Isoxazolyl-Derived 1,4-Dihydroazolo[5,1-c][1,2,4]Triazines: Synthesis and Photochemical Properties

New fluorescent dyes containing an assembled 1,4-dihydroazolo[5,1-c][1,2,4]triazine (DAT) core and an isoxazole ring were synthesized through a reaction between diazopyrazole or diazoimidazoles and isoxazolyl-derived enamines in mild conditions. The photophysical characteristics (maxima absorption and emission, Stokes shifts, fluorescent quantum yields, and fluorescence lifetimes) of the new fluorophores were obtained. The prepared DATs demonstrated emission maxima ranging within 433–487 nm, quantum yields within 6.1–33.3%, and a large Stokes shift. The photophysical characteristics of representative DAT examples were studied in ten different solvents. Specific (hydrogen bonds) and non-specific (dipole–dipole) intermolecular and intramolecular interactions were analyzed using XRD data and spectral experiments. Solvatochromism was analyzed using Lippert–Mataga and Dimroth–Reichardt plots, revealing the relationship between the DAT structure and the nature of solute–solvent interactions. The significant advantages of DATs are the fluorescence of their powders (QY up to 98.7%). DAT-NMe2 10 expressed bright aggregation-induced emission (AIE) behavior in DMSO and THF as the water content increased. The numerous possible variations of the structures of the heterocycles included in the DATs, as well as substituents, create excellent prospects for adjusting their photophysical and physicochemical properties.

We detected the reaction intermediate using 1 H NMR experiments before isolating and characterizing it. This finding introduced some certainty into the question of the reaction mechanism in favor of 1,3-dipolar cycloaddition [38]. This highly efficient process is characterized by mild conditions, high yields, availability, the variability of starting reagents, and the easy isolation of the products. The listed advantages are very important, since DAT 9 is fluorescent and can be considered as a new candidate in the set of fluorosensors due to the sensitivity of its photophysical characteristics to protic or polar solvents.

This Work
The aim of this work was to expand the scope of the reaction between diazoazoles and enamines by synthesizing new DATs 10 and 11 (Scheme 2), bearing an isoxazole heterocycle at position C3 instead of a thiadiazole ring. This study was carried out to synthesize and investigate their photophysical properties (Scheme 2).
The choice of isoxazole was not accidental. Isoxazole rings are very popular in medicinal chemistry [39][40][41][42][43] and it is a structural part of many photoactive compounds [44,45]. The isoxazole electronic structure differs from 1,2,3-thiadiazole, exhibiting more electron saturation at the aromatic cycle. This enables it to change the optical characteristics and sensitivity of DATs depending on the microenvironment [45,46]. To describe the mechanism of heterocyclic backbone formation, several alternatives have been considered. Many comprehensive experimental and theoretical studies have revealed suitable pathways [26][27][28][29][30][31][32][33][34][35]. However, neither experimental results [30,32] nor quantum chemical calculations [26] have given a clear answer to the question of the reaction mechanism and the role of the diazo reagent.
We detected the reaction intermediate using 1 H NMR experiments before isolating and characterizing it. This finding introduced some certainty into the question of the reaction mechanism in favor of 1,3-dipolar cycloaddition [38]. This highly efficient process is characterized by mild conditions, high yields, availability, the variability of starting reagents, and the easy isolation of the products. The listed advantages are very important, since DAT 9 is fluorescent and can be considered as a new candidate in the set of fluorosensors due to the sensitivity of its photophysical characteristics to protic or polar solvents.

This Work
The aim of this work was to expand the scope of the reaction between diazoazoles and enamines by synthesizing new DATs 10 and 11 (Scheme 2), bearing an isoxazole heterocycle at position C3 instead of a thiadiazole ring. This study was carried out to synthesize and investigate their photophysical properties (Scheme 2).
The choice of isoxazole was not accidental. Isoxazole rings are very popular in medicinal chemistry [39][40][41][42][43] and it is a structural part of many photoactive compounds [44,45]. The isoxazole electronic structure differs from 1,2,3-thiadiazole, exhibiting more electron saturation at the aromatic cycle. This enables it to change the optical characteristics and sensitivity of DATs depending on the microenvironment [45,46].

Chemistry
The reaction of 5-diazoazoles 6 and 7a,b and isoxazol-5-yl enamines 3a-d led to the formation of non-aromatic DAT-NMe2 10 and 11a-d in moderate to good yields (Scheme 3) [38]. The process was carried out in dry aprotic solvents at room temperature for 12-24 h. The starting enamines 3a-d [47] and diazoazoles 6 and 7a,b [36] were obtained by procedures described previously. The isolated compounds 10 and 11a-d were characterized by 1 H, 13 С (BB) and HMBC NMR spectra, and by HRESMS and XRD (Supplementary Materials). The 1 Н NMR spectra of DATs-NMe2 10 and 11a-d contain the signals of all proton-containing groups (Figures S1-S5). The most important signal of the 1 H NMR spectra of DATs-NMe2 10 and 11a-d is the C4H triazine ring proton signal, which was recorded at 6.26-6.48 ppm and shifted upfield at 3-4 ppm compared to triazines 4 and 5. The 13 C NMR spectra of derivatives 10 and 11a-d contain a signal of C4 at 67.7-70.1 ppm, in agreement with their sp 3 hybrid state. The NH proton displayed a broad singlet at 9.77-9.78 ppm (for DATs-NMe2 11a,c,d in CDCl3) and 11.98 and 12.10 ppm (for DATs-NMe2 10 and 11b in DMSO-d6), which disappeared when CD3COOD was added.
To refine the molecular structure, DAT-NMe2 11b was studied via single-crystal Xray diffraction analysis. Single crystals of DAT-NMe2 11b were grown from their diluted solutions in acetone via the slow evaporation of the solvent. The molecular structures of the compound are shown in Figures 1 and S1. Compound 11b crystallizes into the centrosymmetric triclinic space group P-1. The asymmetric unit contains two independent molecules, the structure of which is unequal both in bond length and angle values due to the rotation around the C8-C12 bond, the linked bicyclic core, and the isoxazole ring ( Figure  1a). Selected bond lengths and torsion angles are presented in Table S1. The structures in Figure 1 distinctly demonstrate different deviations of the isoxazole ring in different molecules from the bicyclic core, as well as deviations of the phenyl ring plane from the isoxazole ring. The bond lengths are slightly different in the two neighboring molecules; however, their values are lower than the standard ones, indicating that there is a conjugation Scheme 2. Design of new DATs-NMe 2 10 and 11 isoxazole at C4 atom of heterocyclic core.

