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

**Victor V. Fedotov 1,\* , Maria I. Valieva <sup>1</sup> , Olga S. Taniya 1,\* , Semen V. Aminov <sup>1</sup> , Mikhail A. Kharitonov <sup>1</sup> , Alexander S. Novikov <sup>2</sup> , Dmitry S. Kopchuk <sup>1</sup> , Pavel A. Slepukhin <sup>1</sup> , Grigory V. Zyryanov <sup>1</sup> , Evgeny N. Ulomsky <sup>1</sup> , Vladimir L. Rusinov <sup>1</sup> and Valery N. Charushin <sup>1</sup>**

	- **\*** Correspondence: viktor.fedotov@urfu.ru (V.V.F.); olga.tania@urfu.ru (O.S.T.)

**Abstract:** 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 <sup>1</sup>H- and <sup>13</sup>C-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.

**Keywords:** pyrimidine; benzimidazole; aza-Diels–Alder reaction; Povarov reaction; oxidation; fluorescence; aggregation-induced emission; mechanochromic properties

### **1. 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 [5–7], 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 [12–15]. 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 [19–21]. 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 [23–26], is a convenient tool for the construction of six-membered rings with high

**Citation:** Fedotov, V.V.; Valieva, M.I.; Taniya, O.S.; Aminov, S.V.; Kharitonov, M.A.; Novikov, A.S.; Kopchuk, D.S.; Slepukhin, P.A.; Zyryanov, G.V.; Ulomsky, E.N.; et al. 4-(Aryl)-Benzo[4,5]imidazo[1,2 *a*]pyrimidine-3-Carbonitrile-Based Fluorophores: Povarov Reaction-Based Synthesis, Photophysical Studies, and DFT Calculations. *Molecules* **2022**, *27*, 8029. https:// doi.org/10.3390/molecules27228029

Academic Editor: Joseph Sloop

Received: 31 October 2022 Accepted: 15 November 2022 Published: 19 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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. tom 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

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 [23–26], is a convenient tool for the construction of six-membered rings with high molecular complexity via the direct construction of carbon–carbon and carbon–heteroa-

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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 [30–34]. 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]. 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 [30–34]. 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]. We recently reported the synthesis of asymmetric donor-acceptor azoloazine fluoro-

We recently reported the synthesis of asymmetric donor-acceptor azoloazine fluorophors based on 4-heteroaryl-substituted 2-phenyl-2*H*-benzo[4,5]imidazo[1,2-*a*][1,2,3] triazolo[4,5-*e*]pyrimidine via the reaction of nucleophilic aromatic hydrogen substitution (SNH) and studied their microenvironmental sensitivity in the PLICT process (Scheme 1) [42]. phors based on 4-heteroaryl-substituted 2-phenyl-2*H*-benzo[4,5]imidazo[1,2-*a*][1,2,3]triazolo[4,5-*e*]pyrimidine via the reaction of nucleophilic aromatic hydrogen substitution (SNH) and studied their microenvironmental sensitivity in the PLICT process (Scheme 1) [42].

**Scheme 1.** Nucleophilic substitution of hydrogen (SNH) in 2-phenyl-2*H*-benzo[4,5]imidazo[1,2 *a*][1,2,3]triazolo[4,5-*e*]pyrimidine [42]. **Scheme 1.** Nucleophilic substitution of hydrogen (SNH) in 2-phenyl-2*H*-benzo[4,5]imidazo[1,2 *a*][1,2,3]triazolo[4,5-*e*]pyrimidine [42].

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.

#### **2. Results 2. Results**

ods.

#### *2.1. Synthesis 2.1. 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 [43–45] and Lewis acid catalysis [46,47] are traditionally used, as are various modifications, including those involving microwave radiation [48–50]. 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). 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 [43–45] and Lewis acid catalysis [46,47] are traditionally used, as are various modifications, including those involving microwave radiation [48–50]. 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).

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 meth-

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**Scheme 2.** Scope of benzimidazole-2-arylimine **3a–f**. **Scheme 2.** Scope of benzimidazole-2-arylimine **3a**–**f**.

*N*-2-substituted benzimidazoles (Scheme 3).

The structure of all intermediates **3a–f** was confirmed by means of the data from 1H NMR spectroscopy, as well as 13C 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 Materi-The structure of all intermediates **3a**–**f** was confirmed by means of the data from <sup>1</sup>H NMR spectroscopy, as well as <sup>13</sup>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).

als). 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 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). *Molecules* **2022**, *27*, x FOR PEER REVIEW 4 of 25

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

**(Catalysts) X, Equiv Reaction** 

1 Reaction conditions: **3a** (0.10 mmol) and **4** (0.10 mmol); 2 amount of solvent—5 mL; 3 conventional

entry 1 EtOH BF3∙Et2O 0.5 reflux, 5 h 35 entry 2 *i*-PrOH BF3∙Et2O 0.5 reflux, 5 h 46 entry 3 *n*-BuOH BF3∙Et2O 0.5 reflux, 5 h 50 entry 4 Toluene BF3∙Et2O 0.5 reflux, 5 h entry 5 *n*-BuOH BF3∙Et2O 0.5 reflux, 6 h 51 entry 6 AcOH - - reflux, 5 h entry 7 *n*-BuOH Et3N 0.5 reflux, 5 h entry 8 *n*-BuOH BF3∙Et2O 1.0 reflux, 5 h 63 entry 9 *n*-BuOH BF3∙Et2O 1.5 reflux, 5 h 74 entry 10 *n*-BuOH BF3∙Et2O 2.0 reflux, 5 h 76

**Condition** <sup>3</sup> **Yield, %** <sup>4</sup>

**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,

**Scheme 3.** Povarov reaction and rearrangement [49]. heating with an oil bath; and 4 isolated yield. **Scheme 3.** Povarov reaction and rearrangement [49].

**№ Solvent** <sup>2</sup> **Activating Agent** 

entry 9).

**Table 1.** Optimization of the reaction conditions for dihydropyrimidin **5a** 1.

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). 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).

Inspired by this fact, we decided to investigate the possibility of rearrangement in


**Table 1.** Optimization of the reaction conditions for dihydropyrimidin **5a** <sup>1</sup> . **Table 1.** Optimization of the reaction conditions for dihydropyrimidin **5a** 1.

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**Scheme 3.** Povarov reaction and rearrangement [49].

entry 10 *n*-BuOH BF3∙Et2O 2.0 reflux, 5 h 76 1 Reaction conditions: **3a** (0.10 mmol) and **4** (0.10 mmol); 2 amount of solvent—5 mL; 3 conventional <sup>1</sup> Reaction conditions: **3a** (0.10 mmol) and **4** (0.10 mmol); <sup>2</sup> amount of solvent—5 mL; <sup>3</sup> conventional heating with an oil bath; and <sup>4</sup> isolated yield.

heating with an oil bath; and 4 isolated yield. 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-, <sup>1</sup>H-, and <sup>13</sup>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 <sup>1</sup>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 <sup>ν</sup> 2202–2215 cm−<sup>1</sup> (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. 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**. **Scheme 4.** Substrate scope of dihydropyrimidines **5a**–**f**.

