Fernando F. Salomón
    INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina
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  • Nadia C. Vega
    Nadia C. Vega
    INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina
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  • Teodor Parella
    Teodor Parella
    Servei de RMN, Universitat Autonóma de Barcelona, Bellaterra E-08193, Barcelona, Spain
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    Orcidhttp://orcid.org/0000-0002-1914-2709
  • Faustino Eduardo Morán Vieyra
    Faustino Eduardo Morán Vieyra
    INBIONATEC (UNSE-CONICET), Universidad Nacional de Santiago del Estero, RN 9, Km 1125, (4206) Santiago del Estero, Argentina
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  • Claudio D. Borsarelli
    Claudio D. Borsarelli
    INBIONATEC (UNSE-CONICET), Universidad Nacional de Santiago del Estero, RN 9, Km 1125, (4206) Santiago del Estero, Argentina
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  • Claudia Longo
    Claudia Longo
    Institute of Chemistry, University of Campinas, UNICAMP, C. Postal 6154, 13083-970 Campinas, Sao Paulo, Brazil
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  • Mauricio Cattaneo
    Mauricio Cattaneo
    INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina
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  • Néstor E. Katz*
    Néstor E. Katz
    INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina
    *Email: [email protected]. Tel.: (+54)(381)(4200960). Fax: (+54)(381)(4248169).
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    Orcidhttp://orcid.org/0000-0001-5576-2692
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ACS Omega

Cite this: ACS Omega 2020, 5, 14, 8097–8107
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https://doi.org/10.1021/acsomega.0c00243
Published April 2, 2020

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Abstract

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A novel series of complexes of the formula [Ru(4,4′-X2-bpy)2(Mebpy-CN)](PF6)2 (X = −CH3, −OCH3, −N(CH3)2; Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile) have been synthesized and characterized by spectroscopic, electrochemical, and photophysical techniques. Inclusion of the electron-withdrawing substituent −CN at one bpy ligand and different electron-donor groups −X at the 4,4′-positions of the other two bpy ligands produce a fine tuning of physicochemical properties. Redox potentials, electronic absorption maxima, and emission maxima correlate well with Hammett’s σp parameters of X. Quantum mechanical calculations are consistent with experimental data. All the complexes can be anchored through the nitrile moiety of Mebpy-CN over ZnO nanowires in dye-sensitized solar cells that exhibit an improvement of light to electrical energy conversion efficiency as the electronic asymmetry increases in the series.

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Introduction

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Developing new materials for conversion of solar into electrical or chemical energy is a relevant area of research for the substitution of fossil fuels and mitigation of climate change. Dye-sensitized solar cells (DSCs) using n-type semiconductor materials such as TiO2 and sensitizers such as ruthenium complexes are promising low-cost devices based on the discovery of the Grätzel cell. (1) Polypyridylruthenium(II) complexes have been extensively studied since their photophysical and redox properties are suitable for using them as sensitizers in optoelectronic devices and DSCs. (2−4) The commercial dye N719 (5) and the black dye (6) are examples of efficient sensitizers that absorb a wide range of wavenumbers of visible light. Although they are very efficient converters, chemical stability problems arise when considering the possibility of decomposition of the −NCS group, as suggested before. (7)
Tris-substituted bipyridineruthenium(II) complexes are highly stable and a large variety of chemical transformations in the bipyridine rings can be accomplished. It is well known that addition of electron-donor or electron-acceptor groups to the 4- and 4′- positions of bpy (bpy = 2,2′-bipyridine) alter the physicochemical properties of trisbipyridineruthenium complexes. (2) Bpys with electron-donor substituent groups have π orbitals suitable to mix with d orbitals of Ru(II), the net effect being the destabilization of the highest occupied molecular orbital (HOMO). On the other hand, the lowest unoccupied molecular orbital (LUMO) is a π* orbital centered in the bpys and it can be stabilized by introducing electron-acceptor substituents in the bpy rings. The usual procedure to obtain red-emitting Ru(II) complexes is the addition of better π-acceptor ligands than bpy in order to decrease the energy of the LUMO and better σ-donor ligands than bpy in order to increase the energy of the HOMO, such as those complexes described by Hanan et al. (8) The overall result of combining both trends is a decrease of the HOMO–LUMO gap and a shift of light absorption and emission maxima to the near-infrared (NIR) spectrum region, among other physicochemical property changes.
With these modifications, the sensitization of a wide band gap semiconductor at longer wavelengths of the solar spectrum led to the creation of new electrodes in DSC devices, as already reported for TiO2-nanostructured electrodes. (9,10) In recent years, ZnO have become very common in semiconductor devices (11) due to its unique optical, chemical, electrical, and piezoelectrical properties, (11−16) being also a material easy to obtain in different morphologies (films, nanoparticles, and nanowires) (17−19) and having the added possibility of making core–shell structures with other materials to change their optical and electrical properties. (20,21) In the broad spectrum of challenges related to this issue, we address in this work the possibility of improving electron injection efficiency in ruthenium complex DSCs by a careful selection of coligands and using ZnO as a semiconductor, since its sensitization with Ru complexes has received much less attention. ZnO is currently considered as one of the most promising materials to replace the TiO2 semiconductor (22) in solar energy conversion technology. In particular, ZnO nanowires (NWs) in a DSC create direct electron pathways, which lead to an increase of electron diffusion length and lifetime. (22,23) In some of our previous studies, very efficient charge transfer between Ru complexes and ZnO nanowires (24,25) was evidenced, indicating the possibility of exploring new Ru complex/ZnO nanowire structures to develop better DSCs.
In this paper, three new ruthenium(II) polypyridyl complexes of the formula [Ru(4,4′-X2-bpy)2(Mebpy-CN)](PF6)2, where X = −CH3, −OCH3, −N(CH3)2 and Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile, have been synthesized (Scheme 1). Electrochemical, spectroscopic, and photophysical studies have been carried out and the electronic properties can be rationalized by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations. Finally, we demonstrate that ZnO nanowires, grown in fluoride tin oxide (FTO) substrates using a hydrothermal technique, can be sensitized by these complexes anchored to the semiconductor surface through a nitrile group and used as working electrodes in DSCs that perform at low efficiencies but could be improved by increasing the number of anchoring nitrile groups.

Scheme 1

Scheme 1. Molecular Structures of the New Trisbipyridine Ruthenium(II) Complexes
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Results and Discussion

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Syntheses, IR, and NMR Spectra

The precursors Ru(4,4′-X2-bpy)2Cl2 were prepared using procedures similar to those previously reported. (26,27) As described by Pannwitz et al., (27) [Ru(4,4′-{N(CH3)2}2-bpy)2Cl2]Cl was obtained first as a Ru(III) precursor due to the stabilization of higher oxidation states induced by a strong electron-donor group in the bpy ring such as dimethylamine. For this particular complex, trimethylamine was added in the next preparative step in order to reduce Ru(III) to Ru(II). The new complexes 1–3 were prepared by protocols already known. (26)
Fourier transform infrared (FTIR) spectra of complexes 1–3 exhibit a very weak band at 2235 cm–1, which is characteristic of νC≡N, the stretching vibration of the C≡N group of Mebpy-CN. Typical bands corresponding to the vibrational modes of the polypyridyl moieties (4,4′-(X)2-bpy and Mebpy-CN) are observed between 1600 and 1400 cm–1. Figure S2 shows the FTIR spectrum of complex 3 as a representative example. The values of νC≡N obtained using the Raman spectra of complexes 1–3 are identical with those measured by FTIR and almost equal to that of the free ligand, indicating that Mebpy-CN is not coordinated to the metal through the nitrile group.
Complexes 1–3 possess C1 symmetry and all the signals of the atoms of the substituted bpy ligands in the NMR spectra are therefore inequivalent. For this reason, signals of 1H and 13C for the 4,4′-X2-bpy ligands are reported as broad bands. On the other hand, signals for the Mebpy-CN ligand can be assigned straightforwardly by comparison with data previously reported for similar complexes with this ligand. (25) Detailed assignments of 1H-NMR, 13C-NMR, and 15N-NMR signals of complexes 1–3 as well as mass spectroscopy (MS) data can be found in the Syntheses Section. Figures S3–S9 show all the NMR spectra of complex 3 as a representative example and the complete assignments are shown in Figure S10. NMR and MS results confirm the proposed structures depicted in Scheme 1.

Electrochemistry

Redox potentials of complexes 1–3 in CH3CN with 0.1 M TBAH are shown in Table 1. The first two reduction potentials were measured by CV, while the following ones were measured by DPV. The values for the reference complex of formula [Ru(bpy)2(Mebpy-CN)](PF6)2 (25) were measured again under the same conditions. Cyclic and differential pulse voltammograms of complex 3 as a representative example are shown in Figures S11 and S12, respectively.
Table 1. Electrochemical Data for Complexes 1–3 in CH3CN vs SCE
complexE1/2ox(V)E1/2red1(V)E1/2red2(V)E1/2red3(V)a
[Ru(bpy)2(Mebpy-CN)] (PF6)21.28–1.23–1.51–1.772.51
11.24–1.25–1.61–1.862.49
21.06–1.26–1.61–1.762.32
30.64–1.38–1.62–1.932.03
a

ΔE1/2 = E1/2ox – E1/2red1

When the electron-donor strength of the substituent X of 4,4′-(X)2-bpy was increased, a cathodic shift of redox potential of the RuIII/II couple, E1/2ox,is observed. Less pronounced cathodic shifts are also observed for the redox potentials of the ligand reductions E1/2red1, E1/2red2, and E1/2red3. In polypyridylruthenium(II) complexes, when the metal center is oxidized to Ru(III), electron-donor groups in the bpy rings stabilize this latter state by sharing more electron density with the metal. The one-electron reduction process is expected to be centered in the best electron-accepting bpy, Mebpy-CN in this case. Increasing the electron-donor strength of the substituent X in the bpy rings changes the amount of metal-to-ligand π-backbonding, making their reduction more difficult.
The one-electron oxidation process involves removal of an electron from the HOMO, which is a dπ(Ru) orbital. On the other hand, the one-electron reduction process involves addition of an electron to the LUMO, which is centered in Mebpy-CN, as discussed in the Calculations Section. A plot of the difference of potentials of the oxidative and the first reductive processes, ΔE1/2,as a function of Hammett’s sigma parameters σp, (28) shown in Figure 1, is linear and evidences that increasing the donor strength of the substituent group −X destabilizes the HOMO more than the LUMO. This was already reported by Maestri et al. for a series of electron-donor-substituted terpyridines. (29) The net effect is a reduction of the HOMO–LUMO energy gap with the increase of the electron-donor strength of −X , as discussed below.

Figure 1

Figure 1. Difference of potentials, ΔE1/2,of the oxidative and the first reductive processes in [Ru(4,4′-X2-bpy)2(Mebpy-CN)](PF6)2 complexes as a function of Hammett’s σp parameter of X.

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UV–Vis Spectra

The electronic absorption spectra of complexes 1–3 are displayed in Figure 2. The absorption maxima and molar extinction data are summarized in Table S1. For comparative purposes, data for [Ru(bpy)3](PF6)2 and [Ru(bpy)2(Mebpy-CN)](PF6)2 have also been included in the table.

Figure 2

Figure 2. UV–vis spectra of complexes 1 (orange), 2 (blue), and 3 (red) in CH3CN at rt.

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All the complexes show intense absorption bands in the visible region, which can be assigned to metal-to-ligand charge transfer (MLCT) transitions. (2) As shown in Table S1, coordination of a 4,4′-X2-bpy ligand diminishes the wavenumber of the lowest-energy MLCT band in the order of increasing donor strength of −X:
Besides, the asymmetry within the coordination sphere increases in the same order, resulting in splitting of all the visible absorption bands, as compared to [Ru(bpy)3](PF6)2 and [Ru(bpy)2(Mebpy-CN)](PF6)2. The splitting goes from 0.20 eV in complex 1 (λmax = 478 and 444 nm) to 0.24 eV in complex 2 (λmax = 488 and 446 nm) and to 0.44 eV in complex 3 (λmax = 532 and 448 nm). As explained in the Calculations section, the least energetic band can be assigned to a dπ(Ru) → π*(Mebpy-CN) MLCT transition, while the following band can be assigned to a dπ(Ru) → π*(4,4′-X2-bpy) MLCT transition. Absorption of visible light is thus extended to a broader range when increasing the donor strength of −X, making complex 3—with X = −N(CH3)2—almost black in color. The area below the two split bands is almost the same as that of the MLCT bands of [Ru(bpy)3](PF6)2. The maximum frequency of the lowest-lying MLCT band, νMLCT, and the electrochemical energy gap, ΔE1/2 (see Table 1), are correlated linearly, as expected since both values depend on the HOMO–LUMO gap. (30)

Spectroelectrochemistry

UV–vis–NIR spectroelectrochemical measurements for complexes 1–3 are shown in Figure 3. On one-electron oxidation, MLCT bands for all these complexes decrease in intensity, as expected upon changing the metal configuration from d6 to d5. Also, new bands arise at λmax between 300 and 400 nm, which can be assigned to intraligand charge transfer (ILCT) transitions in the oxidized species. Complex 3 exhibits a wide band at λmax from 600 to 1000 nm, which is not present in the other complexes. TD-DFT calculations of 33+ show a set of ligand-to-metal charge transfer (LMCT) transitions from a π(4,4′-DMA-bpy) to a dπ-Ru(III) orbital, as explained below in the Calculations Section. On the other hand, upon one-electron reduction, two new bands appear at λmax around 350 to 370 and 530 to 550 nm. Both are typical of bpy radical formation. (31)

Figure 3

Figure 3. (a) Oxidative difference spectra acquired at an applied potential 100 mV higher than the oxidative potential of the metallic center of complexes 1–3, respectively, in CH3CN solutions. (b) Reductive difference spectra acquired at an applied potential 100 mV lower than the first reductive potential of complexes 1–3, respectively, in CH3CN solutions.

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Photophysical Properties

Table 2 shows the photophysical properties of complexes 1–3 in CH3CN at room temperature, together with the previously studied complex with X = −H. (25) The room-temperature emission spectra of complexes 1–3 are depicted in Figure S13.
Table 2. Photophysical Properties of Complexes 1–3 and [Ru(Bpy)2(Mebpy-CN)](PF6)2 in CH3CN at rt
complexλem [nm]Φemτ [μs]kobs × 10–5 [s–1]kr × 10–5 [s–1]knr × 10–5 [s–1]
[Ru(bpy)2(Mebpy-CN)](PF6)26230.1200.7613.21.5811.6
16660.0240.6714.90.3614.5
27099.3 × 10–30.3231.30.2931.0
37982.0 × 10–40.024650.093465
Time-resolved differential absorption spectra of complexes 1–3, obtained by LFP, are shown in Figure 4. In all cases, bleaching of the MLCT bands and appearance of new bands at λmax = 370 nm are detected. This latter band is associated with ILCT transitions of the Mebpy-CN– radical and is unaffected by the electron-donor ability of −X in the substituted bpys. The spectroelectrochemical measurements described above show that upon one-electron oxidation and reduction of complexes 1–3, similar bands are detected; therefore, the lowest-lying MLCT excited state can be formulated as [RuIII(4,4′-X2-bpy)2(Mebpy-CN·)−]2+. As the electron-donor ability of X increases, the excited state is stabilized and the emission band shifts to longer wavelengths reaching the near-infrared region (see Table 2 and Figure S13). The emission wavenumbers correlate with the MLCT absorption wavenumbers.

Figure 4

Figure 4. Time-resolved differential absorption spectra of 1(top), 2 (middle), and 3 (bottom) in CH3CN solutions at different times (λexc = 370 nm) (left) and the absorbance decay at λ = 370 nm and mono-exponential fitting (right).

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As predicted by the energy gap law, (32) lowering of the energy gap between the excited and ground state is accompanied by an increase in the nonradiative kinetic constant, knr, and consequently a decrease of the excited state lifetime.

Calculations

DFT and TD-DFT calculations allowed us to explain the experimental results that depend on electronic structures. Geometries were assumed to be the same as those of a similar complex determined by crystal structure studies. (25) The molecular orbital energy levels of complexes 1–3 and [Ru(bpy)2(Mebpy-CN)](PF6)2, calculated by DFT, are shown in Figure 5.

Figure 5

Figure 5. Molecular orbital (MO) diagrams calculated by DFT for complexes 1–3 and [Ru(bpy)2(Mebpy-CN)](PF6)2. Different contributions of the groups are marked in different colors: ruthenium in black, 4,4′-(X)2-bpy in red, and Mebpy-CN in blue. The HOMO–LUMO energy gaps are also depicted.

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For all complexes (see Figure 5 and Table S2), the HOMOs are dπ-Ru(II)-type orbitals close in energy with some contribution of π orbitals of 4,4′-(X)2-bpy that increases with increasing electron-donor strength of X. LUMOs for all complexes are delocalized over the Mebpy-CN ligand. A small destabilization of the LUMO with an increase of the electron-donor strength of X is observed due to some mixing with π(4,4′-(X)2-bpy) orbitals. The overall effect of increasing the electron-donor strength of X is a decrease in the HOMO–LUMO gap (see Figure 5). Orbital pictures of HOMOs and LUMOs for complexes 1–3 are displayed in Figure 6. For example, the HOMO of complex 3 has a larger contribution of orbitals localized at the bpy ligand with dimethylamino substituents, indicating higher delocalization between electron-donor-substituted bpys and metal orbitals than that observed for the other two complexes. This is also reflected in Table S2, where participation of X2bpy in the HOMO increases from 10% in [Ru(bpy)2(Mebpy-CN)]2+ to 12% in 1, to 15% in 2, and to 40% in 3.