Chemistry
The reaction of 5-diazoazoles 6 and 7a,b and isoxazol-5-yl enamines 3a-d led to the formation of non-aromatic DAT-NMe 2 10 and 11a-d in moderate to good yields (Scheme 3) [38]. The process was carried out in dry aprotic solvents at room temperature for 12-24 h. The starting enamines 3a-d [47] and diazoazoles 6 and 7a,b [36] were obtained by procedures described previously.

Chemistry
The reaction of 5-diazoazoles 6 and 7a,b and isoxazol-5-yl enamines 3a-d led to the formation of non-aromatic DAT-NMe2 10 and 11a-d in moderate to good yields (Scheme 3) [38]. The process was carried out in dry aprotic solvents at room temperature for 12-24 h. The starting enamines 3a-d [47] and diazoazoles 6 and 7a,b [36] were obtained by procedures described previously. The isolated compounds 10 and 11a-d were characterized by 1 H, 13 С (BB) and HMBC NMR spectra, and by HRESMS and XRD (Supplementary Materials). The 1 Н NMR spectra of DATs-NMe2 10 and 11a-d contain the signals of all proton-containing groups (Figures S1-S5). The most important signal of the 1 H NMR spectra of DATs-NMe2 10 and 11a-d is the C4H triazine ring proton signal, which was recorded at 6.26-6.48 ppm and shifted upfield at 3-4 ppm compared to triazines 4 and 5. The 13 C NMR spectra of derivatives 10 and 11a-d contain a signal of C4 at 67.7-70.1 ppm, in agreement with their sp 3 hybrid state. The NH proton displayed a broad singlet at 9.77-9.78 ppm (for DATs-NMe2 11a,c,d in CDCl3) and 11.98 and 12.10 ppm (for DATs-NMe2 10 and 11b in DMSO-d6), which disappeared when CD3COOD was added.
To refine the molecular structure, DAT-NMe2 11b was studied via single-crystal Xray diffraction analysis. Single crystals of DAT-NMe2 11b were grown from their diluted solutions in acetone via the slow evaporation of the solvent. The molecular structures of the compound are shown in Figures 1 and S1. Compound 11b crystallizes into the centrosymmetric triclinic space group P-1. The asymmetric unit contains two independent molecules, the structure of which is unequal both in bond length and angle values due to the rotation around the C8-C12 bond, the linked bicyclic core, and the isoxazole ring ( Figure  1a). Selected bond lengths and torsion angles are presented in Table S1. The structures in Figure 1 distinctly demonstrate different deviations of the isoxazole ring in different molecules from the bicyclic core, as well as deviations of the phenyl ring plane from the isoxazole ring. The bond lengths are slightly different in the two neighboring molecules; however, their values are lower than the standard ones, indicating that there is a conjugation The isolated compounds 10 and 11a-d were characterized by 1 H, 13 C (BB) and HMBC NMR spectra, and by HRESMS and XRD (Supplementary Materials). The 1 H NMR spectra of DATs-NMe 2 10 and 11a-d contain the signals of all proton-containing groups (Figures S1-S5). The most important signal of the 1 H NMR spectra of DATs-NMe 2 10 and 11a-d is the C4H triazine ring proton signal, which was recorded at 6.26-6.48 ppm and shifted upfield at 3-4 ppm compared to triazines 4 and 5. The 13 C NMR spectra of derivatives 10 and 11a-d contain a signal of C4 at 67.7-70.1 ppm, in agreement with their sp 3 hybrid state. The NH proton displayed a broad singlet at 9.77-9.78 ppm (for DATs-NMe 2 11a,c,d in CDCl 3 ) and 11.98 and 12.10 ppm (for DATs-NMe 2 10 and 11b in DMSO-d 6 ), which disappeared when CD 3 COOD was added.
To refine the molecular structure, DAT-NMe 2 11b was studied via single-crystal X-ray diffraction analysis. Single crystals of DAT-NMe 2 11b were grown from their diluted solutions in acetone via the slow evaporation of the solvent. The molecular structures of the compound are shown in Figure 1 and Figure S1. Compound 11b crystallizes into the centrosymmetric triclinic space group P-1. The asymmetric unit contains two independent molecules, the structure of which is unequal both in bond length and angle values due to the rotation around the C8-C12 bond, the linked bicyclic core, and the isoxazole ring ( Figure 1a). Selected bond lengths and torsion angles are presented in Table S1. The structures in Figure 1 distinctly demonstrate different deviations of the isoxazole ring in different molecules from the bicyclic core, as well as deviations of the phenyl ring plane from the isoxazole ring. The bond lengths are slightly different in the two neighboring molecules; however, their values are lower than the standard ones, indicating that there is a conjugation within the molecule structure. Using a Mercury software package, several short intramolecular contacts were found in the crystal of compound 11b (Figure 1b). The hydrogen bond involving the atom O4 and N6H hydrogen (l = 2.489 Å, R VdW = 2.6 Å) and the two noncovalent bonds formed by atoms O2-N7 (l = 2.909 Å, R VdW = 2.9 Å) and N3-O5 (l = 2.808 Å, R VdW = 2.9 Å) enhance the rigidity of the molecule skeleton ( Figure 1b). A packing structure is formed by the set of parallel layers (Figure 1d,e). The neighboring molecules in the layer form five bonds that connect them together firmly (Figure 1c). The strongest specific bond is the NH-O hydrogen bond, with a length of 1.905 Å. Table S1 shows that the bicyclic core and isoxazole ring in molecule A of the DAT-NMe 2 11b structure are planar, while the phenyl ring deviates at 27.7(5) • . The other molecule demonstrates deviations between the isoxazole and imidazotriazine plains at 57.0 • , while the phenyl ring rejects the isoxazole at 154.5(5) • . within the molecule structure. Using a Mercury software package, several short intramolecular contacts were found in the crystal of compound 11b (Figure 1b). The hydrogen bond involving the atom O4 and N6H hydrogen (l = 2.489 Å, RVdW = 2.6 Å) and the two noncovalent bonds formed by atoms O2-N7 (l = 2.909 Å, RVdW = 2.9 Å) and N3-O5 (l = 2.808 Å, RVdW = 2.9 Å) enhance the rigidity of the molecule skeleton ( Figure 1b). A packing structure is formed by the set of parallel layers (Figure 1d,e). The neighboring molecules in the layer form five bonds that connect them together firmly (Figure 1c). The strongest specific bond is the NH-O hydrogen bond, with a length of 1.905 Å. Table S1 shows that the bicyclic core and isoxazole ring in molecule A of the DAT-NMe2 11b structure are planar, while the phenyl ring deviates at 27.7(5)°. The other molecule demonstrates deviations between the isoxazole and imidazotriazine plains at 57.0°, while the phenyl ring rejects the isoxazole at 154.5(5)°. The distance between layers in the packing was 3.334-3.395 Å (Figure 1d). NMe2 and alkoxycarbonyl groups, as well as phenyl rings, push the layers apart, preventing the formation of stronger contacts. Therefore, the planes of the cyclic fragments can only achieve the partial perturbation of orbitals in parallel layers, preventing strong π‧‧‧π stacking. These spatial peculiarities lead to the formation of a specific 3D structure (Figures 1e and S1). The distance between layers in the packing was 3.334-3.395 Å (Figure 1d). NMe 2 and alkoxycarbonyl groups, as well as phenyl rings, push the layers apart, preventing the formation of stronger contacts. Therefore, the planes of the cyclic fragments can only achieve the partial perturbation of orbitals in parallel layers, preventing strong π· · · π stacking. These spatial peculiarities lead to the formation of a specific 3D structure ( Figure 1e and Figure S1).
