ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence

1H-Imidazole derivatives establish one of the iconic classes of ESIPT-capable compounds (ESIPT = excited state intramolecular proton transfer). This work presents the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LOH,OH) as the first example of ESIPT-capable imidazole derivatives wherein the imidazole moiety simultaneously acts as a proton acceptor and a proton donor. The reaction of LOH,OH with chloroacetone leads to the selective reduction of the imidazolic OH group (whereas the phenolic OH group remains unaffected) and to the isolation of 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LH,OH), a monohydroxy congener of LOH,OH. Both LOH,OH and LH,OH demonstrate luminescence in the solid state. The number of OH···N proton transfer sites in these compounds (one for LH,OH and two for LOH,OH) strongly affects the luminescence mechanism and color of the emission: LH,OH emits in the light green region, whereas LOH,OH luminesces in the orange region. According to joint experimental and theoretical studies, the main emission pathway of both compounds is associated with T1 → S0 phosphorescence and not related to ESIPT. At the same time, LOH,OH also exhibits S1 → S0 fluorescence associated with ESIPT with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety, which is not possible for LH,OH due to the absence of the hydroxy group in the imidazole moiety.

In this manuscript, we report the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L OH,OH ) as the first example of imidazole derivatives wherein the central 1-hydroxy-1H-imidazole moiety simultaneously acts both as a proton acceptor and a proton donor (Scheme 2). Along with the synthesis of L OH,OH , we report the reaction of L OH,OH with chloroacetone leading to the selective formation of a corresponding 1H-imidazole derivative, 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L H,OH ) (Scheme 2), and proceeding without affecting the phenolic hydroxy group. Finally, we present the results of combined comparative experimental and theoretical studies of the emission of L OH,OH and L H,OH and the ESIPT photoreactions in both compounds. Scheme 1. The ESIPT and GSIPT processes in molecules featuring short intramolecular hydrogen bonds.
In this manuscript, we report the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5methyl-2-(pyridin-2-yl)-1H-imidazole (L OH,OH ) as the first example of imidazole derivatives wherein the central 1-hydroxy-1H-imidazole moiety simultaneously acts both as a proton acceptor and a proton donor (Scheme 2). Along with the synthesis of L OH,OH , we report the reaction of L OH,OH with chloroacetone leading to the selective formation of a corresponding 1H-imidazole derivative, 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L H,OH ) (Scheme 2), and proceeding without affecting the phenolic hydroxy group. Finally, we present the results of combined comparative experimental and theoretical studies of the emission of L OH,OH and L H,OH and the ESIPT photoreactions in both compounds.

Synthesis of 1-Hydroxy-4-(2-Hydroxyphenyl
The ESIPT-capable imidazole-based compounds L OH,OH and L H,OH were synthesized using the following reactions (Scheme 3). The first step, i.e., the nitrosation reaction, required the protection of the hydroxy group in ortho hyroxypropiophenone with the benzoyl group [88]. After this, the monoxime B was prepared by the nitrosation of 2-benzoyloxypropiophenone (A) with isopropyl nitrite according to the procedure close to the one reported by Mason [88]. The second step was the construction of the 1-hydroxy-1H-imidazole moiety. The most convenient and widespread method for the synthesis of 1-hydroxy-1H-imidazoles is the condensation of monoxime diketones with aldehydes and ammonia or ammonium acetate [89]. The condensation of the monoxime B with pyridinecarboxaldehyde and ammonia (cf. [83]) led to the isolation of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L OH,OH ). Importantly, the benzoyl protecting group removal occurred at this step along with the simultaneous formation of the imidazole ring. The last step was the conversion of the 1-hydroxy-1H-imidazole derivative L OH,OH to the 1H-imidazole L H,OH . For this conversion, along with various reducing agents (e.g., PCl3, (Ph)3P, trialkylphosphites, TiCl3, etc.), halogen-substituted compounds with electron-withdrawing groups (e.g., BrCH2CO2Me [90] and chloroacetone [91,92]) can be used. The interaction of 1-hydroxy-1H-imidazole with chloroacetone allows the reaction to be carried out under mild conditions through the intermediate formation of a chlorine atom substitution product, followed by its fragmentation to form reduced 1H-imidazole. Importantly, the reaction of L OH,OH with chloroacetone (cf. [93]) proceeded without affecting the phenolic hydroxy group, which greatly simplified the preparation of the 1H-imidazole L H,OH compound. Spectral and structural data for the compounds are given in Supplementary Materials.  The ESIPT-capable imidazole-based compounds L OH,OH and L H,OH were synthesized using the following reactions (Scheme 3). The first step, i.e., the nitrosation reaction, required the protection of the hydroxy group in ortho hyroxypropiophenone with the benzoyl group [88]. After this, the monoxime B was prepared by the nitrosation of 2benzoyloxypropiophenone (A) with isopropyl nitrite according to the procedure close to the one reported by Mason [88]. The second step was the construction of the 1-hydroxy-1H-imidazole moiety. The most convenient and widespread method for the synthesis of 1-hydroxy-1H-imidazoles is the condensation of monoxime diketones with aldehydes and ammonia or ammonium acetate [89]. The condensation of the monoxime B with pyridinecarboxaldehyde and ammonia (cf. [83]) led to the isolation of 1-hydroxy-4-(2hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L OH,OH ). Importantly, the benzoyl protecting group removal occurred at this step along with the simultaneous formation of the imidazole ring. The last step was