4-(Aryl)-Benzo[4,5]imidazo[1,2-a]pyrimidine-3-Carbonitrile-Based Fluorophores: Povarov Reaction-Based Synthesis, Photophysical Studies, and DFT Calculations

A series of novel 4-(aryl)-benzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles were obtained through the Povarov (aza-Diels–Alder) and oxidation reactions, starting from benzimidazole-2-arylimines. Based on the literature data and X-ray diffraction analysis, it was discovered that during the Povarov reaction, [1,3] sigmatropic rearrangement leading to dihydrobenzimidazo[1,2-a]pyrimidines took place. The structures of all the obtained compounds were confirmed based on the data from 1H- and 13C-NMR spectroscopy, IR spectroscopy, and elemental analysis. For all the obtained compounds, their photophysical properties were studied. In all the cases, a positive emission solvatochromism with Stokes shifts from 120 to 180 nm was recorded. Aggregation-Induced Emission (AIE) has been illustrated for compound 6c using different water fractions (fw) in THF. The compounds 6c and 6f demonstrated changes in emission maxima or/and intensities after mechanical stimulation.


Introduction
Azolopyrimidines are ubiquitous heterocyclic systems, particularly important in living organisms as a core of purine bases, and these heterocycles are widely present among biologically active compounds, including those with antiviral [1-4], anticancer [5][6][7], antibacterial [8,9], and antidiabetic activity [10,11]. In addition to a wide range of biological activities, azolopyrimidines are considered promising candidates for important fluorescence applications [12][13][14][15]. Furthermore, strongly electron-withdrawing pyrimidine derivatives have found applications for the synthesis of push-pull molecules and the construction of functionalized π-conjugated materials such as dye-sensitized solar cells [16], non-doped OLED and laser dyes [17], and nonlinear optical materials [18]. Among the methods for the structural modification of azolopyrimidines, the approaches based on the creation of polycyclic fused analogs of azolopyrimidines such as benzo [4,5]imidazo[1,2-a]pyrimidines are of growing interest and significance [19][20][21]. Since polycyclic fused systems with a conjugated planar structure exhibit relevant photophysical properties, they have found applications as phosphors in optoelectronics or as fluorescent dyes for textile and polymer materials [22].
Among the methods for constructing heterocyclic systems is the aza-Diels-Alder [4 + 2] cycloaddition reaction between various dienophiles and N-aryl-substituted imines, which yields a wide range of azaheterocycles. This reaction, also known as the Povarov reaction [23][24][25][26], is a convenient tool for the construction of six-membered rings with high molecular complexity via the direct construction of carbon-carbon and carbon-heteroatom bonds [27]. In addition, the Povarov reaction is considered an important and efficient approach for creating large libraries of bioactive compounds in drug discovery programs [28]. From this point of view, the use of such a powerful synthetic methodology can be useful for the creation of new derivatives of azolopyrimidines, in particular benzo [4,5]imidazo[1, 2a]pyrimidines.
The use of molecules with aggregation-induced emission (AIE) properties, including those with reversible mechanochromism properties, is of great research interest due to their potential applications in biomedical imaging, sensors, and organic light-emitting diodes [29]. Additionally, fluorophores based on acceptor azaheteroarene domains, such as triazoles, oxadiazoles, thiadiazoles, benzothiazoles, quinoxalines, s-or as-triazines, and pyrimidines, are of particular interest [30][31][32][33][34]. Apart from these acceptors, imidazolebased units have been reported as electron acceptors for blue emission acquisition due to their low LUMO energy level [35]. However, the imidazole unit has been less studied for the development of efficient fluorescent materials due to its weak electron-accepting ability [36,37]. Wang et al. reported the synthesis of TPE-substituted phenanthroimidazole derivatives [38]. These compounds exhibited AIE properties as well as an intriguing mechanofluorochromism: after a short-time grinding, the blue emitting in a solid-state fluorophores (with maxima around 438 nm) changed their emission color to sky blue with a maxima near 450 