Push–Pull Derivatives Based on 2,4′-Biphenylene Linker with Quinoxaline, [1,2,5]Oxadiazolo[3,4-B]Pyrazine and [1,2,5]Thiadiazolo[3,4-B]Pyrazine Electron Withdrawing Parts

A series of novel V-shaped quinoxaline, [1,2,5]oxadiazolo[3,4-b]pyrazine and [1,2,5]thiadiazolo[3,4-b]pyrazine push–pull derivatives with 2,4′-biphenylene linker were designed and their electrochemical, photophysical and nonlinear optical properties were investigated. [1,2,5]Oxadiazolo[3,4-b]pyrazine is the stronger electron-withdrawing fragment as shown by electrochemical, and photophysical data. All compounds are emissive in a solid-state (from the cyan to red region of the spectrum) and quinoxaline derivatives are emissions in DCM solution. It has been found that quinoxaline derivatives demonstrate important solvatochromism and extra-large Stokes shifts, characteristic of twisted intramolecular charge transfer excited state as well as aggregation induced emission. The experimental conclusions have been justified by theoretical (TD-)DFT calculations.


Introduction
Push-pull derivatives with a D-π-A structure, where A and D are electron-withdrawing and electron-donating groups and the π represents a π-conjugated system, have been subject to a huge interest in the last decades due to their exceptional linear and nonlinear optical properties [1,2]. The D-A interaction into push-pull structures, also called intramolecular charge transfer (ICT), leads to the formation of low energy molecular orbitals whose electrons can be excited by visible light inducing colored materials [3][4][5][6]. The charge transfer absorption band will be red-shifted with an increase in the ICT strength. The limiting resonance forms of nitroaniline, a typical push-pull structure, are represented in Figure 1. Push-pull molecular materials have found applications as fluorescent sensors [7][8][9][10], dye-sensitized solar cells (DSSC) [11][12][13][14], nonfullerene organic photovoltaic (OPV) materials [15][16][17], and numerous other optoelectronic devices [18][19][20][21]. The intramolecular charge transfer (ICT) in push-pull molecular materials can be modulated either by playing on the A/D couple [1,2,22,23] or by changing the nature and length of the π-conjugated bridge [1,2,24,25]. In this context, π-deficient heterocycles are particularly interesting as A part, as follows: the electron-lone pairs of heteroatoms of the heterocyclic core can be indeed used for protonation, complexation, alkylation, or the formation of a hydrogen bond, modifying their electron-withdrawing character and tuning the optical properties of corresponding push-pull structures by redistribution of charge density [26][27][28][29][30][31].
In the second step, the desired chromophores 10a,b and 11a,b were obtained in good yields through the Suzuki-Miyaura cross-coupling reactions with the appropriate boronic acids 8a,b, as shown in Scheme 2.

Electrochemical Properties
Electrochemical behavior of compounds 9-11 were studied by cyclic voltammetry in dicholromethane (DCM) containing Bu 4 NPF 6 electrolyte at a scan rate of 0.1 V/s. The working electrode was a glassy carbon disk. A Pt wire was used as the counter electrode, and an Ag wire as a reference electrode. Ferrocene was used as an internal reference for potential measurements. The first oxidation/reduction peak potentials and their differences are listed in Table 1. Representative CV diagrams of compounds are shown in Figure 3.
acids 8a,b, as shown in Scheme 2.