the conversion of the 1-hydroxy-1H-imidazole derivative L OH,OH to the 1H-imidazole L H,OH . For this conversion, along with various reducing agents (e.g., PCl 3 , (Ph) 3 P, trialkylphosphites, TiCl 3 , etc.), halogen-substituted compounds with electron-withdrawing groups (e.g., BrCH 2 CO 2 Me [90] and chloroacetone [91,92]) can be used. The interaction of 1-hydroxy-1H-imidazole with chloroacetone allows the reaction to be carried out under mild conditions through the intermediate formation of a chlorine atom substitution product, followed by its fragmentation to form reduced 1H-imidazole. Importantly, the reaction of L OH,OH with chloroacetone (cf. [93]) proceeded without affecting the phenolic hydroxy group, which greatly simplified the preparation of the 1H-imidazole L H,OH compound. Spectral and structural data for the compounds are given in Supplementary Materials. The ESIPT-capable imidazole-based compounds L OH,OH and L H,OH were synthesized using the following reactions (Scheme 3). The first step, i.e., the nitrosation reaction, required the protection of the hydroxy group in ortho hyroxypropiophenone with the benzoyl group [88]. After this, the monoxime B was prepared by the nitrosation of 2-benzoyloxypropiophenone (A) with isopropyl nitrite according to the procedure close to the one reported by Mason [88]. The second step was the construction of the 1-hydroxy-1H-imidazole moiety. The most convenient and widespread method for the synthesis of 1-hydroxy-1H-imidazoles is the condensation of monoxime diketones with aldehydes and ammonia or ammonium acetate [89]. The condensation of the monoxime B with pyridinecarboxaldehyde and ammonia (cf. [83]) led to the isolation of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (L OH,OH ). Importantly, the benzoyl protecting group removal occurred at this step along with the simultaneous formation of the imidazole ring. The last step was the conversion of the 1-hydroxy-1H-imidazole derivative L OH,OH to the 1H-imidazole L H,OH . For this conversion, along with various reducing agents (e.g., PCl3, (Ph)3P, trialkylphosphites, TiCl3, etc.), halogen-substituted compounds with electron-withdrawing groups (e.g., BrCH2CO2Me [90] and chloroacetone [91,92]) can be used. The interaction of 1-hydroxy-1H-imidazole with chloroacetone allows the reaction to be carried out under mild conditions through the intermediate formation of a chlorine atom substitution product, followed by its fragmentation to form reduced 1H-imidazole. Importantly, the reaction of L OH,OH with chloroacetone (cf. [93]) proceeded without affecting the phenolic hydroxy group, which greatly simplified the preparation of the 1H-imidazole L H,OH compound. Spectral and structural data for the compounds are given in Supplementary Materials. The dihydroxy derivative, L OH,OH , crystallizes in the monoclinic space group P2 1 /c (Supplementary Materials, Table S1, Figures S9-S11). There are two crystallographically independent L OH,OH molecules in the crystal structure ( Figure 1). The 2-(pyridin-2yl)imidazole moiety in both independent molecules is practically planar with the torsions smaller than 1 • . On the other hand, the 4-(2-hydroxyphenyl) group deviates from the plane of the imidazole cycle by ca. 7 • in one and by ca. 12 • in another L OH,OH molecule. There are two short intramolecular O-H···N hydrogen bonds in each molecule with the O···N separations of 2.57-2.60 Å.

Tautomeric Forms of L H,OH and L OH,OH : An Introduction
L H,OH and L OH,OH can exist in various tautomeric forms. In this context, for the sake of clarity we introduce the following abbreviations of these forms for further discussions (Scheme 4). L OH,OH has two proton transfer sites and therefore can exist in four tautomeric forms: i) N,N-L OH,OH (no proton transferred, corresponds to the global energy minimum

Tautomeric Forms of L H,OH and L OH,OH : An Introduction
L H,OH and L OH,OH can exist in various tautomeric forms. In this context, for the sake of clarity we introduce the following abbreviations of these forms for further discussions (Scheme 4). L OH,OH has two proton transfer sites and therefore can exist in four tautomeric forms: i) N,N-L OH,OH (no proton transferred, corresponds to the global energy minimum

Absorption Properties of L H,OH and L OH,OH in MeCN
In acetonitrile, both L H,OH and L OH,OH absorb in the ultraviolet domain, with the most intense peak centered at 320 and 342 nm, respectively ( Figure 3). In order to test the relevance of the chosen theory level for quantum chemical computations, theoretical absorption spectra were calculated at the global energy minima of the ground state, S0 N,N (O Ph -H 0.988 Å, O Imid -H 1.010 Å, Table 1) for L OH,OH and S0 N (O Ph -H 0.980 Å) for L OH,OH . The energies and relative intensities of the calculated vertical singlet-to-singlet absorptions are in good agreement with the experimental data ( Figure 3), showing the relevance of the functional and basis set used in this study. The most intense experimental peak corresponds to the first vertical singlet-to-singlet transition (S0 → S1), computed at 336 nm for L H,OH and 348 nm for L OH,OH . In accordance with the experimental spectra, this transition indeed has the highest oscillator strength (ca. 0.5) among the other transitions. In terms of molecular orbitals, S0 → S1 is a HOMO → LUMO transition. For both L H,OH and L OH,OH , HOMO is distributed over hydroxyphenyl and imidazole moieties, while LUMO is located on imidazole and pyridine moieties ( Figure 3). Thus, the S0 → S1 absorption implies charge transfer from the hydroxyphenyl part of the molecule to the pyridine part. Despite there being no visual differences between the HOMO and LUMO of L H,OH and the HOMO and LUMO of L OH,OH , respectively, the most intensive absorption peak of L OH,OH is slightly redshifted compared with that of L H,OH , and the computations fully reproduce this trend. A series of higher lying singlet-to-singlet transitions form the high-energy absorption band centered at ca. 260 nm for both ESIPT-emitters ( Figure 3).

