Strongly Polarized π-Extended 1,4-Dihydropyrrolo[3,2-b]pyrroles Fused with Tetrazolo[1,5-a]quinolines

A straightforward route to 1,4-dihydropyrrolo[3,2-b]pyrroles comprised of two electron-withdrawing quinoline or tetrazolo[1,5-a]quinoline scaffolds has been developed. The versatile multicomponent reaction affording 1,4-dihydropyrrolo[3,2-b]pyrroles combined with intramolecular direct arylation enables assembly of these products in just three steps from anilines with overall yields exceeding 30%. The planarized, ladder-type heteroacenes possess up to 14 conjugated rings. These nominally quadrupolar materials exhibit efficient fluorescence with wavelengths spanning most of the visible spectrum from green–yellow for the dyes possessing biaryl bridges and orange–red for the fully fused systems. In many cases, the fluorescence quantum yields are large, the solvatofluorochromic effects are strong, and the fluorescence is maintained even in crystalline state. Analysis of the electronic structure of these molecular architectures using quantum chemical methods suggests that the character and position of the flanking heterocycle determine the shape of HOMO and LUMO and their extension to N-aryl substituents, influencing the values of molar absorption coefficient. An experimental study of the two-photon absorption (2PA) properties has revealed that it occurs in the 700–800 nm range with apparent deviation from the Laporte parity selection rule, which may be attributed to Hertzberg–Teller contribution to vibronically allowed 2PA transition.


■ INTRODUCTION
Modern chemistry and materials science of complex conjugated functional dyes often implies controlled modulation of the optoelectronic properties achieved by fine-tuning the π-system.Extending the π-conjugation by linking two or more unsaturated hydrocarbons at the periphery of a conjugated architecture is a powerful method for targeted modification of the available π-cloud, leading to changes in the local or global delocalization character. 1The π-electron distribution within a molecule can also be manipulated using both simple substituted aryl groups and exotic heteroaromatic motifs. 2 The 1,4-dihydropyrrolo [3,2-b]pyrrole (DHPP) motif, comprised of two fused pyrrole units in a centrosymmetric orientation, is not only the strongest known electron donor among 10π-electron heteropentalenes but can also be obtained through an easily accessible one-pot synthesis. 3The available facile synthesis of tetraaryl-1,4-dihydropyrrolo [3,2-b]pyrroles (TAPPs), combined with promising photophysical properties, is making TAPP scaffold-based organic chromophores attractive for many research applications. 4−22 The exceptional feature of DHPPs is that a convenient onepot synthesis enables the assembly of the heterocyclic scaffold possessing four arenes linked via biaryl linkages. 23,24The variety of these arenes is quite vast especially considering that by adding just one more synthetic step, e.g., direct arylation, the otherwise weakly coupled dye can be efficiently transformed into a fused planar compound possessing 8, 10, or 12 conjugated aromatic rings. 25These advantages provide organic chemists a unique and versatile toolbox that currently has few if any analogs, at least in the area of centrosymmetric quadrupolar dyes.
Tetrazole is a five-membered, fully conjugated nitrogen-rich 6π-azaheterocycle consisting of a single carbon and four consecutive nitrogen atoms, 26 which render the system very electron deficient.Interestingly, tetrazole has the highest nitrogen content among the stable five-membered heterocycles.Despite the high nitrogen content, tetrazole and most of its derivatives remain relatively stable under the influence of heat or microwave radiation. 27Furthermore, the related compounds are able to withstand a wide range of chemical environments such as strongly acidic and basic media, alkylating agents, dienophiles, as well as oxidizing and reducing conditions. 27Even though tetrazoles 28 and their annulated derivatives have received considerable attention over the past years in fields of medicine, agriculture, and material science, it is still surprising that applications of tetrazoloquinolinecontaining compounds as functional dyes have been rarely investigated to date. 29ur objective is to use unique features of TAPPs' chemistry for the synthesis of large, structurally unique, heavily N-doped nanographenes, possessing tetrazole scaffolds.We reasoned that incorporation of moderately electron-withdrawing moieties such as tetrazolo [1,5-a]quinoline and quinoline at strongly conjugated positions 2 and 5 can be utilized to endow resulting dyes with desired photophysical properties.