Figure 6

Figure 6. Orbital representations of LUMOs (top) and HOMOs (bottom) of complexes 1–3.

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TD-DFT calculations of complexes 1–3 were done to support assignments of electronic transitions. Calculated transitions and experimental UV–vis spectra of complexes 1–3 are depicted in Figure 7. As stated before, a higher amount of splitting of both visible bands is observed when the electron-donor strength of X grows. Since complex 3 has the larger asymmetry, it will be taken as the representative example to explain general features resulting from these calculations. The band at λmax = 532 nm in the spectrum of 3 is mainly assigned to HOMO-2 → LUMO, supporting the MLCT assignment of that band. The broad band at λmax = 448 nm in the experimental spectrum is assigned to a set of transitions from dπ-Ru(II) orbitals (HOMO, HOMO-1, and HOMO-2) to π* ligand orbitals (LUMO, LUMO+1, LUMO+2, and LUMO+3) (see Table S3). As a considerable mixing of π(4,4′-{N(CH3)2}2-bpy) and dπ-Ru(II) orbitals is predicted (see above), these transitions can be assigned as MLCT/ LLCT. Moreover, the band at λmax = 304 nm in the experimental spectrum can be assigned to a transition from HOMO to a 4,4′-(N(CH3)2-bpy-centered orbital (LUMO+8), being mostly MLCT in nature. Five strong transitions (f > 0.1) in the UV region (λmax ≈ 260 nm) confirm the ILCT π → π* assignment of the band at λmax = 261 nm in the experimental spectrum. The calculated extinction molar coefficients are overestimated by a factor of 3, as previously reported for similar complexes. (25)Table S4 and Figure S14 show TD-DFT calculations for the oxidized species 3+ that confirms the assignments discussed in the Spectroelectrochemistry Section.

Figure 7

Figure 7. TD-DFT calculated (straight line) and experimental (dashed line) UV–vis spectra of complexes 1–3.

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Application as Sensitizers for ZnO Nanowire-Based Solar Cells

Figure 8a,b shows the side and front SEM images, respectively, corresponding to a FTO|ZnO NW sample grown by the hydrothermal technique; both images prove that nanowires are formed. Besides, the morphology of the ZnO NWs shows the expected hexagonal wurtzite structure in agreement with the XRD results, shown in Figure 8c, with a mean diameter of 100 nm.

Figure 8

Figure 8. (a) Side and (b) front view SEM images of ZnO NWs grown using the hydrothermal technique. (c) XRD data of FTO|ZnO NWs compared with those of a bulk sample of the ZnO (commercial powder) spectrum.

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Electrodes of FTO|ZnO NWs were characterized by EDAX, UV–vis, and Raman spectra. The EDAX spectrum, shown in Figure S15, indicates only the presence of ZnO, evidencing the purity of this semiconductor. As shown in Figure 8c, the XRD patterns of ZnO NWs, as compared with those of a standard ZnO polycrystalline powder sample, indicate that the NWs grew with a preferential orientation normal to the (002) plane, corresponding to the c axis of the wurtzite structure. Secondary peaks, corresponding to (101) and (102) planes, are not present in the NW structure. The normalized UV–vis spectra of ZnO NWs sensitized with complexes 1–3 are displayed in Figure S16; they present bands in the visible region, which correspond to those previously measured in CH3CN solutions of complexes 1–3, as shown in Figure 2, which are somewhat masked by bands in the UV region due to the ZnO NWs.
Raman spectra of FTO|ZnO NW electrodes sensitized by complexes 1–3 are shown in Figure S17. In all cases, two bands are detected in the nitrile stretching frequency region, νC≡N: one at 2240 cm–1, which is identical with those measured in the FTIR spectra of the solid complexes, as described before, corresponding to free nitriles, and one at 2333 cm–1, which can be assigned to nitrile groups bound to the ZnO surface, indicating chemical adsorption of the complex on the semiconductor. This shift has been observed before when the complex with X = H was adhered to TiO2. (33) The appearance of an unbound dye can be attributed to the presence of only one anchoring nitrile group in all the complexes studied here. Ongoing work on the series [Ru(4,4′-X2-bpy)(Mebpy-CN)2](PF6)2 containing two anchoring nitrile groups will hopefully improve the quantity of sensitizers adsorbed on the surface and consequently the solar cell efficiency. It must be noted that the highest efficiency was detected in our previously studied Ru systems with two nitrile groups adsorbed on TiO2. (33)
The QE spectra of the solar cells, Figure 9a, were acquired under ambient temperature (25 ± 1 °C). The higher efficiencies were observed for incident radiation with the wavelength ranging from 450 to 600 nm, consistent with the UV–visible spectra of sensitized FTO |ZnO NW electrodes (Figure S16). A red shift in the QE band of the electrode is detected when going from 1 to 3, showing that the sample sensitized with complex 3 presents a higher value of efficiency than the other complexes for identical times of immersion and a better response to lower wavelengths. In Figure 9b, I–V curves for all the solar cells are presented: they were obtained under irradiation with a solar simulator (linear sweep voltammetry, 5 mV s–1) at 28 ± 1 °C. Table 3 shows the parameters obtained for these DSCs: under polychromatic irradiation, they all show an overall efficiency, η, of 0.03–0.017 %, open circuit voltages (Voc) in a 0.38–0.26 V range and short circuit current values between 0.17 and 0.14 mA/cm2. Considering these data, the best photoresponse was that of the cell sensitized by complex 3, according to its higher absorption in the NIR region. When comparing these data with the reference N719, the efficiencies were found to be lower by a factor of 10 for our complexes in the same conditions, as expected because of lower adsorption of the dyes, as discussed before. Although the efficiencies obtained are low, our data support the fact that panchromatic shifts in absorption lead to increasing photocurrent generation by modulating the electronic asymmetry by small structural changes in the sensitizer. We hope to improve the cell performances using complexes with two anchoring nitrile groups.

Figure 9

Figure 9. Performance of solar cells assembled with the sensitized FTO|ZnO NWs, Pt counter electrodes, and a liquid electrolyte: (a) quantum efficiency spectra and (b) current–potential (I–V) curves under polychromatic irradiation.

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Table 3. Parameters Obtained from I–V Curves for the Dye-Sensitized ZnO Solar Cells under Polychromatic Irradiation
dyeVoc (V)Isc (mA/cm2)Ptheoa(mW/cm2)Pmaxb (mW/cm2)ffcη (%)d
(1)0.260.143.72.10.580.016
(2)0.220.153.42.20.640.018
(3)0.380.176.63.90.590.030
a

Ptheo = Voc × Isc.

b

Pmax = Imax × Vmax.

c

ff = (Pmax/Ptheo).

d

η = overall efficiency.

It has already been reported that very low efficiencies are obtained when loading ruthenium polypyridyl complexes onto ZnO surfaces with anchoring carboxylate groups. (34) We demonstrate in this work that anchoring similar complexes with nitrile groups onto ZnO nanowires is a promising approach when combined with the occurrence of panchromatic shifting induced by small structural variations in the bipyridine rings as described in this work.

Conclusions

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Changing the electron-donor ability of substituent −X in heteroleptic complexes of the formula [Ru(4,4′-(X)2-bpy)2(Mebpy-CN)]2+, with X = −CH3, −OCH3, and −N(CH3)2, allows us to obtain a fine tuning of the electronic properties of this series of new complexes. Enhancing asymmetry induces absorption over all visible wavelengths and emission with appreciable quantum yields in the NIR region. The observed changes in redox potentials, MLCT absorption and emission maxima, and excited state dynamics can be rationalized using Hammett σp parameters of X, consistent with the important contribution of −X in modulating electron densities and orbital energies in these complexes, which can be used as dyes in DSCs over Zn NWs by anchoring the nitrile group of Mebpy-CN to the semiconductor surface. Enlarging the electron density asymmetry in these molecules by increasing the donor strength of −X gives more pronounced panchromatic shifting and higher photocurrent generation, a concept that may be useful for the development of more efficient DSCs.

Experimental Section

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Materials and Instrumentation

All the chemicals used in this work were of analytical reagent grade and were used as purchased. CH3CN was freshly distilled over KMnO4. Infrared spectra (4000–400 cm–1) of the solids were obtained using KBr pellets with a Perkin-Elmer FTIR RX-I spectrometer. Characterization of the new complexes were carried out in CH3CN solutions by spectroscopic, electrochemical, and photophysical techniques. Construction and instruments used for characterizing the dye-sensitized ZnO nanowire-based solar cells are described below. NMR spectra were obtained in CD3CN with a Bruker 500 MHz equipment operating at a frequency of 500.13 MHz for 1H NMR, 125.75 MHz for 13C NMR, and 50.67 MHz for 15N NMR. Electrospray ionization (ESI) mass spectra were recorded on a Bruker Esquire 6000 MS. Chemical analyses were carried out at INQUIMAE, University of Buenos Aires, Buenos Aires, Argentina, with an estimated error of ±0.5%. Electrochemical measurements were performed with BAS Epsilon EC equipment, using 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAH) as the supporting electrolyte in solutions that were thoroughly degassed with Ar prior to each measurement. A standard three-electrode arrangement was used with Pt as the working and auxiliary electrode and nonaqueous Ag/AgNO3 (0.01 M AgNO3 in CH3CN with 0.1 M of TBAH) as the reference electrode. For reversible electrochemical processes, reported E1/2 values were calculated from cyclic voltammetry (CV) measurements as the averages between the peak values corresponding to the cathodic (Ec) and anodic (Ea) waves: E1/2 = (Ec + Ea)/2. In cases where redox processes were quasi-reversible, potentials were obtained by differential pulse voltammetry (DPV). UV–vis absorption spectra were recorded on a Varian Cary 50 spectrophotometer using 1 cm quartz cells. Absorption spectra of solid films were measured in the same instrument using a solid sample holder for Cary 50/60. Raman spectra were obtained with a DXR spectrometer from Thermo Scientific equipped with a binocular Olympus microscope. Steady-state emission measurements were made at room temperature on a Shimadzu RF-5301 PC spectrofluorometer. Relative radiative quantum yields were calculated by eq 1
(1)
where Φr is the quantum yield of a reference ([Ru(bpy)3](PF6)2 in acetonitrile (Φr = 0.095). (35) UV–vis spectroelectrochemical experiments were performed in CH3CN (0.1 M TBAH) using a Honeycomb spectroelectrochemical cell (Pine Research) with Pt as the working and counter electrode and Ag/AgCl as the reference electrode. Lifetime and transient spectra of the lowest-lying (3)MLCT excited states were obtained with the laser flash photolysis (LFP) setup described previously. (36,37) Luminescence lifetimes were also determined by the time-correlated single photon counting (TCSPC) technique with a Tempro-01 apparatus from HORIBA Jobin Yvon (Glasgow, U.K.) using as the excitation pulse source an ultrafast 450(±15) nm Nanoled operating at 250 kHz. Fluorescence intensity decay was fitted with the fluorescence decay analysis software DAS6 of HORIBA Jobin Yvon by deconvolution of the pulse function using a single-exponential model function. All measurements were performed at room temperature in CH3CN solutions deoxygenated by bubbling high-purity Ar (>99.98, Indura SRL, Argentina) for 20 min. Computational calculations were carried out with Gaussian 03. (38) Molecular geometries were optimized using Becke’s three-parameter hybrid functional (39) with local terms of Lee, Yang, and Parr. (40) For light atoms, a 6-311G(d,p) basis set (41) was chosen, while for the ruthenium atom, LANL2DZ effective core potential with its corresponding basis set was used. (42) The effect of the solvent (CH3CN) was included using the conductor-like polarizable continuum model (CPCM). (43) Frequency calculations were performed to ensure that the optimized structure is a real minimum; no imaginary frequencies were found. No symmetry restrictions were imposed. From optimized structures, molecular orbital energies were determined and TD-DFT calculations were made. The contribution of the different groups of molecules to orbitals and electronic transitions were obtained using version 3.0 of the GaussSum software package. (44)

Syntheses

The precursors [Ru(4,4′-X2-bpy)2Cl2] were prepared by following previously published methods. (26,27) Mebpy-CN was prepared as reported previously. (25) 4,4′-(CH3)2-bpy and 4,4′-(OCH3)2-bpy were purchased from Sigma-Aldrich and 4,4′-{N(CH3)2}2-bpy from Biosynth.

(i) [Ru(4,4′-(CH3)2-bpy)2(Mebpy-CN)](PF6)2 (1)

A mixture of 26 mg of Mebpy-CN (0.13 mmol) and 72 mg of [Ru(4,4′-(CH3)2-bpy)2Cl2] (0.13 mmol) was added to 30 mL of deaerated methanol and heated at reflux under Ar for 4 h. The obtained orange solution was rotaevaporated to dryness, dissolved in a minimum amount of water, and passed through a CM Sephadex C-25 column. The complex was eluted with 1 M HCl, and the collected fractions were rotaevaporated to dryness, dissolved in a minimum amount of water, and precipitated with a concentrated aqueous solution of KPF6 (1 g in 5 mL of water). The mixture was kept in the fridge overnight and filtered; the obtained orange powder was stored in vacuum over P4O10 for 1 day. Yield: 68 mg (54%). Chemical analyses and NMR were coherent with the formula [Ru(4,4′-(CH3)2-bpy)2(Mebpy-CN)](PF6)2.(KPF6)0.5. Anal. calcd (found) for C36H33N7P2.5F15K0.5Ru: C, 41.3 (41.9); H, 3.2(2.5); N, 9.4 (8.1). 1H-NMR (500.13 MHz, CD3CN) 2.56 (s, 12H), 2.58(s, 3H), 7.26 (br, 4H), 7.33 (d, 6.1 MHz, 1H), 7.53 (br, 4H), 7.59 (d, 6.1, 1H), 7.61 (d, 6.0 1H), 7.98 (d, 6.0 Hz, 1H), 8.38 (br, 4H), 8.42 (s, 1H), 8.76 (s, 1H). 13C-NMR (125.75 MHz, CD3CN): 20.5, 20.1, 115.8, 124.9, 125.4, 125.8, 128.2, 128.3, 129.1, 150.4, 150.7, 150.9, 152.3, 152.9, 155.3, 156.5, 158.9. 15N-NMR (50.67 MHz, CD3CN): 241.7, 242.8, 269.0. Positive ESI-MS ion clusters at m/z: 809.8 {M – PF6}+, 332.5 {M – 2PF6}2+.

(ii) [Ru(4,4′-(OCH3)2-bpy)2(Mebpy-CN)](PF6)2 (2)

A mixture of 20 mg of Mebpy-CN (0.10 mmol) and 60 mg of [Ru(4,4′-(OCH3)2-bpy)2Cl2] (0.10 mmol) was added to 30 mL of deaerated methanol and heated at reflux under Ar for 4 h. The obtained reddish-orange solution was rotaevaporated to dryness, dissolved in a minimum amount of water, and passed through a CM Sephadex C-25 column. The complex was eluted with 1 M HCl, and the collected fractions were rotaevaporated to dryness, dissolved in a minimum amount of water, and precipitated with a concentrated aqueous solution of KPF6 (1 g in 5 mL of water). The mixture was kept in the refrigerator overnight and filtered; the obtained dark orange powder was stored in vacuum over P4O10 for 1 day. Yield: 46 mg (45%). Chemical analyses and NMR were coherent with the formula [Ru(4,4′-(OCH3)2-bpy)2(Mebpy-CN)](PF6)2.(KPF6). Anal. calcd (found) for C36H33N7O4P3F18KRu: C, 36.0 (35.9); H, 2.8(2.9); N, 8.2 (10.4). 1H-NMR (500.13 MHz, CD3CN) 2.58 (s, 3H), 4.01 (s, 12H), 6.93 (br, 2H), 7.01 (br, 2H), 7.32 (br, 1H), 7.44 (br, 4H), 7.66 (d, 5.9 Hz, 1H), 7.77 (d, 5.9 Hz, 1H), 7.98 (br, 4H), 8.49 (s, 1H), 8.76 (s, 1H). 13C-NMR (125.75 MHz, CD3CN): 20.1, 56.7, 111.0, 113.7, 121.2, 125.5, 128.6, 149.8, 150.8, 152.2, 152.5, 158.1, 159.0, 167.1, 167.3. 15N-NMR (50.67 MHz, CD3CN): 225.2, 226.6, 244.1, 269.4. Positive ESI-MS ion clusters at m/z: 873.9 {M – PF6}+.