nm. The functionalization of the imidazole-containing domain with a strongly electron-withdrawing cyano-group and a reduced singlet-triplet energy gap, on the other hand, has received special attention as a universal and appealing strategy for creating AIE-active fluorophores, including those with thermally activated delayed fluorescence (TADF) [39]. For instance, the authors of [40] recently developed TADF materials with C3-functionalized cyano-group 2-phenylimidazopyrazine as an acceptor unit linked to either acridine or phenoxazine donor units, and for these fluorophores an EQE of about 12.7% was achieved. In addition, the use of 2-phenylimidazo[1,2-a]pyridine containing cyano-group as an acceptor has been reported as a tool for designing dark blue emitters with a relatively high fluorescence quantum yield [36,41].
Among the methods for constructing heterocyclic systems is the aza-Diels-Alder [4 + 2] cycloaddition reaction between various dienophiles and N-aryl-substituted imines, which yields a wide range of azaheterocycles. This reaction, also known as the Povarov reaction [23][24][25][26], is a convenient tool for the construction of six-membered rings with high molecular complexity via the direct construction of carbon-carbon and carbon-heteroatom bonds [27]. In addition, the Povarov reaction is considered an important and efficient approach for creating large libraries of bioactive compounds in drug discovery programs [28]. From this point of view, the use of such a powerful synthetic methodology can be useful for the creation of new derivatives of azolopyrimidines, in particular benzo [4,5]imidazo[1, 2-a]pyrimidines.
The use of molecules with aggregation-induced emission (AIE) properties, including those with reversible mechanochromism properties, is of great research interest due to their potential applications in biomedical imaging, sensors, and organic light-emitting diodes [29]. Additionally, fluorophores based on acceptor azaheteroarene domains, such as triazoles, oxadiazoles, thiadiazoles, benzothiazoles, quinoxalines, s-or as-triazines, and pyrimidines, are of particular interest [30][31][32][33][34]. Apart from these acceptors, imidazolebased units have been reported as electron acceptors for blue emission acquisition due to their low LUMO energy level [35]. However, the imidazole unit has been less studied for the development of efficient fluorescent materials due to its weak electron-accepting ability [36,37]. Wang et al. reported the synthesis of TPE-substituted phenanthroimidazole derivatives [38]. These compounds exhibited AIE properties as well as an intriguing mechanofluorochromism: after a short-time grinding, the blue emitting in a solid-state fluorophores (with maxima around 438 nm) changed their emission color to sky blue with a maxima near 450 nm. The functionalization of the imidazole-containing domain with a strongly electron-withdrawing cyano-group and a reduced singlet-triplet energy gap, on the other hand, has received special attention as a universal and appealing strategy for creating AIE-active fluorophores, including those with thermally activated delayed fluorescence (TADF) [39]. For instance, the authors of [40] recently developed TADF materials with C3-functionalized cyano-group 2-phenylimidazopyrazine as an acceptor unit linked to either acridine or phenoxazine donor units, and for these fluorophores an EQE of about 12.7% was achieved. In addition, the use of 2-phenylimidazo[1,2-a]pyridine containing cyano-group as an acceptor has been reported as a tool for designing dark blue emitters with a relatively high fluorescence quantum yield [36,41].
Within the frame of current research, we have proposed a new catalyst-free and solvent-free method for obtaining benzimidazole-2-arylimine 3a-f by heating 2-aminobenzimidazoles 1a,b and aromatic aldehydes 2a-c at 130 • C for 3 h. This method afforded desired diene substrates 3a-f in good to excellent yields (83-90%) (Scheme 2).
Herein, we wish to report a synthetic design of novel benzo [4,5]imidazo[1,2-a]pyrimidines bearing cyano-group (instead of a 1,2,3-triazole fragment) via the combination of the Povarov reaction and oxidative aromatization of the resulting dihydro derivatives, as well as studies of their aggregation-induced fluorescence behavior and mechanofluorochromic properties, as well as structure-property correlation studies involving DFT methods.