The analogous concept has been employed to obtain centrosymmetric, planarized tetrazoloquinoline-fused pyrrolo-[3,2-b]pyrroles (TQFPPs).The pivotal difference in these two approaches is that in the case of quinoline-fused dyes, halogen atoms were placed on substituents at positions 2 and 5, whereas for tetrazoloquinoline-fused TAPPs, N-aryl substituents possessed bromine atoms.Dyes 6o and 6p were transformed into TQFPPs 11a and 11b via double intramolecular palladium-catalyzed direct arylation.High yields achieved for all these transformations encouraged us to attempt the synthesis of new TQFPPs incorporating [n]helicenes with hope to obtain the molecules with interesting photophysical properties.To achieve this goal, the chosen building blocks were 1-amino-2-bromonaphthalene and 9amino-10-bromophenanthrene as the arylamine components and tetrazolo [1,5-a]quinoline-4-carbaldehyde bearing hexyl or octyl substituents as the aldehyde.Consequently, the brominated TQPPs 6q and 6r were prepared using appropriate starting materials using our optimized three-component condensation (Scheme 4).
Photophysical Results.Spectroscopic properties of all prepared dyes were measured in toluene (if solubility allowed) (dielectric constant ε = 2.38) and dichloromethane (DCM, ε = 8.93).Spectroscopic properties of representative dyes 4a, 4g, 6a, 6g, 6r, 10a, 10c, 11a, 11c, and 11d are shown in Figures 2  and 3 and summarized in Table 1 (see Table S1 for comprehensive results).The absorption spectra of CQPPs (dyes 4a−4k) are dominated by a strong band at around 390 nm, with the peak extinction coefficient values in the range ε = 15000−30000 M −1 •cm −1 .Comparison of the peak absorption wavelength of CQPPs with that of structurally related 2,5bis(quinolin-2-yl)pyrrolo [3,2-b]pyrroles 25b shows that in toluene the former dyes absorb wavelengths ca.30−50 nm lower.This is attributed to the presence of chloro-substituent located ortho-to DHPP core, which, for steric reasons (confirmed by X-ray, Figure 1b), effectively disturbs ground state electronic coupling between the core and the periphery of the molecule.On the other hand, the absorption maxima of CQPPs are bathochromically shifted by ca. 30 nm compared to those of 2,5-bis(naphth-2-yl)pyrrolo [3,2-b]pyrroles, 24b suggesting that the electron-withdrawing character of quinoline substituents is, nevertheless, clearly manifested.This behavior is even more pronounced for tetrazole-bearing TQPPs, which typically exhibit absorption maxima at 455−464 nm (dyes 6a− 6q), with a vibronic shoulder at ca. 20 nm lower.The only exception from this pattern is dye 6r, for which the vibronic band is stronger than the origin band.The redshift observed for TQPPs was expected due to the significant increase in acceptor character of the substituents at positions 2 and 5 from weakly to strongly electron withdrawing.Additionally, the lack of an ortho-chloro substituent allows better planarization/ electronic coupling in the ground state, which is confirmed by X-ray studies (Figure 1a).TQPPs also exhibit much higher extinction coefficients and fluorescence quantum yields (Φ fl ) than CQPPs, despite the presence of heavy atoms and nitrogen-rich aromatic rings, which often lead to very efficient intersystem crossing and quenching of fluorescence. 32eedless to say, rigid and planar QFPPs and 11a−c display remarkably different characteristics compared to parent TAPP molecules.A significant ≈100 nm bathochromic shift of absorption were observed for dye 10a (when compared to 4h), as well as for dye 11a (when compared to 6i) (Figures 2 and  3).Further consequences of planarization were larger Φ fl (reaching 100% in DCM in the case of dye 11a) and smaller Stokes shifts observed for fused dyes, which are results of more restricted molecular motion comparing to the parent dyes.As expected, the properties of monocoupling product 11d stand in the middle between those of substrates and the bis-coupled products.
As far as the solid-state emission is concerned, both groups of examined quadrupolar dyes, 4 and 6, exhibit strikingly different properties.The analysis of solid-state spectra reveals that in the case of 4d, 4h, and 4i, absorption is only slightly bathochromically shifted, i.e., to 425 nm, whereas emission is hypsochromically shifted (450−475 nm) compared to data collected in toluene.The only exception from this trend is found in the case of 4k ( em max = 515 nm), and it is presumably related to the presence of sterically encumbered substituents (four tert-butyl groups), which impact packing.On the other Scheme 2. Synthesis of 2,5-Bis(tetrazoloquinoline)-1,4diaryl pyrrolo [3,2-b]pyrroles (TQPPs) The Journal of Organic Chemistry hand, dyes 6a, 6e, 6i, and 6o do have bathochromically shifted both absorption and emission to 525 and 575 nm, respectively (see Supporting Information for details).For all these dyes, the fluorescence intensity is moderate ranging from 2.9% to 20%.Two-Photon Absorption. Figure 4 presents the twophoton absorption (2PA) cross-section spectra in GM units (1 GM = 10 −50 cm 4 s −1 photon) of representative dyes 4h, 6h, and 10a in toluene measured with 120 fs laser pulses tuned in the wavelength range λ 2PA = 630−1040 nm (red line, lower horizontal scale) and through the fluorescence excitation method, which consists of scaling the reference-corrected 2PA spectral profiles according to absolute 2PA cross-section values determined at select wavelengths (black symbols).The linear absorption spectra of 4h, 6h, and 10a in toluene are shown for comparison on a 2× expanded wavelength scale (blue line, upper horizontal axis).