Electrochemical properties
Electrochemical behavior of compounds 9-11 were studied by cyclic voltammetry in dicholromethane (DCM) containing Bu4NPF6 electrolyte at a scan rate of 0.1 V/s. The working electrode was a glassy carbon disk. A Pt wire was used as the counter electrode, and an Ag wire as a reference electrode. Ferrocene was used as an internal reference for potential measurements. The first oxidation/reduction peak potentials and their differences are listed in Table 1. Representative CV diagrams of compounds are shown in Figure 3.   Compounds 9b, 10b, and 11b bearing a 9H-carbazol-9-yl fragment as an electrondonating group exhibit a first irreversible oxidation process at, respectively, 1.08 V, 0.87 V, and 1.10 V vs. Fc/Fc + . The measured maximum current intensity is high in comparison with the values obtained for reduction processes, indicating a possible polymerization reaction on the electrode. As a consequence, an irreversible reduction broad peak is detected between −0.67 and −0.84 V on the reverse scan only after the oxidation of compounds ( Figure S1). Upon initial scanning in the negative direction, a reversible reduction is observed at −0.95 and −1.10 V, respectively, for 9b and 10b, and an irreversible reduction is observed at E pc = −2.36 V for 11b. When a donor is a diphenylamino group (compounds 9a, 10a, and 11a), the first irreversible oxidation is shifted towards lower potentials as compared to the respective carbazole analogues 9b, 10b, and 11b. This confirms the superior electrondonating strength of the diphenylamino moiety. For these compounds, the first oxidation is followed by a second one at a higher potential ( Figure S2). The modification of the donor has a very weak influence on the reduction of the diazine (see Table 1 and Figure 3). These trends indicate a weak ground-state interaction between the donor and acceptor.
When comparing the reduction process of the electron-withdrawing fragments, the reduction potential increase in the following order 11 < 10 < 9, indicating that the [1,2,5] oxadiazolo [3,4-b]pyrazine electron-withdrawing fragment is the strongest one. As a result, compound 9a, bearing the strongest electron-donating and electron-withdrawing parts, showed the lowest electronic gap (∆E) and is expected to exhibit the most red-shifted absorption bands across the whole series. Compared to their 1,4-phenylene analogues previously described [39], compounds 9a and 9b exhibited a larger electronic gap (0.65 V and 0.48 V) due to more difficult both oxidation and reduction processes, which indicates a significantly lower ICT.

Photophysical Properties
The UV-Vis and photoluminescence (PL) spectra of compounds 9-11 were measured in DCM solution at room temperature and the data are reported in Table 2. The UV/Vis spectra are provided in Figure 4. Regarding the lowest energetic absorption band, as expected according to electrochemical studies, the diphenylamino derivatives exhibit red-shifted absorption compared to their carbazole analogues. Similarly, as far as the electron-withdrawing part is concerned, the absorption maxima increase in the following order: 11 < 10 < 9 in both the diphenylamino and carbazole series. Compounds 9 and 10 are not luminescent in DCM solution, whereas the quinoxaline derivatives 11a and 11b exhibited green and yellow emissions, respectively, with low quantum yield. The particularly large Stokes shifts (>12,000 cm −1 for 11b) could indicate the presence of a TICT excited state.  In an effort to gain further insight into the photophysical process within push-pull derivatives 11, their emission spectra were registered in a series of aprotic solvents of increasing polarity. The positions of the corresponding emission maxima are reported in Table 3, and the normalized emission spectra registered for 11a, as well as a picture of the solution in the various solvents, can be seen in Figures 5 and 6 (the same data are presented for compound 11b in Figures S3 and S4). Whereas the position of the absorption maximum is not significantly affected by the polarity of the solvent, the position of the emission maxima is bathochromically shifted when the polarity of the solvent, which is estimated according to the Reichardt polarity scale [44], is increased. This pronounced positive emission solvatochromism is characteristic of ICT and is well documented for push-pull materials [45][46][47][48], in particular in the case of biphenylene linkers between A and D parts [49]. Large emission solvatochromism and low emission quantum yield in a polar solvent are characteristics of TICT [50,51].   Aggregation-induced emission (AIE), a concept proposed by Tang and coworkers in 2001 [52,53], induces intensive emission in the solid-state of non-(or slightly) emissive chromophores in solution. A restriction of intramolecular motion is one of the mechanisms of AIE [53]. In order to study the potential AIE properties of compounds 11, their emission spectra were recorded in a mixture of MeCN and water of a different ratio (Figures 7 and S5). The pictures of the solutions under UV irradiation are presented in Figures  8 and S6. For 11a, no emission is observed in pure MeCN, while a dramatic increase in the emission intensity is observed when the water fraction is higher than 80%, inducing a new band centered at 519 nm. The highest intensity is observed for the water fraction of 97%. Compound 11b is slightly emissive in pure MeCN with a yellow emission centered at 570 nm. When the ratio of water is increased, an extinction of the emission is initially observed (a water fraction of 50/60%), followed by the appearance of a new, blue-shifted emission  Aggregation-induced emission (AIE), a concept proposed by Tang and coworkers in 2001 [52,53], induces intensive emission in the solid-state of non-(or slightly) emissive chromophores in solution. A restriction of intramolecular motion is one of the mechanisms of AIE [53]. In order to study the potential AIE properties of compounds 11, their emission spectra were recorded in a mixture of MeCN and water of a different ratio (Figures 7 and S5). The pictures of the solutions under UV irradiation are presented in Figures  8 and S6. For 11a, no emission is observed in pure MeCN, while a dramatic increase in the emission intensity is observed when the water fraction is higher than 80%, inducing a new band centered at 519 nm. The highest intensity is observed for the water fraction of 97%. Compound 11b is slightly emissive in pure MeCN with a yellow emission centered at 570 nm. When the ratio of water is increased, an extinction of the emission is initially observed (a water fraction of 50/60%), followed by the appearance of a new, blue-shifted emission Aggregation-induced emission (AIE), a concept proposed by Tang and coworkers in 2001 [52,53], induces intensive emission in the solid-state of non-(or slightly) emissive chromophores in solution. A restriction of intramolecular motion is one of the mechanisms of AIE [53]. In order to study the potential AIE properties of compounds 11, their emission spectra were recorded in a mixture of MeCN and water of a different ratio (Figure 7 and Figure S5). The pictures of the solutions under UV irradiation are presented in Figure 8 and Figure S6. For 11a, no emission is observed in pure MeCN, while a dramatic increase in the emission intensity is observed when the water fraction is higher than 80%, inducing a new band centered at 519 nm. The highest intensity is observed for the water fraction of 97%. Compound 11b is slightly emissive in pure MeCN with a yellow emission centered at 570 nm. When the ratio of water is increased, an extinction of the emission is initially observed (a water fraction of 50/60%), followed by the appearance of a new, blue-shifted emission band (λ max = 472 nm) with the maximum intensity of 80% of water. For higher water fractions, the emission is slightly blue-shifted and lowered in intensity. band (λmax = 472 nm) with the maximum intensity of 80% of water. For higher water fractions, the emission is slightly blue-shifted and lowered in intensity.  The compounds 9-11 are luminescent solids as measured in the KBr matrix. The emission maxima are listed in Table 2. The spectra and pictures of selected KBr pellets are presented in Figures 9 and 10. The KBr pellets exhibited intense emission from the blue for compound 11b to near-infrared for compound 9a. The emission is red-shifted in the same order as the emission bands recorded in solution. The compounds 9-11 are luminescent solids as measured in the KBr matrix. The emission maxima are listed in Table 2. The spectra and pictures of selected KBr pellets are presented in Figures 9 and 10. The KBr pellets exhibited intense emission from the blue for compound 11b to near-infrared for compound 9a. The emission is red-shifted in the same order as the emission bands recorded in solution.

