Novel D-A-π-A1 Type Organic Sensitizers from 4,7-Dibromobenzo[d][1,2,3]thiadiazole and Indoline Donors for Dye-Sensitized Solar Cells

Two novel D-A-π-A1 metal-free organic dyes of the KEA series containing benzo[d][1,2,3]thiadiazole (isoBT) internal acceptor, indoline donors fused with cyclopentane or cyclohexane rings (D), a thiophene as a π-spacer, and a cyanoacrylate as an anchor part were synthesized. Monoarylation of 4,7-dibromobenzo[d][1,2,3]thiadiazole by Suzuki-Miyamura cross-coupling reaction showed that in the case of indoline and carbazole donors, the reaction was non-selective, i.e., two monosubstituted derivatives were isolated in each case, whereas only one mono-isomer was formed with phenyl- and 2-thienylboronic acids. This was explained by the fact that heterocyclic indoline and carbazole fragments are much stronger donor groups compared to thiophene and benzene, as confirmed by cyclic voltammetry measurements and calculation of HOMO energies of indoline, carbazole, thiophene and benzene molecules. The structure of monoaryl(hetaryl) derivatives was strictly proven by NMR spectroscopy and X-ray diffraction. The optical and photovoltaic properties observed for the KEA dyes showed that these compounds are promising for the creation of solar cells. A comparison with symmetrical benzo[c][1,2,3]thiadiazole dyes WS-2 and MAX114 showed that the asymmetric nature of benzo[d][1,2,3]thiadiazole KEA dyes leads to a hypsochromic shift of the ICT band in comparison with the corresponding benzo[c][1,2,5]thiadiazole isomers. KEA dyes have a narrow HOMO-LUMO gap of 1.5–1.6 eV. Amongst these dyes, KEA321 recorded the best power efficiency (PCE), i.e., 5.17%, which is superior to the corresponding symmetrical benzo[c][1,2,3]thiadiazole dyes WS-2 and MAX114 (5.07 and 4.90%).


Introduction
Recently, the problem of the increasing temperature of the earth has become seriously aggravated, largely due to the use of carbonized energy sources such as coal, oil and gas. The second cause of concern for humanity is the depletion of these energy sources. Therefore, the search for renewable and environmentally friendly energy sources has become an important and urgent task. Solar energy is the most accessible and practically inexhaustible source of energy. In the past few decades, along with well-developed silicon-based photovoltaic devices, research on dye-sensitized solar cells (DSSCs) has been intensively developed due to their relative cheapness, low dye consumption, low-light capability and facile fabrication [1][2][3][4]. The photosensitizer is the most important component of this type of solar cell, as it directly determines their efficiency. There are three main classes of sensitizers that have shown the most outstanding results: ruthenium complexes [5][6][7], porphyrin