Absorption Properties of L H,OH and L OH,OH in MeCN
In acetonitrile, both L H,OH and L OH,OH absorb in the ultraviolet domain, with the most intense peak centered at 320 and 342 nm, respectively ( Figure 3). In order to test the relevance of the chosen theory level for quantum chemical computations, theoretical absorption spectra were calculated at the global energy minima of the ground state, S 0 Table 1) for L OH,OH and S 0 N (O Ph -H 0.980 Å) for L OH,OH . The energies and relative intensities of the calculated vertical singlet-to-singlet absorptions are in good agreement with the experimental data (Figure 3), showing the relevance of the functional and basis set used in this study. The most intense experimental peak corresponds to the first vertical singlet-to-singlet transition (S 0 → S 1 ), computed at 336 nm for L H,OH and 348 nm for L OH,OH . In accordance with the experimental spectra, this transition indeed has the highest oscillator strength (ca. 0.5) among the other transitions. In terms of molecular orbitals, S 0 → S 1 is a HOMO → LUMO transition. For both L H,OH and L OH,OH , HOMO is distributed over hydroxyphenyl and imidazole moieties, while LUMO is located on imidazole and pyridine moieties ( Figure 3). Thus, the S 0 → S 1 absorption implies charge transfer from the hydroxyphenyl part of the molecule to the pyridine part. Despite there being no visual differences between the HOMO and LUMO of L H,OH and the HOMO and LUMO of L OH,OH , respectively, the most intensive absorption peak of L OH,OH is slightly red-shifted compared with that of L H,OH , and the computations fully reproduce this trend. A series of higher lying singlet-to-singlet transitions form the high-energy absorption band centered at ca. 260 nm for both ESIPT-emitters ( Figure 3). Molecules 2023, 28, x FOR PEER REVIEW 6 of 20  a -θ1 is the dihedral angle between the planes of hydroxyphenyl and imidazole moieties. b -θ2 is the dihedral angle between the planes of pyridine and imidazole moieties. c -geometries that are close to the conical intersection between the S0 and S1 states.
It is noteworthy that, in addition to the global energy minimum S0 N,N on the PES of the ground state, L OH,OH has a local minimum S0 T,N (O Ph -H 0.992 Å, O Imid -H 1.595 Å, Figure  4, Table 1), and therefore its corresponding form T,N-L OH,OH can also absorb light. S0 T,N is thermodynamically less favorable than S0 N,N by ca. 17 kJ/mol and is separated from   a -θ 1 is the dihedral angle between the planes of hydroxyphenyl and imidazole moieties. b -θ 2 is the dihedral angle between the planes of pyridine and imidazole moieties. c -geometries that are close to the conical intersection between the S 0 and S 1 states.
It is noteworthy that, in addition to the global energy minimum S 0 N,N on the PES of the ground state, L OH,OH has a local minimum S 0 T,N (O Ph -H 0.992 Å, O Imid -H 1.595 Å, Figure 4, Table 1), and therefore its corresponding form T,N-L OH,OH can also absorb light.