NLO Properties
The second-order NLO properties of compounds 9-11 have been investigated in chloroform using the electric-field induced second harmonic generation (EFISH) method at a non-resonant incident wavelength of 1907 nm. The EFISH method provides an estimation of the NLO response as the scalar product between the permanent dipolar moment of the molecule µ �⃗ and the vector component of, β described as β⫽ [54][55][56]. The data are reported in Table 4. The NLO responses of compounds 9-11 are particularly low and we are close to the limit of detection of the system. It is therefore not reasonable to compare the values between them. For the 1,4-phenylene analogue of compounds 9a, a much higher µβ value was measured (700 × 10 −48 esu) [39]. This indicates that the 1,2-phenylene arrangement drastically reduces the NLO response due to the limited/diminished ICT.

Theoretical Calculation
Molecular structures and electronic properties of V-shaped push-pull chromophores 9-11 were theoretically investigated using the Gaussian 16 software package [57]. First, the geometries were optimized using the DFT B3LYP/6-311+G(2df,p) method in CHCl3. Using the same level of theory, the energies of the frontier molecular orbitals (HOMO/LUMO), ground-state dipole moments (µ), and first hyperpolarizabilities (β) were subsequently calculated. All the calculated data are gathered in Table 5; see the ESI for further information.

NLO Properties
The second-order NLO properties of compounds 9-11 have been investigated in chloroform using the electric-field induced second harmonic generation (EFISH) method at a non-resonant incident wavelength of 1907 nm. The EFISH method provides an estimation of the NLO response as the scalar product between the permanent dipolar moment of the molecule µ �⃗ and the vector component of, β described as β⫽ [54][55][56]. The data are reported in Table 4. The NLO responses of compounds 9-11 are particularly low and we are close to the limit of detection of the system. It is therefore not reasonable to compare the values between them. For the 1,4-phenylene analogue of compounds 9a, a much higher µβ value was measured (700 × 10 −48 esu) [39]. This indicates that the 1,2-phenylene arrangement drastically reduces the NLO response due to the limited/diminished ICT.

Theoretical Calculation
Molecular structures and electronic properties of V-shaped push-pull chromophores 9-11 were theoretically investigated using the Gaussian 16 software package [57]. First, the geometries were optimized using the DFT B3LYP/6-311+G(2df,p) method in CHCl3. Using the same level of theory, the energies of the frontier molecular orbitals (HOMO/LUMO), ground-state dipole moments (µ), and first hyperpolarizabilities (β) were subsequently calculated. All the calculated data are gathered in Table 5; see the ESI for further information.  10a, 10b, 11a, 11b). The picture was taken in the dark upon irradiation with a handheld UV lamp (λ exc = 366 nm).