Synthesis of D-A-π-A1 Dyes
The synthetic strategy for the preparation of the target D-A-π-A1 dyes consisted of a number of steps, as shown in Scheme 1. It included two successive cross-coupling reactions with boronic acids or their esters (Suzuki-Miyamura reaction) and with tributylstannyl derivatives (Stille reaction), followed by the Knoevenagel reaction with cyanoacetic acid tert-butyl ester, and finally, hydrolysis of ester 4 to afford the desired KEA dyes.
Monoarylation of 4,7-dibromobenzo[d] [1,2,3]thiadiazole is the most challenging task and requires special attention in order to achieve the best yields [25]. According to the literature [25], the two most common protocols for the substitution of bromine atoms with aryl groups are the reaction with arylboronic acids or its esters (the Suzuki reaction) and the reaction with trialkylstannates (the Stille reaction). Arylation of 4,7-dibromobenzo[c] [1,2,5]thiadiazole with one equivalent of trialkylstannyl derivative led to low yields (15-35%) of monoarylated heterocycles, since they reacted readily further with the formation of bis-aryl products [24]. Better results for the isolation of mono-substituted derivatives have been achieved by Suzuki cross-coupling reaction; for example, monoarylated derivatives have been isolated in high yields in the reactions of 4,7-dibromobenzo[c] [1,2,5]thiadiazole with arylboronic acid by using the same NCP pincer palladacycle [29]. Also where the reactivity of two bromine atoms in 4,7-dibromo [1,2,5]chalcogenadiazolo [3,4-c]pyridines has differed substantially, regioselective arylation occurred [30,31] and the Suzuki reaction gave a considerably higher yield (72%) compared to the Stille reaction (27%) [24].
Therefore, the behavior of 4,7-dibromobenzo[d] [1,2,3]thiadiazole 1 in thepalladium-catalyzed Suzuki-Miyamura coupling reactions in order to obtain mono-aryl(hetaryl)benzo[d] [1,2,3]thiadiazoles was investigated (Scheme 2). This study, with one equivalent of (9-hexyl-9H-carbazol-3yl)boronic acid and its pinacolate ester 2, included varying the base, solvent and reaction temperature. The results are summarized in Table 1. Monoarylation of 4,7-dibromobenzo[d] [1,2,3]thiadiazole is the most challenging task and requires special attention in order to achieve the best yields [25]. According to the literature [25 and references therein], the two most common protocols for the substitution of bromine atoms with aryl groups are the reaction with arylboronic acids or its esters (the Suzuki reaction) and the reaction with trialkylstannates (the Stille reaction). Arylation of 4,7-dibromobenzo[с] [1,2,5]thiadiazole with one equivalent of trialkylstannyl derivative led to low yields (15-35%) of mono-arylated heterocycles, since they reacted readily further with the formation of bis-aryl products [24]. Better results for the isolation of monosubstituted derivatives have been achieved by Suzuki cross-coupling reaction; for example, monoarylated derivatives have been isolated in high yields in the reactions of 4,7dibromobenzo[с] [1,2,5]thiadiazole with arylboronic acid by using the same NCP pincer palladacycle [29]. Also where the reactivity of two bromine atoms in 4,7-dibromo [1,2,5]chalcogenadiazolo [3,4-c]pyridines has differed substantially, regioselective arylation occurred [30,31] and the Suzuki reaction gave a considerably higher yield (72%) compared to the Stille reaction (27%) [24]. Therefore, the behavior of 4,7-dibromobenzo[d] [1,2,3]thiadiazole 1 in the palladiumcatalyzed Suzuki-Miyamura coupling reactions in order to obtain mono-aryl(hetaryl)benzo[d] [1,2,3]thiadiazoles was investigated (Scheme 2). This study, with one equivalent of (9-hexyl-9H-carbazol-3-yl)boronic acid and its pinacolate ester 2, included varying the base, solvent and reaction temperature. The results are summarized in Table 1 [1,2,3]thiadiazole 1 with one equivalent of boronic acid 2a or its pinacolate ester 3a showed that the reaction was not selective and led to a mixture of two monohetaryl derivatives 7a and 8a and bis-aryl derivative 9a, with yields of these products of around 20%. The nature of the solvent, either type of solvent (THF) or a mixture of solvents that are capable of solubilizing organic substrates (THF, toluene, or dioxane) and inorganic salts (water) did not influence the results of the reaction. The yields for the pinacolate ester 3a were slightly higher than for the corresponding boronic acid 2a (compare Entries 1 and 8 in Table 1). THF gave the best yields of mono-substituted derivatives 7a and 8a (Table 1, Entry 1).