Excitation and Emission Properties of L H,OH and L OH,OH
L H,OH and L OH,OH are non-luminescent in MeCN solution, indicating the possible predominance of various non-radiative deactivation pathways. In the solid state, L H,OH emits in the light green region (Figures 6 and 7). The broad unstructured luminescence band of L H,OH is located in the region 400-750 nm with a maximum at 546 nm. The intensity of this band depends on excitation wavelength: at λex = 400-420 nm, it is three times more intense than at λex = 280-360 nm. However, a change in the excitation energy does not lead to a shift of the emission maximum. L H,OH exhibits a monoexponential photoluminescence decay (Supplementary Materials, Figure S14), indicating that there is likely only one emission mechanism. The lifetime of molecules in the excited state (τ) is 1.10 μs (λex = 300 nm, λdet = 540 nm), so the observed emission is associated with phosphorescence, i.e., with a

Excitation and Emission Properties of L H,OH and L OH,OH
L H,OH and L OH,OH are non-luminescent in MeCN solution, indicating the possible predominance of various non-radiative deactivation pathways. In the solid state, L H,OH emits in the light green region (Figures 6 and 7). The broad unstructured luminescence band of L H,OH is located in the region 400-750 nm with a maximum at 546 nm. The intensity of this band depends on excitation wavelength: at λex = 400-420 nm, it is three times more intense than at λex = 280-360 nm. However, a change in the excitation energy does not lead to a shift of the emission maximum. L H,OH exhibits a monoexponential photoluminescence decay (Supplementary Materials, Figure S14), indicating that there is likely only one emission mechanism. The lifetime of molecules in the excited state (τ) is 1.10 μs (λex = 300 nm, λdet = 540 nm), so the observed emission is associated with phosphorescence, i.e., with a   (Figures 6 and 7). The broad unstructured luminescence band of L H,OH is located in the region 400-750 nm with a maximum at 546 nm. The intensity of this band depends on excitation wavelength: at λ ex = 400-420 nm, it is three times more intense than at λ ex = 280-360 nm. However, a change in the excitation energy does not lead to a shift of the emission maximum. L H,OH exhibits a monoexponential photoluminescence decay (Supplementary Materials, Figure S14), indicating that there is likely only one emission mechanism. The lifetime of molecules in the excited state (τ) is 1.10 µs (λ ex = 300 nm, λ det = 540 nm), so the observed emission is associated with phosphorescence, i.e., with a spin-forbidden triplet-to-singlet transition. The width of the phosphorescence band is associated with the vibrational satellite structure, which involves an interplay of several transitions from the lowest vibrational level of the excited state to various vibrational levels of the ground state.  L OH,OH demonstrates luminescence in the orange region (Figures 6 and 7). As for L H,OH , the emission spectrum is dominated by a broad band at 450-800 nm centered at 568 nm. In contrast to L H,OH , an additional low-energy shoulder at ca. 670 nm appears in the case of L OH,OH , which is responsible for the orange color of luminescence. The emission band is more or less equally intensive when excited at λex = 280-440 nm. The luminescence decay of L OH,OH is multiexponentional and more complex than for L H,OH : the long part of the photoluminescence decay reveals one lifetime in the microsecond range, τ = 1.05 μs (similar to L H,OH ), whereas the short part reveals two lifetimes in the nanosecond range, τ = 2 ns and τ = 21 ns (Supplementary Materials, Figure S15). Thus, L OH,OH shows two emission mechanisms, i.e., phosphorescence and fluorescence. The photoluminescence quantum yield for L H,OH and L OH,OH is less than 1% in the solid state.  L OH,OH demonstrates luminescence in the orange region (Figures 6 and 7). As for L H,OH , the emission spectrum is dominated by a broad band at 450-800 nm centered at 568 nm. In contrast to L H,OH , an additional low-energy shoulder at ca. 670 nm appears in the case of L OH,OH , which is responsible for the orange color of luminescence. The emission band is more or less equally intensive when excited at λex = 280-440 nm. The luminescence decay of L OH,OH is multiexponentional and more complex than for L H,OH : the long part of the photoluminescence decay reveals one lifetime in the microsecond range, τ = 1.05 μs (similar to L H,OH ), whereas the short part reveals two lifetimes in the nanosecond range, τ = 2 ns and τ = 21 ns (Supplementary Materials, Figure S15). Thus, L OH,OH shows two emission mechanisms, i.e., phosphorescence and fluorescence. The photoluminescence quantum yield for L H,OH and L OH,OH is less than 1% in the solid state. L OH,OH demonstrates luminescence in the orange region (Figures 6 and 7). As for L H,OH , the emission spectrum is dominated by a broad band at 450-800 nm centered at 568 nm. In contrast to L H,OH , an additional low-energy shoulder at ca. 670 nm appears in the case of L OH,OH , which is responsible for the orange color of luminescence. The emission band is more or less equally intensive when excited at λ ex = 280-440 nm. The luminescence decay of L OH,OH is multiexponentional and more complex than for L H,OH : the long part of the photoluminescence decay reveals one lifetime in the microsecond range, τ = 1.05 µs (similar to L H,OH ), whereas the short part reveals two lifetimes in the nanosecond range, τ = 2 ns and τ = 21 ns (Supplementary Materials, Figure S15). Thus, L OH,OH shows two emission mechanisms, i.e., phosphorescence and fluorescence. The photoluminescence quantum yield for L H,OH and L OH,OH is less than 1% in the solid state.