It was found that the stirring of the DAT-NMe 2 10, 11a-c in MeOH or in a wet solvent induces the replacement of the NMe 2 group with the MeO or OH group, respectively, and the formation of DATs-OH 12 and 13a and DATs-OMe 14a,b with a good yield (Scheme 4). The reaction proceeds better in the presence of acetic acid or when being heated.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 19 It was found that the stirring of the DAT-NMe2 10, 11a-c in MeOH or in a wet solvent induces the replacement of the NMe2 group with the MeO or OH group, respectively, and the formation of DATs-OH 12 and 13a and DATs-OMe 14a,b with a good yield (Scheme 4). The reaction proceeds better in the presence of acetic acid or when being heated.  The 'one-pot' cycloaddition of 5-diazoimidazole 7b to enamine 3d in wet 1,4-dioxane and further hydrolysis allows us to obtain DAT-OH 13b in a 48% yield (Scheme 5). Thus, this technique can be used for the straightforward synthesis of 4-OH and 4-OMe derivatives of DATs. The prepared DATs-OH 12, 13a,b and DATs-OMe 14a,b were characterized by 1 H, 13 C NMR (including HSQC and HMBC for DAT 13b) (Figures S6-S10), HRESMS, and XRD data. The main sets of signals observed in the 1 H and 13 C spectra are similar to the spectra for DAT-NMe2 10 and 11. The exception is the appearance of the MeO group's singlet at 3.14-3.22 ppm for compounds 14a,b and a doublet of OH (at 6.75-6.96 ppm) and C4H (at 7.71 and 7.80 ppm) with 4 J = 7.8-8. 5 Hz instead of a NMe2 proton singlet. Mass spectra (HRESMS) analysis showed the correct ion peaks [M+H] + suggested by the molecular formulas.
The structure of DAT-OH 13b was confirmed by the XRD data from a single crystal growing in ethanol ( Figure 2). The structure completely agrees with the physical and spectral data. The unit includes one molecule with four intramolecular noncovalent bonds ( Figure 2). This makes the compound structure flatter than molecule 11b and shows a more ordered structure in the package (Figure 2b). This molecular architecture is supported by the many intermolecular hydrogen bonds between the two neighboring molecules in the layers (Figure 2c), taking a zigzag shape (Figure 2d). The 'one-pot' cycloaddition of 5-diazoimidazole 7b to enamine 3d in wet 1,4-dioxane and further hydrolysis allows us to obtain DAT-OH 13b in a 48% yield (Scheme 5). Thus, this technique can be used for the straightforward synthesis of 4-OH and 4-OMe derivatives of DATs.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 19 It was found that the stirring of the DAT-NMe2 10, 11a-c in MeOH or in a wet solvent induces the replacement of the NMe2 group with the MeO or OH group, respectively, and the formation of DATs-OH 12 and 13a and DATs-OMe 14a,b with a good yield (Scheme 4). The reaction proceeds better in the presence of acetic acid or when being heated. The 'one-pot' cycloaddition of 5-diazoimidazole 7b to enamine 3d in wet 1,4-dioxane and further hydrolysis allows us to obtain DAT-OH 13b in a 48% yield (Scheme 5). Thus, this technique can be used for the straightforward synthesis of 4-OH and 4-OMe derivatives of DATs. The prepared DATs-OH 12, 13a,b and DATs-OMe 14a,b were characterized by 1 H, 13 C NMR (including HSQC and HMBC for DAT 13b) (Figures S6-S10), HRESMS, and XRD data. The main sets of signals observed in the 1 H and 13 C spectra are similar to the spectra for DAT-NMe2 10 and 11. The exception is the appearance of the MeO group's singlet at 3.14-3.22 ppm for compounds 14a,b and a doublet of OH (at 6.75-6.96 ppm) and C4H (at 7.71 and 7.80 ppm) with 4 J = 7.8-8. 5 Hz instead of a NMe2 proton singlet. Mass spectra (HRESMS) analysis showed the correct ion peaks [M+H] + suggested by the molecular formulas.
The structure of DAT-OH 13b was confirmed by the XRD data from a single crystal growing in ethanol ( Figure 2). The structure completely agrees with the physical and spectral data. The unit includes one molecule with four intramolecular noncovalent bonds ( Figure 2). This makes the compound structure flatter than molecule 11b and shows a more ordered structure in the package (Figure 2b). This molecular architecture is supported by the many intermolecular hydrogen bonds between the two neighboring molecules in the layers (Figure 2c), taking a zigzag shape (Figure 2d). The prepared DATs-OH 12, 13a,b and DATs-OMe 14a,b were characterized by 1 H, 13 C NMR (including HSQC and HMBC for DAT 13b) (Figures S6-S10), HRESMS, and XRD data. The main sets of signals observed in the 1 H and 13 C spectra are similar to the spectra for DAT-NMe 2 10 and 11. The exception is the appearance of the MeO group's singlet at 3.14-3.22 ppm for compounds 14a,b and a doublet of OH (at 6.75-6.96 ppm) and C4H (at 7.71 and 7.80 ppm) with 4 J = 7.8-8. 5 Hz instead of a NMe 2 proton singlet. Mass spectra (HRESMS) analysis showed the correct ion peaks [M+H] + suggested by the molecular formulas.
The structure of DAT-OH 13b was confirmed by the XRD data from a single crystal growing in ethanol ( Figure 2). The structure completely agrees with the physical and spectral data. The unit includes one molecule with four intramolecular noncovalent bonds ( Figure 2). This makes the compound structure flatter than molecule 11b and shows a more ordered structure in the package (Figure 2b). This molecular architecture is supported by the many intermolecular hydrogen bonds between the two neighboring molecules in the layers (Figure 2c), taking a zigzag shape (Figure 2d). Molecules 2023, 28, x FOR PEER REVIEW 6 of 19 The attempts to aromatize DATs-NMe2 10, 11a to azolo[5,1-c][1,2,4]triazines, as occurred in the reaction of azole diazonium salts with the enamines [25], by the addition of glacial AcOH, as well as catalytic amounts of H2SO4 or equimolar amounts of HBF4, failed. As a result, hydrolysis to the corresponding non-aromatic of DAT-OH 12, 13a proceeded (Scheme 6).