(iii) [Ru(4,4′-{N(CH3)2}2-bpy)2(Mebpy-CN)](PF6)2 (3)

A mixture of 47 mg of [Ru(4,4′-{N(CH3)2}2-bpy)2Cl2]Cl (0.067 mmol), 14 mg of Mebpy-CN (0.071 mmol), and 0.2 mL of triethylamine was added to 10 mL of deaereated ethanol and heated to reflux under Ar for 4 h. The obtained dark red solution was rotaevaporated to a minimum volume, recrystallized with ether, and filtered. The solid was dissolved in 5 mL of water and precipitated with a concentrated aqueous solution of NH4PF6 (1 g in 5 mL of water). The mixture was kept in a refrigerator overnight and filtered; the obtained dark red powder was stored in vacuum over P4O10 for 1 day. Yield: 26 mg (36%). Chemical analyses and NMR were coherent with the formula [Ru(4,4′-{N(CH3)2}2-bpy)2(Mebpy-CN)](PF6)2.H2O. Anal. calc (found) for C40H47N11OP2F12Ru: C, 44.1 (44.1); H, 4.3(4.2); N, 13.4 (14.1). 1H-NMR (500.13 MHz, CD3CN): 2.56 (s, 3H), 3.13 (s, 12H), 3.17 (s, 12H), 6.49 (br, 2H), 6.63 (br, 2H), 7.03 (br, 2H), 7.23 (br, 2H), 7.29 (d, 1H), 7.51 (br, 2H), 7.53 (br, 2H), 7.63 (d, 1H), 7.76 (d, 1H), 8.10 (d, 1H), 8.47 (s, 1H), 8.71 (s, 1H). 13C-NMR (125.75 MHz, CD3CN): 20.1, 39.1, 105.8, 109.3, 121.1, 124.0, 124.9, 128.0, 148.2, 148.9, 149.5, 150.6, 152.3, 154.2, 156.6, 157.0, 159.4. 15N-NMR (50.67 MHz, CD3CN): 68.0, 204.1, 205.9, 252.0, 272.2. Positive ESI-MS ion clusters at m/z: 925.9 {M – PF6}+, 390.3 {M – 2PF6}2+.

FTO |ZnO NWs

For ZnO NW growth, glass-FTO substrates (Sigma-Aldrich) of 2 × 1.5 cm were used. They were initially cleaned in ultrasonic baths of ethanol and acetone for 3 min. Then, seeds of ZnO were made on glass/FTO using a spin-coating deposit of an ethanolic solution of 20 mM zinc acetate dihydrate (spinning at 1500 rpm during 1 min) followed by drying at 125 °C for 1 min. This procedure was repeated 5 times, and finally the substrates were heated at 125 °C for 4 h to improve the seed adherence. In a second step, ZnO NWs were synthesized by a solvothermal method: ZnO-seeded substrates were put in 25 mL polytetrafluoroethylene (PTFE) vessels, which were then filled with a solution of distilled water (DW), hexamethylenetetramine (HMTA), and zinc nitrate dihydrate (0.5 M, aqueous solutions). The vessels were placed inside a stainless steel autoclave and heated at 125 °C for 4 h. The autoclaves were then cooled down to room temperature, and after an hour, each sample was taken out of the vessel, rinsed with DW, and dried in air.
Characterization of the FTO|ZnO NWs was carried out using a Zeiss Supra 55VP scanning electron microscope (SEM). X-ray powder diffraction analyses between 30 and 40° were carried out in an X-ray diffraction (XRD) RIGAKU Mini Flex 600 instrument using Cu Kα1 radiation. UV–visible, and Raman measurements of films were made in the equipment already described.

Ru Complexes/ZnO Solar Cells

In order to assemble the DSCs, the FTO/ZnO NW samples were heated at 100 °C for 60 min to remove traces of water and then immersed in solutions of complexes 1–3 in CH3CN (C = 1.2 mM) for 24 h. Samples were rinsed with DW and dried in air to obtain the different sensitized electrodes. They were characterized by Raman and UV–vis absorption techniques.
Solar cells were subsequently assembled with an area of 1 cm2 as described in Figure S1: a glass-FTO with a layer of Pt (deposited by sputtering) was used as a counter electrode, and a solution 0.5 M KI/0.005 M I2 in acetonitrile was used as the electrolyte. Acetonitrile was chosen as the solvent since most data for DSCs are reported in this media, (3) and no experimental evidence of adsorption of this solvent on the ZnO surface has been obtained. Double-sided tape (thickness of 60 μm) was used as an electrode separator. Solar cells were characterized using I–V (current–potential) curves and quantum efficiency (QE) measurements as described previously. (33)
The performance of each DSC for solar energy conversion was evaluated using the quantum efficiency/incident photon to charge carrier efficiency (QE/IPCE) Newport measurement kit, consisting of a 300 W ozone-free Xe lamp, chopper, monochromator, detector, and other accessories. The cells were irradiated using a “homemade” solar simulator, which was assembled with a metallic vapor lamp (Metalarc Sylvania HIS-YHX 400 W). Considering the distance from the lamp and solar cell (15 cm), the polychromatic irradiance was estimated to be 130 ± 5 mW cm–2.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00243.

  • Construction of DSCs, FTIR spectrum of complex 3, 1D and 2D NMR spectra and assignments of complex 3, UV–vis absorption data of complexes 1–3, CV and DPV curves of complex 3, emission spectra of complexes 1–3, TD-DFT calculations and assignments of complexes 1–3 and 3+, EDAX of ZnO NWs, UV–vis absorption spectra of bare and sensitized FTO |ZnO NWs, and Raman spectra of sensitized FTO |ZnO NWs (PDF).

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Author Information

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  • Corresponding Author
    • Néstor E. Katz - INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina;  Orcidhttp://orcid.org/0000-0001-5576-2692;  Email: [email protected]
  • Authors
    • Fernando F. Salomón - INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina
    • Nadia C. Vega - INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina;  Present Address: Facultad de Ciencias Exactas y Tecnología, Laboratorio de Física del Sólido, INFINOA (CONICET-UNT), Universidad Nacional de Tucumán, 4000 San Miguel de Tucumán, Argentina
    • Teodor Parella - Servei de RMN, Universitat Autonóma de Barcelona, Bellaterra E-08193, Barcelona, Spain;  Orcidhttp://orcid.org/0000-0002-1914-2709
    • Faustino Eduardo Morán Vieyra - INBIONATEC (UNSE-CONICET), Universidad Nacional de Santiago del Estero, RN 9, Km 1125, (4206) Santiago del Estero, Argentina
    • Claudio D. Borsarelli - INBIONATEC (UNSE-CONICET), Universidad Nacional de Santiago del Estero, RN 9, Km 1125, (4206) Santiago del Estero, Argentina
    • Claudia Longo - Institute of Chemistry, University of Campinas, UNICAMP, C. Postal 6154, 13083-970 Campinas, Sao Paulo, Brazil
    • Mauricio Cattaneo - INQUINOA (UNT- CONICET), Instituto de Química Física, Facultad de Bioquímica, Química y Farmacia, Universidad Nacional de Tucumán, Ayacucho 471, (T4000INI) San Miguel de Tucumán, Argentina
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We thank CONICET (Grant PIP-2015-098), FONCyT (Grant PICT-2016-0553), and UNT (Grant PIUNT 26D/620) for financial support. F.F.S. and N.C.V. thank CONICET for fellowships. M.C., F.E.M.V., C.D.B., and N.E.K. are members of the Research Career (CONICET). T.P. acknowledges financial support for this research provided by Spanish MINECO (Project PGC2018-095808-B-I00). We thank Dr. Miriam Pérez (UAB) for MS measurements, Dr. Mónica Tirado (FACET, UNT) for letting us use their facilities, and Dr. Carolina Navarro (FQByF, UNT) for XRD measurements. We also thank Bioq. Luciano Martinez and Ing. Hernán Esquivel (CIME-CONICET-UNT) for SEM and EDS measurements.