In all three dyes, the 2PA shows distinct features with the maximum cross-section values, σ 2PA = 50 GM (770 nm) and 43 GM (660 nm) in 4h, σ 2PA = 250 GM (775 nm) in 6h, and σ 2PA = 5 GM (930 nm) and 25 GM (700 nm) in 10a.The experimental observation that the TQPP 6h shows almost 1 order of magnitude larger peak σ 2PA value compared to the CQPP and QFPP-type compounds correlates well with a stronger-acting electron-withdrawing ability of the attached tetrazole moiety.As a further testimony to the quadrupolar nature of these chromophores, none of the 2PA spectral profiles coincide with underlying 1PA transition profiles.Nevertheless, the ratio of σ 2PA vs ε 1PA (dashed black lines) remains finite even for the longest wavelength part of the spectra.Such apparent deviation from the Laporte parity selection rule has been previously observed for a number of nominally inversion-symmetric diketopyrrolopyrroles 33 and is most likely due to Hertzberg−Teller contribution to vibronically allowed 2PA transition. 22It is worth to point out that much larger values of σ 2PA were reached for various strongly pyrrole derivatives bearing strongly polarized donor−acceptor structure. 34Computational Studies.DFT and TDDFT/M06/6-31G(d,p) calculations with the optimization of molecular structures in the ground electronic state S 0 and the lowest electronic excited state S 1 were performed for the description of the spectroscopic properties of molecules 4, 6, 10, and 11 (for results, see Table 2).These dyes have been chosen since they constitute model dyes for the four groups of compounds studied in this project.The calculations also included the molecule 10-x−regioisomer 10, which is topologically similar to 11.The M06 calculation method was chosen as it best reproduces the experimental results (Table S7 and Figure S7).Calculations were performed using the Gaussian 16 package. 35he effect of solvent was described in the PCM procedure.The SOC elements were also calculated using the Orca program. 36able 2 contains the calculated energies and oscillator strengths for the absorption (S 0 → S 1 ) and fluorescence (S 1 → S 0 ), characterizing structures optimized in the S 0 and S 1 states.Two of the considered molecules, 4 and 6, are nonplanar systems.The angle between the planes of the peripheral arenes possessing electron-withdrawing character and the DHPP center in 4, optimized in the S 0 ground state, is 43°, and is 20°i n the case of compound 6.In the S 1 excited state, the angle values slightly decrease to 32°and 18°, respectively.The planes between N-aryl substituents also form large angles with the DHPP plane: 43°and 45°in the S 0 and S 1 states of molecule 4, and 48°and 49°in the case of 6, respectively, which is corroborated by X-ray data (Figure 1).
The computational results, given in Table 2, capture the experimental facts, such as the red shifts of spectra of dye 6 relative to TAPP 4 and of dye 11 relative to TAPP 10 (i.e., in the context of the changing the acceptor), as well as 10 relative to 4 and 11 relative to 6 (i.e., in context fused vs nonfused system).
The calculations reveal that electronic transitions between the S 0 and S 1 states in all tested compounds are described by the electronic configuration {HOMO, LUMO}, where HOMO and LUMO have a specific structure, resembling "exciplex" The Journal of Organic Chemistry EDA systems.Meaning, the HOMO orbital is created from the HOMO of the DHPP donor with an admixture of HOMO orbitals of both acceptors, and the LUMO orbitals are created from a combination of LUMO orbitals of both acceptors with an admixture of the donor's LUMO orbital (see Figures 5 and  S8; the orbitals of individual elements are shown in Table S8).Interestingly, formation of fully π-conjugated systems 10 and 11 is accompanied by spreading both HOMO and LUMO into N-aryl substituents (see Figure 5).