The structures of the obtained dihydropyrimidines **5a–f** were confirmed by means of IR-, 1H-, and 13C-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 1H 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– 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<sup>2</sup> to four equivalents resulted in the complete conversion of **5a** within 1 h (Table 2, entry 8).

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 avail-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).

ability 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 eval-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 <sup>1</sup>H-NMR, <sup>13</sup>C-NMR, IR-spectroscopy, and elemental analysis (Supplementary Materials). In particular, in the <sup>1</sup>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 <sup>13</sup>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 <sup>1</sup>H- and <sup>13</sup>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−<sup>1</sup> were observed.

uated 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 entry 8).


**Table 2.** Optimization of the oxidation reactions for dihydropyrimidin **5a** <sup>1</sup> . **Table 2.** Optimization of the oxidation reactions for dihydropyrimidin **5a** 1.

**Table 2.** Optimization of the oxidation reactions for dihydropyrimidin **5a** 1.

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 (Table 2,

MnO2 to four equivalents resulted in the complete conversion of **5a** within 1 h (Table 2,

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entry 8).

1 Reaction conditions: **5a** (0.10 mmol); 2 amount of solvent—5 mL; 3 X equivalent of oxidant; 4 conventional heating with an oil bath; 5 in accordance with TLC; and 6 isolated yield. <sup>1</sup> Reaction conditions: **5a** (0.10 mmol); <sup>2</sup> amount of solvent—5 mL; <sup>3</sup> X equivalent of oxidant; <sup>4</sup> conventional heating with an oil bath; <sup>5</sup> in accordance with TLC; and <sup>6</sup> isolated yield. (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**. **Scheme 5.** Scope of the 4-(aryl)benzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitriles **6a**–**f**.

**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 1H-NMR, 13C-NMR, IR-spectroscopy, 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 1H-NMR, 13C-NMR, IR-spectroscopy, 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.

and elemental analysis (Supplementary Materials). In particular, in the 1H-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 13C-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 1H- and 13C-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

and elemental analysis (Supplementary Materials). In particular, in the 1H-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 13C-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 1H- and 13C-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

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 unsubsti-

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 unsubsti-

**Figure 1.** Molecular structure of **6c**. position of C4 of the pyrimidine ring, which indicates the possibility of the rearrangement in the herein reported systems.

**Figure 1.** Molecular structure of **6c.**

tuted benzimidazole-2-arylimine (Figure 1).

*Molecules* **2022**, *27*, x FOR PEER REVIEW 7 of 25

ν 2227–2230 см−1 were observed.

tuted benzimidazole-2-arylimine (Figure 1).

ν 2227–2230 см−1 were observed.

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 The proposed mechanism of the interaction between benzimidazole-2-arylimines **3a**–**f** and 3-morpholinoacrylonitrile (**4**), based on the reactivity of these substrates and literature data [23,26,49], is shown in Scheme 6. The proposed mechanism of the interaction between benzimidazole-2-arylimines **3a– f** and 3-morpholinoacrylonitrile (**4**), based on the reactivity of these substrates and literature data [23,26,49], is shown in Scheme 6.

**Scheme 6.** Plausible reaction mechanisms of dihydropyrimidines **5a**–**f** formation and [1,3] sigmatropic rearrangement.

As a first stage, the benzimidazole-2-arylimines **3a**–**f** are activated via the interaction with BF3·Et2O, 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**.

In addition, we discovered that all of the 4-(aryl)benzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles **6a**–**f** obtained are fluorescent in solution and solid form. Therefore, photophysical studies of the obtained products **6a**–**f** were carried out.

### *2.2. Photophysical Studies*

#### 2.2.1. Absorption/Fluorescence Studies in Solution and Solvent Effect

All the obtained fluorophores were soluble in concentrations less than 2 <sup>×</sup> <sup>10</sup>−<sup>5</sup> 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).


**Table 3.** Data of photophysical properties of fluorophores (**6a**–**f**) (10−<sup>5</sup> M) in THF solvent.

<sup>1</sup> Absorption spectra were measured at r.t. in THF in range from 230 to 500 nm; <sup>2</sup> emission spectra were measured at r.t. in THF; <sup>3</sup> weighted average decay time <sup>τ</sup>av <sup>=</sup> <sup>Σ</sup> (τ<sup>i</sup> <sup>×</sup> <sup>α</sup><sup>i</sup> ) in THF (LED 370 nm); and <sup>4</sup> absolute quantum yields were measured using the Integrating Sphere of the Horiba FluoroMax-4 at r.t. in THF.

Emission spectra for all the compounds were measured at low concentrations of 10−<sup>5</sup> M to avoid any concentration-dependent dimerization and fluorescence quenching. All the graphs were normalized for comparative analysis (Figure 2). Emission spectra for all the compounds were measured at low concentrations of 10−<sup>5</sup> M to avoid any concentration-dependent dimerization and fluorescence quenching. All the graphs were normalized for comparative analysis (Figure 2).

**Figure 2.** Absorption (**a**) and emission (**b**) spectra of fluorophores **6a–f** in THF (c = 10<sup>−</sup>5 M). **Figure 2.** Absorption (**a**) and emission (**b**) spectra of fluorophores **6a**–**f** in THF (c = 10−<sup>5</sup> M).

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 × 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→S<sup>2</sup> and S0→S<sup>1</sup> transitions. In this case, all the compounds show a dominant absorption band due to the transition S<sup>0</sup> <sup>→</sup> <sup>S</sup><sup>2</sup> with <sup>ε</sup>M < 14.5 <sup>×</sup> 104 M−<sup>1</sup> cm−<sup>1</sup> .

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

Сompounds **6a–f** with variation of electron-donating fragments (4-methoxyphenyl, 4-(dimethylamino)phenyl and anthracen-9-yl) based on the 3-cyanosubstituted benzo [4,5]imidazo [1,2-*a*]pyrimidine, including those substituted with fluorine atoms in positions 7,8 implies that the solvent polarity may influence the electronic state properties of

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

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 τav = Σ (τi × αi) (Table S1). Overall, the average fluorescence lifetime (τav) 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%.

the chromophore (See Section 2.2.5. Theoretical Calculations.

in the 520–610 nm wavelength range (Figure S3).