Spectroscopic Properties in a Chloroform Solution
The structures of the synthesized compound do not have an extended π-conjugated framework because the two heteroaromatic cycles (imidazole or pyrazole) are divided by a non-aromatic dihydro-1,2,4-triazine cycle and linked via a σ bond with another heteroaromatic (isoxazole). The lateral substituents are both electron-withdrawing (COOR) and electron-donating groups (NMe2, OMe, OH at C4 atom of the dihydrotriazine ring and The attempts to aromatize DATs-NMe 2 10, 11a to azolo[5,1-c][1,2,4]triazines, as occurred in the reaction of azole diazonium salts with the enamines [25], by the addition of glacial AcOH, as well as catalytic amounts of H 2 SO 4 or equimolar amounts of HBF 4 , failed. As a result, hydrolysis to the corresponding non-aromatic of DAT-OH 12, 13a proceeded (Scheme 6).  The attempts to aromatize DATs-NMe2 10, 11a to azolo[5,1-c][1,2,4]triazines, as occurred in the reaction of azole diazonium salts with the enamines [25], by the addition of glacial AcOH, as well as catalytic amounts of H2SO4 or equimolar amounts of HBF4, failed. As a result, hydrolysis to the corresponding non-aromatic of DAT-OH 12, 13a proceeded (Scheme 6).