Before turning to calculations that will help us identify the emission pathways, it is worthwhile to make a visual inspection of the possible number and nature of the photoluminescence mechanisms by comparing the spectra of L H,OH and L OH,OH . As mentioned above, both compounds exhibit phosphorescence with similar lifetimes in the order of one microsecond. Owing to the close wavelength of the maxima of the most intense band (546 nm for L H,OH and 568 nm for L OH,OH ), we can assume that this band implies the same emission mechanism for both compounds. The shoulder appearing at ca. 670 nm in the case of L OH,OH may be responsible for the short lifetimes and can therefore be attributed to fluorescence. The absence of this shoulder for L H,OH may indicate that the fluorescence mechanism observed for L OH,OH cannot be realized for L H,OH . We hypothesize that this fluorescence mechanism is somehow related to the O Imid -H···N Py proton transfer site, which is absent for L H,OH .

Elucidation of the Fluorescence and Phosphorescence Mechanisms for L H,OH and L OH,OH
Geometry optimizations of the excited states were performed in order to establish the photoluminescence mechanisms for L H,OH and L OH,OH and to verify our predictions from the previous paragraph. The PEC of the first triplet excited state of L H,OH reveals two minima, T 1 N and T 1 T ( Figure 5). The T 1 N optimized geometry is characterized by a slightly enlarged O Ph -H distance (1.006 Å for T 1 N vs. 0.980 Å for S 0 N ) and a shortened O Ph ···N Imid hydrogen bond length (2.550 Å for T 1 N vs. 2.627 Å for S 0 N ) compared with the S 0 N relaxed geometry. The calculated T 1 N → S 0 N phosphorescence wavelength (578 nm) is in excellent agreement with the maximum of the intensive emission band (568 nm). According to the analysis of the frontier molecular orbitals, T 1 N → S 0 N is LUMO → HOMO transition ( Figure 8). LUMO is a π*-orbital that is equally located on pyridine and imidazole moieties, whereas HOMO is a π-orbital that is majorly located on hydroxyphenyl and imidazole parts of the molecule. Therefore, the observed T 1 N → S 0 N phosphorescence is associated with charge transfer from the pyridine moiety to the hydroxyphenyl moiety (this is directly opposite to the S 0 N → S 1 N absorption mechanism discussed above). Although the second minimum on the PEC of the T 1 state, T 1 T (O Ph -H 1.841 Å, Figure 5), is thermodynamically more stable than T 1 N by ca. 16 kJ/mol, the energy barrier separating T 1 N and T 1 T is as high as ca. 14 kJ/mol, which impedes efficient ESIPT in the triplet manifold. Furthermore, the computed T 1 T → S 0 T phosphorescence wavelength (1095 nm) is largely overestimated compared with the position of the phosphorescence band. Thus, we attribute the observed phosphorescence of L H,OH with τ = 1.05 µs to the T 1 N → S 0 N transition of the N-L H,OH form, which is not related to the ESIPT process.
Having established the phosphorescence mechanism (T 1 N → S 0 N ) for L H,OH , the following question arises: how can the molecules of L H,OH populate the T 1 state? Classically, in most compounds the triplet manifold is populated after S 0 → S 1 excitation followed by S 1 → T 1 intersystem crossing. Returning to our discussion of absorption properties, the S 0 N → S 1 N vertical absorption is computed at 336 nm for L H,OH (Figure 3). At the same time, the phosphorescence band of L H,OH in the region 450-750 nm is predominantly excited at λ ex = 400-420 nm. Obviously, such low energies cannot lead to the population of the S 1 state. Therefore, we suggest that in the case of L H,OH there is a direct population of the triplet manifold from the ground state, S 0 N → T 1 N , since only triplets can be populated with λ ex = 400-420 nm (λ calc. S0-T1 = 462 nm, λ calc. S0-T2 = 395 nm). However, the classical mechanism of populating the T 1 state (S 0 N → S 1 N → T 1 N ) is also feasible when molecules are excited with high energy quanta (λ ex < 336 nm).
In contrast to the triplet manifold, ESIPT is possible for the singlet manifold of L H,OH . After S 0 N → S 1 N excitation, the ESIPT process is barrierless in the S 1 state. There are no minima on the PEC of the first singlet excited state, as shown in Figure 5. A non-constrained geometry optimization of the S 1 state directly leads to a non-planar geometry near the conical intersection (CI) between the S 0 and S 1 states (Figure 9b). According to the literature, ESIPT is often coupled with the radiationless deactivation via twisted intramolecular charge transfer (TICT) states of a non-planar biradicaloid nature [83,85,[94][95][96][97][98][99]. This non-planarity arises from the twisting around a double-like bond between proton-donating and protonaccepting moieties (around the C Ph -C Imid bond in our case). Subsequent ultrafast internal conversion via S 0 /S 1 CI results in the non-radiative deactivation of the excited twisted phototautomer. Since L H,OH does not luminesce in solution and weakly luminesces in the solid state, we believe that this non-radiative deactivation is the predominant photophysical process for L H,OH , which is responsible for emission quenching. It should be noted that the precise geometry of the CI between the S 0 and S 1 states can only be optimized using ab initio methods such as CASSCF, CASPT2 or NEVPT2. However, our TDDFT optimization of the S 1 state leads to the oscillations around the CI geometry, which may serve as an indirect evidence of its existence. Figure 9b shows the geometry at the optimization step closest to the real CI geometry (with the lowest S 0 -S 1 energy gap of only 2.2 kJ/mol; the dihedral angle between the proton-donating hydroxyphenyl and proton-accepting imidazole moieties reaches 85 • at this geometry).