Synthesis
Arylimines (the diene component) and various dienophiles are the classical substrates used for the Povarov reaction (the aza-Diels-Alder reaction). For the preparation of arylimines, Brønsted acid catalysis [43][44][45] and Lewis acid catalysis [46,47] are traditionally used, as are various modifications, including those involving microwave radiation [48][49][50]. Within the frame of current research, we have proposed a new catalyst-free and solvent-free method for obtaining benzimidazole-2-arylimine 3a-f by heating 2-aminobenzimidazoles 1a,b and aromatic aldehydes 2a-c at 130 °C for 3 h. This method afforded desired diene substrates 3a-f in good to excellent yields (83-90%) (Scheme 2). The structure of all intermediates 3a-f was confirmed by means of the data from 1 H NMR spectroscopy, as well as 13 C NMR spectroscopy, IR spectroscopy, and elemental analysis. These data were also considered for the identification of previously undescribed benzimidazole-2-arylimines 3a, 3c-f (Figures S4-S8 and S21-S23, Supplementary Materials).
It is worth mentioning that Chen et al. previously reported an unprecedented in situ [1,3] sigmatropic rearrangement that resulted in 4,10-dihydropyrimido[1,2-a]benzimidazoles [49]. Additionally, the same rearrangement was observed by us in the case of using N-2-substituted benzimidazoles (Scheme 3). The structure of all intermediates 3a-f was confirmed by means of the data from 1 H NMR spectroscopy, as well as 13 C NMR spectroscopy, IR spectroscopy, and elemental analysis. These data were also considered for the identification of previously undescribed benzimidazole-2-arylimines 3a, 3c-f (Figures S4-S8 and S21-S23, Supplementary Materials).
It is worth mentioning that Chen et al. previously reported an unprecedented in situ [1,3] sigmatropic rearrangement that resulted in 4,10-dihydropyrimido[1,2-a]benzimidazoles [49]. Additionally, the same rearrangement was observed by us in the case of using N-2substituted benzimidazoles (Scheme 3). Inspired by this fact, we decided to investigate the possibility of rearrangement in the case of unsubstituted benzimidazole-2-arylimine. To test this possibility, derivatives 3a-f were used as diene substrates in the Povarov reaction, and 3-morpholinoacrylonitrile 4 was chosen as an EWG-dienophile (Table 1). A careful literature survey revealed that Scheme 3. Povarov reaction and rearrangement [49].
Inspired by this fact, we decided to investigate the possibility of rearrangement in the case of unsubstituted benzimidazole-2-arylimine. To test this possibility, derivatives 3a-f were used as diene substrates in the Povarov reaction, and 3-morpholinoacrylonitrile 4 was chosen as an EWG-dienophile (Table 1). A careful literature survey revealed that the most commonly used catalysts for this type of reaction are Brønsted acids [51,52] and Lewis acids [27,53]. However, there are examples of using basic catalysts [49] as well as electrochemical methods [23]. To optimize the synthetic procedure for the reaction between benzimidazole-2-arylimine 3a and 3-morpholinoacrylonitrile 4, leading to the target, 4-(4-(dimethylamino)phenyl)-1,4-dihydrobenzo [4,5]imidazo[1,2-a]pyrimidine-3-carbonitrile 5a was chosen. Next, the influence of the nature of the solvents and activating agents, their amounts, as well as the reaction time, on the yields of the target product was assessed ( Table 1). The obtained results clearly demonstrated that BF 3 ·Et 2 O was the best activating agent when used at an amount of 1.5 equivalents in n-BuOH for 5 h (Table 1, entry 9).
Inspired by this fact, we decided to investigate the possibility of rearrangement in the case of unsubstituted benzimidazole-2-arylimine. To test this possibility, derivatives 3a-f were used as diene substrates in the Povarov reaction, and 3-morpholinoacrylonitrile 4 was chosen as an EWG-dienophile (Table 1). A careful literature survey revealed that the most commonly used catalysts for this type of reaction are Brønsted acids [51,52] and Lewis acids [27,53]. However, there are examples of using basic catalysts [49] as well as electrochemical methods [23]. To optimize the synthetic procedure for the reaction between benzimidazole-2-arylimine 3a and 3-morpholinoacrylonitrile 4, leading to the target, 4-(4-(dimethylamino)phenyl)-1,4-dihydrobenzo [4,5]imidazo[1,2-a]pyrimidine-3-carbonitrile 5a was chosen. Next, the influence of the nature of the solvents and activating agents, their amounts, as well as the reaction time, on the yields of the target product was assessed ( Table 1). The obtained results clearly demonstrated that BF3•Et2O was the best activating agent when used at an amount of 1.5 equivalents in n-BuOH for 5 h (Table 1, entry 9). As a next step, by using the optimized reaction conditions, we have prepared a series of annulated dihydropyrimidines 5a-f in moderate to good yields (59-74%) (Scheme 4).
The structures of the obtained dihydropyrimidines 5a-f were confirmed by means of IR-, 1 H-, and 13 C-NMR spectroscopy as well as elemental analysis data. Due to the very low solubility of derivatives 5a-f a mixture of CDCl 3 −CF 3 COOD (v/v = 10:1) was used as a solvent for NMR measurements. All the prepared compounds provided satisfactory analytical data. The signals H-4 are the characteristic ones for the products 5a-f in the corresponding 1 H NMR spectra. It should be noted that in compounds 5a,b and 5d,e the H-4 signals are located at δ 6.16-6.42 ppm, whereas for the derivatives 5c and 5f, bearing an anthracene fragment, the H-4 proton shifts downfield to the region of δ 7.60-7.63 ppm. Apparently, it occurs due to the deshielding effect of the H-4 proton because of the presence of the anthracene substituent. In the IR spectra, for all the series of dihydropyrimidines 5a-f the characteristic stretching vibrations of (-C≡N) bonds are observed at ν 2202-2215 cm −1 (see Supplementary Materials).
The Povarov reaction is a versatile and efficient method to access the tetrahydroquinoline scaffolds [26], and, as a rule, the research on this reaction is limited only by the availability of such systems. At the same time, the oxidative aromatization products of the Povarov reaction may be of interest from the point of view of studying their properties, in particular their photophysical ones. Therefore, as a next step, the aromatization of these novel dihydropyrimidine systems 5a-f was carried out.
Molecules 2022, 27, x FOR PEER REVIEW 5 of 25 As a next step, by using the optimized reaction conditions, we have prepared a series of annulated dihydropyrimidines 5a-f in moderate to good yields (59-74%) (Scheme 4).