The conditions for the synthesis of monohetaryl derivatives 7a and 8a were further used for several boronic acids 2 and esters 3 (Scheme 3). It was found that in the case of indoline donors 2b-d, the reaction remained non-selective; two monosubstituted derivatives, 7b-d and 8b-d, were isolated in all these cases with yields of about 20% (Table 2). Compounds 8c, 8d and 9d were not isolated in a pure state due to the difficulty of purification; nevertheless, their structure was proven by 1  As is known [32,33], the Suzuki reaction mechanism includes the formation of an intermediate complex of palladium with halide and boronic acid, in which electrons are transferred from the metal to the halide and from the boronic acids to the metal. In order to evaluate the donor ability of the substituent of the studied boronic acids to donate electrons, the oxidation potentials of donor 10 are shown in Table 3, as determined by cyclic voltammetry (CV) (see Supplementary Materials). It was established that all compounds were oxidized irreversibly on platinum and glassy carbon electrodes, and the values of the oxidation potentials practically did not depend on the type of electrode. Oxidation peaks of donors 10a,c,d were observed by CV in DMF solutions (0.1 M Bu 4 NClO 4 ), whereas for 10e and 10f, having a higher oxidation potential, peaks were obtained in acetonitrile solution. To estimate the energies of the highest occupied molecular orbital (E HOMO ), we used estimated peak onset values (E ox onset ). The values of E ox onset were calculated relative to the potential of the reversible oxidation of ferrocene/ferrocenium (Fc/Fc + ) redox pair, the absolute potential of which was taken as −5.1 eV [34,35]. The values of E HOMO presented in the Table 3 were calculated according to Equation (1) (see below).   The cyclic voltammetry showed that heterocyclic fragments 10a, 10c and 10d had much higher EHOMO values, which indicated that they were much stronger donor groups compared to benzene 10e and thiophene 10f. It can be assumed that the relative ease of electron transfer in the intermediate complex may be one of the factors contributing to the formation of a mixture of monoisomers 7a-d and 8a-d, in contrast to thienyl-and phenylboronic acids 2e,f.  a Here E ox onset and E red onset are a linear extrapolation of the low reduction potential side of the first oxidation or reduction wave respectively to the base line relative to Fc/Fc + respectively. b Energies of HOMO frontier orbital were calculated according to Equation (1), see below.
The cyclic voltammetry showed that heterocyclic fragments 10a, 10c and 10d had much higher EHOMO values, which indicated that they were much stronger donor groups compared to benzene 10e and thiophene 10f. It can be assumed that the relative ease of electron transfer in the intermediate complex may be one of the factors contributing to the formation of a mixture of monoisomers 7a-d and 8a-d, in contrast to thienyl-and phenylboronic acids 2e,f.  a Here E ox onset and E red onset are a linear extrapolation of the low reduction potential side of the first oxidation or reduction wave respectively to the base line relative to Fc/Fc + respectively. b Energies of HOMO frontier orbital were calculated according to Equation (1), see below.
The cyclic voltammetry showed that heterocyclic fragments 10a, 10c and 10d had much higher EHOMO values, which indicated that they were much stronger donor groups compared to benzene 10e and thiophene 10f. It can be assumed that the relative ease of electron transfer in the intermediate complex may be one of the factors contributing to the formation of a mixture of monoisomers 7a-d and 8a-d, in contrast to thienyl-and phenylboronic acids 2e,f.  a Here E ox onset and E red onset are a linear extrapolation of the low reduction potential side of the first oxidation or reduction wave respectively to the base line relative to Fc/Fc + respectively. b Energies of HOMO frontier orbital were calculated according to Equation (1), see below.
The cyclic voltammetry showed that heterocyclic fragments 10a, 10c and 10d had much higher EHOMO values, which indicated that they were much stronger donor groups compared to benzene 10e and thiophene 10f. It can be assumed that the relative ease of electron transfer in the intermediate complex may be one of the factors contributing to the formation of a mixture of monoisomers 7a-d and 8a-d, in contrast to thienyl-and phenylboronic acids 2e,f.   a Here E ox onset and E red onset are a linear extrapolation of the low reduction potential side of the first oxidation or reduction wave respectively to the base line relative to Fc/Fc + respectively. b Energies of HOMO frontier orbital were calculated according to Equation (1), see below.
The cyclic voltammetry showed that heterocyclic fragments 10a, 10c and 10d had much higher EHOMO values, which indicated that they were much stronger donor groups compared to benzene 10e and thiophene 10f. It can be assumed that the relative ease of electron transfer in the intermediate complex may be one of the factors contributing to the formation of a mixture of monoisomers 7a-d and 8a-d, in contrast to thienyl-and phenyl- a Here E ox onset and E red onset are a linear extrapolation of the low reduction potential side of the first oxidation or reduction wave respectively to the base line relative to Fc/Fc + respectively. b Energies of HOMO frontier orbital were calculated according to Equation (1), see below.
The cyclic voltammetry showed that heterocyclic fragments 10a, 10c and 10d had much higher E HOMO values, which indicated that they were much stronger donor groups compared to benzene 10e and thiophene 10f. It can be assumed that the relative ease of electron transfer in the intermediate complex may be one of the factors contributing to the formation of a mixture of monoisomers 7a-d and 8a-d, in contrast to thienyl-and phenylboronic acids 2e,f.