Elucidation of the Fluorescence and Phosphorescence Mechanisms for L H,OH and L OH,OH
Geometry optimizations of the excited states were performed in order to establish the photoluminescence mechanisms for L H,OH and L OH,OH and to verify our predictions from the previous paragraph. The PEC of the first triplet excited state of L H,OH reveals two minima, T1 N and T1 T ( Figure 5). The T1 N optimized geometry is characterized by a slightly enlarged O Ph -H distance (1.006 Å for T1 N vs. 0.980 Å for S0 N ) and a shortened O Ph ···N Imid hydrogen bond length (2.550 Å for T1 N vs. 2.627 Å for S0 N ) compared with the S0 N relaxed geometry. The calculated T1 N → S0 N phosphorescence wavelength (578 nm) is in excellent agreement with the maximum of the intensive emission band (568 nm). According to the analysis of the frontier molecular orbitals, T1 N → S0 N is LUMO → HOMO transition ( Figure  8). LUMO is a π*-orbital that is equally located on pyridine and imidazole moieties, whereas HOMO is a π-orbital that is majorly located on hydroxyphenyl and imidazole parts of the molecule. Therefore, the observed T1 N → S0 N phosphorescence is associated with charge transfer from the pyridine moiety to the hydroxyphenyl moiety (this is directly opposite to the S0 N → S1 N absorption mechanism discussed above). Although the second minimum on the PEC of the T1 state, T1 T (O Ph -H 1.841 Å, Figure 5), is thermodynamically more stable than T1 N by ca. 16 kJ/mol, the energy barrier separating T1 N and T1 T is as high as ca. 14 kJ/mol, which impedes efficient ESIPT in the triplet manifold. Furthermore, the computed T1 T → S0 T phosphorescence wavelength (1095 nm) is largely overestimated compared with the position of the phosphorescence band. Thus, we attribute the observed phosphorescence of L H,OH with τ = 1.05 μs to the T1 N → S0 N transition of the N-L H,OH form, which is not related to the ESIPT process. Having established the phosphorescence mechanism (T1 N → S0 N ) for L H,OH , the following question arises: how can the molecules of L H,OH populate the T1 state? Classically, in most compounds the triplet manifold is populated after S0 → S1 excitation followed by S1 → T1 intersystem crossing. Returning to our discussion of absorption properties, the S0 N → S1 N vertical absorption is computed at 336 nm for L H,OH (Figure 3). At the same time, the phosphorescence band of L H,OH in the region 450-750 nm is predominantly excited at λex = 400-420 nm. Obviously, such low energies cannot lead to the population of the S1 state. Therefore, we suggest that in the case of L H,OH there is a direct population of the triplet manifold from the ground state, S0 N → T1 N , since only triplets can be populated with λex = 400-420 nm (λcalc. S0-T1 = 462 nm, λcalc. S0-T2 = 395 nm). However, the classical mechanism of populating the T1 state (S0 N → S1 N → T1 N ) is also feasible when molecules are excited with high energy quanta (λex < 336 nm).
In contrast to the triplet manifold, ESIPT is possible for the singlet manifold of L H,OH . After S0 N → S1 N excitation, the ESIPT process is barrierless in the S1 state. There are no minima on the PEC of the first singlet excited state, as shown in Figure 5. A non-constrained geometry optimization of the S1 state directly leads to a non-planar geometry near the conical intersection (CI) between the S0 and S1 states (Figure 9b). According to the literature, ESIPT is often coupled with the radiationless deactivation via twisted intramolecular charge transfer (TICT) states of a non-planar biradicaloid nature [83,85,[94][95][96][97][98][99]. This non-planarity arises from the twisting around a double-like bond between proton-donating and proton-accepting moieties (around the C Ph -C Imid bond in our case). Subsequent ultrafast internal conversion via S0/S1 CI results in the non-radiative deactivation of the excited twisted phototautomer. Since L H,OH does not luminesce in solution and weakly luminesces in the solid state, we believe that this non-radiative deactivation is the predominant photophysical process for L H,OH , which is responsible for emission quenching. It should be noted that the precise geometry of the CI between the S0 and S1 states can only be optimized using ab initio methods such as CASSCF, CASPT2 or NEVPT2. However, our TDDFT optimization of the S1 state leads to the oscillations around the CI geometry, which may serve as an indirect evidence of its existence. Figure 9b shows the geometry at the optimization step closest to the real CI geometry (with the lowest S0-S1 energy gap of only 2.2 kJ/mol; the dihedral angle between the proton-donating hydroxyphenyl and proton-accepting imidazole moieties reaches 85° at this geometry).  Figure 4). However, these three minima do not lead to emission for the following reasons. Firstly, the population of the T 1 N,T minimum after S 0 N,N → T 1 N,N excitation is kinetically restricted due to the high energy barrier between the T 1 N,N and T 1 N,T minima (ca. 14 kJ/mol). Secondly, although the energy barriers for the T 1 N,N → T 1 T,N and T 1 N,N → T 1 T,T ESIPT processes are significantly lower (ca. 1 kJ/mol), the calculated T 1 T,N → S 0 T,N and T 1 T,T → S 0 T,T phosphorescence wavelengths (959 and 1301 nm, respectively) are located in the infrared region and hugely overestimated compared with the experimental phosphorescence band. Owing to the fact that we do not observe luminescence in the infrared region, the molecules that populate the T 1 T,N and T 1 T,T minima most likely deactivate non-radiatively, for example via S 0 /T 1 conical intersections. Thus, among four possible radiative deactivation channels in the triplet manifold associated with four energy minima, only one (T 1 N,N → S 0 N,N ) takes place according to the experimental data.