Scheme 4. Substrate scope of dihydropyrimidines 5a-f.
The structures of the obtained dihydropyrimidines 5a-f were confirmed by means of IR-, 1 H-, and 13 C-NMR spectroscopy as well as elemental analysis data. Due to the very low solubility of derivatives 5a-f a mixture of CDCl3−CF3COOD (v/v = 10:1) was used as a solvent for NMR measurements. All the prepared compounds provided satisfactory analytical data. The signals H-4 are the characteristic ones for the products 5a-f in the corresponding 1 H NMR spectra. It should be noted that in compounds 5a,b and 5d,e the H-4 signals are located at δ 6.16-6.42 ppm, whereas for the derivatives 5c and 5f, bearing an anthracene fragment, the H-4 proton shifts downfield to the region of δ 7.60-7.63 ppm. Apparently, it occurs due to the deshielding effect of the H-4 proton because of the presence of the anthracene substituent. In the IR spectra, for all the series of dihydropyrimidines 5a-f the characteristic stretching vibrations of (-C≡N) bonds are observed at ν 2202-2215 см −1 (see Supplementary Materials).
The Povarov reaction is a versatile and efficient method to access the tetrahydroquinoline scaffolds [26], and, as a rule, the research on this reaction is limited only by the availability of such systems. At the same time, the oxidative aromatization products of the Povarov reaction may be of interest from the point of view of studying their properties, in particular their photophysical ones. Therefore, as a next step, the aromatization of these novel dihydropyrimidine systems 5a-f was carried out.
By using compound 5a as a key heterocyclic substrate, the most suitable solvent for the oxidation reaction was selected (Table 2). Thus, DMF seems to be the most suitable solvent for the reaction since substrate 5a has good solubility in this solvent. Moreover, the boiling point of DMF makes it possible to carry out the reaction at high temperatures. As a first step, the blank experiments without oxidation agents ( Table 2 entries 1-4) were carried out. It was found that heating the substrate 5a in DMF resulted in the formation of the oxidation product 6a in some amounts (according to TLC data), possibly, due to the oxidation in the ambient air. However, even after the prolonged heating (12 h) at the evaluated 140 ℃ temperature, the complete conversion of compound 5a to the target product 6a was not observed. The use of mild oxidizing agents at 120 °C, such as MnO2, reduced Scheme 4. Substrate scope of dihydropyrimidines 5a-f. By using compound 5a as a key heterocyclic substrate, the most suitable solvent for the oxidation reaction was selected (Table 2). Thus, DMF seems to be the most suitable solvent for the reaction since substrate 5a has good solubility in this solvent. Moreover, the boiling point of DMF makes it possible to carry out the reaction at high temperatures. As a first step, the blank experiments without oxidation agents ( Table 2 entries 1-4) were carried out. It was found that heating the substrate 5a in DMF resulted in the formation of the oxidation product 6a in some amounts (according to TLC data), possibly, due to the oxidation in the ambient air. However, even after the prolonged heating (12 h) at the evaluated 140 • C temperature, the complete conversion of compound 5a to the target product 6a was not observed. The use of mild oxidizing agents at 120 • C, such as MnO2, reduced the reaction time to 6 h ( Table 2 entries 5-8). Subsequently, the increase in the amount of MnO 2 to four equivalents resulted in the complete conversion of 5a within 1 h ( Table 2, entry 8).
All derivatives 6a-f were obtained with comparable yields, which indicates an insignificant influence of the nature of the substituents on the oxidation process. All the synthesized compounds were fully characterized by 1 H-NMR, 13 C-NMR, IR-spectroscopy, and elemental analysis (Supplementary Materials). In particular, in the 1 H-NMR spectra, the aromatic proton signals were observed at δ 5.00-9.41 ppm, whereas the aliphatic proton signals were observed at δ 3.11-3.96 ppm. In the 13 C-NMR spectra, (hetero)aryl carbon nuclei are located at δ 94.6-162.2 ppm, while signals corresponding to aliphatic carbon were observed at δ 39.6-55.6 ppm. It should be emphasized that for the difluoro derivatives 6d-f in both the 1 H-and 13 C-NMR spectra, a characteristic multiplicity was observed, due to the spin-spin interaction of the H-F and C-F nuclei. It is also interesting that in the IR spectra of compounds 6a-f, the characteristic stretching vibrations of (-C≡N) bonds at ν 2227-2230 cm −1 were observed. the reaction time to 6 h ( Table 2 entries 5-8). Subsequently, the increase in the amount of MnO2 to four equivalents resulted in the complete conversion of 5a within 1 h (  This newly developed methodology was then used to synthesize a series of new 4-(aryl)benz [4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles 6b-f with yields in the range of 80-90% (Scheme 5). Scheme 5. Scope of the 4-(aryl)benzo [4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles 6a-f.
All derivatives 6a-f were obtained with comparable yields, which indicates an insignificant influence of the nature of the substituents on the oxidation process. All the synthesized compounds were fully characterized by 1 H-NMR, 13  the reaction time to 6 h ( Table 2 entries 5-8). Subsequently, the increase in the amount of MnO2 to four equivalents resulted in the complete conversion of 5a within 1 h (  This newly developed methodology was then used to synthesize a series of new 4-(aryl)benz [4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles 6b-f with yields in the range of 80-90% (Scheme 5).
As previously stated, an unprecedented in situ [1,3] sigmatropic rearrangement was reported for the related N-10 substituted systems. However, the spectral data obtained for compounds 5a-f and 6a-f do not allow one to determine the position of the Ar substituent in the dihydropyrimidine system with certainty. Single crystal X-ray diffraction analysis was performed on compound 6c to confirm the structure of the obtained compound and to prove the hypotheses about the possibility of rearrangement in the case of unsubstituted benzimidazole-2-arylimine (Figure 1).
According to the XRD data, in compound 6c, the (Ar) substituent is located in the position of C4 of the pyrimidine ring, which indicates the possibility of the rearrangement in the herein reported systems. reported for the related N-10 substituted systems. However, the spectral data obtained for compounds 5a-f and 6a-f do not allow one to determine the position of the Ar substituent in the dihydropyrimidine system with certainty. Single crystal X-ray diffraction analysis was performed on compound 6c to confirm the structure of the obtained compound and to prove the hypotheses about the possibility of rearrangement in the case of unsubstituted benzimidazole-2-arylimine (Figure 1). According to the XRD data, in compound 6c, the (Ar) substituent is located in the position of C4 of the pyrimidine ring, which indicates the possibility of the rearrangement in the herein reported systems.
to prove the hypotheses about the possibility of rearrangement in the case of unsubstituted benzimidazole-2-arylimine (Figure 1). According to the XRD data, in compound 6c, the (Ar) substituent is located in the position of C4 of the pyrimidine ring, which indicates the possibility of the rearrangement in the herein reported systems.
As a first stage, the benzimidazole-2-arylimines 3a-f are activated via the interaction with BF 3 ·Et 2 O, resulting in the formation of activated complex A. At the next stage, there is an asynchronous concerted process interaction of the intermediate A with 3-morpholinoacrylonitrile (4) through an ephemeral transition state B resulting in the formation of a tetrahydropyrimide system C. The removal of the morpholine molecule results in system D, which undergoes [1,3] sigmatropic rearrangement and yields derivatives 5a-f.