Spectroscopic Properties in a Chloroform Solution
The structures of the synthesized compound do not have an extended π-conjugated framework because the two heteroaromatic cycles (imidazole or pyrazole) are divided by a non-aromatic dihydro-1,2,4-triazine cycle and linked via a σ bond with another heteroaromatic (isoxazole). The lateral substituents are both electron-withdrawing (COOR) and electron-donating groups (NMe2, OMe, OH at C4 atom of the dihydrotriazine ring and  The structures of the synthesized compound do not have an extended π-conjugated framework because the two heteroaromatic cycles (imidazole or pyrazole) are divided by a non-aromatic dihydro-1,2,4-triazine cycle and linked via a σ bond with another heteroaromatic (isoxazole). The lateral substituents are both electron-withdrawing (COOR) and electron-donating groups (NMe 2 , OMe, OH at C4 atom of the dihydrotriazine ring and the Me group in isoxazole). The XRD data revealed the complicated character of the 3D structure, where the isoxazole rings of some molecules of DAT-NMe 2 11b can form a conjugation with the bicyclic core ( Figure 1). However, other molecules are strongly repelled by this ring from the central core's plane and therefore have slight conjugation within the molecule. The abundance of heteroatoms in the cyclic fragments and lateral substitutions with mobile electrons, and the presence of NH and OH groups, can provide DATs with specific photophysical properties, such as sensitivity to the microenvironment and the ability to induce differences in the investigated set of compounds.
DATs are soluble in organic solvents. Only hydroxy derivative 13a shows limited solubility in non-polar solvents, while DAT-OH 13b is insoluble in toluene, in contrast to its NMe 2 or OMe congeners. DAT solutions are colorless; however, they exhibit blue or blue-green fluorescence upon UV irradiation. The absorption spectra of DATs-NMe 2 10 and 11a-c,e, DATs-OH 12, 13a,b, and DATs-OMe 14a,b show the same absorption spectra profile, with one band in the 321-384 nm range (Table 1 and Figure 3a). Emission maxima of DATs-NMe 2 are in the range of 433 to 488 nm (Table 1 and Figure 3b). DAT-NMe 2 10 with pyrazole in the bicyclic core displays significant hypsochromic shifts in the absorption and emission maxima (1205-3899 cm −1 and 1863-2603 cm −1 , respectively) ( Figure S3). by this ring from the central core's plane and therefore have slight conjugation within the molecule. The abundance of heteroatoms in the cyclic fragments and lateral substitutions with mobile electrons, and the presence of NH and OH groups, can provide DATs with specific photophysical properties, such as sensitivity to the microenvironment and the ability to induce differences in the investigated set of compounds.
DATs are soluble in organic solvents. Only hydroxy derivative 13a shows limited solubility in non-polar solvents, while DAT-OH 13b is insoluble in toluene, in contrast to its NMe2 or OMe congeners. DAT solutions are colorless; however, they exhibit blue or bluegreen fluorescence upon UV irradiation. The absorption spectra of DATs-NMe2 10 and 11a-c,e, DATs-OH 12, 13a,b, and DATs-OMe 14a,b show the same absorption spectra profile, with one band in the 321-384 nm range (Table 1 and Figure 3a). Emission maxima of DATs-NMe2 are in the range of 433 to 488 nm (Table 1 and Figure 3b). DAT-NMe2 10 with pyrazole in the bicyclic core displays significant hypsochromic shifts in the absorption and emission maxima (1205-3899 cm −1 and 1863-2603 cm −1 , respectively) ( Figure S3).
The DAT quantum yields (QYs) vary from 3.4 to 33.3%. DAT-NMe2 10 shows the lowest QY in a chloroform solution, while DAT-OH 13b is not fluorescent at all ( Figure  S3). It is worth emphasizing the significant Stokes shift of DATs 11a-d and 13 (Table 1), demonstrating the large differences between the ground (GS) and excited (ES) states in the electronic structure and revealing a partial intramolecular charge transfer (ICT) upon vertical excitation. The phenyl ring at the C5 isoxazole does not increase the ICT, since the maxima absorption and emission are closed to the analogous structure, bearing a Me substituent. This means that the rotation of this structure fragment prevents it from participating in the molecule's conjugation channel. These conclusions are in agreement with the XRD data analysis (vide supra). It should be mentioned that the DATs 10-14 displayed significant hypsochromic shifts in the absorption and emission maxima in comparison with the maxima of similar derivatives 9 (Scheme 1), bearing a thiadiazole moiety at the С4 atom of the azolotriazine core [38]. However, the DATs 9 molar absorption coefficients are 1.4 fold and QYs 2.0-3.8 fold lower than DATs' 10-14 absorption and emission intensity characteristics. Thus, this position of the fluorophore molecule is effective for the tuning of its electronic state and photophysical properties [48]. The DAT quantum yields (QYs) vary from 3.4 to 33.3%. DAT-NMe 2 10 shows the lowest QY in a chloroform solution, while DAT-OH 13b is not fluorescent at all ( Figure  S3). It is worth emphasizing the significant Stokes shift of DATs 11a-d and 13 (Table 1), demonstrating the large differences between the ground (GS) and excited (ES) states in the electronic structure and revealing a partial intramolecular charge transfer (ICT) upon vertical excitation. The phenyl ring at the C5 isoxazole does not increase the ICT, since the maxima absorption and emission are closed to the analogous structure, bearing a Me substituent. This means that the rotation of this structure fragment prevents it from participating in the molecule's conjugation channel. These conclusions are in agreement with the XRD data analysis (vide supra).
It should be mentioned that the DATs 10-14 displayed significant hypsochromic shifts in the absorption and emission maxima in comparison with the maxima of similar derivatives 9 (Scheme 1), bearing a thiadiazole moiety at the C4 atom of the azolotriazine core [38]. However, the DATs 9 molar absorption coefficients are 1.4 fold and QYs 2.0-3.8 fold lower than DATs' 10-14 absorption and emission intensity characteristics. Thus, this position of the fluorophore molecule is effective for the tuning of its electronic state and photophysical properties [48].  10, 11a-c,e, 13a,b, and 14a,b in CHCl 3 (c = 5 × 10 −5 M for adsorption and c = 5 × 10 −6 M for emission).

Entry
Compd. The fluorescence lifetimes of the DATs were measured in chloroform at room temperature using the time-correlated single photon counting technique ( Table 2). All decay ( Figure  S4) can be fitted well with the double exponential decay function, exhibiting the coexistence of two fluorescent species in the solvent. DAT-OH 13a demonstrated the longest fluorescence lifetime, followed by DAT-OMe 14b and DAT-NMe 2 11c. DAT-NMe 2 10 had the shortest fluorescence lifetime and the highest velocity of non-radiative energy dissipation. Table 2. The fluorescence lifetime (<τ> f ) and radiative (k r ) and non-radiative (k nr ) decay rate constants of DATs 10a, 11a-c, 13a, and 14a,b solutions in CHCl 3 .