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 τav = Σ (τ<sup>i</sup> × α<sup>i</sup> ) (Table S1). Overall, the average fluorescence lifetime (τav) 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%.

Compounds **6a**–**f** with variation of electron-donating fragments (4-methoxyphenyl, 4-(dimethylamino)phenyl and anthracen-9-yl) based on the 3-cyanosubstituted benzo [4,5]imidazo [1,2-*a*]pyrimidine, including those substituted with fluorine atoms in positions 7,8 implies that the solvent polarity may influence the electronic state properties of the chromophore (See Section 2.2.5. Theoretical Calculations).

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).

#### 2.2.2. 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.


**Table 4.** Optical properties of the compounds **6a**–**f** in the solid state and in PVA film.

<sup>1</sup> Absolute quantum yields were measured using the Integrating Sphere of the Horiba FluoroMax-4 at r.t. in film/powder form.

In the manufacture of OLED devices, thin films of compounds are applied in layers; therefore, it is necessary to conduct optical studies with thin films of materials [54]. To examine the emission in the films, thin films of PVA with integrated fluorophores **6** were deposited on quartz plates, and their emission spectra were measured by using the integrating sphere. In all the spectra, the emission maxima were observed at about 545 nm and were quite similar to the ones collected in THF solution. Thus, the absence of an anomalous red

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. **6e** 542 12.0 509 19.3 **6f** 540 34.5 525 3.4 1 Absolute quantum yields were measured using the Integrating Sphere of the Horiba FluoroMax-4 at r.t. in film/powder form.

**λemmax, nm Φ***f***, (%) 1 λemmax, nm Φ***f***, (%) 1**

**Table 4.** Optical properties of the compounds **6a–f** in the solid state and in PVA film.

**№ In PVA Film In Powder** 

**6a** 546 4.8 572 20.5 **6b** 545 49.6 517 17.8 **6c** 546 25.6 511 3.9 **6d** 545 13.9 626 8.3

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

In the manufacture of OLED devices, thin films of compounds are applied in layers; therefore, it is necessary to conduct optical studies with thin films of materials [54]. To examine the emission in the films, thin films of PVA with integrated fluorophores **6** were deposited on quartz plates, and their emission spectra were measured by using the integrating sphere. In all the spectra, the emission maxima were observed at about 545 nm and were quite similar to the ones collected in THF solution. Thus, the absence of an anomalous red 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

*Molecules* **2022**, *27*, x FOR PEER REVIEW 10 of 25

2.2.2. Solid State Fluorescence Studies

solid state.

in solutions.

**Figure 4.** Emission spectra of dyes **6a–f** in PVA films (**a**) and photographs of the samples under daylight and 365 nm UV irradiation (**b**). **Figure 4.** Emission spectra of dyes **6a**–**f** in PVA films (**a**) and photographs of the samples under daylight and 365 nm UV irradiation (**b**).

#### 2.2.3. Aggregation Studies 2.2.3. 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 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

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

(Table S8, Figure 5). Apparent changes in the mean fluorescence lifetime (τav) 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 in-

terpret the AIE process (See Section 2.2.5. Theoretical Calculations).

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 (τav) 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). *Molecules* **2022**, *27*, x FOR PEER REVIEW 12 of 25

**Figure 5.** Emission spectra of **6c** in different ratios of THF–water (v/v) mixtures (**a**). Plot of I/I0 versus water fraction (vol%), where I0 is the fluorescence intensity in pure THF and emission images of the **6c** in different water fraction mixtures under 365 nm UV illumination (λex = 365 nm) with the concentration of 10<sup>−</sup>5 M (**b**). **Figure 5.** Emission spectra of **6c** in different ratios of THF–water (*v*/*v*) mixtures (**a**). Plot of I/I<sup>0</sup> versus water fraction (vol%), where I<sup>0</sup> is the fluorescence intensity in pure THF and emission images of the **6c** in different water fraction mixtures under 365 nm UV illumination (λex = 365 nm) with the concentration of 10−<sup>5</sup> M (**b**).

#### 2.2.4. Mechanochromic Properties 2.2.4. 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). 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 CH2Cl2, yellow crystals were formed (Figure 7a) and a slight shift of the emission 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.

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 in-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).

creased with fluorescence intensity (Figures 6b and 7b, Table S9).

**Figure 6.** (**a**) Emission spectra of **6c** in solid states (λex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH2Cl2 (3). (**b**) Emission spectra of **6f** in solid states (λex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH2Cl2 (3). **Figure 6.** (**a**) Emission spectra of **6c** in solid states (λex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH2Cl<sup>2</sup> (3). (**b**) Emission spectra of **6f** in solid states (λex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH2Cl<sup>2</sup> (3). **Figure 6.** (**a**) Emission spectra of **6c** in solid states (λex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH2Cl2 (3). (**b**) Emission spectra of **6f** in solid states (λex = 350 nm): as prepared (1), after grinding (2), and after treatment with CH2Cl2 (3).

**Figure 7.** Photographs of **6c** (**a**) and **6f** (**b**) taken under 365 nm UV irradiation. **Figure 7.** Photographs of **6c** (**a**) and **6f** (**b**) taken under 365 nm UV irradiation. **Figure 7.** Photographs of **6c** (**a**) and **6f** (**b**) taken under 365 nm UV irradiation.

depends on the molecular stacking structures in the solid state [59].

Most probably, the fluorescence response of the samples **6c** and **6f** during grinding Most probably, the fluorescence response of the samples **6c** and **6f** during grinding Most probably, the fluorescence response of the samples **6c** and **6f** during grinding depends on the molecular stacking structures in the solid state [59].

#### depends on the molecular stacking structures in the solid state [59]. 2.2.5. Theoretical Calculations

are shown in Figure 8 and in Table 5.

2.2.5. 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 2.2.5. 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 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).

(FMOs), and descriptors—charge-transfer indices (CT-indexes). The electron density distributions of the boundary molecular orbitals of FMO **6a–f** (FMOs), and descriptors—charge-transfer indices (CT-indexes). The electron density distributions of the boundary molecular orbitals of FMO **6a–f** The electron density distributions of the boundary molecular orbitals of FMO **6a**–**f** are shown in Figure 8 and in Table 5.

are shown in Figure 8 and in Table 5. **Table 5.** HOMO/LUMO based on the functionality B3LYP/6-311G\* in the THF phase.


**Figure 8.** Energy gaps of fluorophores **6a–f** in THF phase. **Figure 8.** Energy gaps of fluorophores **6a**–**f** in THF phase.

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**. 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**.