Therefore, the electronic transitions between the S 0 and S 1 states in all the tested compounds are qualitatively of the same nature: a multicenter CT transition with a charge shift between the central DHPP scaffold and two symmetrically arranged acceptor moieties at the periphery.Referring to the symbolic record of the CT transition energy E CT = I D − E A + E int (D,A,R), it implies that the quantitative differences in the spectroscopic properties of these molecules are dictated by the interplay of several factors: differences in the electron affinity of acceptors and differences in interaction energy, resulting from variability in the mutual orientation of the donor and acceptors and the role played by the N-aryl substituents.The red shifts of the absorption and fluorescence spectra of 6 relative to 4 (and 11 relative to dye 10) are in correspondence with the change in the electron affinity of the acceptor (≈1800 cm −1 , Table S8).
However, differences in the electron affinity of the acceptor are not the only reason for differences in spectroscopic properties of the tested molecules.In the case of such complex molecules, with the possibility of rotation of the components around the bonds connecting them, an important factor influencing the spectroscopic properties of these compounds is the mutual orientation of the planes of the individual elements.An interesting observation can be derived from probing the changes in the energy and the oscillator strength of the absorption depending on the mutual orientation of the donor and acceptors using compound 4 as an example (Figure 6).According to the results of the optimization of the molecular structure, the energy minimum for this compound corresponds to the geometry in which the angle between the planes of acceptors and donor is 43°.That means that in this geometry, a balance is achieved between the stabilizing role of interactions between donor and acceptors and the destabilizing role of steric interactions.This dye absorbs in DCM at 424 nm with an oscillator strength of 0.815 (Table 2).Both quantities change when moving away from equilibrium by changing the donor−acceptor angle (see Figure 6).In the case of orthogonality of the donor and acceptor planes (the system practically is not stabilized by the interactions between them), the absorption transition would be at higher energy at 418 nm with zero oscillator strength.In turn, in the planar case, the interaction of the donor with the acceptors is the strongest, which is reflected in the absorption at 460 nm with f = 1.389.The planar arrangement is destabilized by strong steric interactions (see the potential energy curve in Figure 6).Collectively, these results highlight that those attractive spectroscopic properties, such as low transition energy and high oscillator strength should emerge upon the formation of a fused system, i.e., 10.The photophysical data gathered in Tables 1 and S7 corroborate this prediction.
The values of absorption energy and oscillator strength for compound 10 (Table 2) are close to those predicted based on calculations for the planarized version of 4 (Figure 6).This is not true, however, for the pair 6 and 11.Despite the red shift of 11 relative to 6, the oscillator strength of the transition between S 0 and S 1 in 11 is f = 1.02 and is smaller than the oscillator strength in 6 ( f = 1.79).These computational predictions are fully corroborated by comparison of experimental molar absorption coefficients (Tables 1 and S7).Therefore, when creating a fused system, an additional circumstance appears that affects its spectroscopic properties.
The oscillator strength is a quantity strongly determined by the shape of the molecular orbitals.Therefore, the reasons for Scheme 3. Synthesis of Ladder-Type Quinoline-Fused Pyrrolo [3,2-b]

pyrroles (QFPPs) via Intramolecular Double Direct Arylation
The Journal of Organic Chemistry such discrepancies can be sought in differences in the extension of the charge distribution to atoms of moieties originating from electron-withdrawing moieties, which occurs in fused dyes.To confirm this, calculations were performed for the compound 10-x (Figure 7), which is an isomer of 10 with a geometric structure analogous to 11 (for comparison of the both structures see Table S3).The calculated oscillator strength (f = 0.748) for fused system in geometry (10-x) is smaller than oscillator strength (f = 0.815) for nonfused molecule 4 and smaller than f = 1.203 for fused 10.Therefore, the relationships between the transition energies and oscillator strengths of 10-x and 4 are similar as the relations between 11 with 6 (see Table 2).Comparison of the orbitals 10 and 10-x is given in Table S9 and indicates that the orbitals of 10-x are more spread to the N-aryl substituents than the orbital of 10.