Proof of the Structure of Monobromo Derivatives 7 and 8 by NMR Spectroscopy and X-ray Diffraction
A set of two-dimensional NMR spectra was recorded in order to correlate the signals in the 1 H and 13 C spectra. The key interatomic interactions for both isomers 7b and 8b are shown in Figure 1. For product 8b, a proton doublet at C6 is observed at 7.53 ppm with an interaction in HMBC spectra with the quaternary carbon 8 of the donor fragment (highlighted in red), while for the second proton of the isothiadiazole ring at C5 at 7.83 ppm, this interaction is absent. An additional confirmation of this proved the interaction in HMBC spectra of protons at C9 and C11 of the donor fragment with the quaternary carbon 7 of isothiadiazole (highlighted in green). Similarly, for isomer 7b, key interactions in HMBC spectra of the proton at C5, which manifests itself at 7.55 ppm, with the quaternary carbon 8 of the donor (red highlight), as well as protons at C9 and C11 of the donor fragment with carbon 4 of isothiadiazole (green highlight), are observed. The absence of cross peaks in the HMBC spectrum of compound 7b related to the interaction of the proton at C6 and the nodal carbon 8 of the donor, as well as the absence of NOE interaction between protons at C6 and C9/C11 as well as the presence of one between the protons C5 and C9/C11 (blue arrows) additionally confirm the correlation of signals.  The use of NMR spectroscopy made it possible to combine the results of XRD and HMBC data, and perform a complete correlation of signals in the 1 H and 13 C spectra for dibromide 1 (Figure 1) and isomers 7b, 8b (see Supplementary Materials). As such, we were able to establish that the introduction of a donor substituent led to an upfield shift of the signals from both the nearest proton and the corresponding carbon of the benzo[d] [1,2,3]thiadiazole system compared to the parent 4,7-dibromobenzo[d] [1,2,3]thiadiazole 1. In this case, the signal of carbon, which had undergone substitution, was shifted to a lower field, and the signal of carbon with the remaining bromine atom had a chemical shift ±3 ppm relative to the corresponding dibromide 1 chemical shift (Table 4). At the same time, the chemical shifts of carbons bearing a bromine atom (C7 for 7; C4 for 8) were characterized by the largest upfield shifts compared to the rest of the signals of aromatic hydrocarbons. Based on the above conclusions, an unambiguous determination The use of NMR spectroscopy made it possible to combine the results of XRD and HMBC data, and perform a complete correlation of signals in the 1 H and 13 C spectra for dibromide 1 (Figure 1) and isomers 7b, 8b (see Supplementary Materials). As such, we were able to establish that the introduction of a donor substituent led to an upfield shift of the signals from both the nearest proton and the corresponding carbon of the benzo[d] [1,2,3]thiadiazole system compared to the parent 4,7-dibromobenzo[d][1,2,3]thiadiazole 1. In this case, the signal of carbon, which had undergone substitution, was shifted to a lower field, and the signal of carbon with the remaining bromine atom had a chemical shift ±3 ppm relative to the corresponding dibromide 1 chemical shift (Table 4). At the same time, the chemical shifts of carbons bearing a bromine atom (C7 for 7; C4 for 8) were characterized by the largest upfield shifts compared to the rest of the signals of aromatic hydrocarbons. Based on the above conclusions, an unambiguous determination of the structure of all obtained mono-substituted compounds 7 and 8 was made. For compounds 8c, 8d, 9d, which could not be isolated in pure form, 1 H NMR spectra, as well as MALDI-TOF spectra were recorded (see Supplementary Materials), on the basis of which their formation could be confirmed. Thus, for compound 8d, the doublet corresponding to the proton at C6 appeared at 7.65 ppm, and the proton doublet at C5 was observed at 7.85 ppm, while the proton signals at C5/6, recorded for isomer 7d, were observed as a minor product. Additional confirmation could serve as a change of chemical shifts of protons at the nodal carbon atoms 12/12 of the donor fragment: for 7d, two multiplets were observed in the regions of 3.92-4.01 and 4.86-4.93 ppm, and for 8d for the main product, in the regions of 4.65-4.74 ppm and 5.37-5.45 ppm. In this case, the 1 H NMR spectrum of the bis-derivative 9d was characterized by the presence of all four multiplets in the given regions, integrating as single protons.
In the case of compound 8c, the difference in chemical shifts of key protons at C5/6 was not so characteristic, since the chemical the shift of one of the doublets was the same for both isomers; however, the doublet signal of the second proton for 8c appeared in a weaker field, at 7.86 ppm corresponding to a proton at C6, and a doublet at 7.86 ppm characterized the proton at C5. In the high-field region of proton signals at C12/12', the difference in chemical shifts for both isomers was not as critical as in the case of the 7d-8d pair, but its presence also served as additional evidence for the formation of the 8c isomer.
The structures of a few monobromo derivatives 7 and 8 were confirmed by single crystal X-ray diffraction study of compounds 7b, 8b, 7e and 7f (Figure 2).