We did not plot the PES of the S 1 state for L OH,OH because geometry optimizations of the S 1 state with almost all initial guess structures directly lead to the non-planar near-CI geometry and oscillate around it, proving that most of the molecules that are excited to the S 1 state deactivate non-radiatively through a conical intersection. A typical evolution of (i) the energy, (ii) dihedral angle θ between the planes of hydroxyphenyl and hydroxyimidazole parts and (iii) the S 0 -S 1 energy gap during the geometry optimization is shown in Figure 10. Starting from the planar geometry with the O Ph -H distance of 0.95 Å, this distance tends to increase during each optimization cycle. In parallel with the energy stabilization, the S 0 -S 1 energy gap decreases during the optimization process. At the O Ph -H distance of 1.75 Å, the dihedral angle θ starts to increase drastically and reaches 55 • at the near-CI geometry with the S 0 -S 1 energy gap of only 7.3 kJ/mol. After the 16th optimization cycle, the optimization process starts oscillating around this near-CI geometry.
However, there is one exemption to the above-mentioned trend of radiationless deactivation via CI for L OH,OH . The geometry of the T,N-L OH,OH form can be successfully optimized in the S 1 state without falling into S 0 /S 1 CI. The corresponding S 1 T,N → S 0 T,N transition (λ calc. = 731 nm, f = 0.0367) is in accordance with the position of the low-energy shoulder in the experimental luminescence spectrum of L OH,OH . This transition represents charge transfer from the π*-orbital located on pyridine moiety (LUMO) to the π-orbital located on both hydroxyimidazole and hydroxyphenyl moieties (HOMO, Figure 8) However, there is one exemption to the above-mentioned trend of radiationless deactivation via CI for L OH,OH . The geometry of the T,N-L OH,OH form can be successfully optimized in the S1 state without falling into S0/S1 CI. The corresponding S1 T,N → S0 T,N transition (λcalc. = 731 nm, f = 0.0367) is in accordance with the position of the low-energy shoulder in the experimental luminescence spectrum of L OH,OH . This transition represents charge transfer from the π*-orbital located on pyridine moiety (LUMO) to the π-orbital located on both hydroxyimidazole and hydroxyphenyl moieties (HOMO, Figure 8). Thus, short lifetimes of the excited states observed for L OH,OH (τ = 2 ns and τ = 21 ns) are due to the S1 T,N → S0 T,N fluorescence. Now it becomes obvious that the same low-energy shoulder does not appear for L H,OH due to the lack of the O Imid -H···N Py proton transfer site. Summing up, two major emission channels have been established for L OH,OH : (i) T1 N,N → S0 N,N phosphorescence of the N,N-L OH,OH form related to the most intensive emission band at 500-800 nm; and (ii) S1 T,N → S0 T,N fluorescence of the T,N-L OH,OH form related to the low-energy shoulder at ca. 670 nm ( Figure 11).  Figure 10. Evolution of energy, dihedral angle θ and the S0-S1 energy gap during the geometry optimization of the S1 state for L OH,OH . The number of the optimization cycle is shown near the energy curve (black).
However, there is one exemption to the above-mentioned trend of radiationless deactivation via CI for L OH,OH . The geometry of the T,N-L OH,OH form can be successfully optimized in the S1 state without falling into S0/S1 CI. The corresponding S1 T,N → S0 T,N transition (λcalc. = 731 nm, f = 0.0367) is in accordance with the position of the low-energy shoulder in the experimental luminescence spectrum of L OH,OH . This transition represents charge transfer from the π*-orbital located on pyridine moiety (LUMO) to the π-orbital located on both hydroxyimidazole and hydroxyphenyl moieties (HOMO, Figure 8). Thus, short lifetimes of the excited states observed for L OH,OH (τ = 2 ns and τ = 21 ns) are due to the S1 T,N → S0 T,N fluorescence. Now it becomes obvious that the same low-energy shoulder does not appear for L H,OH due to the lack of the O Imid -H···N Py proton transfer site. Summing up, two major emission channels have been established for L OH,OH : (i) T1 N,N → S0 N,N phosphorescence of the N,N-L OH,OH form related to the most intensive emission band at 500-800 nm; and (ii) S1 T,N → S0 T,N fluorescence of the T,N-L OH,OH form related to the low-energy shoulder at ca. 670 nm ( Figure 11).