■ CONCLUSIONS
We have provided experimental evidence that the efficient synthesis of quinoline derivatives enables not only preparation of 1,4-dihydropyrrrolo [3,2-b]pyrroles possessing planarized structures based on tetrazole but also previously inaccessible large planar N-doped nanographenes based on quinolines.The  The Journal of Organic Chemistry emission range of these prepared centrosymmetric, polarized TAPPs can be modulated from green to yellow, orange, and red, and their efficiency are typically moderate to high.Planarization of the chromophore structure results in considerable red shift of the emission wavelength (quinolinederived dyes show strong yellow fluorescence and dyes possessing two tetrazoloquinoline units exhibit red emission).The two-photon absorption spectrum follows, in most cases, the Laporte rule, which is in accord with the inversionsymmetric structure, with the peak cross-section values reaching σ 2PA = 250 GM in the case of TAPP possessing two tetrazoloquinoline substituents at positions 2 and 5. Critically, subtle changes in geometry of these π-extended dyes have a profound effect on their photophysics.Computational studies suggest that intersystem crossing yield is responsible for variation for modulation of fluorescence intensity, thus confirming the notion that planarization of the geometry of polarized centrosymmetric dyes affords control over the molecular excited state.The change in the absorption strength between weakly coupled (biaryl bridges) and fused quadrupolar, centrosymmetric dyes is different for quinoline derivatives and for tetrazoloquinoline derivatives.In the latter case, the molar absorption coefficient actually decreases after planarization of the entire chromophore.The computational investigation enabled us to postulate that this striking difference has its origin in different orientation of the electron-deficient heterocyclic moiety, which causes the spread of the localization of both HOMO and LUMO on N-aryl moiety leading in turn to decrease in molar absorptivity.The relative position of singlet and triplet excited states plays a dominant role in determining the fate of the molecules after excitation.In particular, it is responsible for relatively small fluorescence quantum yield for bis(quinolinyl)dyes.Collectively, this work demonstrates that a tetrazole scaffold can be efficiently incorporated into the structure of quadrupolar, centrosymmetric functional dyes furnishing the latter with outstanding photophysical characteristics.
■ EXPERIMENTAL SECTION General Information.All chemicals were bought from Sigma-Aldrich, TCI, Ambeed, and AlfaAesar and were used as received unless otherwise noted.All solvents used for reactions were analysis grade and were used without further purification.2-Chloroquinoline-3-carbaldehyde derivatives 1, 37 tetrazolo-[1,5-a]quinoline-4-carbaldehyde 5, 38 2-bromo-4-dodecylaniline, 39 and 9-amino-10-bromophenanthrene 40 were synthe-sized according to literature procedures.All reactions requiring heating were carried out using an oil bath.Reaction progress was monitored by thin layer chromatography (TLC), which was performed on aluminum foil plates, covered with Silica gel 60 F254 (Merck).The identity and purity of prepared compounds were proved by 1 H NMR and 13  All melting points for crystalline products were measured with automated melting point apparatus EZ-MELT and were given without correction.
Spectrophotometric grade solvents were used without further purification.All photophysical studies were performed with freshly prepared, air equilibrated solutions at room temperature.Steady-state fluorescence measurements were performed in standard 1 cm quartz cuvettes with dilute solutions (10 −6 M, optical density <0.1) to minimize inner filter effects and/or aggregation.Absorption spectra were measured using a UV−vis Shimadzu UV-3600i Plus spectrophotometer.The calculation of molar absorption coefficient was conducted from Beer−Lambert's law equation.Emission spectra were measured using Edinburgh Instruments FS5 spectrofluorometer equipped with photomultiplier Hamamatsu R123456.Fluorescence quantum yields (Φ) were calculated from the equation: where A denotes absorbance, n is a refractive index of a solvent, and S is an integrated fluorescence intensity.
The procedure for the synthesis of compounds 4b−4l is similar to that of compound 4a.
The procedure for the synthesis of compounds 6b−6r is similar to that of compound 6a.
The procedure for the synthesis of compounds 10b and 10c is similar to that of compound 10a.

Figure 4 .
Figure 4. 2PA cross-section spectra of 4h, 6h, and 10a in toluene solution in GM (red line, left vertical and lower horizontal scale).Black symbols�absolute 2PA cross sections measured at select wavelengths.Blue line�molar extinction spectra in toluene (blue line, upper horizontal axis).Dashed black line�ratio between the 2PA and 1PA spectral profiles (in arbitrary units).

Figure 5 .
Figure 5. Shape of HOMO and LUMO orbitals of investigated dyes.Similarity and differences between HOMO and LUMO orbitals of nonfused and fused dyes, 6 and 11 − orbitals of 11 are spreading to N-aryl substituents.

Figure 6 .
Figure 6.Potential energy curve of molecule 4 in S 0 state as a function of changes in the mutual orientation of the planes of the DHPP donor and acceptors.At the top, the absorption energy values together with the oscillator strengths, corresponding to the minimum geometry and extreme cases of coplanar and orthogonal donor and acceptor planes.

Figure 7 .
Figure 7. Structure of dye 10-x designed for computational purpose.