Absorption/Fluorescence Studies in Solution and Solvent Effect
All the obtained fluorophores were soluble in concentrations less than 2 × 10 −5 M both in nonpolar (cyclohexane and toluene) and in weakly and strongly apolar aprotonic solvents (THF, acetonitrile, DMSO). Additionally, all the compounds have exhibited an intense fluorescence in solution. Taking into account the subsequent study of the phenomenon of aggregation-induced emission (AIE), THF, which is located at the interface between nonpolar and polar solvents with an average value of orientational polarizability, was chosen as the optimal aprotonic solvent (∆f = 0.21). The results of the photophysical studies are presented below (Table 3). Emission spectra for all the compounds were measured at low concentrations of 10 −5 M to avoid any concentration-dependent dimerization and fluorescence quenching. All the graphs were normalized for comparative analysis (Figure 2).

Molecules 2022, 27, x FOR PEER REVIEW 9 of 25
Emission spectra for all the compounds were measured at low concentrations of 10 −5 M to avoid any concentration-dependent dimerization and fluorescence quenching. All the graphs were normalized for comparative analysis (Figure 2).  The absorption spectra of the fluorophores 6a-f are presented by two absorption bands with different intensities at maximum wavelengths in the 220-300 nm and 350-500 nm ranges, which correspond to S0→S2 and S0→S1 transitions. In this case, all the compounds show a dominant absorption band due to the transition S0 → S2 with εМ < 14.5 × 104 M −1 cm −1 .
The emission spectra of the fluorophores 6a-f are presented by the solid unstructured emission bands with maximums from 520 to 567 nm, referring to the excited ICT-state in a polar aprotic solvent [36]. A significant bathochromic shift was observed for the two 2dimethylaminophenyl substituted imidazopyrimidine fluorophores 6a,d, which have The absorption spectra of the fluorophores 6a-f are presented by two absorption bands with different intensities at maximum wavelengths in the 220-300 nm and 350-500 nm ranges, which correspond to S 0 →S 2 and S 0 →S 1 transitions. In this case, all the compounds show a dominant absorption band due to the transition S 0 → S 2 with εM < 14.5 × 104 M −1 cm −1 .
The emission spectra of the fluorophores 6a-f are presented by the solid unstructured emission bands with maximums from 520 to 567 nm, referring to the excited ICT-state in a polar aprotic solvent [36]. A significant bathochromic shift was observed for the two 2-dimethylaminophenyl substituted imidazopyrimidine fluorophores 6a,d, which have some of the most energetically favorable states among the obtained series of fluorophores (3.39 eV for 6a and 3.24 eV for 6b) (See Section 2.2.5. Theoretical Calculations). The fluorescence lifetimes of the investigated compounds 6a-f exhibited a two-exponential decay in THF. The lifetime of the excited state of the fluorophores was measured at r.t. in THF using a nanosecond LED with an excitation wavelength of 370 nm. The average lifetime was calculated using the expression τ av = Σ (τ i × α i ) (Table S1). Overall, the average fluorescence lifetime (τ av ) ranged from 1.59 ns (lowest for 6d) to 8.78 ns (highest for 6c) ( Table 3). The compounds were characterized by large Stokes shift values (<140 nm), while the quantum yield values in THF were not higher than 7.5%.
We studied the emission characteristics of 6a-f compounds in various solvents (Tables S2-S7). Indeed, the effect of the solvent polarity was observed for the chromophores of the entire series with Stokes shifts from 120 to 180 nm. However, only for anthracenyl substituted fluorophores, upon the increasing solvent polarity in a row from nonpolar cyclohexane to the polar DMSO and MeCN, the emission bands of the fluorophores 6c,f became broad and significantly shifted to the red region, which agrees with the character of strong intramolecular charge transfer (ICT) and is confirmed by the values of theoretically calculated descriptors. Interestingly, in a study of the AIE effect, fluorophore 6f showed a solvatochromic shift in the THF-water binary system of 10-90% water content in the 520-610 nm wavelength range ( Figure S3).

Solid State Fluorescence Studies
The emission spectra of fluorophores 6a-f in the powder/film as well as the experimental data are presented in Table 4 and Figures 3 and 4. Interestingly, only the dimethoxyphenyl-substituted fluorophores 6a and 6d exhibited a redshifted emission in a powder when compared to the spectra in THF solution, implying specific π-π interactions in the solid state. shift in the solid emission demonstrates the useful role of the cyano-group in the phenylimidazopyridine chromophore for restraining the formation of heavy J-aggregates in the solid state [55]. In contrast to the emission in powder, the 6b-f samples in the PVA film showed a significant improvement in fluorescence along with an up to 50% increase in quantum yields, which demonstrates the existence of AIE effects similar to those in solutions.