Synthesis and Characterization of the Dyes
Synthesis of the target dyes in the KEA321 and KEA337 series was carried out using Stille cross-coupling reaction of 4-substituted 7-dibromobenzo[d] [1,2,3]thiadiazole with π-spacer 5 (Scheme 4). This stage was carried out under conditions of Stille reactions catalyzed by PdCl 2 (PPh 3 ) 2 in toluene followed by saponification with HCl to remove dioxolane protection. After this, the products were introduced into the Knoevenagel reaction with cyanoacetic acid tert-butyl ester with subsequent hydrolysis resulting in the final structures KEA321 and KEA337, which, after thorough chromatographic purification, were used in solar cell device fabrication.

Optical Properties
For the obtained dyes KEA321 and KEA327, UV-Vis spectra were recorded in a DCM solution at a concentration of 5.5 × 10 −5 M. Table 5 shows the absorption maxima (λ max ) and the corresponding extinction coefficients (ε) for KEA dyes, as well as for isomeric dyes

Optical Properties
For the obtained dyes KEA321 and KEA327, UV-Vis spectra were recorded in a DCM solution at a concentration of 5.5 × 10 −5 M. Table 5 shows the absorption maxima (λmax) and the corresponding extinction coefficients (ε) for KEA dyes, as well as for isomeric dyes  Both new dyes had two absorption maxima; however, for the KEA337 compound,  Both new dyes had two absorption maxima; however, for the KEA337 compound, due to the proximity of both maxima, their merging was observed. The short-wavelength absorption band at 400 nm, which for KEA337 was identified as a shoulder, and for KEA321 was characterized by a high extinction coefficient, corresponded to π/π* electron transition, and long-wavelength absorption maxima in the range of 450-500 nm indicated the presence of an intramolecular charge transfer (ICT) process between donor and acceptor fragments (Figure 4). For all four dyes, π/π* transition absorption peaks were observed in a narrow wavelength range, i.e., 394-401 nm, which characterizes close values of the energy level gap. The long-wavelength absorption maxima obeyed the previously established regularity [27]

Optical Properties
For the obtained dyes KEA321 and KEA327, UV-Vis spectra were recorded in a DCM solution at a concentration of 5.5 × 10 −5 M. Table 5 shows the absorption maxima (λmax) and the corresponding extinction coefficients (ε) for KEA dyes, as well as for isomeric dyes MAX114 and WS-2 based on benzo[c][1,2,5]thiadiazole ( Figure 3). Both new dyes had two absorption maxima; however, for the KEA337 compound, due to the proximity of both maxima, their merging was observed. The short-wavelength absorption band at 400 nm, which for KEA337 was identified as a shoulder, and for KEA321 was characterized by a high extinction coefficient, corresponded to π/π* electron transition, and long-wavelength absorption maxima in the range of 450-500 nm indicated the presence of an intramolecular charge transfer (ICT) process between donor and acceptor fragments (Figure 4). For all four dyes, π/π* transition absorption peaks were observed in a narrow wavelength range, i.e., 394-401 nm, which characterizes close values of the energy level gap. The long-wavelength absorption maxima obeyed the previously established regularity [27]

Electrochemical Properties
To estimate the energy values of the frontier orbitals and to determine the stability of the particles formed during electron transfer, cyclic voltammetry patterns (CV curves) of KEA321 and KEA337 were measured. The first stages of the electrooxidation (EO) and electroreduction (ER) curves of the studied compounds are shown in Figure 5, and the potential values and calculated frontier orbitals energies are summarized in Table 6.