**Table 5.** HOMO/LUMO based on the functionality B3LYP/6-311G\* in the THF phase. **Compound HOMO, eV LUMO, eV ΔE, eV 6a** −5.43 −2.04 3.39 Thus, based on theoretical calculations and experimental data, one can present a general model of the studied fluorophores consisting of a donor methoxyphenyl/dimethylaminophenyl/anthracenyl fragment (Ar, blue) and an acceptor 3-cyano-substituted benzo[4,5] imidazo[1,2-*a*]pyrimidine domain (red), including substituted fluorine atoms at positions 7 and 8 (Figure 9). *Molecules* **2022**, *27*, x FOR PEER REVIEW 15 of 25

**6b** −5.96 −2.13 3.83

**Figure 9.** Donor-acceptor structure 4-aryl-substituted benzo[4,5]imidazo[1,2-*a*]pyrimidine chromophors **6a–f**. **Figure 9.** Donor-acceptor structure 4-aryl-substituted benzo[4,5]imidazo[1,2-*a*]pyrimidine chromophors **6a**–**f**.

To obtain a deeper understanding of the correlation between charge transfer and fluorophore structures, additional calculations of CT-indices were performed [60]. The

**Table 6.** Calculated dipole moments for model structures in ground and excited multiplicity states

**Dipole Moment in Excited Multiplicity State (Debye)** 

**6b** 3.1676 4.3108 0.978 0.62822 −0.326 **6e** 4.0313 1.9340 0.927 0.62287 −0.550 **6a** 3.1463 9.2304 3.722 0.50976 0.617 **6d** 6.5432 13.7974 3.832 0.50975 0.678 **6c** 3.8859 1.8867 4.406 0.26547 2.599 **6f** 1.7563 3.5274 4.523 0.24338 2.707

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 Sr 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 Sr 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

**D(Å) Sr (a.u.) t (Å)** 

**Dipole Moment in Ground Multiplicity State (Debye)** 

and estimated indexes related to hole-electron distribution (CT-indexes).

phores in the Multiwfn program [61].

the calculated values of D at t > 0.

**Compound** 

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, Sr, and t) presented in Table 6 were calculated for all fluorophores in the Multiwfn program [61].


**Table 6.** Calculated dipole moments for model structures in ground and excited multiplicity states and estimated indexes related to hole-electron distribution (CT-indexes).

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<sup>r</sup> 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<sup>r</sup> 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.

#### *2.3. Crystallography*

According to the XRD data, two independent molecules of the compound **6c** crystallize with a molecule of CH2Cl<sup>2</sup> in the centrosymmetric space group of the triclinic system. In the result, the structurally independent unit C51H30Cl2N<sup>8</sup> (M = 825.73 g/mol) was used for all calculations. The molecule CH2Cl<sup>2</sup> 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).

*Molecules* **2022**, *27*, x FOR PEER REVIEW 16 of 25

*2.3. Crystallography* 

*2.3. Crystallography* 

VdW radii, Figure 11).

VdW radii, Figure 11).

**Figure 10.** The compound **6c** in the thermal ellipsoid at the 50% probability level. **Figure 10.** The compound **6c** in the thermal ellipsoid at the 50% probability level. **Figure 10.** The compound **6c** in the thermal ellipsoid at the 50% probability level.

According to the XRD data, two independent molecules of the compound **6c** crystallize with a molecule of CH2Cl2 in the centrosymmetric space group of the triclinic system. In the result, the structurally independent unit C51H30Cl2N8 (M = 825.73 g/mol) was used for all calculations. The molecule CH2Cl2 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

According to the XRD data, two independent molecules of the compound **6c** crystallize with a molecule of CH2Cl2 in the centrosymmetric space group of the triclinic system. In the result, the structurally independent unit C51H30Cl2N8 (M = 825.73 g/mol) was used for all calculations. The molecule CH2Cl2 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

**Figure 11.** π-π-contacts in the crystal of the compound **6c**. **Figure 11. Figure 11.**π-ππ--contacts in the crystal of the compound π-contacts in the crystal of the compound **6c 6c**..

### **3. Materials and Methods**

#### *3.1. 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 <sup>1</sup>H- and <sup>13</sup>C-NMR spectra were acquired on a Bruker DRX-400 instrument (Karlsruhe, Germany) (400 and 101 MHz, respectively), utilizing DMSO-*d*6, CDCl3, and CF3COOD 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].

General procedure for the synthesis of N-(4-arylidene)-1H-benzo[d]imidazol-2-amine (**3a**,**c** and **3d**–**f**).

Corresponding 1*H*-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.

4-Dimethylaminobenzylidene-1H-benzo[*d*]imidazol-2-amine (**3a**). Yellow powder (2.37 g, yield 90%), m.p. 245–247 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3051, 1614, 1584, 1443, 1415, 1167. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 3.04 (6H, s, -N(CH3)2), 6.82 (2H, d, *J* = 8.6 Hz, H-2<sup>0</sup> , H-60 ), 7.09–7.15 (2H, m, H-5, H-6), 7.29–7.44 (1H, m, H-4), 7.43–7.57 (1H, m, H-7), 7.86 (2H, d, *J* = 8.4 Hz, H-30 , H-50 ), 9.24 (1H, s, N=CH), 12.36 (1H, s, NH). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 40.1 (2C), 66.8, 111.1, 112.1 (2C), 118.5, 121.8 (2C), 123.0, 132.0, 134.6, 143.1, 153.8, 157.5, 164.9 Calcd for C16H16N4: C 72.70, H 6.10, N 21.20; found: C 72.63, H 6.15, N 21.22.

N-(Anthracen-9-ylidene)-1H-benzo[*d*]imidazol-2-amine (**3c**). Orange powder (2.73 g, yield 85%), m.p. 277–279 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3046, 1790, 1620, 1553, 1517, 1338. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 7.15–7.22 (2H, m, H-5, H-6), 7.44–7.51 (1H, m, H-4), 7.56–7.65 (3H, m, H-7, 2xHAN), 7.67–7.73 (2H, m, 2xHAN), 8.17 (2H, d, *J* = 8.3 Hz, 2xHAN), 8.80 (1H, s, HAN), 9.13 (2H, d, *J* = 9.0 Hz, 2xHAN), 10.74 (1H, s, N=CH), 12.73 (1H, s, NH). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 111.2, 118.9, 122.1, 124.6, 124.8, 125.8, 128.3, 129.4, 130.9, 131.0, 132.9, 134.4, 142.5, 156.1, 163.9. Calcd for C22H15N3: C 82.22, H 4.70, N 13.08; found: C 82.25, H 4.66, N 13.03.