Aggregation Studies
The phenomenon of aggregation-induced emission (AIE) is usually associated with the well-known Mie scattering effect and is a signal of nanoaggregate formation [56]. The AIE properties of the 6a-f dyes were investigated using different water fractions (fw) in THF. As shown in Table 3, anthracenyl substituted fluorophore 6c almost does not emit in pure THF with a fluorescence quantum yield of less than 0.1%. However, when the water content in the THF solution was increased to 60%, a new green emission band with a maximum at 555 nm was observed for this dye. At the same time, the emission intensity increased approximately two-fold. In addition, the absorption spectra of 6c with a water fraction of 60% did not coincide with the spectra of pure THF and contained an additional absorption peak in the 425-500 nm range, which may be associated with light scattering due to the formation of nanoaggregates ( Figure 5) [57]. In addition, the time-resolved fluorescence curves of 6c in pure THF and with a water fraction of 60% did not coincide

Aggregation Studies
The phenomenon of aggregation-induced emission (AIE) is usually associated with the well-known Mie scattering effect and is a signal of nanoaggregate formation [56]. The AIE properties of the 6a-f dyes were investigated using different water fractions (fw) in THF. As shown in Table 3, anthracenyl substituted fluorophore 6c almost does not emit in pure THF with a fluorescence quantum yield of less than 0.1%. However, when the water content in the THF solution was increased to 60%, a new green emission band with a maximum at 555 nm was observed for this dye. At the same time, the emission intensity increased approximately two-fold. In addition, the absorption spectra of 6c with a water fraction of 60% did not coincide with the spectra of pure THF and contained an additional absorption peak in the 425-500 nm range, which may be associated with light scattering due to the formation of nanoaggregates ( Figure 5) [57]. In addition, the time-resolved fluorescence curves of 6c in pure THF and with a water fraction of 60% did not coincide (Table S8, Figure 5). Apparent changes in the mean fluorescence lifetime (τ av ) of 6c from 6.9 ns in THF to 8.8 ns after the addition of water were observed. The experimental results of the effect of the nature of solvents and the values of the theoretically calculated descriptors are consistent with the fluorescence enhancement behavior of 6c and indicate that the AIE process is accompanied by the formation of molecular aggregates. The optimized 6c geometries for the ground and excited states in the THF were calculated to interpret the AIE process (See Section 2.2.5. Theoretical Calculations).

Mechanochromic Properties
In general, non-planar push-pull luminophores with AIE properties tend to show mechanochromic response [58]. As shown above, fluorophores 6c,f turned out to be AIEactive; their emission maximums were different in the solid state and in aggregate (Tables  3 and 4); therefore, these two fluorophores were selected as the most suitable candidates for the study of mechanochromic properties. As crystalline samples, anthracenyl substituted fluorophores 6c and 6f were obtained with low emission intensities, QYs of 3.9% (6c) and 3.4% (6f), and emission maxima of 511 and 525 nm, respectively (Table 3).
After grinding with a mortar and pestle, the fluorescence emission of compounds 6c and 6f was measured. As it turned out, the compounds demonstrated different responses to mechanical (grinding) stimulation. Thus, the grinding of the yellow powder 6c led to a red-shift of the fluorescence spectra by 31 nm (the red line) and a decrease in fluorescence intensity (Figures 6a and 7a, Table S9). Additionally, after the resuspension of the sample from CH2Cl2, yellow crystals were formed ( Figure 7a) and a slight shift of the emission peak to the blue region was recorded.
The 6f derivative was obtained as yellow crystals with poor emission intensity ( Table  3). The grinding of the crystals of 6f resulted in a bright yellow powder (Figure 7b), along with a low red-shift of the fluorescence by 10 nm (the red line) with the same fluorescence intensity. (Figure 6b). Interestingly, after resuspension of the sample in CH2Cl2, a mixture of crystals and powder formed, as well as a slightly blue-shifted emission peak that increased with fluorescence intensity (Figures 6b and 7b, Table S9).

Mechanochromic Properties
In general, non-planar push-pull luminophores with AIE properties tend to show mechanochromic response [58]. As shown above, fluorophores 6c,f turned out to be AIE-active; their emission maximums were different in the solid state and in aggregate (Tables 3 and 4); therefore, these two fluorophores were selected as the most suitable candidates for the study of mechanochromic properties. As crystalline samples, anthracenyl substituted fluorophores 6c and 6f were obtained with low emission intensities, QYs of 3.9% (6c) and 3.4% (6f), and emission maxima of 511 and 525 nm, respectively (Table 3).
After grinding with a mortar and pestle, the fluorescence emission of compounds 6c and 6f was measured. As it turned out, the compounds demonstrated different responses to mechanical (grinding) stimulation. Thus, the grinding of the yellow powder 6c led to a red-shift of the fluorescence spectra by 31 nm (the red line) and a decrease in fluorescence intensity (Figures 6a and 7a, Table S9). Additionally, after the resuspension of the sample from CH 2 Cl 2 , yellow crystals were formed ( Figure 7a) and a slight shift of the emission peak to the blue region was recorded.
The 6f derivative was obtained as yellow crystals with poor emission intensity ( Table 3). The grinding of the crystals of 6f resulted in a bright yellow powder (Figure 7b), along with a low red-shift of the fluorescence by 10 nm (the red line) with the same fluorescence intensity. (Figure 6b). Interestingly, after resuspension of the sample in CH 2 Cl 2 , a mixture of crystals and powder formed, as well as a slightly blue-shifted emission peak that increased with fluorescence intensity (Figures 6b and 7b, Table S9).  Most probably, the fluorescence response of the samples 6c and 6f during grinding depends on the molecular stacking structures in the solid state [59].

