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Article

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

by
Nikita A. Shekhovtsov
1,*,
Elena B. Nikolaenkova
2,
Alexey A. Ryadun
1,
Denis G. Samsonenko
1,
Alexsei Ya. Tikhonov
2,* and
Mark B. Bushuev
1,*
1
Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3, Acad. Lavrentiev Ave., Novosibirsk 630090, Russia
2
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of Russian Academy of Sciences, 9, Acad. Lavrentiev Ave., Novosibirsk 630090, Russia
*
Authors to whom correspondence should be addressed.
Molecules 2023, 28(4), 1793; https://doi.org/10.3390/molecules28041793
Submission received: 27 January 2023 / Revised: 7 February 2023 / Accepted: 9 February 2023 / Published: 14 February 2023
(This article belongs to the Special Issue Recent Developments in the Synthesis and Functionalization of Nitrogen Heterocycles)
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Abstract

:
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.

1. Introduction

Aromatic and heteroaromatic compounds featuring strong intramolecular hydrogen bonds of the O–H···Y and N–H···Y types (Y = O, NR) can manifest photoinduced intramolecular proton transfer reactions (Scheme 1) [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. The photoexcitation of such molecules in their most stable, or normal (N), form leads to the electron density redistribution, followed by the excited state intramolecular proton transfer (ESIPT) reaction yielding the excited state tautomeric form (T). Radiative and non-radiative processes proceeding in the tautomeric form convert this excited state form into the ground state. The last step in this sequence of processes is the ground state intramolecular proton transfer (GSIPT) reaction, converting the tautomeric form to the normal one.
The ESIPT photoreaction (Scheme 1) is highly sensitive to substituents [15,16,17,18,19,20,21,22,23,24,25,26,27] and coordinated metal ions [28,29,30,31,32,33], protonation/deprotonation [34,35,36,37,38,39,40,41], the state of aggregation [42,43], the polarity of solvent [44,45,46,47,48] and the presence of various analytes [49,50,51,52,53,54,55,56]. If the excited state tautomerization (normal-to-tautomeric) is barrierless, the only form to emit is the tautomeric one, which typically luminesces with rather large Stokes shift [57,58]. In the case of barriers on excited state potential energy surfaces, the molecule can be trapped in a local minimum of the normal form, leading to the emission of the normal form. Modifying the barrier height in the excited state, one can achieve dual emission associated with the luminescence of both forms [59,60,61,62,63,64,65,66]. The sensitivity of ESIPT-capable compounds to various stimuli makes them an appealing platform for numerous applications [67,68,69].
1H-Imidazoles, 1,3-oxazoles, 1,3-thiazoles and their benzannulated congeners are often used in the design of ESIPT-fluorophores [70,71,72,73,74,75,76,77,78,79,80,81,82]. Normally, when decorated with such proton-donating groups as unsubstituted or substituted 2-hydroxyphenyl groups in the α-position to aza-atoms, their free nitrogen atoms act as proton acceptors during the ESIPT process [70,71,72,73,74,75,76,77,78,79,80,81,82]. Recently we proposed a new approach in the design of imidazole-based ESIPT-fluorophores in which we switched the role of the imidazole cycle to the one of a proton donor by introducing the hydroxy group in the position 1 and the pyridin-2-yl group in the position 2 of the imidazole ring [83,84,85,86,87]. Importantly, both roles of the imidazole ring in ESIPT-fluorophores, i.e., the proton acceptor and the proton donor ones, can be combined in a single molecule if we introduce the proton-donating 2-hydroxyphenyl group in the position 4 and the proton accepting pyridin-2-yl group in the position 2 of the 1-hydroxy-1H-imidazole moiety. In this case, the molecule will feature two spatially separated ESIPT-sites with two short O–H···N hydrogen bonds therein.
In this manuscript, we report the synthesis of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LOH,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 LOH,OH, we report the reaction of LOH,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 (LH,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 LOH,OH and LH,OH and the ESIPT photoreactions in both compounds.