The Solvatochromic Behaviour of DATs
The UV-Vis absorption and fluorescence spectra of DATs 10 and 11 in 10 solvents are shown in Figure 4 and Table S4. The obtained results demonstrate the solvent's effect on the optical properties of the investigated compounds. The absorption maxima change within a small range (5-7 nm), while the molar extinction coefficient diminishes significantly, passing from toluene to the DMSO. The most unexpected result is the difference in the photophysical properties of DAT-NMe 2 10 and DAT-NMe 2 11c, 13a, and 14b. DAT-NMe 2 10 exhibited emissions only in protic solvents (EtOH, MeOH, ethylene glycol (EG)) and chloroform. This clearly indicates that solvents capable of forming specific interactions with the dye can promote fluorescence. Moreover, the greater the redshift of the emission maximum, the stronger these interactions are. Therefore, the largest shift is observed in the DMSO-water mixture (v/v, 1/9), while the smallest is registered in chloroform. The QY has rather random values, ranging from 6 to 15%. Ethylene glycol's influence is not only as a protic solvent, but also as a viscous one. Therefore, the QY increased compared to methanol 2.5-fold.
The absorption maxima are weakly dependent on the nature of the solvent, unlike DAT-NMe 2 10. DATs 11c, 13a, and 14b obviously exhibit positive solvato(fluoro)chromism and a red shift in the polar solvents. For example, in DMSO, these displacements are 1156, 706, and 1057 cm −1 for DATs 11c, 13a, 14b, respectively, passing from non-polar toluene, or THF for DAT 13a. The best QYs were registered in non-polar toluene for 11c and 14b and THF for compound 13a. In contrast to DAT-NMe 2 10, the QYs of the compounds 11c, 13a, and 14b decrease in alcohol in comparison with both polar and non-polar solvents. The Stokes shift is large, ranging within 7295-8783 cm −1 (with the maximum value provided by acetonitrile for DAT-NMe 2 11c).
Molecules 2023, 28, x FOR PEER REVIEW 9 of 19  The obtained results demonstrate the significant influence of the nature of the solvent used on the DATs' fluorescence characteristics. Therefore, the solvatochromic behavior of DATs 11c, 13a, and 14b was analyzed using Lippert-Mataga [49][50][51] (Equation (S1)) and Dimroth-Reichardt (E T (30)), (Equation (S6)) [52][53][54][55] solvent polarity plots ( Figure 5). The Lippert-Mataga Equation (S1) is based on the correlation of the energy difference between the ground and excited states (Stokes shift) and the solvent's orientation polarizability (∆f). This expression of the Stokes shift only takes into account dipole-dipole interaction. The solution's polarizability and the influence of specific interactions are neglected.
ism and a red shift in the polar solvents. For example, in DMSO, these displacements are 1156, 706, and 1057 cm −1 for DATs 11c, 13a, 14b, respectively, passing from non-polar toluene, or THF for DAT 13a. The best QYs were registered in non-polar toluene for 11c and 14b and THF for compound 13a. In contrast to DAT-NMe2 10, the QYs of the compounds 11c, 13a, and 14b decrease in alcohol in comparison with both polar and non-polar solvents. The Stokes shift is large, ranging within 7295-8783 cm −1 (with the maximum value provided by acetonitrile for DAT-NMe2 11c).
The obtained results demonstrate the significant influence of the nature of the solvent used on the DATs' fluorescence characteristics. Therefore, the solvatochromic behavior of DATs 11c, 13a, and 14b was analyzed using Lippert-Mataga [49][50][51] (Equation (S1)) and Dimroth-Reichardt (ET(30)), Equation (S6)) [52][53][54][55] solvent polarity plots ( Figure 5). The Lippert-Mataga Equation (S1) is based on the correlation of the energy difference between the ground and excited states (Stokes shift) and the solvent's orientation polarizability (∆f). This expression of the Stokes shift only takes into account dipole-dipole interaction. The solution's polarizability and the influence of specific interactions are neglected. The correlation coefficients obtained from linear correlations (R 2 ) were estimated using the maximum number of solvents with the aim of finding the best linearity. The values deviating from linearity were subtracted from the plots. As a result, good linearity (R 2 = 0.89-0.96) was obtained with the exception of MeCN and EG for DAT-NMe2 11c and THF and DMF for 13a and 14b. The Lippert-Mataga plot shows good linearity, indicating an increase in the solvatochromic shift as the solvent polarity increases (Figure 5a). The positive slope obtained for each of the DATs under consideration exhibits a larger excitedstate dipole moment than the ground-state dipole moment [49]. It is suggested that the dielectric interaction of the surrounding solvents is very responsible for the observed spectral shifts of the studied fluorophores. The slope values demonstrate that DAT-NMe2 11c is significantly less dependent on the solvent orientation polarizability (∆f) than DATs 13a and 14b. Therefore, a dipole-dipole interaction between the solute and the solvents for compound 11c is less important than for compounds 14b and 13a.
The The correlation coefficients obtained from linear correlations (R 2 ) were estimated using the maximum number of solvents with the aim of finding the best linearity. The values deviating from linearity were subtracted from the plots. As a result, good linearity (R 2 = 0.89-0.96) was obtained with the exception of MeCN and EG for DAT-NMe 2 11c and THF and DMF for 13a and 14b. The Lippert-Mataga plot shows good linearity, indicating an increase in the solvatochromic shift as the solvent polarity increases (Figure 5a). The positive slope obtained for each of the DATs under consideration exhibits a larger excitedstate dipole moment than the ground-state dipole moment [49]. It is suggested that the dielectric interaction of the surrounding solvents is very responsible for the observed spectral shifts of the studied fluorophores. The slope values demonstrate that DAT-NMe 2 11c is significantly less dependent on the solvent orientation polarizability (∆f) than DATs 13a and 14b. Therefore, a dipole-dipole interaction between the solute and the solvents for compound 11c is less important than for compounds 14b and 13a.
The advantage of the Dimroth-Reichardt method over the Lippert-Mataga method is the incorporation of both the solvent polarity and hydrogen bonding in the solvent parameter. The Dimroth-Reihardt plot of the Stokes shift for DAT-NMe 2 11c via the ET(30) solvent polarity parameter (Figure 5b) displays a better correlation (R 2 = 0.93) than that proposed by the Lippert-Mataga equation (excepting MeCN and EG). The Dimroth-Reichardt plots for DATs 13a and 14b showed a slight decrease in the linearity obtained, with the exception of MeCN and EtOAc (R 2 = 0.91 and 0.92, respectively). Moreover, the slope of DAT-NMe 2 11c is higher than those of DATs 13a and 14b, which were obtained according to the Dimroth-Reichardt correlation. This result confirms the conclusions stated on the basis of the Lippert-Mataga equation. Hydrogen bonding is very important for the solvatochromic behaviour of DAT-NMe 2 11c. Compound 11c is more sensitive to the specific interaction with solvents than 13a and 14b.