Electrochemical Properties
To estimate the energy values of the frontier orbitals and to determine the stability of the particles formed during electron transfer, cyclic voltammetry patterns (CV curves) of KEA321 and KEA337 were measured. The first stages of the electrooxidation (EO) and electroreduction (ER) curves of the studied compounds are shown in Figure 5, and the potential values and calculated frontier orbitals energies are summarized in Table 6.
The potentials of the ER and EO peaks of KEA321 and KEA337 were close to each other, the ER in both cases proceeded irreversibly, and the EO peak of KEA321 had a quasireversible character, in contrast to the irreversible EO peak of KEA337 (Table 6). To calculate the energies of the lowest unoccupied molecular orbital (E LUMO ) and the highest occupied molecular orbital (E HOMO ), we used estimates of the peak onset values ER (E red onset ) and EO (E ox onset ), respectively. The values of E red onset and E ox onset were calculated relative to the potential of the reversible oxidation of ferrocene/ferrocenium (Fc/Fc + ) redox pair, the absolute potential of which equaled −5.1 eV [34,35]. We used Equations (1) and (2) The obtained E LUMO values of both compounds were above the energy level of TiO 2 semiconductor (−4.2 eV) [38], while the E HOMO values were below the I − /I 3 − (−4.9 eV) level [39]. Thus, the electrochemical characteristics obtained for the KEA dyes allowed us to state that these compounds are promising for the creation of solar cells.

Electrochemical Properties
To estimate the energy values of the frontier orbitals and to determine the stability o the particles formed during electron transfer, cyclic voltammetry patterns (CV curves) o KEA321 and KEA337 were measured. The first stages of the electrooxidation (EO) and electroreduction (ER) curves of the studied compounds are shown in Figure 5, and th potential values and calculated frontier orbitals energies are summarized in Table 6.   a Here E ox onset and E red onset are a linear extrapolation of the low reduction potential side of the first oxidation or reduction wave respectively to the base line relative to Fc/Fc + respectively. b Energies of frontier orbitals were calculated according to Equations (1) and (2). c E g = E LUMO − E HOMO .

Photovoltaic Performance
The new dyes based on benzo[d] [1,2,3]thiadiazole were used to construct DSSCs with a two-layer photoanode made of TiO 2 (4.5 µm scattering layer and 9 µm transparent layer), I − /I 3 − electrolyte, and a Pt film as the counter electrode (See Supporting Information for the preparation of DSSCs). The resulting cells were tested under AM1.5 G irradiation (100 mW cm −2 ); the photovoltaic parameters of the best devices compared to the MAX114 and WS-2 isomeric dyes are shown in Table 7, and the current-voltage characteristics of the KEA series compounds are shown in Figure 6 (statistics of photovoltaic performance of DSSCs fabricated with KEA dyes are given in Supporting Information). Despite higher values of the molar extinction coefficient in the UV-Vis spectrum obtained for the WS-2 dye, its J SC is somewhat lower than for the benzo[d] [1,2,3]thiadiazole analogue of KEA321. At the same time, for a pair of compounds based on the hexahydro-1H-carbazole donor block, the trend in J SC changes is somewhat more predictable: the dye with benzo[c] [1,2,5]thiadiazole acceptor MAX114, which has a four times higher value of ε, showed J SC of 0.5 mA cm −2 , which is higher than its KEA337 isomer. The V OC values for DSSCs based on the new KEA series are higher than those for isomeric analogs with a symmetric acceptor (WS-2 and MAX114), probably, due to reduced recombination through the blocking effect of the dye.

Analytical Instruments
The solution UV-visible absorption spectra were recorded using an OKB Spektr SF-2000 UV/Vis/NIR spectrophotometer (Saint-Petersburg, Russia) controlled with SF-2000 software. All samples were measured in a 1 cm quartz cell at room temperature with a 5.5 × 10 −5 mol/mL concentration in DCM. The melting points were determined on a Kofler 1 13 Figure 6. Current density-voltage curves of DSSCs based on the dyes of KEA series at AM1.5G, 100 mW cm −2 .