References

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    Bessho, T.; Yoneda, E.; Yum, J.-H.; Guglielmi, M.; Tavernelli, I.; Imai, H.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. New Paradigm in Molecular Engineering of Sensitizers for Solar Cell Applications. J. Am. Chem. Soc. 2009, 131, 5930– 5934,  DOI: 10.1021/ja9002684
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    New Paradigm in Molecular Engineering of Sensitizers for Solar Cell Applications
    Bessho, Takeru; Yoneda, Eiji; Yum, Jun-Ho; Guglielmi, Matteo; Tavernelli, Ivano; Imai, Hachiro; Rothlisberger, Ursula; Nazeeruddin, Mohammad K.; Gratzel, Michael
    Journal of the American Chemical Society (2009), 131 (16), 5930-5934CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A novel thiocyanate-free cyclometalated Ru sensitizer for solar cells was developed. Upon anchoring to nanocryst. TiO2 films, it exhibits a remarkable incident monochromatic photon-to-current conversion efficiency of 83%. The solar cell employing a liq.-based electrolyte exhibits a short-circuit photocurrent d. of 17 mA/cm2, an open-circuit voltage of 800 mV, and a fill factor of 0.74, corresponding to an overall conversion efficiency of 10.1% at std. AM 1.5 sunlight. To understand the structural, electronic and optical properties of the cyclometallated Ru sensitizer, DFT and time-dependent DFT (TDDFT), were used. The results show that the HOMO is located mostly on Ru and the cyclometalated ligand, while the LUMO is on 4-carboxylic acid-4'-carboxylate-2,2'-bipyridine. MOs anal. confirmed the exptl. assignment of redox potentials and TDDFT calcns. allowed assignment of the visible absorption bands. The present findings provide new design criteria for the next generation of Ru sensitizers and promotes engineering of new sensitizers that interact effectively with the I-/I3- redox couple.
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    Pal, A. K.; Nag, S.; Ferreira, J. G.; Brochery, V.; La Ganga, G.; Santoro, A.; Serroni, S.; Campagna, S.; Hanan, G. S. Red-Emitting [Ru(bpy)2(N-N)]2+ Photosensitizers:Emission from a Ruthenium(II) to 2,2′-Bipyridine 3MLCT State in the Presence of Neutral Ancillary “Super Donor” Ligands. Inorg. Chem. 2014, 53, 1679– 1689,  DOI: 10.1021/ic4028332
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    Red-Emitting [Ru(bpy)2(N-N)]2+ Photosensitizers: Emission from a Ruthenium(II) to 2,2'-Bipyridine 3MLCT State in the Presence of Neutral Ancillary "Super Donor" Ligands
    Pal, Amlan K.; Nag, Samik; Ferreira, Janaina G.; Brochery, Victor; La Ganga, Giuseppina; Santoro, Antonio; Serroni, Scolastica; Campagna, Sebastiano; Hanan, Garry S.
    Inorganic Chemistry (2014), 53 (3), 1679-1689CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The synthesis and characterization of a novel family of [RuII(bpy)2(N-N)](PF6)2 (bpy = 2,2'-bipyridine) complexes are reported, where N-N = pyridine/pyrimidine/pyrazine functionalized in different positions with the electron-donating bicyclic hexahydropyrimidopyrimidine (hpp) unit. Bidentate ligands 1a-5a were synthesized in good to high yields (55-96%). The corresponding complexes 1b, 2b, and 5b were prepd. in n-butanol, while complexes 3b and 4b were prepd. in a mixt. of n-butanol and water (1/1, vol./vol.) in modest to good yields (23-76%). Both ligand and complex structures were fully characterized by a variety of techniques, including x-ray crystallog. In cyclic voltammetric studies, all the complexes exhibit a RuIII/II couple, which is ∼500 mV less pos. than the RuIII/II couple in Ru(bpy)32+. The 1MLCT and 3MLCT states of all of the complexes (530-560 nm/732-745 nm) are shifted bathochromically in comparison to those of Ru(bpy)32+ (450 nm/620 nm). These values are in good agreement with DFT and TD-DFT calcns.
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    A low-cost, high-efficiency solar cell based on dye-sensitized colloidal titanium dioxide films
    O'Regan, Brian; Graetzel, Michael
    Nature (London, United Kingdom) (1991), 353 (6346), 737-40CODEN: NATUAS; ISSN:0028-0836.
    A photoelectrochem. cell was fabricated from low- to medium-purity materials through low-cost processes, which exhibits a com. realistic energy-conversion efficiency. The device is based on a 10-μm-thick, optically transparent film of TiO2 particles, coated with a monolayer of a charge-transfer dye to sensitize the film for light harvesting. The device harvests a high proportion of the incident solar energy flux (46%) and shows exceptionally high efficiencies for the conversion of incident photons to elec. current (>80%). The overall light-to-elec. energy conversion yield is 7.1-8.9% in simulated solar light and 12% in diffuse daylight. The c.d. >12 mA/cm2 and exceptional stability (sustaining ≥5 million turnover without decompn.), and low cost, make practical applications feasible.
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    Photoelectrochemical cells
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    A review discussing the historical background, and present status and development prospects of new generation of photoelectrochem. cells.
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    Özgür, Ü.; Hofstetter, D.; Morkoç, H. ZnO Devices and Applications : A Review of Current Status and Future Prospects. Proc. IEEE 2010, 98, 1255– 1268,  DOI: 10.1109/JPROC.2010.2044550
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    ZnO devices and applications: a review of current status and future prospects
    Ozgur, Umit; Hofstetter, Daniel; Morkoc, Hadis
    Proceedings of the IEEE (2010), 98 (7), 1255-1268CODEN: IEEPAD; ISSN:0018-9219. (Institute of Electrical and Electronics Engineers)
    A review. ZnO is an attractive material for applications in electronics, photonics, acoustics, and sensing. In optical emitters, its high exciton binding energy (60 meV) gives ZnO an edge over other semiconductors such as GaN if reproducible and reliable p-type doping in ZnO were to be achieved, which currently remains to be the main obstacle for realization of bipolar devices. On the electronic side, ZnO holds some potential in transparent thin film transistors (TFTs) owing to its high optical transmittivity and high cond. Among the other promising areas of application for ZnO are acoustic wave devices, due to large electromech. coupling in ZnO, and devices utilizing nanowires/nanorods such as biosensors and gas sensors and solar cells, since it is relatively easy to produce such forms of ZnO nanostructures, which have good charge carrier transport properties and high cryst. quality. Despite the significant progress made, there is still a no. of important issues that need to be resolved before ZnO can be transitioned to com. use, not to mention the stiff competition it is facing with GaN, which is much more mature in terms of devices. In this paper, recent progress in device applications of ZnO is discussed and a review of crit. issues for realization of ZnO-based devices is given.
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    Rodnyi, P. A.; Khodyuk, I. V. Optical and Luminescence Properties of Zinc Oxide ( Review ). Opt. Spectrosc. 2011, 111, 776– 785,  DOI: 10.1134/S0030400X11120216
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    Optical and luminescence properties of zinc oxide (Review)
    Rodnyi, P. A.; Khodyuk, I. V.
    Optics and Spectroscopy (2011), 111 (5), 776-785CODEN: OPSUA3; ISSN:0030-400X. (MAIK Nauka/Interperiodica)
    We generalize and systematize basic exptl. data on optical and luminescence properties of ZnO single crystals, thin films, powders, ceramics, and nanocrystals. We consider and study mechanisms by which two main emission bands occur, a short-wavelength band near the fundamental absorption edge and a broad long-wavelength band, the max. of which usually lies in the green spectral range. We det. a relationship between the two luminescence bands and study in detail the possibility of controlling the characteristics of ZnO by varying the max. position of the short-wavelength band. We show that the optical and luminescence characteristics of ZnO largely depend on the choice of the corresponding impurity and the parameters of the synthesis and subsequent treatment of the sample. Prospects for using zinc oxide as a scintillator material are discussed. Addnl., we consider exptl. results that are of principal interest for practice.
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    Lee, J.-H.; Ko, K.-H.; Park, B.-O. Electrical and Optical Properties of ZnO Transparent Conducting Films by the Sol – Gel Method. J. Cryst. Growth 2003, 247, 119– 125,  DOI: 10.1016/S0022-0248(02)01907-3
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    Electrical and optical properties of ZnO transparent conducting films by the sol-gel method
    Lee, Jin-Hong; Ko, Kyung-Hee; Park, Byung-Ok
    Journal of Crystal Growth (2003), 247 (1-2), 119-125CODEN: JCRGAE; ISSN:0022-0248. (Elsevier Science B.V.)
    The effect of drying conditions and the first- and second-heat treatment on the structural, elec. and optical properties of ZnO thin films prepd. by the sol-gel method was investigated. ZnO films were deposited on silica glass substrates by the spin-coating method. Zinc acetate dihydrate, 2-methoxyethanol and monoethanolamine were used as a starting material, solvent and stabilizer, resp. ZnO films were dried at 350°C and then heated at 600°C showed an extremely sharp (002) peak in the XRD patterns. The more films were oriented preferentially along (002) direction, the greater their elec. and optical properties became. By applying the second-heat treatment in nitrogen with 5% hydrogen at 500°C, the grain size of films was increased, the resistivity value was 0.099 Ω cm and optical transmittances became higher than 85% in the visible range.
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    Wisz, G.; Virt, I.; Sagan, P.; Potera, P.; Yavorskyi, R. Structural, Optical and Electrical Properties of Zinc Oxide Layers Produced by Pulsed Laser Deposition Method. Nanoscale Res. Lett. 2017, 12, 253,  DOI: 10.1186/s11671-017-2033-9
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    Structural, Optical and Electrical Properties of Zinc Oxide Layers Produced by Pulsed Laser Deposition Method
    Wisz G; Virt I; Sagan P; Potera P; Yavorskyi R; Virt I; Yavorskyi R
    Nanoscale research letters (2017), 12 (1), 253 ISSN:1931-7573.
    The structural, optical, and electrical properties of zinc oxide (ZnO) layers manufactured at different process conditions were investigated. ZnO epitaxial layers were grown on silicon, glass, and ITO/glass substrates by pulsed laser deposition (PLD) technique. The influence of power beam, substrate temperature, and deposition time on films properties was analysed. Morphological features of the film surface were investigated by scanning electron microscopy. A structural study shown planar orientation of films at low temperatures of substrate, but the columnar type of growth originated in temperature enhances. Electrical properties were determined in the temperature range 300-500 K. It was shown that the type of films conductivity is metallic and it is limited by charge transfer across grain boundaries.
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    He, J. H.; Hsin, C. L.; Liu, J.; Chen, L. J.; Wang, Z. L. Piezoelectric Gated Diode of a Single ZnO Nanowire. Adv. Mater. 2007, 19, 781– 784,  DOI: 10.1002/adma.200601908
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    Piezoelectric gated diode of a single ZnO nanowire
    He, H., Jr.; Hsin, Cheng L.; Liu, Jin; Chen, Lih J.; Wang, Zhong L.
    Advanced Materials (Weinheim, Germany) (2007), 19 (6), 781-784CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A ZnO nanowire behaves like a rectifier under bending strain, as demonstrated by its current-voltage characteristics. This is interpreted with the consideration of a piezoelectricity-induced potential energy barrier at the interface of the conductive tip and nanowire. Under appropriate bending and voltage control, each NW could correspond to a device element for random-access-memory, diode, and force-sensor applications.
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    Emanetoglu, N. W.; Gorla, C.; Liu, Y.; Liang, S.; Lu, Y. Epitaxial ZnO Piezoelectric Thin Films for Saw Filters. Mater. Sci. Semicond. Process. 1999, 2, 247– 252,  DOI: 10.1016/S1369-8001(99)00022-0
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    Epitaxial ZnO piezoelectric thin films for SAW filters
    Emanetoglu, N. W.; Gorla, C.; Liu, Y.; Liang, S.; Lu, Y.
    Materials Science in Semiconductor Processing (1999), 2 (3), 247-252CODEN: MSSPFQ; ISSN:1369-8001. (Elsevier Science Ltd.)
    ZnO is a wide bandgap semiconductor material with high piezoelec. coupling coeffs. It can be used for making low-loss surface acoustic wave (SAW) filters operating at high frequency. The authors report MOCVD growth of epitaxial ZnO thin films on R-plane sapphire substrates. The crystallinity and orientation of the films, as well as the epitaxial relation between the ZnO films and the R-plane Al2O3 substrate were studied using x-ray diffraction techniques. An atomically sharp interface structure was revealed by high-resoln. TEM. Surface morphol. was studied using field emission SEM. SAW filters with 10 and 16 μm wavelength were fabricated. Low insertion loss and high piezoelec. coupling coeff., up to 6%, were achieved. The acoustic velocities range from 4050 to 5800 m/s, varying as a function of ZnO film thickness.
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    Vega, N. C.; Wallar, R.; Caram, J.; Grinblat, G.; Tirado, M.; LaPierre, R. R.; Comedi, D. ZnO Nanowire Co-Growth on SiO2 and C by Carbothermal Reduction and Vapour Advection. Nanotechnology 2012, 23, 275602,  DOI: 10.1088/0957-4484/23/27/275602
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    ZnO nanowire co-growth on SiO2 and C by carbothermal reduction and vapour advection
    Vega, N. C.; Wallar, R.; Caram, J.; Grinblat, G.; Tirado, M.; La Pierre, R. R.; Comedi, D.
    Nanotechnology (2012), 23 (27), 275602/1-275602/11CODEN: NNOTER; ISSN:1361-6528. (Institute of Physics Publishing)
    Vertically aligned ZnO nanowires (NWs) were grown on Au-nanocluster-seeded amorphous SiO2 films by the advective transport and deposition of Zn vapors obtained from the carbothermal reaction of graphite and ZnO powders. Both the NW vol. and visible-to-UV photoluminescence ratio were found to be strong functions of, and hence could be tailored by, the (ZnO + C) source-SiO2 substrate distance. We observe C flakes on the ZnO NWs/SiO2 substrates which exhibit short NWs that developed on both sides. The SiO2 and C substrates/NW interfaces were studied in detail to det. growth mechanisms. NWs on Au-seeded SiO2 were promoted by a rough ZnO seed layer whose formation was catalyzed by the Au clusters. In contrast, NWs grew without any seed on C. A correlation comprising three orders of magnitude between the visible-to-UV photoluminescence intensity ratio and the NW vol. is found, which results from a characteristic Zn partial pressure profile that fixes both O deficiency defect concn. and growth rate.
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    Grinblat, G.; Capeluto, M. G.; Tirado, M.; Bragas, A. V.; Comedi, D. Hierarchical ZnO Nanostructures : Growth Mechanisms and Surface Correlated Photoluminescence. Appl. Phys. Lett. 2012, 100, 233116,  DOI: 10.1063/1.4724195
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    Hierarchical ZnO nanostructures: Growth mechanisms and surface correlated photoluminescence
    Grinblat, G.; Capeluto, M. G.; Tirado, M.; Bragas, A. V.; Comedi, D.
    Applied Physics Letters (2012), 100 (23), 233116/1-233116/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    ZnO nanowires were grown by vapor-transport and deposition on Au nanocluster covered fused and thermal SiO2 and c-Si. The nanowire size and d. depended strongly on the substrate type. By decreasing the O2 to local Zn partial pressure ratio, the growth pattern changed to nanocombs and nanosheets. ZnO nanohedgehogs were found on bare c-Si. The authors observe a remarkable correlation between the defect to exciton photoluminescence intensity ratio and the nanostructures sp. surface areas. Changes in strain and O deficiency defects at surfaces are behind the obsd. morphol. changes, one to two-dimensional growth transition, and corresponding luminescence. (c) 2012 American Institute of Physics.
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    Vega, N. C.; Tirado, M.; Comedi, D.; Rodriguez, A.; Rodriguez, T.; Hughes, G. M.; Grovenor, C. R. M.; Audebert, F. Electrical , Photoelectrical and Morphological Properties of ZnO Nanofiber Networks Grown on SiO2 and on Si Nanowires. Mater. Res. 2013, 16, 597– 602,  DOI: 10.1590/S1516-14392013005000030
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    Electrical, photoelectrical and morphological properties of ZnO nanofiber networks grown on SiO2 and on Si nanowires
    Vega, Nadia Celeste; Tirado, Monica; Comedi, David; Rodriguez, Andres; Rodriguez, Tomas; Hughes, Gareth M.; Grovenot, Chris R. M.; Audebert, Fernando
    Materials Research (Sao Carlos, Brazil) (2013), 16 (3), 597-602CODEN: MAREFW; ISSN:1516-1439. (Materials Research)
    ZnO nanofiber networks (NFNs) were grown by vapor transport method on Si-based substrates. One type of substrate was SiO2 thermally grown on Si and another consisted of a Si wafer onto which Si nanowires (NWs) had been grown having Au nanoparticles catalysts. The ZnO-NFN morphol. was obsd. by SEM on samples grown at 600 and 720 °C substrate temp., while an focused ion beam was used to study the ZnO NFN/Si NWs/Si and ZnO NFN/SiO2 interfaces. Photoluminescence, elec. conductance and photoconductance of ZnO-NFN was studied for the sample grown on SiO2. The photoluminescence spectra show strong peaks due to exciton recombination and lattice defects. The ZnO-NFN presents quasi-persistent photocond. effects and ohmic I-V characteristics which become nonlinear and hysteretic as the applied voltage is increased. The elec. conductance as a function of temp. can be described by a modified three dimensional variable hopping model with nanometer-ranged typical hopping distances.
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    Vega, N. C.; Marin, O.; Tosi, E.; Grinblat, G.; Mosquera, E.; Moreno, M. S.; Tirado, M.; Comedi, D. The Shell Effect on the Room-Temperature Photoluminescence from ZnO/MgO Core/Shell Nanowires: Exciton-Phonon Coupling and Strain. Nanotechnology 2017, 28, 275702,  DOI: 10.1088/1361-6528/aa7454
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    The shell effect on the room temperature photoluminescence from ZnO/MgO core/shell nanowires: exciton-phonon coupling and strain
    Vega, N. C.; Marin, O.; Tosi, E.; Grinblat, G.; Mosquera, E.; Moreno, M. S.; Tirado, M.; Comedi, D.
    Nanotechnology (2017), 28 (27), 275702/1-275702/9CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
    The room temp. photoluminescence from ZnO/MgO core/shell nanowires (NWs) grown by a simple two-step vapor transport method was studied for various MgO shell widths (w). Two distinct effects induced by the MgO shell were clearly identified. The first one, related to the ZnO/MgO interface formation, is evidenced by strong enhancements of the zero-phonon and first phonon replica of the excitonic emission, which are accompanied by a total suppression of its second phonon replica. This effect can be explained by the redn. of the band bending within the ZnO NW core that follows the removal of atm. adsorbates and assocd. surface traps during the MgO growth process on one hand, and a reduced exciton-phonon coupling as a result of the mech. stabilization of the outermost ZnO NW monolayers by the MgO shell on the other hand. The second effect is the gradual increase of the excitonic emission and decrease in the defect related emission by up to two and one orders of magnitude, resp., when w is increased in the ∼3-17 nm range. Uniaxial strain build-up within the ZnO NW core with increasing w, as detected by x-ray diffraction measurements, and photocarrier tunneling escape from the ZnO core through the MgO shell enabled by defect-states are proposed as possible mechanisms involved in this effect. These findings are expected to be of key significance for the efficient design and fabrication of ZnO/MgO NW heterostructures and devices.
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    Grinblat, G.; Bern, F.; Barzola-quiquia, J.; Tirado, M.; Comedi, D.; Esquinazi, P. Luminescence and Electrical Properties of Single ZnO/MgO Core/Shell Nanowires. Appl. Phys. Lett. 2014, 104, 103113,  DOI: 10.1063/1.4868648
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    Luminescence and electrical properties of single ZnO/MgO core/shell nanowires
    Grinblat, Gustavo; Bern, Francis; Barzola-Quiquia, Jose; Tirado, Monica; Comedi, David; Esquinazi, Pablo
    Applied Physics Letters (2014), 104 (10), 103113/1-103113/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
    To neutralize the influence of the surface of ZnO nanowires for photonics and optoelectronic applications, we have covered them with insulating MgO film and individually contacted them for elec. characterization. Such a metal-insulator-semiconductor-type nanodevice exhibits a high diode ideality factor of 3.4 below 1 V. MgO shell passivates ZnO surface states and provides confining barriers to electrons and holes within the ZnO core, favoring excitonic UV radiative recombination, while suppressing defect-related luminescence in the visible and improving elec. cond. The results indicate the potential use of ZnO/MgO nanowires as a convenient building block for nano-optoelectronic devices. (c) 2014 American Institute of Physics.
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    Anta, J. A.; Guillén, E.; Tena-Zaera, R. ZnO-Based Dye-Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 11413– 11425,  DOI: 10.1021/jp3010025