5,6-Difluoro-N-(4-dimethylaminobenzylidene)-1H-benzo[*d*]imidazol-2-amine (**3d**). Yellow powder (2.61 g, yield 87%), m.p. 294–296 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3045, 1636, 1614, 1549, 1353, 1155. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 3.07 (6H, s, -N(CH3)2), 6.83 (2H d, *J* = 8.6 Hz, H-20 , H-60 ), 7.31–7.61 (2H, m, H-4, H-7), 7.86 (2H, d, *J* = 8.5 Hz, H-30 , H-50 ), 9.19 (1H, s, N=CH), 12.59 (1H, s, NH). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 31.2 (2C), 99.2, 105.9, 112.1 (2C), 122.7, 129.9 (d, *J* = 8.9 Hz) 132.2 (2C), 138.7 (d, *J* = 9.7 Hz), 144.6 (d, *J* = 249.0 Hz), 145.1 (d, *J* = 227.2 Hz), 154.0 (2C), 159.2, 165.3. <sup>19</sup>F-NMR (376 MHz, DMSO-*d6*) *δ* (ppm) −145.9 (d, *J* = 22.2 Hz), -145.14 (d, *J* = 20.8 Hz). Calcd for C16H14F2N4: C 63.99, H 4.70, N 18.66; found: C 63.81, H 4.73, N 18.53.

5,6-Difluoro-N-(4-methoxybenzylidene)-1H-benzo[*d*]imidazol-2-amine (**3e**). Yellow powder (2.58 g, yield 90%), m.p. 254–256 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3062, 1593, 1568, 1509, 1453, 1256. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 3.86 (3H, s, OCH3), 7.12 (2H d, *J* = 8.4 Hz, H-20 , H-60 ), 7.37–7.62 (2H, m, H-4, H-7), 8.01 (2H, d, *J* = 8.3 Hz, H-30 , H-5 0 ), 9.32 (1H, s, N=CH), 12.79 (1H, s, NH). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 55.6, 99.0, 105.8, 114.6 (2C), 127.8, 129.7, 137.7 (2C), 138.0, 146.5 (d, *J* = 237.6 Hz), 146.6 (d, *J* = 237.2 Hz), 157.8, 163.2, 165.1. <sup>19</sup>F-NMR (376 MHz, DMSO-*d6*) *δ* (ppm) -145.3, -144.3. Calcd for C15H11F2N3O: C 62.72, H 3.86, N 13.23; found: C 62.65, H 3.91, N 13.17.

5,6-Difluoro-N-(Anthracen-9-ylidene)-1H-benzo[*d*]imidazol-2-amine (**3f**). Orange powder (3.00 g, yield 84%), m.p. 282–284 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3145, 1666, 1553, 1479, 1452, 1199. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 7.52–7.76 (6H, m, H-4, 5xHAN), 8.20 (2H, d, *J* = 8.4 Hz, 2xHAN), 8.89 (1H, s, HAN), 8.96–9.09 (2H, m, H-7, HAN), 10.63 (1H, s, N=CH), 13.15 (1H, s, NH). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 99.9 (d, *J* = 22.5 Hz), 106.7 (d, *J* = 19.8 Hz), 124.0, 125.0, 126.3, 128.9, 129.76, 129.83, 129.9, 130.4 (d, *J* = 11.5 Hz), 131.2, 131.4, 131.6, 131.9, 133.7, 135.7, 138.6 (d, *J* = 10.9 Hz), 147.2 (d, *J* = 237.7 Hz), 147.4 (d, *J* = 238.0 Hz), 158.2, 164.9, 194.7. <sup>19</sup>F-NMR (376 MHz, DMSO-*d6*) *δ* (ppm) -144.7 (d, *J* = 21.8 Hz), -143.54 (d, *J* = 22.1 Hz). Calcd for C22H13F2N3: C 73.94, H 3.67, N 11.76; found: C 74.03, H 3.53, N 11.46.

General procedure for the synthesis of 4-(aryl)-1,2-dihydrobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitriles (**5a**–**c**).

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 BF3·Et2O 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.

4-(Dimethylaminophenyl)-1,2-dihydrobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**5a**). White powder (2.33 g, yield 74%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3071, 2805, 2215, 1621, 1578, 1459. <sup>1</sup>H-NMR (400 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 3.36 (6H, s, -N(CH3)2), 6.42 (1H, s, H-4), 6.97 (1H, d, *J* = 8.3 Hz, H-6), 7.35 (1H, t, *J* = 7.9 Hz, H-7), 7.50–7.56 (2H, m, H-8, H-2), 7.64–7.73 (5H, m, H-9, H-20 , H-30 , H-50 H-60 ), 11.30 (1H, s, -NH). <sup>13</sup>C{1H}-NMR (100 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 47.6 (2C), 57.3, 87.6, 111.5, 114.1, 114.4, 122.4 (2C), 126.7, 127.3, 127.8, 128.6, 129.7 (2C), 135.8, 138.4, 141.6, 143.6. Calcd for C19H17N5: C 72.36, H 5.43, N 22.21; found: C 72.45, H 5.51, N 22.04.

4-(4-Methoxyphenyl)-1,2-dihydrobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**5b**). White powder (1.90 g, yield 63%), m.p. 270–272 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3376, 3109, 2209, 1659, 1624, 1254. <sup>1</sup>H-NMR (400 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 3.82 (3H, s, OCH3), 6.16 (1H, s, H-4), 6.89–7.05 (3H, m, H-6, H-3<sup>0</sup> , H-50 ), 7.25 (1H, t, *J* = 7.8 Hz, H-7), 7.28–7.36 (3H, m, H-2, H-20 , H-60 ), 7.40 (1H, t, *J* = 7.8 Hz, H-8), 7.62 (1H, d, *J* = 8.2 Hz, H-9). <sup>13</sup>C{1H}-NMR (100 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 55.6 (2C), 58.1, 89.3, 111.9, 114.0, 115.4 (2C), 125.4, 126.5, 127.5, 128.0, 128.6 (2C), 129.4, 133.8, 142.6, 161.3. Analytical calculated for C18H14N4O: C 71.51, H 4.67, N 18.53; found: C 71.58, H 4.61, N 18.45.

4-(Anthracen-9-yl)-1,2-dihydrobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**5c**). White powder (2.23 g, yield 60%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3062, 2635, 2202, 1666, 1502, 1447. <sup>1</sup>H-NMR (400 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 6.11 (1H, d, *J* = 8.4 Hz, HAN), 6.86 (1H, t, *J* = 8.0 Hz, H-6), 7.21– 7.27 (1H, m, H-7), 7.42–7.55 (3H, m, H-2, 2xHAN), 7.60 (1H, s, H-4), 7.64–7.70 (1H, m, H-8), 7.76–7.85 (2H, m, 2xHAN), 7.97 (1H, s, HAN), 8.05–8.12 (1H m, HAN), 8.21 (1H, d, *J* = 8.5 Hz, H-9), 8.49 (1H, d, *J* = 9.1 Hz, HAN), 8.72 (1H, s, HAN). <sup>13</sup>C{1H}-NMR (100 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 53.3, 88.8, 112.0, 113.8, 114.1, 120.3, 120.5, 121.5, 125.9, 126.1, 126.3, 127.0, 128.2, 128.3, 129.1, 129.9, 130.3, 130.9, 131.2, 131.3, 131.5, 132.0, 133.4, 135.5, 141.8. Calcd for C25H16N4: C 80.63, H 4.33, N 15.04; found: C 80.53, H 4.42, N 15.05.