Spectroscopic Properties in a Solid State
The fluorescence of the DAT powders was measured using an integrating sphere. The absolute QYs of the investigated DATs are in the range of 9.8 to 98.7% ( Figure 6 and Table 3): most of them are higher than the QY in CHCl 3 (1.2-3.0-fold). The maximum emission wavelengths in the solid state are very close to those obtained in chloroform for DAT 11a, 13a, and 14a: they are redshifted by 1300 cm −1 and 722 cm −1 for DAT-NMe 2 11c and DAT-OMe 14b, respectively. Compounds 11c and 14b exhibited the lowest QYs among the compounds demonstrating fluorescence in a solid state. In addition, their structure has the same substituents, a decorated heterocycle ring in the molecular structure: they are differentiated only by NMe 2 and OMe groups on the C4 atom.
with the exception of MeCN and EtOAc (R 2 = 0.91 and 0.92, respectively). Moreover, the slope of DAT-NMe2 11c is higher than those of DATs 13a and 14b, which were obtained according to the Dimroth-Reichardt correlation. This result confirms the conclusions stated on the basis of the Lippert-Mataga equation. Hydrogen bonding is very important for the solvatochromic behaviour of DAT-NMe2 11c. Compound 11c is more sensitive to the specific interaction with solvents than 13a and 14b.

Spectroscopic Properties in a Solid State
The fluorescence of the DAT powders was measured using an integrating sphere. The absolute QYs of the investigated DATs are in the range of 9.8 to 98.7% ( Figure 6 and Table  3): most of them are higher than the QY in CHCl3 (1.2-3.0-fold). The maximum emission wavelengths in the solid state are very close to those obtained in chloroform for DAT 11a, 13a, and 14a: they are redshifted by 1300 cm −1 and 722 cm −1 for DAT-NMe2 11с and DAT-OMe 14b, respectively. Compounds 11c and 14b exhibited the lowest QYs among the compounds demonstrating fluorescence in a solid state. In addition, their structure has the same substituents, a decorated heterocycle ring in the molecular structure: they are differentiated only by NMe2 and OMe groups on the C4 atom.

Study of DATs' Aggregation-Induced Emission (AIE) and Aggregation-Induced Enhancement (AIEE)
The behavior of a fluorophore in aquatic environments plays an important role in determining the scope of the application, including such important fields as biology, medicine, and ecology [56][57][58][59][60]. Neutral organic dyes and fluorophores are poorly soluble in water and aqueous media. When using binary mixtures of an organic solvent with water, they are able to form suspensions. This process can cause two opposite photophysical behaviors: aggregation-caused fluorescence quenching (ACQ) or aggregation-induced   11a,c, 13a, and 14a,

Study of DATs' Aggregation-Induced Emission (AIE) and Aggregation-Induced Enhancement (AIEE)
The behavior of a fluorophore in aquatic environments plays an important role in determining the scope of the application, including such important fields as biology, medicine, and ecology [56][57][58][59][60]. Neutral organic dyes and fluorophores are poorly soluble in water and aqueous media. When using binary mixtures of an organic solvent with water, they are able to form suspensions. This process can cause two opposite photophysical behaviors: aggregation-caused fluorescence quenching (ACQ) or aggregation-induced emission (AIE). The reason behind ACQ is the increased redistribution of energy between the excited and unexcited molecules and the emergence of new channels for the loss of excitation energy during the aggregation. However, some compounds have a specific molecular architecture that prevents this energy loss: this is due to the presence of structural elements that induce the restriction of intramolecular motion (RIM) [56][57][58]. This can include bulky groups, structural peculiarities that decrease intermolecular interaction, the donors and acceptors of hydrogen bonds, and the insertion of molecules of solvents into the molecular packing structure.
Preliminary experiments in a DMSO-H 2 O mixture (v/v, 1/9) (Table 4) 10, 11a,c, 13a, and 14a,   Thus, a sample of DAT-NMe 2 10 in a mixture of DMSO-H 2 O and THF-H 2 O became fluorescent when 20% of water was added. The fluorescence intensity decreases, but even at the ratio of DMSO-H 2 O and THF-H 2 O (v/v, 99:1), the QY holds at 6%. DAT-NMe 2 11c had the opposite behavior, as the QY in DMSO-H 2 O showed a sharp increase as the water content increased up to 10%; then, there was a gradual decrease to 6%. In the THF-H 2 O mixture, the QY of DAT-NMe 2 11c exhibited a sharp decrease (2-fold), and then the compounds held the fluorescence intensity at this level up to 50% water content and at the end slowly decreased. This research clearly shows that DAT-NMe 2 10 is a new AIE gene, has a strong effect, and can be used in aquatic environments.