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    ZnO-Based Dye-Sensitized Solar Cells
    Anta, Juan A.; Guillen, Elena; Tena-Zaera, Ramon
    Journal of Physical Chemistry C (2012), 116 (21), 11413-11425CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    A review. ZnO was one of the first metal oxides used in dye-sensitized solar cells. It exhibits a unique combination of potentially interesting properties such as high bulk electron mobility and probably the richest variety of nanostructures based on a very wide range of synthesis routes. However, in spite of the huge amt. of literature produced in the past few years, the reported efficiencies of ZnO-based solar cells are still far from their TiO2 counterparts. The origin of this striking difference in performance is analyzed and discussed with the perspective of future applications of ZnO in dye-sensitized solar cells and related devices. In this regard, a change of focus of the current research on ZnO-based dye-sensitized solar cells (from morphol. to surface control) is suggested.
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    Zhang, Q.; Dandeneau, C. S.; Zhou, X.; Cao, G. ZnO Nanostructures for Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, 4087– 4108,  DOI: 10.1002/adma.200803827
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    ZnO Nanostructures for Dye-Sensitized Solar Cells
    Zhang, Qifeng; Dandeneau, Christopher S.; Zhou, Xiaoyuan; Cao, Guozhong
    Advanced Materials (Weinheim, Germany) (2009), 21 (41), 4087-4108CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review of recent developments in the use of ZnO nanostructures for dye-sensitized solar cells (DSSCs). Carefully designed and fabricated nanostructured ZnO films are advantageous for use as a DSSC photoelectrode since they offer larger surface areas than bulk film material, direct electron pathways, or effective light-scattering centers, and, when combined with TiO2, produce a core-shell structure that decreases the combination rate. The limitations of ZnO-based DSCs are also discussed and several possible methods are proposed so as to expand the knowledge of ZnO to TiO2, motivating further improvement in the power-conversion efficiency of DSCs.
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  24. 24
    Vega, N. C.; Mecchia Ortiz, J. H.; Tirado, M.; Katz, N. E.; Comedi, D. ZnO Nanowire Sensitization with Ru Polypyridyl Complexes : Charge-Transfer Probed by Spectral and Relaxation Photocurrent Measurements. Mater. Res. Express 2018, 5, 075020  DOI: 10.1088/2053-1591/aacfa5
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    24
    ZnO nanowire sensitization with Ru polypyridyl complexes: charge-transfer probed by spectral and relaxation photocurrent measurements
    Vega, N. C.; Ortiz, Juan H. Mecchia; Tirado, M.; Katz, Nestor E.; Comedi, D.
    Materials Research Express (2018), 5 (7), 075020/1-075020/12CODEN: MREAC3; ISSN:2053-1591. (IOP Publishing Ltd.)
    Dye-sensitized ZnO nanowire (NW) electrodes were fabricated using Ru polypyridyl complexes that use nitrile instead of carboxylic group as anchoring unit to the NW surfaces. The complexes formula is [Ru(bpy)3-x(Mebpy-CN)x]2+ (x = 1-3, bpy = 2,2'-bipyridine, Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile). The ZnO NWs were grown by a vapor transport method on insulating SiO2/Si substrates. The sensitized ZnO NW electrodes were studied by electron microscopy, Raman and PL spectroscopies, and spectral and relaxation photocurrent measurements. The Raman spectra confirm that the complexes were effectively anchored to the ZnO NWs through one of the pendant nitrile groups of the bipyridyl ligands. The nanostructured morphol. of the NW electrodes was maintained so that their light trapping characteristics were preserved. The Ru complexes were found to be excellent sensitizers of the ZnO NWs, improving by orders of magnitude their photocurrent in the visible region. The Fe-based complex of formula [Fe(Mebpy-CN)3](PF6)2 was also tested; however it did not show any sensitizing effect. An order of magnitude shortening of the persistent photocurrent relaxation times (after the illumination is interrupted) was found to occur upon successful sensitization of the ZnO NWs with the Ru complexes. This effect is interpreted in terms of hole traps at ∼1 eV above the ZnO valence band edge, which are lowered by ∼50-60 meV in the soaked samples due to screening of the trap centers provided by the extra photoexcited charge carriers transferred from the sensitizing complex to the NWs.
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  25. 25
    Mecchia Ortiz, J. H.; Vega, N.; Comedi, D.; Tirado, M.; Romero, I.; Fontrodona, X.; Parella, T.; Morán Vieyra, F. E.; Borsarelli, C. D.; Katz, N. E. Improving the Photosensitizing Properties of Ruthenium Polypyridyl Complexes Using 4-Methyl-2,2′-bipyridine-4′-carbonitrile as an Auxiliary Ligand. Inorg. Chem. 2013, 52, 4950– 4962,  DOI: 10.1021/ic302594b
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    Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Mixed Phosphine 2,2′-Bipyridine Complexes of Ruthenium. Inorg. Chem. 2002, 17, 3334– 3341,  DOI: 10.1021/ic50190a006
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    Pannwitz, A.; Prescimone, A.; Wenger, O. S. Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents. Eur. J. Inorg. Chem. 2017, 609– 615,  DOI: 10.1002/ejic.201601403
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    Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents
    Pannwitz, Andrea; Prescimone, Alessandro; Wenger, Oliver S.
    European Journal of Inorganic Chemistry (2017), 2017 (3), 609-615CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Complexes of the type [Ru(bpy)2pyimH]2+ [bpy = 2,2'-bipyridine; pyimH = 2-(2-pyridyl)imidazole] with various substituents on the bpy ligands can act as photoreductants. Their reducing power in the ground state and in the long-lived 3MLCT excited state is increased significantly upon deprotonation, and they can undergo proton-coupled electron transfer (PCET) in the ground and excited state. PCET with both the proton and electron originating from a single donor resembles hydrogen atom transfer (HAT) and can be described thermodynamically by formal bond dissocn. free energies (BDFEs). Whereas the class of complexes studied herein has long been known, their N-H BDFEs were not detd. even though this is important in view of assessing their reactivity. The authors' study demonstrates that the N-H BDFEs in the 3MLCT excited states are 34-52 kcal mol-1 depending on the chem. substituents at the bpy spectator ligands. Specifically, the authors report on the electrochem. and PCET thermochem. of three heteroleptic complexes in 1:1 (vol./vol.) CH3CN/H2O with CF3, tBu, and NMe2 substituents on the bpy ligands.
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    Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91, 165– 195,  DOI: 10.1021/cr00002a004
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    A survey of Hammett substituent constants and resonance and field parameters
    Hansch, Corwin; Leo, A.; Taft, R. W.
    Chemical Reviews (Washington, DC, United States) (1991), 91 (2), 165-95CODEN: CHREAY; ISSN:0009-2665.
    Included in this review is an anal. of newer methods which can supplant this classic procedure for detn. of the title consts., 283 refs.
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    Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill Thompson, A. M. W. Complexes of the Ruthenium(II)-2,2’:6’,2’’-terpyridine Family. Effect of Electron-Accepting and -Donating Substituents on the Photophysical and Electrochemical Properties. Inorg. Chem. 1995, 34, 2759– 2767,  DOI: 10.1021/ic00114a039
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    Complexes of the Ruthenium(II)-2,2':6',2''-terpyridine Family. Effect of Electron-Accepting and -Donating Substituents on the Photophysical and Electrochemical Properties
    Maestri, Mauro; Armaroli, Nicola; Balzani, Vincenzo; Constable, Edwin C.; Thompson, Alexander M. W. Cargill
    Inorganic Chemistry (1995), 34 (10), 2759-67CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    We have investigated the luminescence properties of 14 [Ru(tpy-X)(tpy-Y)]2+ complexes (tpy = 2,2':6',2''-terpyridine; X = Y = MeSO2, Cl, H, Ph, EtO, OH, or Me2N; X = H, Y = MeSO2; X = OH, Y = MeSO2; X = Cl, Y = EtO; X = OH, Y = Ph; X = MeSO2, Y = Me2N; X = Cl, Y = Me2N; X = OH, Y = Me2N; Me = CH3; Et = C2H5; Ph = C6H5). All the complexes examd. display a strong luminescence in rigid matrix at 77 K, with lifetimes in the 1-10 μs time scale. The energy of the emission max. is red shifted for both electron-accepting and electron-donating substituents compared to that of the parent Ru(tpy)22+ complex. At room temp., electron-accepting substituents increase the luminescence quantum yield and the excited state lifetime, whereas electron-donating substituents show an opposite effect. The temp. dependence of the emission lifetime has been investigated for some representative complexes, and the role played by activated and activation-less nonradiative transitions is examd. It is shown that the values of rate consts. for radiationless decay from the luminescent excited state to the ground state are governed not only by the energy gap but also by the nature of the substituents, which presumably affects the changes in the equil. displacement or frequency between the two levels. Correlations of the electrochem. redox potentials, the Hammett σ parameter, and the energy of the luminescent level are reported and discussed. Such correlations show that electron-accepting substituents have a larger stabilization effect on the LUMO π* ligand-centered orbital than on the HOMO π(t2g) metal orbital, whereas electron-donating substituents cause a larger destabilization on the HOMO π(t2g) metal orbital than on the LUMO π* ligand-centered orbital. Heteroleptic complexes carrying an electron-accepting group and an electron-donating group always show lower emission energies when compared with the parent homoleptic complexes because the π* orbital of the tpy-A ligand is stabilized, and the tpy-D ligand destabilizes the metal-centered π(t2g) orbitals.
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    Dodsworth, E. S.; Lever, A. B. P. Correlation of Electronic Charge Transfer Transitions and Electrochemical Potentials. The Bispyrazine(tetracarbonyl)molybdenum(0) System in Various Solvents. Chem. Phys Lett. 1984, 112, 567– 570,  DOI: 10.1016/0009-2614(84)85780-2
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    Correlation of electronic charge transfer transitions and electrochemical potentials. The bipyrazine(tetracarbonyl)molybdenum(0) system in various solvents
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    Chemical Physics Letters (1984), 112 (6), 567-70CODEN: CHPLBC; ISSN:0009-2614.
    By using a free-energy diagram, a relation is drawn between an optical charge transfer energy and the elec. potentials of the donor and acceptor orbitals concerned. The charge transfer spectroscopy and elec. potentials of the title complex were studied in various solvents. A linear correlation, with neg. slope, was obsd. between the difference in oxidn. and redn. potentials and an metal-ligand charge transfer transition. Using some addnl. solvent data, a no. of useful parameters were derived in a fashion which would not be possible through consideration of either technique alone.
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    Wallin, S.; Davidsson, J.; Modin, J.; Hammarström, L. Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast Interligand Randomization of the MLCT State. J. Phys Chem. A 2005, 109, 4697– 4704,  DOI: 10.1021/jp0509212
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    Femtosecond transient absorption anisotropy study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. ultrafast interligand randomization of the MLCT state
    Wallin, Staffan; Davidsson, Jan; Modin, Judit; Hammarstroem, Leif
    Journal of Physical Chemistry A (2005), 109 (21), 4697-4704CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    It is known that the relaxed excited state of [Ru(bpy)3]2+ is best described as a metal to ligand charge transfer (MLCT) state having one formally reduced bipyridine and two neutral. Previous reports have suggested [Malone, R. et al. J. Chem. Phys. 1991, 95, 8970] that the electron "hops" from ligand to ligand in the MLCT state with a time const. of about 50 ps in acetonitrile. However, we have done transient absorption anisotropy measurements indicating that already after one picosecond, the mol. has no memory of which bipyridine was initially photo-selected, which suggests an ultrafast interligand randomization of the MLCT state.
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    Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Application of the Energy Gap Law to the Decay of Charge-transfer Excited States. J. Am. Chem. Soc. 1982, 104, 630– 632,  DOI: 10.1021/ja00366a051
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    Application of the energy gap law to the decay of charge-transfer excited states
    Caspar, Jonathan V.; Kober, Edward M.; Sullivan, B. Patrick; Meyer, Thomas J.
    Journal of the American Chemical Society (1982), 104 (2), 630-2CODEN: JACSAT; ISSN:0002-7863.
    Investigation of a new series of mono- and bis-2,2'-bipyridyl or 1,10-phenanthroline complexes of Os(II) which exhibit long-lived, emitting, charge transfer excited states is reported. The compds. exhibit systematic changes in excited state properties, with changes in structure including excited state potentials, emission lifetimes, and emission max. In particular, a linear dependence of ln(knr) (knr is the excited state non-radiative decay rate) upon the emission energy max. is demonstrated, in agreement with the energy gap law.
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    Mecchia Ortiz, J. H.; Longo, C.; Katz, N. E. Polypyridyl Ruthenium Complexes Containing Anchoring Nitrile Groups as TiO2 Sensitizers for Application in Solar Cells. Inorg. Chem. 2015, 55, 69– 72,  DOI: 10.1016/j.inoche.2015.03.009
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    Polypyridyl ruthenium complexes containing anchoring nitrile groups as TiO2 sensitizers for application in solar cells
    Mecchia Ortiz, Juan H.; Longo, Claudia; Katz, Nestor E.
    Inorganic Chemistry Communications (2015), 55 (), 69-72CODEN: ICCOFP; ISSN:1387-7003. (Elsevier B.V.)
    Ru polypyridyl complexes [Ru(bpy)3-x(Mebpy-CN)x]2 + (x = 1,2 and 3, bpy = 2,2'-bipyridine, Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile) can be used as visible dyes in novel solar cells formed with a porous TiO2 film (1 cm2), Pt counter-electrode and I/iodide as the redox mediator electrolyte dissolved in a polymeric matrix. These complexes can be anchored over the surface of nanocryst. TiO2 through nitrile groups, as evidenced by Raman spectra of the adsorbed species. Irradiated by a solar simulator (67 mW cm- 2), the cells assembled with the Ru complexes with x = 2 and 3 as TiO2 sensitizers exhibit almost identical current-potential curves, with short-circuit photocurrents of 1.25 mA cm- 2, fill factors of 0.5 and overall efficiencies around 0.44%. The Ru complex with x = 1 and a similar Re complex did not perform as well as sensitizers. These data were consistent with results obtained from quantum efficiency curves and impedance spectra. Complexes with nitrile groups as anchoring entities are promising candidates for designing efficient DSCCs.
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    Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Studies of the Adsorption Process of Ru Complexes in Nanoporous ZnO Electrodes. Langmuir 2000, 16, 4688– 4694,  DOI: 10.1021/la9912702
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    Studies of the Adsorption Process of Ru Complexes in Nanoporous ZnO Electrodes
    Keis, Karin; Lindgren, Jan; Lindquist, Sten-Eric; Hagfeldt, Anders
    Langmuir (2000), 16 (10), 4688-4694CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Nanostructured ZnO films consisting of 150-nm sized particles were dye-sensitized with different ruthenium complexes. Incident photon-to-current conversion efficiencies were found to be dependent on dye concn. and on residence time of the ZnO film in the dye soln. Results obtained using IR and Raman spectroscopies together with traditional two-electrode measurements using a sandwich-type cell gave evidence of dye agglomeration in the nanostructured ZnO film. Possible reasons for the agglomeration process in ZnO films are discussed.
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    Ishida, H.; Tobita, S.; Hasegawa, Y.; Katoh, R.; Nozaki, K. Recent Advances in Instrumentation for Absolute Emission Quantum Yield Measurements. Coord. Chem. Rev. 2010, 254, 2449– 2458,  DOI: 10.1016/j.ccr.2010.04.006
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    Recent advances in instrumentation for absolute emission quantum yield measurements
    Ishida, Hitoshi; Tobita, Seiji; Hasegawa, Yasuchika; Katoh, Ryuzi; Nozaki, Koichi
    Coordination Chemistry Reviews (2010), 254 (21-22), 2449-2458CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. The fluorescence and phosphorescence quantum yields (Φ) of std. solns. were re-evaluated based on an abs. method using an integrating sphere equipped with a multichannel spectrometer. In detail the Φ value of ruthenium(II) tris(2,2'-bipyridine) complex was examd. which were often used as the stds. in the detn. of quantum yields of transition-metal complexes. This revealed that the Φ values for [Ru(bpy)3]2+ were 0.063 in deaerated H2O, 0.040 in aerated H2O, 0.095 in deaerated CH3CN, and 0.018 in aerated CH3CN, resp., which are significantly higher than the previously accepted values. Also the tech. aspects in the detn. of abs. emission quantum yields for lanthanide complexes and those of org. crystals of anthracene were examd. For the accurate detn. for lanthanide complexes, special care must be taken in the spectroscopic measurements because of their narrow absorption and emission bands. For org. crystals, the fluorescence quantum yields are reduced due to reabsorption, chem. impurities, and structural defects. The observations for highly purified anthracene crystals revealed that the lower limit value of Φ was 0.64.
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    Cattaneo, M.; Fagalde, F.; Katz, N. E.; Borsarelli, C. D.; Parella, T. pH-Induced Luminescence Changes of Chromophore-Quencher Tricarbonylpolypyridylrhenium(I) Complexes with 4-Pyridinealdazine. Eur. J. Inorg. Chem. 2007, 3, 5323– 5332,  DOI: 10.1002/ejic.200700860
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    Cattaneo, M.; Fagalde, F.; Borsarelli, C. D.; Katz, N. E. Improvement of the Dynamic Range of pH Sensing by Using a Luminescent Tricarbonylpolypyridylrhenium(I) Complex with Three Different Protonation Sites. Inorg. Chem. 2009, 48, 3012– 3017,  DOI: 10.1021/ic802203d
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    Improvement of the Dynamic Range of pH Sensing by Using a Luminescent Tricarbonylpolypyridylrhenium(I) Complex with Three Different Protonation Sites
    Cattaneo, Mauricio; Fagalde, Florencia; Borsarelli, Claudio D.; Katz, Nestor E.
    Inorganic Chemistry (2009), 48 (7), 3012-3017CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    [Re(4,4'-(CO2H)2-bpy)(CO)3(4,4'-bpy)](CF3SO3), 1 (4,4'-(CO2H)2-bpy = 4,4'-dicarboxyl-2,2'-bipyridine, 4,4'-bpy = 4,4'-bipyridine), synthesized and characterized by spectroscopic techniques, displays a strong dependence of its photophys. properties on pH. From both emission intensity and lifetime measurements at different pH values, three values for the protonation consts. of the excited states were detd. (pKa1* = 1.8 ± 0.1, pKa2* = 3.9 ± 0.1, and pKa3* = 5.6 ± 0.1). The unusual bell-shaped variations of these photophys. properties can be accounted for by the changes of energy level orderings induced by each protonation, as confirmed by time-dependent d. functional theory (TD-DFT) calcns. Since the soly., stability, and dynamic range of pH sensing by 1 were improved with respect to similar tricarbonylpolypyridylrhenium(I) complexes, 1 can be used as an efficient mol. switch of the on-off-on type.
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    Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02. Gaussian, Inc.: Wallingford, CT, 2004.
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    Density-functional thermochemistry. III. The role of exact exchange
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    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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    Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785– 789,  DOI: 10.1103/PhysRevB.37.785
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    Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density
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    A correlation-energy formula due to R. Colle and D. Salvetti (1975), in which the correlation energy d. is expressed in terms of the electron d. and a Laplacian of the 2nd-order Hartree-Fock d. matrix, is restated as a formula involving the d. and local kinetic-energy d. On insertion of gradient expansions for the local kinetic-energy d., d.-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calcns. on a no. of atoms, pos. ions, and mols., of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
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    A contracted Gaussian basis set (6-311G**) is developed by optimizing exponents and coeffs. at the Moller-Plesset (MP) second-order level for the ground states of first-row atoms. This has a triple split in the valence s and p shells together with a single set of uncontracted polarization functions on each atom. The basis is tested by computing structures and energies for some simple mols. at various levels of MP theory and comparing with expt.
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  • Abstract