NLO Properties
The second-order NLO properties of compounds 9-11 have been investigated in chloroform using the electric-field induced second harmonic generation (EFISH) method at a non-resonant incident wavelength of 1907 nm. The EFISH method provides an estimation of the NLO response as the scalar product between the permanent dipolar moment of the molecule → µ and the vector component of, β described as β // [54][55][56]. The data are reported in Table 4. The NLO responses of compounds 9-11 are particularly low and we are close to the limit of detection of the system. It is therefore not reasonable to compare the values between them. For the 1,4-phenylene analogue of compounds 9a, a much higher µβ value was measured (700 × 10 −48 esu) [39]. This indicates that the 1,2-phenylene arrangement drastically reduces the NLO response due to the limited/diminished ICT.

Theoretical Calculation
Molecular structures and electronic properties of V-shaped push-pull chromophores 9-11 were theoretically investigated using the Gaussian 16 software package [57]. First, the geometries were optimized using the DFT B3LYP/6-311+G(2df,p) method in CHCl 3 . Using the same level of theory, the energies of the frontier molecular orbitals (HOMO/LUMO), ground-state dipole moments (µ), and first hyperpolarizabilities (β) were subsequently calculated. All the calculated data are gathered in Table 5; see the ESI for further information. All data calculated at the DFT level by using the Gaussian ® 16 software package and DFT B3LYP/6-311+G(2df,p) method in CHCl 3 . The first hyperpolarizabilities β(−2ω,ω,ω) were calculated at 1907 nm. The electronic absorption spectra, the longest-wavelength absorption maxima and the corresponding electron transitions were calculated using TD-DFT (n states = 8) B3LYP/6-311+G(2df,p).
The optimized geometries of 9-11 are shown in Figure 11A. As can be seen, the central biphenylene π-linker, interconnecting the N,N-diphenylamino, and pyrazine-derived moieties, adopts twisted geometry with the central torsion angle between 20 • and 60 • . Carbazole derivatives b showed generally larger torsion angles. The heterocyclic moiety at position 2 is always turned out by about 45 • , independently of the particular derivative. The bond length alternation of both 1,4-phenylene moieties of the π-linker was investigated, revealing the Bird index I 6 and the quinoid character (δr) within the range of 91-96/2-5% [58,59]. For unsubstituted benzene, the I 6 and δr are equal to 100 and 0, respectively. The calculated values imply that both 1,4-phenylene moieties in 9-11 are slightly polarized. Alternation of the heterocyclic acceptor significantly affected the ground state dipole moment of V-shaped chromophores. The largest ones were calculated for furazanopyrazine derivatives 9, while replacement of the fused terminal oxa/thiadiazolo fragment by benzene diminished the dipole moment significantly as seen for 11.  12 18 All data calculated at the DFT level by using the Gaussian ® 16 software package and DFT B3LYP/6-311+G(2df,p) method in CHCl3. The first hyperpolarizabilities β(−2ω,ω,ω) were calculated at 1907 nm. The electronic absorption spectra, the longest-wavelength absorption maxima and the corresponding electron transitions were calculated using TD-DFT (nstates = 8) B3LYP/6-311+G(2df,p).
The optimized geometries of 9-11 are shown in Figure 11A. As can be seen, the central biphenylene π-linker, interconnecting the N,N-diphenylamino, and pyrazine-derived moieties, adopts twisted geometry with the central torsion angle between 20 and 60°. Carbazole derivatives b showed generally larger torsion angles. The heterocyclic moiety at position 2 is always turned out by about 45°, independently of the particular derivative. The bond length alternation of both 1,4-phenylene moieties of the π-linker was investigated, revealing the Bird index I6 and the quinoid character (δr) within the range of 91-96/2-5% [58,59]. For unsubstituted benzene, the I6 and δr are equal to 100 and 0, respectively. The calculated values imply that both 1,4-phenylene moieties in 9-11 are slightly polarized. Alternation of the heterocyclic acceptor significantly affected the ground state dipole moment of V-shaped chromophores. The largest ones were calculated for furazanopyrazine derivatives 9, while replacement of the fused terminal oxa/thiadiazolo fragment by benzene diminished the dipole moment significantly as seen for 11.  The calculated energies of the frontier molecular orbitals (E HOMO/LUMO ) and their differences E are summarized in Table 5. The latter quantity correlates tightly with the electrochemical gaps ( Figure S7). The oxadiazolopyrazine acceptor moiety in 9a-b im-parts the strongest ICT with the lowest calculated HOMO-LUMO gaps as compared to chromophores 10 and 11. When comparing the triads of chromophores a and b, N,Ndiphenylamino-substituted derivatives a possess lower HOMO-LUMO gaps than the corresponding carbazole derivatives b. Figure 9B shows HOMO and LUMO localizations in the particular derivatives; an obvious charge-separation is seen for 9a and 9b. For  N,N-diphenylamino-substituted derivatives 10a and 11a are the situation similar, which is in contrast to carbazoles 10b and 11b. In these derivatives, both HOMO and LUMO are predominantly cumulated over the carbazole's nitrogen atom, which results in their diminished ICT character and larger ∆E values.
Fundamental optical properties of 9-11 were investigated by TD-DFT CAM-B3LYP/6-311+G(2df,p) (nstates = 8) method. The calculated electronic absorption spectra along with the experimental ones are visualized in Figure S8. Both spectra feature the same shape and number of peaks and mostly differ in the position of the longest-wavelength absorption maxima. The λ max values listed in Table 5 showed a very tight correlation with the experimental ones (Table 1)-see Figure S9. According to the aforementioned discussion on the HOMO-LUMO gap, oxadiazolopyrazine chromophores 9a-b possess the most bathochromically shifted absorption maxima. Hence, both O→S replacement and fusing of benzene rings as in 10 and 11 shift the absorption maxima hypsochromically. An inspection of the transition forming the particular bands revealed that the longest wavelength absorption bands of chromophores 9a-b, 10a, and 11a are generated by the HOMO→LUMO transition and these can be designed as CT bands. The corresponding high-energy bands (blue shifted) involve also the HOMO-1→LUMO and the HOMO→LUMO+1 transitions. On the contrary, the absorption of chromophores 10b and 11b is dominated by the HOMO-2→LUMO transition, with a weak contribution of the HOMO→LUMO transition seen for 10b. These observations agree with their largest HOMO-LUMO gaps and the most hypsochromically shifted spectra.
The first order hyperpolarizability β(-2ω, ω, ω) calculated at 1907 nm is listed in Table 5. As can be seen, the largest polarizabilities were calculated for chromophores 9a and 10a, similarly to the EFISH experiment. Oxadiazolopyrazine, or eventually thiadiazolopyrazine, in combination with an N,N-diphenylamino substituent, is the most useful electron acceptor/donor pair in V-shaped NLOphores 9-11.