General Information
Elemental analysis was performed with a EuroEA3000 analyzer using standard technique. The IR spectra were recorded in KBr on a Bruker Vector-22 spectrometer. 1 H and 13 C NMR spectra were recorded on Bruker AV-400 (400.13 and 100.61 MHz) and Bruker DRX-500 (500.13 and 125.76 MHz) spectrometers using the residual signals of the solvent (CDCl 3 ) at 7.24 ppm for 1 H and 76.9 ppm for 13 C with respect to TMS as the internal stan-

X-ray Crystallography
Diffraction data for single-crystal L OH,OH were obtained at 291 K on an automated four-circle Agilent Xcalibur diffractometer equipped with an area AtlasS2 detector (graphite monochromator, λ(MoKα) = 0.71073 Å, ω-scans with a step 0.25 • ). Integration, absorption correction, and determination of unit cell parameters were performed using the CrysAlisPro program package [100]. The structure was solved by dual space algorithm (SHELXT [101]) and refined by the full-matrix least squares technique (SHELXL [102]) in the anisotropic approximation (except hydrogen atoms). Positions of hydrogen atoms were calculated geometrically and refined in the riding model. The crystallographic data and details of the structure refinements are summarized in Supplementary Materials (Table S1). CCDC 2237906 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at http://www.ccdc.cam.ac.uk/structures/ (accessed on 26 January 2023).

Computational Details
The quantum chemical calculations presented in this study were conducted using density functional theory (DFT), time-dependent DFT (TDDFT) and Tamm-Dancoff approximated DFT (TDADFT) methods in Gaussian 16 software package [103]. We used the hybrid exchange-correlation functional PBE0 [104] since our previous studies demonstrated its satisfying performance in modeling photophysical and photochemical properties of organic ESIPT-emitters [83,85]. Compared with probably the best known hybrid functional B3LYP, PBE0 provides absorption energies that are closer to the experimental data, while B3LYP tends to red-shift some vertical absorptions for L H,OH and L OH,OH (Supplementary Materials, Figures S12 and S13). The 6-31 + G(d) basis set was used for all atoms [105][106][107][108][109]. Absorption spectra were calculated on ground state geometries using TDDFT. Singlet excited state geometries (S 1 ) as well as S 1 -S 0 fluorescence energies were also determined using the TDDFT approach. The optimizations of the lowest triplet excited state (T 1 ) geometries of L H,OH and L OH,OH were carried out by an unrestricted DFT (uDFT) method. Subsequent single-point TDADFT computations on T 1 optimized geometries revealed T 1 -S 0 phosphorescence energies. The use of TDADFT rather than TDDFT in the latter case is justified by the fact that the Tamm-Dancoff approximation tends to strongly correct the computed triplet state energies comparatively to TDDFT. Relaxed T 1 state geometries can also be obtained using TDDFT or TDADFT approaches; however, the uDFT method is more preferable because it requires much less computational cost. In the case of absorption spectra, the solvent effects of acetonitrile molecules were considered by the polarizable continuum model (PCM), and all other computations were performed in the gas phase. The D3 version of Grimme's dispersion with Becke-Johnson damping was employed for each calculation. Potential energy curves (PECs) and surfaces (PESes) of the desired states (S 0 , S 1 , T 1 ) along the proton transfer reaction were plotted by scanning the O...H bond distance between 0.95 and 2.00 Å with a step of 0.05 Å. All frequencies in the harmonic approximation for the calculated global minimum energy geometries were positive, confirming that the optimized molecular geometries correspond to the real minima on the potential energy surfaces. The atomic coordinates of all optimized geometries are given in Supplementary Materials (Tables S3-S16). The geometries and molecular orbitals were visualized using ChemCraft software [110].
A slight structural difference between these two compounds leads to significant changes in their photoluminescence response. L H,OH emits in the light green region, while L OH,OH luminesces in the orange region. According to our computations, both emitters share the same emission mechanism, i.e., phosphorescence of the normal form of the molecule (T 1 N → S 0 N for the N-L H,OH form and T 1 N,N → S 0 N,N for the N,N-L OH,OH form), which is not related to ESIPT. After the ESIPT process, both compounds can decay non-radiatively through S 0 /S 1 and S 0 /T 1 conical intersections, which explains their low photoluminescence quantum yield. The phosphorescence band is the most intensive for both compounds. However, L OH,OH also exhibits fluorescence of the T,N-L OH,OH form, S 1 T,N → S 0 T,N , with one proton transferred from the hydroxyimidazole moiety to the pyridine moiety. This fluorescence mechanism is responsible for the appearance of the low-energy shoulder in the emission spectrum of L OH,OH . Thus, owing to the presence of two proton transfer sites, L OH,OH appears to be a rare example of ESIPT-emitters that exhibit fluorescence and phosphorescence simultaneously.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28041793/s1, Tables S1-S16 and Figures S1-S15: characterization data and quantum chemical calculations data. Data Availability Statement: Data will be made available on request.