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    Scheme 1

    Scheme 1. Molecular Structures of the New Trisbipyridine Ruthenium(II) Complexes
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    Figure 1

    Figure 1. Difference of potentials, ΔE1/2,of the oxidative and the first reductive processes in [Ru(4,4′-X2-bpy)2(Mebpy-CN)](PF6)2 complexes as a function of Hammett’s σp parameter of X.

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    Figure 2

    Figure 2. UV–vis spectra of complexes 1 (orange), 2 (blue), and 3 (red) in CH3CN at rt.

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    Figure 3

    Figure 3. (a) Oxidative difference spectra acquired at an applied potential 100 mV higher than the oxidative potential of the metallic center of complexes 1–3, respectively, in CH3CN solutions. (b) Reductive difference spectra acquired at an applied potential 100 mV lower than the first reductive potential of complexes 1–3, respectively, in CH3CN solutions.

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    Figure 4

    Figure 4. Time-resolved differential absorption spectra of 1(top), 2 (middle), and 3 (bottom) in CH3CN solutions at different times (λexc = 370 nm) (left) and the absorbance decay at λ = 370 nm and mono-exponential fitting (right).

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    Figure 5

    Figure 5. Molecular orbital (MO) diagrams calculated by DFT for complexes 1–3 and [Ru(bpy)2(Mebpy-CN)](PF6)2. Different contributions of the groups are marked in different colors: ruthenium in black, 4,4′-(X)2-bpy in red, and Mebpy-CN in blue. The HOMO–LUMO energy gaps are also depicted.

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    Figure 6

    Figure 6. Orbital representations of LUMOs (top) and HOMOs (bottom) of complexes 1–3.

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    Figure 7

    Figure 7. TD-DFT calculated (straight line) and experimental (dashed line) UV–vis spectra of complexes 1–3.

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    Figure 8

    Figure 8. (a) Side and (b) front view SEM images of ZnO NWs grown using the hydrothermal technique. (c) XRD data of FTO|ZnO NWs compared with those of a bulk sample of the ZnO (commercial powder) spectrum.

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    Figure 9

    Figure 9. Performance of solar cells assembled with the sensitized FTO|ZnO NWs, Pt counter electrodes, and a liquid electrolyte: (a) quantum efficiency spectra and (b) current–potential (I–V) curves under polychromatic irradiation.

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  • References

    This article references 44 other publications.