General Information
All reagents and solvents were obtained from commercial sources and dried by using standard procedures before use. Starting materials 5 and 7 were prepared according to the earlier reported procedure [60]. The 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE-600 instruments using Me 4 Si as an internal standard. Elemental analysis was carried on a Eurovector EA 3000 automated analyzer. Melting points were determined on Boetius combined heating stages and were not corrected. The chromatographic purification of compounds was achieved with silica gel Alfa Aesar 0.040-0.063 mm (230-400 mesh), eluting with CH 2 Cl 2 /hexane (1:2, v/v). The progress of reactions and the purity of compounds were checked by TLC on Sorbfil plates (Russia), in which the spots were visualized with UV light (λ 254 or 365 nm). IR spectra of samples (solid powders) were recorded on a Spectrum One Fourier transform IR spectrometer (Perkin Elmer, Waltham, MA, USA) equipped with a diffuse reflectance attachment (DRA) in the frequency range 4000 ÷ 400 cm −1 . Spectrum processing and band intensity determination were carried out using the special software supplied with the spectrometer.

Electrochemical Characterization
The electrochemical studies of the compounds were performed with a home-designed 3-electrodes cell (WE: glassy carbon disk, RE: Ag wire, Ce: Pt). Ferrocene was added at the end of each experiment to determine redox potential values.

Photophysical Characterization
The absorption spectra of the samples were detected with a JASCO V-650 instrument, whereas the emission spectra were detected by a Horiba Fluoromax spectrophotometer. UV/Vis and fluorescence spectra were recorded by using standard 1 cm quartz cells. Compounds were excited at their absorption maxima in solution (band of lowest energy) to record the emission spectra. The Φ F values were calculated by using a well-known procedure with 9,10-diphenylethynylanthracene in cyclohexane as a standard (Φ F = 1.00) [61]. Stokes shifts were calculated by considering the lowest energy absorption band. Experimental details on EFISH measurements are described elsewhere [62].