Chemistry
1 H and 13 C NMR spectra were recorded on a Bruker Avance II 400 (400 and 100 MHz, respectively) spectrometer or a Bruker Avance NEO 600 (600 and 150 MHz, respectively) spectrometer, equipped with the broadband gradient Prodigy Cryoprobe and using DMSOd 6 or CDCl 3 as the solvent and TMS as an internal standard. 1 H NMR data are reported as the chemical shift in parts per million, multiplicity (s, singlet; br. s, broadened singlet; d, doublet; t, triplet; q, quartet; m, multiplet), coupling constant in hertz, and number of protons. The concerted application of 1 H-13 C 2D heteronuclear experiments HSQC and HMBC was used for distinguishing carbon and proton resonances. Mass spectra were recorded with a Shimadzu GCMS-QP 2010 "Ultra" (Kyoto, Japan) mass spectrometer using the electron impact (EI) ionization technique (40-200 • C, 70 eV). Spectra of exact mass were acquired on a quadrupole orthogonal acceleration time-of-flight mass spectrometer (Synapt G2 HDMS, Waters, Milford, MA, USA). Samples were infused at 3 uL/min and spectra were obtained in positive (or negative) ionization mode with a resolution of 15,000 (FWHM), using leucine enkephalin as a lock mass. The abbreviation [M] + refers to the molecular ion. Elemental analysis was performed on a PerkinElmer 2400 II CHNS-analyzer. All melting points were determined with a Stuart SMP3 apparatus.
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.

Photophysical Study
UV-Vis absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer (Kyoto, Japan). Fluorescence of the sample solutions was measured using a Hitachi F-7000 spectrophotometer (Tokyo, Japan). The absorption and emission spectra were recorded in toluene, dioxane, CH 2 Cl 2 , CHCl 3 , THF, EtOH, EtOAc, acetone, MeCN, DMF, and DMSO using 10.00 mm quartz cells. The excitation wavelength was at the absorption maxima. Atmospheric oxygen contained in solutions was not removed. Concentration of the compounds in the solution was 5.0 × 10 −5 M and 5.0 × 10 −6 M for absorption and fluorescence measurements, respectively. The relative fluorescence quantum yields (QY) were determined using quinine sulfate (c = 5 × 10 −5 M) in 0.1 M H 2 SO 4 as a standard (Φ F = 0.546).
AIE/AIEE phenomena study: A stock solution of the investigated compound in THF (or DMSO) (5 × 10 −4 M) was prepared and an aliquot (1.0 mL) of this solution transferred to a 25 mL volumetric flask. After addition of a calculated volume of pure solvent, water was added at once to prepare mixtures with water content in the range of 0-90 vol% and final concentration 5 × 10 −5 M. The UV-Vis and fluorescence measurements of the resultant solutions were then performed immediately. The absolute quantum yield for the solid-state and time-resolution study was recorded on a Horiba FlouroMax 4 Spectrofluorometer (Kyoto, Japan) with a Quanta-ϕ integrating sphere using the FluorEssence 3.5 Software (Kyoto, Japan).

Conclusions
In summary, we synthesized new assemblies of 1,4-dihydroazolotriazines and isoxazoles and characterized them using 1 H and 13 C NMR, HRESMS, and elemental analysis. The DATs obtained demonstrated the stability of their bicyclic systems, although C4-NMe 2 is easily substituted by the MeO and OH groups. The DATs demonstrate emissions in organic solvents ranging from 433 to 487 nm, with QYs of 6.1-33.3% and a high Stokes shift (up to150 nm). Studies of the XRD data of the two DAT patterns demonstrate their tendency to form supramolecular architectures due to the ability to form a large amount of intraand intermolecular NCIs. New non-aromatic azolotriazines demonstrated fluorescence in powder with a high quantum yield up to 95%.
DATs exhibited different sensitivities to the microenvironment depending on the structure of the azole ring involved in the central bicyclic scaffold. Imidazole derivatives 11 showed positive solvato(fluoro)chromism, with the best QYs in non-polar toluene (38%). The pyrazole derivative 10 only exhibited fluorescence in protic solvents and chloroform. Moreover, 1,4-dihydropyrasolotriazine had an excellent AIE effect in a DMSO-H 2 O and THF-H 2 O binary mixture with the increasing of QYs up to 16 and 13%, respectively. Analysis of the DAT behavior in different solvents with Lippert-Mataga and Dimroth-Reichardt plots established the strong influence of the nature of C4 substituents. Thus, the DATs' photophysical properties are very sensitive to structural fragments and can be tuned by various types of modification. Structural diversity can be provided by using different starting heterocyclic diazo compounds and enamines and employing the nucleophilic substitution of the NMe 2 group by numerous natural and bioactive compounds.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28073192/s1. 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy; UV-Vis and fluorescent spectra and photophysical characteristics. X-ray data of compounds 11b and 13b. These experiments were accomplished on the automated X-ray diffractometer «Xcalibur 3» with CCD detector on standard procedure (MoKα-irradiation, graphite monochromator, ω-scans with 1o step at T = 295(2) K). Empirical absorption correction was applied. The solution and refinement of the structures were accomplished with using Olex2 program package [64]. The structure was solved with the Superflip [65] structure solution program using Charge Flipping and refined by ShelXL by full-matrix least-squared method in the anisotropic approximation for non-hydrogen atoms [66]. The H-atoms at C-H bonds were placed in the calculated positions, the H-atoms at N-H bonds were refined independently in isotropic approximation. CCDC 2225550 for 11b and CCDC 2238176 for 13b can be obtained free of charge from the Cambridge Crystallographic Data Centre via link www.ccdc.cam.ac.uk/data_request/cif.