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      A review. The dye-sensitized solar cells provide a tech. and economically credible alternative concept to present-day p-n junction photovoltaic devices. In contrast to the conventional systems where the semiconductor assume both the task of light absorption and charge carrier transport the two functions are sepd. here. Light is absorbed by a sensitizer, which is anchored to the surface of a wide band semiconductor. Charge sepn. takes place at the interface via photoinduced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. The use of sensitizers having a broad absorption band in conjunction with oxide films of nanocryst. morphol. permits to harvest a large fraction of sunlight. Nearly quant. conversion of incident photon into elec. current is achieved over a large spectral range extending from the UV to the near-IR region. Overall solar (std. air-mass 1.5) to current conversion efficiencies of >10% have been reached. There are good prospects to produce these cells at lower cost than conventional devices. Here we present the current state of the field, discuss new concepts of the dye-sensitized nanocryst. solar cell including heterojunction variants, and analyze the perspectives for the future development of the technol.
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      Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-Sensitized Solar Cells. Chem. Rev. 2010, 110, 6595– 6663,  DOI: 10.1021/cr900356p
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      Dye-Sensitized Solar Cells
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      Chemical Reviews (Washington, DC, United States) (2010), 110 (11), 6595-6663CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review on dye-sensitized solar cells (DSCs). Some brief notes on solar energy in general and DSC in particular are given, followed by a discussion of the operational principles of DSC (energetics and kinetics). Then, the development of material components and some specific exptl. techniques to characterize DSC are described. The current status of module development is also discussed, and finally a brief future outlook is given.
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      Sharma, K.; Sharma, V.; Sharma, S. S. Dye-Sensitized Solar Cells : Fundamentals and Current Status. Nanoscale Res. Lett. 2018, 13, 381,  DOI: 10.1186/s11671-018-2760-6
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      Dye-Sensitized Solar Cells: Fundamentals and Current Status
      Sharma Khushboo; Sharma Vinay; Sharma S S
      Nanoscale research letters (2018), 13 (1), 381 ISSN:1931-7573.
      Dye-sensitized solar cells (DSSCs) belong to the group of thin-film solar cells which have been under extensive research for more than two decades due to their low cost, simple preparation methodology, low toxicity and ease of production. Still, there is lot of scope for the replacement of current DSSC materials due to their high cost, less abundance, and long-term stability. The efficiency of existing DSSCs reaches up to 12%, using Ru(II) dyes by optimizing material and structural properties which is still less than the efficiency offered by first- and second-generation solar cells, i.e., other thin-film solar cells and Si-based solar cells which offer ~ 20-30% efficiency. This article provides an in-depth review on DSSC construction, operating principle, key problems (low efficiency, low scalability, and low stability), prospective efficient materials, and finally a brief insight to commercialization.
      >> More from SciFinder ®
      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BB3crlvFGnuw%253D%253D&md5=01acdd55f404c169b5be9189e56fffd3
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      Nazeeruddin, M. K.; Humphry-Baker, R.; Liska, P.; Grätzel, M. Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell. J. Phys. Chem. B 2003, 107, 8981– 8987,  DOI: 10.1021/jp022656f
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      Investigation of Sensitizer Adsorption and the Influence of Protons on Current and Voltage of a Dye-Sensitized Nanocrystalline TiO2 Solar Cell
      Nazeeruddin, Md. K.; Humphry-Baker, R.; Liska, P.; Graetzel, M.
      Journal of Physical Chemistry B (2003), 107 (34), 8981-8987CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      FTIR spectra of [Ru(dcbpyH2)2(NCS)2] (N 3), (Bu4N)2[Ru(dcbpyH)2(NCS)2] (N 719), and (Bu4N)4[Ru(dcbpy)2(NCS)2] (N 712) complexes measured as solid samples in photoacoustic mode display fine resoln. of IR bands and exhibit differences between the cis and the trans carboxylic acid groups. The interaction between N 3, N 719, and N 712 sensitizers with nanocryst. TiO2 film was studied by ATR-FTIR spectroscopy. These complexes are being anchored onto the TiO2 surface in bridging coordination mode using two out of their four carboxylic acid groups, which are trans to the NCS ligand. The effect of protons on both the short circuit photocurrent and the open circuit photovoltage of dye-sensitized nanocryst. solar cells was scrutinized. For the std. electrolyte formulation employed and TiCl4 treated mesoporous TiO2 films, the monoprotonated form of the N3 dye exhibited superior power conversion efficiency under AM 1.5 sun compared to the four, two, and zero proton sensitizers.
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      Nazeeruddin, M. K.; Péchy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grätzel, M. Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO2-Based Solar Cells. J. Am. Chem. Soc. 2001, 123, 1613– 1624,  DOI: 10.1021/ja003299u
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      Engineering of Efficient Panchromatic Sensitizers for Nanocrystalline TiO2-Based Solar Cells
      Nazeeruddin, Mohammad K.; Pechy, Peter; Renouard, Thierry; Zakeeruddin, Shaik M.; Humphry-Baker, Robin; Comte, Pascal; Liska, Paul; Cevey, Le; Costa, Emiliana; Shklover, Valery; Spiccia, Leone; Deacon, Glen B.; Bignozzi, Carlo A.; Graetzel, Michael
      Journal of the American Chemical Society (2001), 123 (8), 1613-1624CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A new series of panchromatic ruthenium(II) sensitizers derived from carboxylated terpyridyl complexes of tris-thiocyanato Ru(II) have been developed. Black dye contg. different degrees of protonation {(C2H5)3NH}[Ru(H3tcterpy)(NCS)3] (1), {(C4H9)4N}2[Ru(H2tcterpy)(NCS)3] (2), {(C4H9)4N}3[Ru(Htcterpy)(NCS)3] (3), and {(C4H9)4N}4[Ru(tcterpy)(NCS)3] (4) (tcterpy = 4,4',4''-tricarboxy-2,2':6',2''-terpyridine) have been synthesized and fully characterized by UV-vis, emission, IR, Raman, NMR, cyclic voltammetry, and X-ray diffraction studies. The crystal structure of complex 2 confirms the presence of a RuIIN6 central core derived from the terpyridine ligand and three N-bonded thiocyanates. Intermol. H-bonding between carboxylates on neighboring terpyridines gives rise to 2-D H-bonded arrays. The absorption and emission maxima of the black dye show a bathochromic shift with decreasing pH and exhibit pH-dependent excited-state lifetimes. The red-shift of the emission maxima is due to better π-acceptor properties of the acid form that lowers the energy of the CT excited state. The low-energy metal-to-ligand charge-transfer absorption band showed marked solvatochromism due to the presence of thiocyanate ligands. The Ru(II)/(III) oxidn. potential of the black dye and the ligand-based redn. potential shifted cathodically with decreasing no. of protons and showed more reversible character. The adsorption of complex 3 from methoxyacetonitrile soln. onto transparent TiO2 films was interpreted by a Langmuir isotherm yielding an adsorption equil. const., Kads, of (1.0 ± 0.3) × 105 Μ-1. The amt. of dye adsorbed at monolayer satn. was (nα = 6.9 ± 0.3) × 10-8 mol/mg of TiO2, which is around 30% less than that of the cis-di(thiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) complex. The black dye, when anchored to nanocryst. TiO2 films achieves very efficient sensitization over the whole visible range extending into the near-IR region up to 920 nm, yielding over 80% incident photon-to-current efficiencies (IPCE). Solar cells contg. the black dye were subjected to anal. by a photovoltaic calibration lab. (NREL, U.S.A.) to det. their solar-to-elec. conversion efficiency under std. AM 1.5 sunlight. A short circuit photocurrent d. obtained was 20.5 mA/cm2, and the open circuit voltage was 0.72 V corresponding to an overall conversion efficiency of 10.4%.
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    7. 7
      Bessho, T.; Yoneda, E.; Yum, J.-H.; Guglielmi, M.; Tavernelli, I.; Imai, H.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. New Paradigm in Molecular Engineering of Sensitizers for Solar Cell Applications. J. Am. Chem. Soc. 2009, 131, 5930– 5934,  DOI: 10.1021/ja9002684
      7
      New Paradigm in Molecular Engineering of Sensitizers for Solar Cell Applications
      Bessho, Takeru; Yoneda, Eiji; Yum, Jun-Ho; Guglielmi, Matteo; Tavernelli, Ivano; Imai, Hachiro; Rothlisberger, Ursula; Nazeeruddin, Mohammad K.; Gratzel, Michael
      Journal of the American Chemical Society (2009), 131 (16), 5930-5934CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A novel thiocyanate-free cyclometalated Ru sensitizer for solar cells was developed. Upon anchoring to nanocryst. TiO2 films, it exhibits a remarkable incident monochromatic photon-to-current conversion efficiency of 83%. The solar cell employing a liq.-based electrolyte exhibits a short-circuit photocurrent d. of 17 mA/cm2, an open-circuit voltage of 800 mV, and a fill factor of 0.74, corresponding to an overall conversion efficiency of 10.1% at std. AM 1.5 sunlight. To understand the structural, electronic and optical properties of the cyclometallated Ru sensitizer, DFT and time-dependent DFT (TDDFT), were used. The results show that the HOMO is located mostly on Ru and the cyclometalated ligand, while the LUMO is on 4-carboxylic acid-4'-carboxylate-2,2'-bipyridine. MOs anal. confirmed the exptl. assignment of redox potentials and TDDFT calcns. allowed assignment of the visible absorption bands. The present findings provide new design criteria for the next generation of Ru sensitizers and promotes engineering of new sensitizers that interact effectively with the I-/I3- redox couple.
      >> More from SciFinder ®
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    8. 8
      Pal, A. K.; Nag, S.; Ferreira, J. G.; Brochery, V.; La Ganga, G.; Santoro, A.; Serroni, S.; Campagna, S.; Hanan, G. S. Red-Emitting [Ru(bpy)2(N-N)]2+ Photosensitizers:Emission from a Ruthenium(II) to 2,2′-Bipyridine 3MLCT State in the Presence of Neutral Ancillary “Super Donor” Ligands. Inorg. Chem. 2014, 53, 1679– 1689,  DOI: 10.1021/ic4028332
      8
      Red-Emitting [Ru(bpy)2(N-N)]2+ Photosensitizers: Emission from a Ruthenium(II) to 2,2'-Bipyridine 3MLCT State in the Presence of Neutral Ancillary "Super Donor" Ligands
      Pal, Amlan K.; Nag, Samik; Ferreira, Janaina G.; Brochery, Victor; La Ganga, Giuseppina; Santoro, Antonio; Serroni, Scolastica; Campagna, Sebastiano; Hanan, Garry S.
      Inorganic Chemistry (2014), 53 (3), 1679-1689CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The synthesis and characterization of a novel family of [RuII(bpy)2(N-N)](PF6)2 (bpy = 2,2'-bipyridine) complexes are reported, where N-N = pyridine/pyrimidine/pyrazine functionalized in different positions with the electron-donating bicyclic hexahydropyrimidopyrimidine (hpp) unit. Bidentate ligands 1a-5a were synthesized in good to high yields (55-96%). The corresponding complexes 1b, 2b, and 5b were prepd. in n-butanol, while complexes 3b and 4b were prepd. in a mixt. of n-butanol and water (1/1, vol./vol.) in modest to good yields (23-76%). Both ligand and complex structures were fully characterized by a variety of techniques, including x-ray crystallog. In cyclic voltammetric studies, all the complexes exhibit a RuIII/II couple, which is ∼500 mV less pos. than the RuIII/II couple in Ru(bpy)32+. The 1MLCT and 3MLCT states of all of the complexes (530-560 nm/732-745 nm) are shifted bathochromically in comparison to those of Ru(bpy)32+ (450 nm/620 nm). These values are in good agreement with DFT and TD-DFT calcns.
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      O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737– 740,  DOI: 10.1038/353737a0
      9
      A low-cost, high-efficiency solar cell based on dye-sensitized colloidal titanium dioxide films
      O'Regan, Brian; Graetzel, Michael
      Nature (London, United Kingdom) (1991), 353 (6346), 737-40CODEN: NATUAS; ISSN:0028-0836.
      A photoelectrochem. cell was fabricated from low- to medium-purity materials through low-cost processes, which exhibits a com. realistic energy-conversion efficiency. The device is based on a 10-μm-thick, optically transparent film of TiO2 particles, coated with a monolayer of a charge-transfer dye to sensitize the film for light harvesting. The device harvests a high proportion of the incident solar energy flux (46%) and shows exceptionally high efficiencies for the conversion of incident photons to elec. current (>80%). The overall light-to-elec. energy conversion yield is 7.1-8.9% in simulated solar light and 12% in diffuse daylight. The c.d. >12 mA/cm2 and exceptional stability (sustaining ≥5 million turnover without decompn.), and low cost, make practical applications feasible.
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    10. 10
      Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338– 344,  DOI: 10.1038/35104607
      10
      Photoelectrochemical cells
      Gratzel, Michael
      Nature (London, United Kingdom) (2001), 414 (6861), 338-344CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      A review discussing the historical background, and present status and development prospects of new generation of photoelectrochem. cells.
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    11. 11
      Özgür, Ü.; Hofstetter, D.; Morkoç, H. ZnO Devices and Applications : A Review of Current Status and Future Prospects. Proc. IEEE 2010, 98, 1255– 1268,  DOI: 10.1109/JPROC.2010.2044550
      11
      ZnO devices and applications: a review of current status and future prospects
      Ozgur, Umit; Hofstetter, Daniel; Morkoc, Hadis
      Proceedings of the IEEE (2010), 98 (7), 1255-1268CODEN: IEEPAD; ISSN:0018-9219. (Institute of Electrical and Electronics Engineers)
      A review. ZnO is an attractive material for applications in electronics, photonics, acoustics, and sensing. In optical emitters, its high exciton binding energy (60 meV) gives ZnO an edge over other semiconductors such as GaN if reproducible and reliable p-type doping in ZnO were to be achieved, which currently remains to be the main obstacle for realization of bipolar devices. On the electronic side, ZnO holds some potential in transparent thin film transistors (TFTs) owing to its high optical transmittivity and high cond. Among the other promising areas of application for ZnO are acoustic wave devices, due to large electromech. coupling in ZnO, and devices utilizing nanowires/nanorods such as biosensors and gas sensors and solar cells, since it is relatively easy to produce such forms of ZnO nanostructures, which have good charge carrier transport properties and high cryst. quality. Despite the significant progress made, there is still a no. of important issues that need to be resolved before ZnO can be transitioned to com. use, not to mention the stiff competition it is facing with GaN, which is much more mature in terms of devices. In this paper, recent progress in device applications of ZnO is discussed and a review of crit. issues for realization of ZnO-based devices is given.
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    12. 12
      Rodnyi, P. A.; Khodyuk, I. V. Optical and Luminescence Properties of Zinc Oxide ( Review ). Opt. Spectrosc. 2011, 111, 776– 785,  DOI: 10.1134/S0030400X11120216
      12
      Optical and luminescence properties of zinc oxide (Review)
      Rodnyi, P. A.; Khodyuk, I. V.
      Optics and Spectroscopy (2011), 111 (5), 776-785CODEN: OPSUA3; ISSN:0030-400X. (MAIK Nauka/Interperiodica)
      We generalize and systematize basic exptl. data on optical and luminescence properties of ZnO single crystals, thin films, powders, ceramics, and nanocrystals. We consider and study mechanisms by which two main emission bands occur, a short-wavelength band near the fundamental absorption edge and a broad long-wavelength band, the max. of which usually lies in the green spectral range. We det. a relationship between the two luminescence bands and study in detail the possibility of controlling the characteristics of ZnO by varying the max. position of the short-wavelength band. We show that the optical and luminescence characteristics of ZnO largely depend on the choice of the corresponding impurity and the parameters of the synthesis and subsequent treatment of the sample. Prospects for using zinc oxide as a scintillator material are discussed. Addnl., we consider exptl. results that are of principal interest for practice.
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    13. 13
      Lee, J.-H.; Ko, K.-H.; Park, B.-O. Electrical and Optical Properties of ZnO Transparent Conducting Films by the Sol – Gel Method. J. Cryst. Growth 2003, 247, 119– 125,  DOI: 10.1016/S0022-0248(02)01907-3
      13
      Electrical and optical properties of ZnO transparent conducting films by the sol-gel method
      Lee, Jin-Hong; Ko, Kyung-Hee; Park, Byung-Ok
      Journal of Crystal Growth (2003), 247 (1-2), 119-125CODEN: JCRGAE; ISSN:0022-0248. (Elsevier Science B.V.)
      The effect of drying conditions and the first- and second-heat treatment on the structural, elec. and optical properties of ZnO thin films prepd. by the sol-gel method was investigated. ZnO films were deposited on silica glass substrates by the spin-coating method. Zinc acetate dihydrate, 2-methoxyethanol and monoethanolamine were used as a starting material, solvent and stabilizer, resp. ZnO films were dried at 350°C and then heated at 600°C showed an extremely sharp (002) peak in the XRD patterns. The more films were oriented preferentially along (002) direction, the greater their elec. and optical properties became. By applying the second-heat treatment in nitrogen with 5% hydrogen at 500°C, the grain size of films was increased, the resistivity value was 0.099 Ω cm and optical transmittances became higher than 85% in the visible range.
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    14. 14
      Wisz, G.; Virt, I.; Sagan, P.; Potera, P.; Yavorskyi, R. Structural, Optical and Electrical Properties of Zinc Oxide Layers Produced by Pulsed Laser Deposition Method. Nanoscale Res. Lett. 2017, 12, 253,  DOI: 10.1186/s11671-017-2033-9
      14
      Structural, Optical and Electrical Properties of Zinc Oxide Layers Produced by Pulsed Laser Deposition Method
      Wisz G; Virt I; Sagan P; Potera P; Yavorskyi R; Virt I; Yavorskyi R
      Nanoscale research letters (2017), 12 (1), 253 ISSN:1931-7573.
      The structural, optical, and electrical properties of zinc oxide (ZnO) layers manufactured at different process conditions were investigated. ZnO epitaxial layers were grown on silicon, glass, and ITO/glass substrates by pulsed laser deposition (PLD) technique. The influence of power beam, substrate temperature, and deposition time on films properties was analysed. Morphological features of the film surface were investigated by scanning electron microscopy. A structural study shown planar orientation of films at low temperatures of substrate, but the columnar type of growth originated in temperature enhances. Electrical properties were determined in the temperature range 300-500 K. It was shown that the type of films conductivity is metallic and it is limited by charge transfer across grain boundaries.
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    15. 15
      He, J. H.; Hsin, C. L.; Liu, J.; Chen, L. J.; Wang, Z. L. Piezoelectric Gated Diode of a Single ZnO Nanowire. Adv. Mater. 2007, 19, 781– 784,  DOI: 10.1002/adma.200601908
      15
      Piezoelectric gated diode of a single ZnO nanowire
      He, H., Jr.; Hsin, Cheng L.; Liu, Jin; Chen, Lih J.; Wang, Zhong L.
      Advanced Materials (Weinheim, Germany) (2007), 19 (6), 781-784CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A ZnO nanowire behaves like a rectifier under bending strain, as demonstrated by its current-voltage characteristics. This is interpreted with the consideration of a piezoelectricity-induced potential energy barrier at the interface of the conductive tip and nanowire. Under appropriate bending and voltage control, each NW could correspond to a device element for random-access-memory, diode, and force-sensor applications.
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    16. 16
      Emanetoglu, N. W.; Gorla, C.; Liu, Y.; Liang, S.; Lu, Y. Epitaxial ZnO Piezoelectric Thin Films for Saw Filters. Mater. Sci. Semicond. Process. 1999, 2, 247– 252,  DOI: 10.1016/S1369-8001(99)00022-0
      16
      Epitaxial ZnO piezoelectric thin films for SAW filters
      Emanetoglu, N. W.; Gorla, C.; Liu, Y.; Liang, S.; Lu, Y.
      Materials Science in Semiconductor Processing (1999), 2 (3), 247-252CODEN: MSSPFQ; ISSN:1369-8001. (Elsevier Science Ltd.)
      ZnO is a wide bandgap semiconductor material with high piezoelec. coupling coeffs. It can be used for making low-loss surface acoustic wave (SAW) filters operating at high frequency. The authors report MOCVD growth of epitaxial ZnO thin films on R-plane sapphire substrates. The crystallinity and orientation of the films, as well as the epitaxial relation between the ZnO films and the R-plane Al2O3 substrate were studied using x-ray diffraction techniques. An atomically sharp interface structure was revealed by high-resoln. TEM. Surface morphol. was studied using field emission SEM. SAW filters with 10 and 16 μm wavelength were fabricated. Low insertion loss and high piezoelec. coupling coeff., up to 6%, were achieved. The acoustic velocities range from 4050 to 5800 m/s, varying as a function of ZnO film thickness.
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    17. 17
      Vega, N. C.; Wallar, R.; Caram, J.; Grinblat, G.; Tirado, M.; LaPierre, R. R.; Comedi, D. ZnO Nanowire Co-Growth on SiO2 and C by Carbothermal Reduction and Vapour Advection. Nanotechnology 2012, 23, 275602,  DOI: 10.1088/0957-4484/23/27/275602
      17
      ZnO nanowire co-growth on SiO2 and C by carbothermal reduction and vapour advection
      Vega, N. C.; Wallar, R.; Caram, J.; Grinblat, G.; Tirado, M.; La Pierre, R. R.; Comedi, D.
      Nanotechnology (2012), 23 (27), 275602/1-275602/11CODEN: NNOTER; ISSN:1361-6528. (Institute of Physics Publishing)
      Vertically aligned ZnO nanowires (NWs) were grown on Au-nanocluster-seeded amorphous SiO2 films by the advective transport and deposition of Zn vapors obtained from the carbothermal reaction of graphite and ZnO powders. Both the NW vol. and visible-to-UV photoluminescence ratio were found to be strong functions of, and hence could be tailored by, the (ZnO + C) source-SiO2 substrate distance. We observe C flakes on the ZnO NWs/SiO2 substrates which exhibit short NWs that developed on both sides. The SiO2 and C substrates/NW interfaces were studied in detail to det. growth mechanisms. NWs on Au-seeded SiO2 were promoted by a rough ZnO seed layer whose formation was catalyzed by the Au clusters. In contrast, NWs grew without any seed on C. A correlation comprising three orders of magnitude between the visible-to-UV photoluminescence intensity ratio and the NW vol. is found, which results from a characteristic Zn partial pressure profile that fixes both O deficiency defect concn. and growth rate.
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    18. 18
      Grinblat, G.; Capeluto, M. G.; Tirado, M.; Bragas, A. V.; Comedi, D. Hierarchical ZnO Nanostructures : Growth Mechanisms and Surface Correlated Photoluminescence. Appl. Phys. Lett. 2012, 100, 233116,  DOI: 10.1063/1.4724195
      18
      Hierarchical ZnO nanostructures: Growth mechanisms and surface correlated photoluminescence
      Grinblat, G.; Capeluto, M. G.; Tirado, M.; Bragas, A. V.; Comedi, D.
      Applied Physics Letters (2012), 100 (23), 233116/1-233116/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      ZnO nanowires were grown by vapor-transport and deposition on Au nanocluster covered fused and thermal SiO2 and c-Si. The nanowire size and d. depended strongly on the substrate type. By decreasing the O2 to local Zn partial pressure ratio, the growth pattern changed to nanocombs and nanosheets. ZnO nanohedgehogs were found on bare c-Si. The authors observe a remarkable correlation between the defect to exciton photoluminescence intensity ratio and the nanostructures sp. surface areas. Changes in strain and O deficiency defects at surfaces are behind the obsd. morphol. changes, one to two-dimensional growth transition, and corresponding luminescence. (c) 2012 American Institute of Physics.
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    19. 19
      Vega, N. C.; Tirado, M.; Comedi, D.; Rodriguez, A.; Rodriguez, T.; Hughes, G. M.; Grovenor, C. R. M.; Audebert, F. Electrical , Photoelectrical and Morphological Properties of ZnO Nanofiber Networks Grown on SiO2 and on Si Nanowires. Mater. Res. 2013, 16, 597– 602,  DOI: 10.1590/S1516-14392013005000030
      19
      Electrical, photoelectrical and morphological properties of ZnO nanofiber networks grown on SiO2 and on Si nanowires
      Vega, Nadia Celeste; Tirado, Monica; Comedi, David; Rodriguez, Andres; Rodriguez, Tomas; Hughes, Gareth M.; Grovenot, Chris R. M.; Audebert, Fernando
      Materials Research (Sao Carlos, Brazil) (2013), 16 (3), 597-602CODEN: MAREFW; ISSN:1516-1439. (Materials Research)
      ZnO nanofiber networks (NFNs) were grown by vapor transport method on Si-based substrates. One type of substrate was SiO2 thermally grown on Si and another consisted of a Si wafer onto which Si nanowires (NWs) had been grown having Au nanoparticles catalysts. The ZnO-NFN morphol. was obsd. by SEM on samples grown at 600 and 720 °C substrate temp., while an focused ion beam was used to study the ZnO NFN/Si NWs/Si and ZnO NFN/SiO2 interfaces. Photoluminescence, elec. conductance and photoconductance of ZnO-NFN was studied for the sample grown on SiO2. The photoluminescence spectra show strong peaks due to exciton recombination and lattice defects. The ZnO-NFN presents quasi-persistent photocond. effects and ohmic I-V characteristics which become nonlinear and hysteretic as the applied voltage is increased. The elec. conductance as a function of temp. can be described by a modified three dimensional variable hopping model with nanometer-ranged typical hopping distances.
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    20. 20
      Vega, N. C.; Marin, O.; Tosi, E.; Grinblat, G.; Mosquera, E.; Moreno, M. S.; Tirado, M.; Comedi, D. The Shell Effect on the Room-Temperature Photoluminescence from ZnO/MgO Core/Shell Nanowires: Exciton-Phonon Coupling and Strain. Nanotechnology 2017, 28, 275702,  DOI: 10.1088/1361-6528/aa7454
      20
      The shell effect on the room temperature photoluminescence from ZnO/MgO core/shell nanowires: exciton-phonon coupling and strain
      Vega, N. C.; Marin, O.; Tosi, E.; Grinblat, G.; Mosquera, E.; Moreno, M. S.; Tirado, M.; Comedi, D.
      Nanotechnology (2017), 28 (27), 275702/1-275702/9CODEN: NNOTER; ISSN:1361-6528. (IOP Publishing Ltd.)
      The room temp. photoluminescence from ZnO/MgO core/shell nanowires (NWs) grown by a simple two-step vapor transport method was studied for various MgO shell widths (w). Two distinct effects induced by the MgO shell were clearly identified. The first one, related to the ZnO/MgO interface formation, is evidenced by strong enhancements of the zero-phonon and first phonon replica of the excitonic emission, which are accompanied by a total suppression of its second phonon replica. This effect can be explained by the redn. of the band bending within the ZnO NW core that follows the removal of atm. adsorbates and assocd. surface traps during the MgO growth process on one hand, and a reduced exciton-phonon coupling as a result of the mech. stabilization of the outermost ZnO NW monolayers by the MgO shell on the other hand. The second effect is the gradual increase of the excitonic emission and decrease in the defect related emission by up to two and one orders of magnitude, resp., when w is increased in the ∼3-17 nm range. Uniaxial strain build-up within the ZnO NW core with increasing w, as detected by x-ray diffraction measurements, and photocarrier tunneling escape from the ZnO core through the MgO shell enabled by defect-states are proposed as possible mechanisms involved in this effect. These findings are expected to be of key significance for the efficient design and fabrication of ZnO/MgO NW heterostructures and devices.