4-(4-(Dimethylamino)phenyl)-7,8-difluoro-1,2-dihydrobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**5d**). White powder (2.28 g, yield 65%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3106, 2886, 2216, 1658, 1495, 1463. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 2.88 (6H, s, -N(CH3)2), 6.25 (1H, s, H-4), 6.69 (2H, d, *J* = 8.3 Hz, H-3<sup>0</sup> , H-50 ), 6.98 (1H, dd, *J* = 10.5, 7.3 Hz, H-6), 7.20 (2H, d, *J* = 8.3 Hz, H-20 , H-60 ), 7.42 (1H, dd, *J* = 11.2, 7.3 Hz, H-9), 7.58 (1H, s, H-2), 11.14 (1H, s, -NH). <sup>13</sup>C{1H}-NMR (100 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 47.4 (2C), 57.5, 87.7, 101.1 (d, *J* = 25.4 Hz), 104.2 (d, *J* = 24.1 Hz), 113.8, 122.7 (2C), 123.1 (d, *J* = 12.7 Hz), 124.6 (d, *J* = 13.1 Hz), 129.8 (2C), 135.6, 137.8, 142.9, 143.9, 149.7 (dd, *J* = 253.1, 15.0 Hz), 150.5 (dd, *J* = 251.6, 15.4 Hz). Calcd for C19H15F2N5: C 64.95, H 4.30, N 19.93; found: C 64.78, H 4.47, N 19.87.

7,8-Difluoro-4-(4-methoxyphenyl)-1,2-dihydrobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**5e**). White powder (1.99 g, yield 59%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3109, 2213, 1586, 1462, 1374, 1252. <sup>1</sup>H-NMR (400 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 3.84 (3H, s, OCH3), 6.09 (1H, s, H-4), 6.76–6.83 (1H, m, H-6), 6.98 (2H, d, *J* = 8.6 Hz, H-30 , H-50 ), 7.28–7.35 (3H, m, H-2, H-20 , H-60 ), 7.48–7.55 (1H, m, H-9). <sup>13</sup>C{1H}-NMR (100 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 55.6, 58.4, 89.4, 101.4 (d, *J* = 24.5 Hz), 103.5 (d, *J* = 24.1 Hz), 114.9, 115.7 (2C), 123.7 (d, *J* = 10.2 Hz), 125.5 (d, *J* = 11.8 Hz), 126.7, 128.6 (2C), 133.5, 143.8, 148.7 (dd, *J* = 249.3, 14.4 Hz), 149.6 (dd, *J* = 249.4, 13.7 Hz), 161.6. Calcd for C18H12F2N4O: C 63.90, H 3.58, N 16.56; found: C 63.83, H 3.61, N 16.38.

4-(Anthracen-9-yl)-7,8-difluoro-1,2-dihydrobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**5f**). White powder (2.57 g, yield 63%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3069,

2204, 1636, 1465, 1384, 1268. <sup>1</sup>H-NMR (400 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 5.80–5.96 (1H, m, H-6), 7.42–7.56 (3H, m, 3x HAN), 7.63 (1H, s, H-4), 7.70 (1H, t, *J* = 7.5 Hz, HAN), 7.77–7.87 (2H, m, H-9, HAN), 7.95 (1H, s, HAN), 8.11–8.18 (1H, m, HAN), 8.26 (1H, d, *J* = 8.5 Hz, HAN), 8.46 (1H, d, *J* = 9.1 Hz, HAN), 8.78 (1H, s, H-2). <sup>13</sup>C{1H}-NMR (100 MHz, CDCl<sup>3</sup> + 0.1 mL CF3COOD) *δ* (ppm) 53.5, 77.4, 89.0, 101.4 (d, *J* = 24.8 Hz), 103.5 (d, *J* = 24.5 Hz), 120.0, 120.2, 120.4, 123.8 (d, *J* = 10.1 Hz), 124.3 (d, *J* = 11.1 Hz), 126.0, 126.3, 129.4, 130.2, 130.3, 131.0, 131.3, 131.4, 131.5, 132.0, 133.8, 135.3, 143.0, 149.2 (dd, *J* = 248.1, 11.8 Hz), 150.0 (dd, *J* = 256.5, 19.6 Hz). Calcd for C25H14F2N4: C 73.52, H 3.46, N 13.72; found: C 73.63, H 3.49, N 13.58.

General procedure for the synthesis of 4-(aryl)benzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitriles (**6a**–**f**).

To a stirred solution of the appropriate derivatives **5a**–**f** (0.005 mol, 1 equivalent) in DMF (30 mL), MnO<sup>2</sup> (1.74 g, 0.02 mol, 4 equivalent) was added. The resulting mixture was stirred for 2 h at 130 ◦C (oil bath temperature) in an open air atmosphere until TLC (EtOAc as eluent) indicated total consumption of starting dihydropyrimidines **5a**–**f**. The reaction mixture was filtered through ceolite, the filtrate was poured into 150 mL of water, and the solid product was collected by filtration to give the expected pure product.

4-(4-(Dimethylamino)phenyl)benzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**6a**). Orange powder (1.33 g, yield 85%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 2232, 1604, 1538, 1400, 1372, 1189. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 3.10 (6H, s, -N(CH3)2), 6.84 (1H, d, *J* = 8.4 Hz, H-6), 7.00 (2H, d, *J* = 8.4 Hz, H-30 , H-50 ), 7.21 (1H, t, *J* = 7.9 Hz, H-7), 7.54 (1H, t, *J* = 7.8 Hz, H-8), 7.60 (2H, d, *J* = 8.4 Hz, H-20 , H-60 ), 7.90 (1H, d, *J* = 8.2 Hz, H-9), 9.04 (1H, s, H-2). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 39.6 (2C), 94.6, 111.6 (2C), 114.8, 115.1, 116.1, 119.9, 122.2, 126.8, 127.6, 129.6 (2C), 144.6, 150.1, 152.3, 155.3, 156.7. Calcd for C19H15N5: C 72.83, H 4.82, N 22.35; found: C 72.71, H 5.06, N 22.23.