Theoretical Calculations
The DFT-calculations were performed in order to evaluate the donor-acceptor properties and the nature of intramolecular charge transfer based on the obtained optimized model structures of fluorophores 6a-f in the ground and excited states in the solvent phase, energy levels and electron density distribution in frontier molecular orbitals (FMOs), and descriptors-charge-transfer indices (CT-indexes).
The electron density distributions of the boundary molecular orbitals of FMO 6a-f are shown in Figure 8 and in Table 5.  Most probably, the fluorescence response of the samples 6c and 6f during grinding depends on the molecular stacking structures in the solid state [59].

Theoretical Calculations
The DFT-calculations were performed in order to evaluate the donor-acceptor properties and the nature of intramolecular charge transfer based on the obtained optimized model structures of fluorophores 6a-f in the ground and excited states in the solvent phase, energy levels and electron density distribution in frontier molecular orbitals (FMOs), and descriptors-charge-transfer indices (CT-indexes).
The electron density distributions of the boundary molecular orbitals of FMO 6a-f are shown in Figure 8 and in Table 5. Most probably, the fluorescence response of the samples 6c and 6f during grinding depends on the molecular stacking structures in the solid state [59].

Theoretical Calculations
The DFT-calculations were performed in order to evaluate the donor-acceptor properties and the nature of intramolecular charge transfer based on the obtained optimized model structures of fluorophores 6a-f in the ground and excited states in the solvent phase, energy levels and electron density distribution in frontier molecular orbitals (FMOs), and descriptors-charge-transfer indices (CT-indexes).
The electron density distributions of the boundary molecular orbitals of FMO 6a-f are shown in Figure 8 and in Table 5.  The highest occupied molecular orbitals (HOMOs) of the anthracenyl subst dyes 6c,f delocalized exclusively on the donor group, whereas the acceptor group on the 3-cyano substituted benzo [4,5]imidazo [1,2-a]pyrimidine domain is responsi the contribution to the lowest unoccupied molecular orbitals (LUMOs). Charge de zation was less pronounced in the electron density distribution in the FMO for dim aminophenyl substituted fluorophores 6a,d. In fact, there was no delocalization o tron density for methoxyphenyl substituted samples 6b,e. Thus, based on theoretical calculations and experimental data, one can present eral model of the studied fluorophores consisting of a donor methoxyphenyl/dime minophenyl/anthracenyl fragment (Ar, blue) and an acceptor 3-cyano-subst benzo [4,5]imidazo[1,2-a]pyrimidine domain (red), including substituted fluorine ato positions 7 and 8 ( Figure 9). The highest occupied molecular orbitals (HOMOs) of the anthracenyl substituted dyes 6c,f delocalized exclusively on the donor group, whereas the acceptor group based on the 3-cyano substituted benzo [4,5]imidazo [1,2-a]pyrimidine domain is responsible for the contribution to the lowest unoccupied molecular orbitals (LUMOs). Charge delocalization was less pronounced in the electron density distribution in the FMO for dimethylaminophenyl substituted fluorophores 6a,d. In fact, there was no delocalization of electron density for methoxyphenyl substituted samples 6b,e.
To obtain a deeper understanding of the correlation between charge transfer a fluorophore structures, additional calculations of CT-indices were performed [60]. T corresponding indices (D, Sr, and t) presented in Table 6 were calculated for all fluo phores in the Multiwfn program [61]. To obtain a deeper understanding of the correlation between charge transfer and fluorophore structures, additional calculations of CT-indices were performed [60]. The corresponding indices (D, S r , and t) presented in Table 6 were calculated for all fluorophores in the Multiwfn program [61]. The highest D index values [60], as the distance between the centers of gravity of the donor and acceptor, were 4.4 and 4.5 Å for anthracenyl substituted 6c and 6f, respectively, which result from the highest degree of intramolecular charge transfer. The S r index introduced by Tozer in 2008 [62] gives a good correlation between the value of the Stokes shift and the CT junction value; that is, the smaller S r corresponds to the larger Stokes shifts.
The lowest values of this index correspond to compounds 6c,f, as confirmed by studies of the solvatochromic effect with the highest values of the 168-181 nm Stokes shift. The index t > 0 confirms the very fact of charge separation (CD) between the chromophore donor and acceptor due to charge excitation. Thus, the analysis of CT indices confirmed the ICT process for the anthracenyl and dimethylaminophenyl substituted chromophores 6a,d and 6c,f, and also made it possible to predict a significant overlap between the centroids of the positive charge of the donor and the negative charge of the acceptor, representing the zones of increase and decrease in electron density upon excitation, based on the calculated values of D at t > 0.