Analytical Instruments
The solution UV-visible absorption spectra were recorded using an OKB Spektr SF-2000 UV/Vis/NIR spectrophotometer (Saint-Petersburg, Russia) controlled with SF-2000 software. All samples were measured in a 1 cm quartz cell at room temperature with a 5.5 × 10 −5 mol/mL concentration in DCM. The melting points were determined on a Kofler hot-stage apparatus and were uncorrected. 1 H and 13 C NMR spectra were taken with a Bruker AM-300 machine (Bruker Ltd., Moscow, Russia) with TMS as the standard. J values are given in Hz. MS spectra (EI, 70 eV) were obtained with a Finnigan MAT INCOS 50 instrument (Thermo Finnigan LLC, San Jose, CA, USA). High-resolution MS spectra were measured on a Bruker micrOTOF II instrument using electrospray ionization (ESI). The measurement was operated in a positive ion mode (interface capillary voltage −4500 V) or in a negative ion mode (3200 V); the mass range was from m/z 50 to m/z 3000 Da; external or internal calibration was performed with Electrospray Calibrant Solution (Fluka Chemicals Ltd., Gillingham, UK). A syringe injection was used for solutions in acetonitrile, methanol, or water (flow rate 3 µL·min −1 ). Nitrogen was applied as a dry gas; the interface temperature was set at 180 • C. IR spectra were measured with a Bruker "Alpha-T" instrument (Bruker, Billerica, MA, USA) in KBr pellets. Electrochemical measurements were carried out in a dry argon atmosphere using an IPC Pro MF potentiostat (Econix, Russia). The redox properties of compounds were determined using cyclic voltammetry in a three-electrode electrochemical system. A three-electrode system consisting of platinum as the working electrode with an area of 0.8 mm 2 , platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode was employed. The reduction and oxidation potentials were determined in MeCN or in DMF, using 0.1 mol L −1 n-Bu 4 NClO 4 as the supporting electrolyte. The cyclic voltammetry (CV) measurements were performed with the use of scan rates of 0.1-5.0 V s −1 . The first reduction/oxidation potentials were referenced to the internal standard redox couple Fc/Fc + . Ferrocene was added to each sample solution at the end of the experiment and employed for calibration.

X-ray Analysis
X-ray diffraction data were collected at 100 K on a four-circle Rigaku Synergy S diffractometer equipped with a HyPix600HE area-detector (kappa geometry, shutterless ω-scan technique), using graphite monochromatized Cu K α -radiation. The intensity data were integrated and corrected for absorption and decay by the CrysAlisPro program [45]. The structure was solved by direct methods using SHELXT and refined on F 2 using SHELXL-2018 [46] in the OLEX2 program [47]. All non-hydrogen atoms were refined with individual anisotropic displacement parameters. All hydrogen atoms were placed in ideal calculated positions and refined as riding atoms with relative isotropic displacement parameters. A rotating group model was applied for methyl groups. The Cambridge Crystallographic Data Centre contains the supplementary crystallographic data for this paper No. CCDC 2112091 (7e), 2116443 (7f), 2061783 (7b), 2061784 (8b). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (accessed on 3 February 2022) (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk). Crystal data and structure refinement for these compounds are given in Tables 8 and 9.

Conclusions
In summary, two new organic D-A-π-A1 dyes of the KEA series were designed and synthesized. In order to prepare the precursors for these dyes, i.e., unsymmetrical monoaryl(hetaryl) derivatives of benzo[d] [1,2,3]thiadiazole, the Suzuki-Miyamura crosscoupling reaction between 4,7-dibromobenzo[d] [1,2,3]thiadiazole and aryl(hetaryl)boronic acids and their esters was thoroughly investigated. Fused indoline and carbazole derivatives formed two isomers, whereas phenyl-and 2-thienylboronic acids formed only one 4-substituted compound. Cyclic voltammetry measurements showed that fused indoline and carbazole had much higher E HOMO values than benzene and thiophene, which indicated that they were much stronger donors, which should facilitate electron transfer in the intermediate complex, contributing to the formation of a mixture of monoisomers, in contrast to thienyl-and phenylboronic acids. The structure of mono derivatives was rigorously proven by NMR spectroscopy and X-ray analysis. The KEA dyes had a very narrow energy gap and the position of E HOMO and E LUMO was suitable for the effective use of these dyes in solar cells. The asymmetric nature of benzo[d] [1,2,3]thiadiazole KEA dyes in comparison with the symmetrical benzo[c] [1,2,5]thiadiazole isomers led to a hypsochromic shift in the ICT band and lower extinction coefficients which suggested that that the symmetry breaking of the acceptor benzo[d] [1,2,3]thiadiazole fragment reduced the efficiency of intramolecular charge transfer for compounds of the KEA series. KEA dyes had a narrow HOMO-LUMO gap of 1.5-1.6 eV, which apparently led to the better power efficiency (PCE), i.e., 5.17%, for KEA321, which was superior to the corresponding symmetrical benzo[c] [1,2,3]thiadiazole dyes WS-2 and MAX114 (5.07 and 4.90%). Thus, it was shown that the exchange of a symmetrical benzo[c] [1,2,5]thiadiazole ring to an asymmetrical benzo[d] [1,2,3]thiadiazole can lead to promising photovoltaic properties.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27134197/s1, Supporting Information including 1 H and 13 C NMR spectra for novel compounds.