4-(4-Methoxyphenyl)benzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**6b**). Yellow powder (1.25 g, yield 83%), m.p. 233–235 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 2230, 1667, 1473, 1091, 1058, 1020. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 3.95 (3H, s, OCH3), 6.54 (1H, d, *J* = 8.5 Hz, H-6), 7.20 (1H, t, *J* = 7.8 Hz, H-7), 7.34 (2H, d, *J* = 8.7 Hz, H-30 , H-50 ), 7.55 (1H, t, *J* = 7.7 Hz, H-8), 7.77 (2H, d, *J* = 8.5 Hz, H-20 , H-60 ), 7.92 (1H, d, *J* = 8.1 Hz, H-9), 9.11 (1H, s, H-2). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 55.6, 95.0, 114.8, 115.1 (2C), 115.6, 120.0, 121.1, 122.5, 127.0, 127.4, 130.1 (2C), 144.6, 149.8, 155.1, 155.9, 161.9. Calcd for C18H12N4O: C 71.99, H 4.03, N 18.66; found: C 71.80, H 3.91, N 18.70.

4-(Anthracen-9-yl)benzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**6c**). Yellow powder (1.66 g, yield 90%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3051, 2227, 1621, 1483, 1446, 1350. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 5.29 (1H, d, *J* = 8.4 Hz, H-6), 6.72–6.80 (1H, m, H-7), 7.39 (1H, t, *J* = 7.3 Hz, H-8), 7.46–7.53 (2H, m, 2xHAN), 7.62–7.73 (4H, m, 4xHAN), 7.92 (1H, d, *J* = 8.3 Hz, H-9), 8.38 (2H, d, *J* = 8.5 Hz, 2xHAN), 9.20 (1H, s, HAN), 9.37 (1H, s, H-2). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 97.2, 113.5, 115.0, 120.1, 121.0, 123.0, 123.7 (2C), 126.5 (2C), 126.6, 127.0, 128.6 (2C), 128.8 (2C), 129.3 (2C), 130.6 (2C), 132.1, 144.7, 149.9, 153.0, 155.4. Calcd for C25H14N4: C 81.06, H 3.81, N 15.13; found: C 80.94, H 3.58, N 15.12.

4-(4-(Dimethylamino)phenyl)-7,8-difluorobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**6d**). Orange powder (1.41 g, yield 81%), m.p. 284–286 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3082, 2225, 1603, 1438, 1398, 1377. <sup>1</sup>H-NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 3.11 (6H, s, -N(CH3)2), 6.63 (1H, t, *J* = 9.3 Hz, H-6), 7.02 (2H, d, *J* = 8.4 Hz, H-30 , H-50 ), 7.61 (2H, d, *J* = 8.3 Hz, H-20 , H-60 ), 8.02 (1H, t, *J* = 9.2 Hz, H-9), 9.07 (1H, s, H-2). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 95.0, 103.1 (d, *J* = 24.4 Hz), 107.1 (d, *J* = 19.9 Hz), 111.5 (2C), 113.7, 115.4, 122.7 (d, *J* = 10.7 Hz), 129.5 (2C), 140.8 (d, *J* = 11.6 Hz), 145.3 (dd, *J* = 241.7, 15.4 Hz), 149.1 (dd, *J* = 245.8, 14.8 Hz), 151.2, 152.5, 155.4, 156.1. Calcd for C19H13F2N4: C 65.32, H 3.75, N 20.05; found: C 65.53, H 3.89, N 19.92.

7,8-Difluoro-4-(4-methoxyphenyl)benzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**6e**). Beige powder (1.34 g, yield 80%), m.p. 246–248 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3046, 2229, 1595, 1530, 1490, 1101. <sup>1</sup>H NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 3.96 (3H, s, OCH3), 6.31

(1H, dd, *J* = 10.8, 7.3 Hz, H-6), 7.37 (2H, d, *J* = 8.3 Hz, H-30 , H-50 ), 7.77 (2H, d, J = 8.3 Hz, H-20 , H-60 ), 8.05 (1H, dd, *J* = 10.8, 7.5 Hz, H-9), 9.15 (1H, s, H-2). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 55.6, 95.6, 103.0 (d, *J* = 24.4 Hz), 107.5 (d, *J* = 19.8 Hz), 115.2, 115.3 (2C), 120.3, 122.7 (d, *J* = 10.9 Hz), 130.2 (2C), 140.9 (d, *J* = 11.8 Hz), 145.7 (dd, *J* = 242.2, 15.6 Hz), 149.3 (dd, *J* = 245.8, 15.0 Hz), 151.0, 155.5, 155.6, 162.2. Calcd for C18H10F2N4O: C 64.29, H 3.00, N 16.66; found: C 64.35, H 3.19, N 16.52.

4-(Anthracen-9-yl)-7,8-difluorobenzo[4,5]imidazo[1,2-*a*]pyrimidine-3-carbonitrile (**6f**). Yellow powder (1.71 g, yield 84%), m.p. > 300 ◦C. FT-IR (neat) νmax (cm−<sup>1</sup> ): 3088, 2230, 1594, 1505, 1465, 1074. <sup>1</sup>H NMR (400 MHz, DMSO-*d6*) *δ* (ppm) 5.00 (1H, t, *J* = 8.9 Hz, H-6), 7.52 (2H, t, *J* = 7.7 Hz, 2xHAN), 7.67 (2H, t, *J* = 7.5 Hz, 2xHAN), 7.75 (2H, d, *J* = 8.8 Hz, 2xHAN), 8.07 (1H, t, *J* = 9.3 Hz, H-9), 8.38 (2H, d, *J* = 8.6 Hz, 2xHAN), 9.23 (1H, s, H-2), 9.41 (1H, s, HAN). <sup>13</sup>C{1H}-NMR (100 MHz, DMSO-*d*6) *δ* (ppm) 98.0, 101.5 (d, *J* = 24.5 Hz), 107.9 (d, *J* = 19.9 Hz), 114.7, 119.9, 121.9 (d, *J* = 10.8 Hz), 123.6 (2C), 126.6 (2C), 128.7 (2C), 129.1 (2C), 129.3 (2C), 130.5 (2C), 132.5, 141.1 (d, *J* = 11.8 Hz), 145.9 (dd, *J* = 243.2, 15.6 Hz), 149.3 (dd, *J* = 246.6, 14.9 Hz), 151.3, 152.6, 155.7. Calcd for C25H12F2N4: C 73.89, H 2.98, N 13.79; found: C 74.05, H 2.85, N 13.62.