Crystallography
According to the XRD data, two independent molecules of the compound 6c crystallize with a molecule of CH 2 Cl 2 in the centrosymmetric space group of the triclinic system. In the result, the structurally independent unit C 51 H 30 Cl 2 N 8 (M = 825.73 g/mol) was used for all calculations. The molecule CH 2 Cl 2 is disordered and demonstrates the high magnitude of the anisotropic displacement parameters. The geometry of independent heterocyclic molecules differs only slightly, primarily in the dihedral angles between the heterocyclic and anthracene planes. The general geometry of the molecule was shown in Figure 10. The mean bond distances and angles in the molecules are close to expectations. The heterocyclic and anthracene parts of the molecule are non-conjugated due to high dihedral angles between their planes. In the crystal some polar CArH . . . NC-contacts are observed with participation of the CN-group, in particular, H(9A) . . . N(2) [x − 1, y + 1, z] 2.66 Å (on a scale of 0.09 Å less than the sum of the VdW radii) and N(2A) . . . H(19A) [−x, 1 − y, 1 − z] 2.71 Å (on the order of 0.04 Å less than the sum of the VdW radii ). The π-π-contacts in the crystal are presented only as shortened π-π-contact between the heterocycle and anthracene moiety C(5A) . . . C (17) at a distance of 3.336(4) Å (0.064 Å less than the sum of the VdW radii, Figure 11). angles between their planes. In the crystal some polar CArH…NC-contacts are observed with participation of the CN-group, in particular, H(9A)…N(2) [x − 1, y + 1, z] 2.66 Å (on a scale of 0.09 Å less than the sum of the VdW radii) and N(2A)…H(19A) [−x, 1 − y, 1 − z] 2.71 Å (on the order of 0.04 Å less than the sum of the VdW radii ). The π-π-contacts in the crystal are presented only as shortened π-π-contact between the heterocycle and anthracene moiety C(5A)…C (17) at a distance of 3.336(4) Å (0.064 Å less than the sum of the VdW radii, Figure 11).   with participation of the CN-group, in particular, H(9A)…N(2) [x − 1, y + 1, z] 2.66 Å (on a scale of 0.09 Å less than the sum of the VdW radii) and N(2A)…H(19A) [−x, 1 − y, 1 − z] 2.71 Å (on the order of 0.04 Å less than the sum of the VdW radii ). The π-π-contacts in the crystal are presented only as shortened π-π-contact between the heterocycle and anthracene moiety C(5A)…C (17) at a distance of 3.336(4) Å (0.064 Å less than the sum of the VdW radii, Figure 11).  . π-π-contacts in the crystal of the compound 6c. Figure 11. π-π-contacts in the crystal of the compound 6c.

Chemical Experiment
Commercial reagents were obtained from Sigma-Aldrich, Acros Organics, or Alfa Aesar and used without any preprocessing. All workup and purification procedures were carried out using analytical-grade solvents. One-dimensional 1 H-and 13 C-NMR spectra were acquired on a Bruker DRX-400 instrument (Karlsruhe, Germany) (400 and 101 MHz, respectively), utilizing DMSO-d 6 , CDCl 3 , and CF 3 COOD as solvents and an external reference, respectively. Chemical shifts are expressed in δ (parts per million, ppm) values, and coupling constants are expressed in hertz (Hz). The following abbreviations are used for the multiplicity of NMR signals: s, singlet; d, doublet; t, triplet; dd, doublet of doublet; m, multiplet; and AN, anthracene. IR spectra were recorded on a Bruker α spectrometer equipped with a ZnSe ATR accessory. Elemental analysis was performed on a PerkinElmer PE 2400 elemental analyzer (Waltham, MA, USA). Melting points were determined on a Stuart SMP3 (Staffordshire, UK) and are uncorrected. The monitoring of the reaction progress was performed using TLC on Sorbfil plates (Imid LTD, Russia, Krasnodar) (the eluent is EtOAc). The spectral characteristics of the compound 3b correspond to the data [63]. The compound 3-Morpholinoacrylonitrile (4) was prepared according to a literature procedure [64].
Corresponding 1H-benzo[d]imidazol-2-amine 1a,b (0.01 mol) was mixed with corresponding aldehydes 2a,c and 2d-f (0.0105 mol) and the mixture was heated at 130 • C for 3 h. The reaction mixture was cooled to room temperature and ground up to give the expected pure product. To a suspension of the corresponding derivative 3a-c (0.01 mol, 1 equivalent) in 30 mL of n-BuOH, 1.88 mL (0.015 mol., 1.5 equiv.) of BF 3 ·Et 2 O was added. To the resulting solution, 1.38 g (0.01 mol, 1 equivalent) of 3-morpholinoacrylonitrile (4) was added. The reaction mixture was heated in an oil bath at 130 • C for 5 h. The resulting mixture was cooled to room temperature and stirred for 15 min. The obtained precipitate was filtered off and washed with i-PrOH, water, and acetone to give the expected pure product.

Conclusions
In summary, we have designed and synthesized a series of novel 4-(aryl)-benzo [4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles by successive transformations, including the preparation of benzimidazole-2-arylimines, the Povarov reaction, and the oxidation of dihydrobenzo [4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles. Based on the literature data and X-ray diffraction analysis, it was found that during the Povarov reaction, [1,3] sigmatropic rearrangement occurred. The structure of the synthesized compounds is unambiguously confirmed by the set of spectral data. For the derivatives 6a-f, the ordinary photophysical properties such as absorption, emission, lifetime, and QY in solution, as well as emission and QY in powder, were studied. For the chromophore 6c, Aggregation-Induced Emission (AIE) has been illustrated using different water fractions (fw) in THF. Finally, the mechanofluorochromic properties of derivatives 6c and 6f were investigated, and the response to mechanical stimulation with changing emission maxima or/and intensity was recorded. The significant photophysical properties and availability of 4-(aryl)benzo [4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles pave the way for future applications in biology, medicine, ecology, and photonics.