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    21. 21
      Grinblat, G.; Bern, F.; Barzola-quiquia, J.; Tirado, M.; Comedi, D.; Esquinazi, P. Luminescence and Electrical Properties of Single ZnO/MgO Core/Shell Nanowires. Appl. Phys. Lett. 2014, 104, 103113,  DOI: 10.1063/1.4868648
      21
      Luminescence and electrical properties of single ZnO/MgO core/shell nanowires
      Grinblat, Gustavo; Bern, Francis; Barzola-Quiquia, Jose; Tirado, Monica; Comedi, David; Esquinazi, Pablo
      Applied Physics Letters (2014), 104 (10), 103113/1-103113/4CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      To neutralize the influence of the surface of ZnO nanowires for photonics and optoelectronic applications, we have covered them with insulating MgO film and individually contacted them for elec. characterization. Such a metal-insulator-semiconductor-type nanodevice exhibits a high diode ideality factor of 3.4 below 1 V. MgO shell passivates ZnO surface states and provides confining barriers to electrons and holes within the ZnO core, favoring excitonic UV radiative recombination, while suppressing defect-related luminescence in the visible and improving elec. cond. The results indicate the potential use of ZnO/MgO nanowires as a convenient building block for nano-optoelectronic devices. (c) 2014 American Institute of Physics.
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    22. 22
      Anta, J. A.; Guillén, E.; Tena-Zaera, R. ZnO-Based Dye-Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 11413– 11425,  DOI: 10.1021/jp3010025
      22
      ZnO-Based Dye-Sensitized Solar Cells
      Anta, Juan A.; Guillen, Elena; Tena-Zaera, Ramon
      Journal of Physical Chemistry C (2012), 116 (21), 11413-11425CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      A review. ZnO was one of the first metal oxides used in dye-sensitized solar cells. It exhibits a unique combination of potentially interesting properties such as high bulk electron mobility and probably the richest variety of nanostructures based on a very wide range of synthesis routes. However, in spite of the huge amt. of literature produced in the past few years, the reported efficiencies of ZnO-based solar cells are still far from their TiO2 counterparts. The origin of this striking difference in performance is analyzed and discussed with the perspective of future applications of ZnO in dye-sensitized solar cells and related devices. In this regard, a change of focus of the current research on ZnO-based dye-sensitized solar cells (from morphol. to surface control) is suggested.
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    23. 23
      Zhang, Q.; Dandeneau, C. S.; Zhou, X.; Cao, G. ZnO Nanostructures for Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, 4087– 4108,  DOI: 10.1002/adma.200803827
      23
      ZnO Nanostructures for Dye-Sensitized Solar Cells
      Zhang, Qifeng; Dandeneau, Christopher S.; Zhou, Xiaoyuan; Cao, Guozhong
      Advanced Materials (Weinheim, Germany) (2009), 21 (41), 4087-4108CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review of recent developments in the use of ZnO nanostructures for dye-sensitized solar cells (DSSCs). Carefully designed and fabricated nanostructured ZnO films are advantageous for use as a DSSC photoelectrode since they offer larger surface areas than bulk film material, direct electron pathways, or effective light-scattering centers, and, when combined with TiO2, produce a core-shell structure that decreases the combination rate. The limitations of ZnO-based DSCs are also discussed and several possible methods are proposed so as to expand the knowledge of ZnO to TiO2, motivating further improvement in the power-conversion efficiency of DSCs.
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    24. 24
      Vega, N. C.; Mecchia Ortiz, J. H.; Tirado, M.; Katz, N. E.; Comedi, D. ZnO Nanowire Sensitization with Ru Polypyridyl Complexes : Charge-Transfer Probed by Spectral and Relaxation Photocurrent Measurements. Mater. Res. Express 2018, 5, 075020  DOI: 10.1088/2053-1591/aacfa5
      24
      ZnO nanowire sensitization with Ru polypyridyl complexes: charge-transfer probed by spectral and relaxation photocurrent measurements
      Vega, N. C.; Ortiz, Juan H. Mecchia; Tirado, M.; Katz, Nestor E.; Comedi, D.
      Materials Research Express (2018), 5 (7), 075020/1-075020/12CODEN: MREAC3; ISSN:2053-1591. (IOP Publishing Ltd.)
      Dye-sensitized ZnO nanowire (NW) electrodes were fabricated using Ru polypyridyl complexes that use nitrile instead of carboxylic group as anchoring unit to the NW surfaces. The complexes formula is [Ru(bpy)3-x(Mebpy-CN)x]2+ (x = 1-3, bpy = 2,2'-bipyridine, Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile). The ZnO NWs were grown by a vapor transport method on insulating SiO2/Si substrates. The sensitized ZnO NW electrodes were studied by electron microscopy, Raman and PL spectroscopies, and spectral and relaxation photocurrent measurements. The Raman spectra confirm that the complexes were effectively anchored to the ZnO NWs through one of the pendant nitrile groups of the bipyridyl ligands. The nanostructured morphol. of the NW electrodes was maintained so that their light trapping characteristics were preserved. The Ru complexes were found to be excellent sensitizers of the ZnO NWs, improving by orders of magnitude their photocurrent in the visible region. The Fe-based complex of formula [Fe(Mebpy-CN)3](PF6)2 was also tested; however it did not show any sensitizing effect. An order of magnitude shortening of the persistent photocurrent relaxation times (after the illumination is interrupted) was found to occur upon successful sensitization of the ZnO NWs with the Ru complexes. This effect is interpreted in terms of hole traps at ∼1 eV above the ZnO valence band edge, which are lowered by ∼50-60 meV in the soaked samples due to screening of the trap centers provided by the extra photoexcited charge carriers transferred from the sensitizing complex to the NWs.
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    25. 25
      Mecchia Ortiz, J. H.; Vega, N.; Comedi, D.; Tirado, M.; Romero, I.; Fontrodona, X.; Parella, T.; Morán Vieyra, F. E.; Borsarelli, C. D.; Katz, N. E. Improving the Photosensitizing Properties of Ruthenium Polypyridyl Complexes Using 4-Methyl-2,2′-bipyridine-4′-carbonitrile as an Auxiliary Ligand. Inorg. Chem. 2013, 52, 4950– 4962,  DOI: 10.1021/ic302594b
      There is no corresponding record for this reference.
    26. 26
      Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Mixed Phosphine 2,2′-Bipyridine Complexes of Ruthenium. Inorg. Chem. 2002, 17, 3334– 3341,  DOI: 10.1021/ic50190a006
      There is no corresponding record for this reference.
    27. 27
      Pannwitz, A.; Prescimone, A.; Wenger, O. S. Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents. Eur. J. Inorg. Chem. 2017, 609– 615,  DOI: 10.1002/ejic.201601403
      27
      Ruthenium(II)-Pyridylimidazole Complexes as Photoreductants and PCET Reagents
      Pannwitz, Andrea; Prescimone, Alessandro; Wenger, Oliver S.
      European Journal of Inorganic Chemistry (2017), 2017 (3), 609-615CODEN: EJICFO; ISSN:1434-1948. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Complexes of the type [Ru(bpy)2pyimH]2+ [bpy = 2,2'-bipyridine; pyimH = 2-(2-pyridyl)imidazole] with various substituents on the bpy ligands can act as photoreductants. Their reducing power in the ground state and in the long-lived 3MLCT excited state is increased significantly upon deprotonation, and they can undergo proton-coupled electron transfer (PCET) in the ground and excited state. PCET with both the proton and electron originating from a single donor resembles hydrogen atom transfer (HAT) and can be described thermodynamically by formal bond dissocn. free energies (BDFEs). Whereas the class of complexes studied herein has long been known, their N-H BDFEs were not detd. even though this is important in view of assessing their reactivity. The authors' study demonstrates that the N-H BDFEs in the 3MLCT excited states are 34-52 kcal mol-1 depending on the chem. substituents at the bpy spectator ligands. Specifically, the authors report on the electrochem. and PCET thermochem. of three heteroleptic complexes in 1:1 (vol./vol.) CH3CN/H2O with CF3, tBu, and NMe2 substituents on the bpy ligands.
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    28. 28
      Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent Constants and Resonance and Field Parameters. Chem. Rev. 1991, 91, 165– 195,  DOI: 10.1021/cr00002a004
      28
      A survey of Hammett substituent constants and resonance and field parameters
      Hansch, Corwin; Leo, A.; Taft, R. W.
      Chemical Reviews (Washington, DC, United States) (1991), 91 (2), 165-95CODEN: CHREAY; ISSN:0009-2665.
      Included in this review is an anal. of newer methods which can supplant this classic procedure for detn. of the title consts., 283 refs.
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    29. 29
      Maestri, M.; Armaroli, N.; Balzani, V.; Constable, E. C.; Cargill Thompson, A. M. W. Complexes of the Ruthenium(II)-2,2’:6’,2’’-terpyridine Family. Effect of Electron-Accepting and -Donating Substituents on the Photophysical and Electrochemical Properties. Inorg. Chem. 1995, 34, 2759– 2767,  DOI: 10.1021/ic00114a039
      29
      Complexes of the Ruthenium(II)-2,2':6',2''-terpyridine Family. Effect of Electron-Accepting and -Donating Substituents on the Photophysical and Electrochemical Properties
      Maestri, Mauro; Armaroli, Nicola; Balzani, Vincenzo; Constable, Edwin C.; Thompson, Alexander M. W. Cargill
      Inorganic Chemistry (1995), 34 (10), 2759-67CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      We have investigated the luminescence properties of 14 [Ru(tpy-X)(tpy-Y)]2+ complexes (tpy = 2,2':6',2''-terpyridine; X = Y = MeSO2, Cl, H, Ph, EtO, OH, or Me2N; X = H, Y = MeSO2; X = OH, Y = MeSO2; X = Cl, Y = EtO; X = OH, Y = Ph; X = MeSO2, Y = Me2N; X = Cl, Y = Me2N; X = OH, Y = Me2N; Me = CH3; Et = C2H5; Ph = C6H5). All the complexes examd. display a strong luminescence in rigid matrix at 77 K, with lifetimes in the 1-10 μs time scale. The energy of the emission max. is red shifted for both electron-accepting and electron-donating substituents compared to that of the parent Ru(tpy)22+ complex. At room temp., electron-accepting substituents increase the luminescence quantum yield and the excited state lifetime, whereas electron-donating substituents show an opposite effect. The temp. dependence of the emission lifetime has been investigated for some representative complexes, and the role played by activated and activation-less nonradiative transitions is examd. It is shown that the values of rate consts. for radiationless decay from the luminescent excited state to the ground state are governed not only by the energy gap but also by the nature of the substituents, which presumably affects the changes in the equil. displacement or frequency between the two levels. Correlations of the electrochem. redox potentials, the Hammett σ parameter, and the energy of the luminescent level are reported and discussed. Such correlations show that electron-accepting substituents have a larger stabilization effect on the LUMO π* ligand-centered orbital than on the HOMO π(t2g) metal orbital, whereas electron-donating substituents cause a larger destabilization on the HOMO π(t2g) metal orbital than on the LUMO π* ligand-centered orbital. Heteroleptic complexes carrying an electron-accepting group and an electron-donating group always show lower emission energies when compared with the parent homoleptic complexes because the π* orbital of the tpy-A ligand is stabilized, and the tpy-D ligand destabilizes the metal-centered π(t2g) orbitals.
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    30. 30
      Dodsworth, E. S.; Lever, A. B. P. Correlation of Electronic Charge Transfer Transitions and Electrochemical Potentials. The Bispyrazine(tetracarbonyl)molybdenum(0) System in Various Solvents. Chem. Phys Lett. 1984, 112, 567– 570,  DOI: 10.1016/0009-2614(84)85780-2
      30
      Correlation of electronic charge transfer transitions and electrochemical potentials. The bipyrazine(tetracarbonyl)molybdenum(0) system in various solvents
      Dodsworth, Elaine S.; Lever, A. B. P.
      Chemical Physics Letters (1984), 112 (6), 567-70CODEN: CHPLBC; ISSN:0009-2614.
      By using a free-energy diagram, a relation is drawn between an optical charge transfer energy and the elec. potentials of the donor and acceptor orbitals concerned. The charge transfer spectroscopy and elec. potentials of the title complex were studied in various solvents. A linear correlation, with neg. slope, was obsd. between the difference in oxidn. and redn. potentials and an metal-ligand charge transfer transition. Using some addnl. solvent data, a no. of useful parameters were derived in a fashion which would not be possible through consideration of either technique alone.
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    31. 31
      Wallin, S.; Davidsson, J.; Modin, J.; Hammarström, L. Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast Interligand Randomization of the MLCT State. J. Phys Chem. A 2005, 109, 4697– 4704,  DOI: 10.1021/jp0509212
      31
      Femtosecond transient absorption anisotropy study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. ultrafast interligand randomization of the MLCT state
      Wallin, Staffan; Davidsson, Jan; Modin, Judit; Hammarstroem, Leif
      Journal of Physical Chemistry A (2005), 109 (21), 4697-4704CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      It is known that the relaxed excited state of [Ru(bpy)3]2+ is best described as a metal to ligand charge transfer (MLCT) state having one formally reduced bipyridine and two neutral. Previous reports have suggested [Malone, R. et al. J. Chem. Phys. 1991, 95, 8970] that the electron "hops" from ligand to ligand in the MLCT state with a time const. of about 50 ps in acetonitrile. However, we have done transient absorption anisotropy measurements indicating that already after one picosecond, the mol. has no memory of which bipyridine was initially photo-selected, which suggests an ultrafast interligand randomization of the MLCT state.
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    32. 32
      Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. Application of the Energy Gap Law to the Decay of Charge-transfer Excited States. J. Am. Chem. Soc. 1982, 104, 630– 632,  DOI: 10.1021/ja00366a051
      32
      Application of the energy gap law to the decay of charge-transfer excited states
      Caspar, Jonathan V.; Kober, Edward M.; Sullivan, B. Patrick; Meyer, Thomas J.
      Journal of the American Chemical Society (1982), 104 (2), 630-2CODEN: JACSAT; ISSN:0002-7863.
      Investigation of a new series of mono- and bis-2,2'-bipyridyl or 1,10-phenanthroline complexes of Os(II) which exhibit long-lived, emitting, charge transfer excited states is reported. The compds. exhibit systematic changes in excited state properties, with changes in structure including excited state potentials, emission lifetimes, and emission max. In particular, a linear dependence of ln(knr) (knr is the excited state non-radiative decay rate) upon the emission energy max. is demonstrated, in agreement with the energy gap law.
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    33. 33
      Mecchia Ortiz, J. H.; Longo, C.; Katz, N. E. Polypyridyl Ruthenium Complexes Containing Anchoring Nitrile Groups as TiO2 Sensitizers for Application in Solar Cells. Inorg. Chem. 2015, 55, 69– 72,  DOI: 10.1016/j.inoche.2015.03.009
      33
      Polypyridyl ruthenium complexes containing anchoring nitrile groups as TiO2 sensitizers for application in solar cells
      Mecchia Ortiz, Juan H.; Longo, Claudia; Katz, Nestor E.
      Inorganic Chemistry Communications (2015), 55 (), 69-72CODEN: ICCOFP; ISSN:1387-7003. (Elsevier B.V.)
      Ru polypyridyl complexes [Ru(bpy)3-x(Mebpy-CN)x]2 + (x = 1,2 and 3, bpy = 2,2'-bipyridine, Mebpy-CN = 4-methyl-2,2'-bipyridine-4'-carbonitrile) can be used as visible dyes in novel solar cells formed with a porous TiO2 film (1 cm2), Pt counter-electrode and I/iodide as the redox mediator electrolyte dissolved in a polymeric matrix. These complexes can be anchored over the surface of nanocryst. TiO2 through nitrile groups, as evidenced by Raman spectra of the adsorbed species. Irradiated by a solar simulator (67 mW cm- 2), the cells assembled with the Ru complexes with x = 2 and 3 as TiO2 sensitizers exhibit almost identical current-potential curves, with short-circuit photocurrents of 1.25 mA cm- 2, fill factors of 0.5 and overall efficiencies around 0.44%. The Ru complex with x = 1 and a similar Re complex did not perform as well as sensitizers. These data were consistent with results obtained from quantum efficiency curves and impedance spectra. Complexes with nitrile groups as anchoring entities are promising candidates for designing efficient DSCCs.
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    34. 34
      Keis, K.; Lindgren, J.; Lindquist, S.-E.; Hagfeldt, A. Studies of the Adsorption Process of Ru Complexes in Nanoporous ZnO Electrodes. Langmuir 2000, 16, 4688– 4694,  DOI: 10.1021/la9912702
      34
      Studies of the Adsorption Process of Ru Complexes in Nanoporous ZnO Electrodes
      Keis, Karin; Lindgren, Jan; Lindquist, Sten-Eric; Hagfeldt, Anders
      Langmuir (2000), 16 (10), 4688-4694CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Nanostructured ZnO films consisting of 150-nm sized particles were dye-sensitized with different ruthenium complexes. Incident photon-to-current conversion efficiencies were found to be dependent on dye concn. and on residence time of the ZnO film in the dye soln. Results obtained using IR and Raman spectroscopies together with traditional two-electrode measurements using a sandwich-type cell gave evidence of dye agglomeration in the nanostructured ZnO film. Possible reasons for the agglomeration process in ZnO films are discussed.
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    35. 35
      Ishida, H.; Tobita, S.; Hasegawa, Y.; Katoh, R.; Nozaki, K. Recent Advances in Instrumentation for Absolute Emission Quantum Yield Measurements. Coord. Chem. Rev. 2010, 254, 2449– 2458,  DOI: 10.1016/j.ccr.2010.04.006
      35
      Recent advances in instrumentation for absolute emission quantum yield measurements
      Ishida, Hitoshi; Tobita, Seiji; Hasegawa, Yasuchika; Katoh, Ryuzi; Nozaki, Koichi
      Coordination Chemistry Reviews (2010), 254 (21-22), 2449-2458CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. The fluorescence and phosphorescence quantum yields (Φ) of std. solns. were re-evaluated based on an abs. method using an integrating sphere equipped with a multichannel spectrometer. In detail the Φ value of ruthenium(II) tris(2,2'-bipyridine) complex was examd. which were often used as the stds. in the detn. of quantum yields of transition-metal complexes. This revealed that the Φ values for [Ru(bpy)3]2+ were 0.063 in deaerated H2O, 0.040 in aerated H2O, 0.095 in deaerated CH3CN, and 0.018 in aerated CH3CN, resp., which are significantly higher than the previously accepted values. Also the tech. aspects in the detn. of abs. emission quantum yields for lanthanide complexes and those of org. crystals of anthracene were examd. For the accurate detn. for lanthanide complexes, special care must be taken in the spectroscopic measurements because of their narrow absorption and emission bands. For org. crystals, the fluorescence quantum yields are reduced due to reabsorption, chem. impurities, and structural defects. The observations for highly purified anthracene crystals revealed that the lower limit value of Φ was 0.64.
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    36. 36
      Cattaneo, M.; Fagalde, F.; Katz, N. E.; Borsarelli, C. D.; Parella, T. pH-Induced Luminescence Changes of Chromophore-Quencher Tricarbonylpolypyridylrhenium(I) Complexes with 4-Pyridinealdazine. Eur. J. Inorg. Chem. 2007, 3, 5323– 5332,  DOI: 10.1002/ejic.200700860
      There is no corresponding record for this reference.
    37. 37
      Cattaneo, M.; Fagalde, F.; Borsarelli, C. D.; Katz, N. E. Improvement of the Dynamic Range of pH Sensing by Using a Luminescent Tricarbonylpolypyridylrhenium(I) Complex with Three Different Protonation Sites. Inorg. Chem. 2009, 48, 3012– 3017,  DOI: 10.1021/ic802203d
      37
      Improvement of the Dynamic Range of pH Sensing by Using a Luminescent Tricarbonylpolypyridylrhenium(I) Complex with Three Different Protonation Sites
      Cattaneo, Mauricio; Fagalde, Florencia; Borsarelli, Claudio D.; Katz, Nestor E.
      Inorganic Chemistry (2009), 48 (7), 3012-3017CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      [Re(4,4'-(CO2H)2-bpy)(CO)3(4,4'-bpy)](CF3SO3), 1 (4,4'-(CO2H)2-bpy = 4,4'-dicarboxyl-2,2'-bipyridine, 4,4'-bpy = 4,4'-bipyridine), synthesized and characterized by spectroscopic techniques, displays a strong dependence of its photophys. properties on pH. From both emission intensity and lifetime measurements at different pH values, three values for the protonation consts. of the excited states were detd. (pKa1* = 1.8 ± 0.1, pKa2* = 3.9 ± 0.1, and pKa3* = 5.6 ± 0.1). The unusual bell-shaped variations of these photophys. properties can be accounted for by the changes of energy level orderings induced by each protonation, as confirmed by time-dependent d. functional theory (TD-DFT) calcns. Since the soly., stability, and dynamic range of pH sensing by 1 were improved with respect to similar tricarbonylpolypyridylrhenium(I) complexes, 1 can be used as an efficient mol. switch of the on-off-on type.
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    38. 38
      Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02. Gaussian, Inc.: Wallingford, CT, 2004.
      There is no corresponding record for this reference.
    39. 39
      Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648– 5652,  DOI: 10.1063/1.464913
      39
      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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    40. 40
      Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785– 789,  DOI: 10.1103/PhysRevB.37.785
      40
      Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density
      Lee, Chengteh; Yang, Weitao; Parr, Robert G.
      Physical Review B: Condensed Matter and Materials Physics (1988), 37 (2), 785-9CODEN: PRBMDO; ISSN:0163-1829.
      A correlation-energy formula due to R. Colle and D. Salvetti (1975), in which the correlation energy d. is expressed in terms of the electron d. and a Laplacian of the 2nd-order Hartree-Fock d. matrix, is restated as a formula involving the d. and local kinetic-energy d. On insertion of gradient expansions for the local kinetic-energy d., d.-functional formulas for the correlation energy and correlation potential are then obtained. Through numerical calcns. on a no. of atoms, pos. ions, and mols., of both open- and closed-shell type, it is demonstrated that these formulas, like the original Colle-Salvetti formulas, give correlation energies within a few percent.
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    41. 41
      Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-Consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650– 654,  DOI: 10.1063/1.438955
      41
      Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions
      Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A.
      Journal of Chemical Physics (1980), 72 (1), 650-4CODEN: JCPSA6; ISSN:0021-9606.
      A contracted Gaussian basis set (6-311G**) is developed by optimizing exponents and coeffs. at the Moller-Plesset (MP) second-order level for the ground states of first-row atoms. This has a triple split in the valence s and p shells together with a single set of uncontracted polarization functions on each atom. The basis is tested by computing structures and energies for some simple mols. at various levels of MP theory and comparing with expt.
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    42. 42
      Hay, P. J.; Wadt, W. R. Ab Initio Effective Core Potentials for Molecular Calculations. Potentials for the Transition Metal Atoms Sc to Hg. J. Chem. Phys. 1985, 82, 270– 283,  DOI: 10.1063/1.448799
      42
      Ab initio effective core potentials for molecular calculations. Potentials for the transition metal atoms scandium to mercury
      Hay, P. Jeffrey; Wadt, Willard R.
      Journal of Chemical Physics (1985), 82 (1), 270-83CODEN: JCPSA6; ISSN:0021-9606.
      Ab initio effective core potentials (ECP's) were generated to replace the Coulomb, exchange, and core-orthogonality effects of the chem. inert core electron in the transition metal atoms Sc to Hg. For the second and third transition series relative ECP's were generated which also incorporate the mass-velocity and Darwin relativistic effects into the potential. The ab initio ECP's should facilitate valence electron calcns. on mols. contg. transition-metal atoms with accuracies approaching all-electron calcns. at a fraction of the computational cost. Analytic fits to the potentials are presented for use in multicenter integral evaluation. Gaussian orbital valence basis sets are developed for the (3d,4s,4p), (4d,5s,5p), and (5d,6s,6p) orbitals of the first, second, and third transition series atoms, resp. All-electron and valence-electron at. excitation energies are also compared for the low-lying states of Sc-Hg, and the valence-electron calcns. reproduce the all-electron excitation energies (typically within a few tenths of an eV).
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    43. 43
      Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995– 2001,  DOI: 10.1021/jp9716997
      43
      Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model
      Barone, Vincenzo; Cossi, Maurizio
      Journal of Physical Chemistry A (1998), 102 (11), 1995-2001CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      A new implementation of the conductor-like screening solvation model (COSMO) in the GAUSSIAN94 package is presented. It allows Hartree-Fock (HF), d. functional (DF) and post-HF energy, and HF and DF gradient calcns.: the cavities are modeled on the mol. shape, using recently optimized parameters, and both electrostatic and nonelectrostatic contributions to energies and gradients are considered. The calcd. solvation energies for 19 neutral mols. in water are found in very good agreement with exptl. data; the solvent-induced geometry relaxation is studied for some closed and open shell mols., at HF and DF levels. The computational times are very satisfying: the self-consistent energy evaluation needs a time 15-30% longer than the corresponding procedure in vacuo, whereas the calcn. of energy gradients is only 25% longer than in vacuo for medium size mols.
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    44. 44
      O’Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. Cclib: A Library for Package-Independent Computational Chemistry Algorithms. J. Comput. Chem. 2008, 29, 839– 845,  DOI: 10.1002/jcc.20823
      44
      Software news and updates cclib: a library for package-independent computational chemistry algorithms
      O'Boyle, Noel M.; Tenderholt, Adam L.; Langner, Karol M.
      Journal of Computational Chemistry (2008), 29 (5), 839-845CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
      There are now a wide variety of packages for electronic structure calcns., each of which differs in the algorithms implemented and the output format. Many computational chem. algorithms are only available to users of a particular package despite being generally applicable to the results of calcns. by any package. Here we present cclib, a platform for the development of package-independent computational chem. algorithms. Files from several versions of multiple electronic structure packages are automatically detected, parsed, and the extd. information converted to a std. internal representation. A no. of population anal. algorithms have been implemented as a proof of principle. In addn., cclib is currently used as an input filter for two GUI applications that analyze output files: PyMOlyze and GaussSum.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00243.

    • Construction of DSCs, FTIR spectrum of complex 3, 1D and 2D NMR spectra and assignments of complex 3, UV–vis absorption data of complexes 1–3, CV and DPV curves of complex 3, emission spectra of complexes 1–3, TD-DFT calculations and assignments of complexes 1–3 and 3+, EDAX of ZnO NWs, UV–vis absorption spectra of bare and sensitized FTO |ZnO NWs, and Raman spectra of sensitized FTO |ZnO NWs (PDF).


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