### *3.2. Crystallography Experiment*

The XRD analyses were carried out using equipment of the Center for Joint Use "Spectroscopy and Analysis of Organic Compounds" at the Postovsky Institute of Organic Synthesis of the Russian Academy of Sciences (Ural Branch). The experiment was carried out on a standard procedure (MoKα-irradiation, graphite monochromator, ω-scans with 1<sup>0</sup> step at T = 295(2) K) on an automated X-ray diffractometer Xcalibur 3 with a CCD detector. Empirical absorption correction was applied. The solution and refinement of the structures were accomplished using the Olex program package [65]. The structures were solved by the method of the intrinsic phases in the ShelXT program and refined by the ShelXL by full-matrix least-squares method for non-hydrogen atoms [66]. The H atoms were placed in the calculated positions and refined in isotropic approximation.

Crystal Data for C51H30Cl2N<sup>8</sup> (M = 825.73 g/mol): triclinic, space group P-1, a = 8.5135(4) Å, b = 10.4646(5) Å, c = 22.9053(12) Å, α= 88.784(4)◦ , β= 85.741(4)◦ , γ = 82.301(4)◦ , V = 2016.53(17) Å<sup>3</sup> , Z = 2, T = 295(2) K, µ(MoKα) = 0.210 mm−<sup>1</sup> , Dcalc = 1.360 g/cm<sup>3</sup> , 20,634 reflections measured (7.384◦ ≤ 2Θ ≤ 60.982◦ ), 10,876 unique (Rint = 0.0577, Rsigma = 0.0845), which were used in all calculations. The final R<sup>1</sup> = 0.0767, wR<sup>2</sup> = 0.1916 (I > 2σ(I)) and R<sup>1</sup> = 0.1463, wR<sup>2</sup> = 0.2616 (all data). Largest peak/hole difference is 0.34/−0.35.

The XRD data were deposited in the Cambridge Structural Database with the number CCDC 2215090. This data can be requested free of charge via www.ccdc.cam.ac.uk (accessed on 17 November 2022).

### *3.3. DFT Calculations*

The quantum chemical calculations were performed at the B3LYP/6-31G\*//PM6 level of theory using the Gaussian-09 program package (M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Had DJF. Gaussian 09, Revision C.01. Wallingford, CT 2010). No symmetry restrictions were applied during the geometry optimization procedure. The solvent effects were taken into account using the SMD (solvation model based on density) continuum solvation model suggested by Truhlar et al. [67] for THF. The Hessian matrices were calculated for all optimized model structures to prove the location of correct minima on the potential energy surface (no imaginary frequencies were found in all cases). The Chemcraft program http://www.chemcraftprog.com/ (accessed on 17 November 2022) was used for visualization. The hole-electron analysis was carried out in Multiwfn program (version 3.7) [61]. The Cartesian atomic coordinates for all optimized equilibrium model structures are presented in the attached xyz-files.

#### **4. 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.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules27228029/s1, Table S1: Fluorescence lifetime of probes **6a**–**<sup>f</sup>** (C = 2 <sup>×</sup> <sup>10</sup>−<sup>6</sup> M) in THF; Table S2: Orientation polarizability for solvents (∆f), absorption and fluorescence emission maxima (λabs, λem, nm), and Stokes shift (nm, cm−<sup>1</sup> ) of **6a** in different solvents; Table S3: Orientation polarizability for solvents (∆f), absorption and fluorescence emission maxima (λabs, λem, nm), and Stokes shift (nm, cm−<sup>1</sup> ) of **6b** in different solvents; Table S4: Orientation polarizability for solvents (∆f), absorption and fluorescence emission maxima (λabs, λem, nm), and Stokes shift (nm, cm−<sup>1</sup> ) of **6c** in different solvents; Table S5: Orientation polarizability for solvents (∆f), absorption and fluorescence emission maxima (λabs, λem, nm) and Stokes shift (nm, cm−<sup>1</sup> ) of **6d** in different solvents; Table S6: Orientation polarizability for solvents (∆f), absorption and fluorescence emission maxima (λabs, λem, nm) and Stokes shift (nm, cm−<sup>1</sup> ) of **6e** in different solvents; Table S7: Orientation polarizability for solvents (∆f), absorption and fluorescence emission maxima (λabs, λem, nm) and Stokes shift (nm, cm−<sup>1</sup> ) of **6f** in different solvents; Table S8: Fluorescence lifetime of probe **6c** (C = 2 <sup>×</sup> <sup>10</sup>−<sup>6</sup> M) in THF/water mixtures with water fractions 0/60 (vol%); Table S9: Mechanochromic properties of probes **6a**–**f**; Table S10: Crystal data and structure refinement for **6c**; Table S11: Fractional Atomic Coordinates (×10<sup>4</sup> ) and Equivalent Isotropic Displacement Parameters (Å2 <sup>×</sup> <sup>10</sup><sup>3</sup> ) for **6c**; Table S12: Anisotropic Displacement Parameters (Å2 <sup>×</sup> <sup>10</sup><sup>3</sup> ) for **6c**; Table S13: Bond Lengths for **6c**; Table S14: Bond Angles for **6c**; Table S15: Torsion Angles for **6c**; Table S16: Hydrogen Atom Coordinates (Å <sup>×</sup> <sup>10</sup><sup>4</sup> ) and Isotropic Displacement Parameters (Å2 <sup>×</sup> <sup>10</sup><sup>3</sup> ) Table S17: for **6c**; Atomic Occupancy for 6c; Figure S1: Solvent effect of **6c** and **6f**; Figure S2: UV-Vis absorption spectra of **6c** in THF/water mixtures with water fractions 0/60% (**A**). Time-resolved emission decay curves of **6c** in THF/water mixtures with water fractions 0/60% (**B**); Figure S3: Solvent effect for **6f** in THF/water; Figures S4–S20: <sup>1</sup>H- and <sup>13</sup>C-NMR spectra of compounds **3a**,**c**, **3d**–**f**, **5a**–**f**, and **6a**–**f**; Figures S21–S29 IR spectra of compounds **3a**,**c**, **3d**–**f**, **5a**–**f**, and **6a**–**f**.

**Author Contributions:** Synthesis, V.V.F., M.A.K. and S.V.A.; methodology, V.V.F., E.N.U. and V.L.R.; writing—original draft preparation, V.V.F., M.I.V. and O.S.T.; writing—review and editing, E.N.U., V.L.R., G.V.Z. and V.N.C.; photophysical studies, M.I.V. and O.S.T.; visualization, A.S.N. and D.S.K.; quantum chemical calculations, A.S.N.; crystallographic investigation, P.A.S.; supervision, V.L.R., G.V.Z. and V.N.C.; project administration, V.L.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research funding from the Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority-2030 Program) is gratefully acknowledged.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are contained within the article.

**Acknowledgments:** The team of authors would like to thank the Laboratory for Comprehensive Research and Expert Evaluation of Organic Materials under the direction of O.S. Eltsov.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds **3a**–**c**, **4**, **5a**–**f**, and **6a**–**f** are available from the authors.

### **References**