2. Results and Discussion

2.1. Synthesis of 1-Hydroxy-4-(2-Hydroxyphenyl)-5-Methyl-2-(Pyridin-2-yl)-1H-Imidazole (LOH,OH) and 4-(2-Hydroxyphenyl)-5-Methyl-2-(Pyridin-2-yl)-1H-Imidazole (LH,OH)

The ESIPT-capable imidazole-based compounds LOH,OH and LH,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 (LOH,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 LOH,OH to the 1H-imidazole LH,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 LOH,OH with chloroacetone (cf. [93]) proceeded without affecting the phenolic hydroxy group, which greatly simplified the preparation of the 1H-imidazole LH,OH compound. Spectral and structural data for the compounds are given in Supplementary Materials.

2.2. X-ray Single Crystal Structure of 1-Hydroxy-4-(2-Hydroxyphenyl)-5-Methyl-2-(Pyridin-2-yl)-1H-Imidazole (LOH,OH)

The dihydroxy derivative, LOH,OH, crystallizes in the monoclinic space group P21/c (Supplementary Materials, Table S1, Figures S9–S11). There are two crystallographically independent LOH,OH molecules in the crystal structure (Figure 1). The 2-(pyridin-2-yl)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 LOH,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.
The LOH,OH molecules are assembled into corrugated ribbons running along the c axis through weak C–H···O hydrogen bonds (Figure 2). The ribbons are further gathered into 3D supramolecular structure via C–H···C and C–H···H–C van der Waals interactions (Supplementary Materials, Figures S9–S11).

2.3. Tautomeric Forms of LH,OH and LOH,OH: An Introduction

LH,OH and LOH,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). LOH,OH has two proton transfer sites and therefore can exist in four tautomeric forms: i) N,N-LOH,OH (no proton transferred, corresponds to the global energy minimum and to the X-ray crystal structure), (ii) N,T-LOH,OH (one proton transferred from the hydroxyphenyl moiety to the hydroxyimidazole moiety), (iii) T,N-LOH,OH (one proton transferred from the hydroxyimidazole moiety to the pyridine moiety), (iv) T,T-LOH,OH (both protons transferred). LH,OH has only one proton transfer site and can exist in two tautomeric forms: (i) N-LH,OH (no proton transferred) and (ii) T-LH,OH (one proton transferred). The same abbreviations are used for the energy minima of ground and excited states, e.g., S0N,N, S1T,N, T1T, etc.

2.4. Absorption Properties of LH,OH and LOH,OH in MeCN

In acetonitrile, both LH,OH and LOH,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, S0N,N (OPh–H 0.988 Å, OImid–H 1.010 Å, Table 1) for LOH,OH and S0N (OPh–H 0.980 Å) for LOH,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 LH,OH and 348 nm for LOH,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 LH,OH and LOH,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 LH,OH and the HOMO and LUMO of LOH,OH, respectively, the most intensive absorption peak of LOH,OH is slightly red-shifted compared with that of LH,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).
It is noteworthy that, in addition to the global energy minimum S0N,N on the PES of the ground state, LOH,OH has a local minimum S0T,N (OPh–H 0.992 Å, OImid–H 1.595 Å, Figure 4, Table 1), and therefore its corresponding form T,N-LOH,OH can also absorb light. S0T,N is thermodynamically less favorable than S0N,N by ca. 17 kJ/mol and is separated from S0N,N by an energy barrier of ca. 20 kJ/mol. Although such a low barrier may indicate coexistence of the N,N-LOH,OH and T,N-LOH,OH tautomeric forms in solution, the fact that the experimental absorption spectrum is completely reproduced by the transitions of the N,N-LOH,OH form points to the very small contribution of the T,N-LOH,OH form to the absorption spectrum. In the case of LH,OH, there is only one minimum on the PEC of the ground state, S0N (Figure 5).

2.5. Excitation and Emission Properties of LH,OH and LOH,OH

LH,OH and LOH,OH are non-luminescent in MeCN solution, indicating the possible predominance of various non-radiative deactivation pathways. In the solid state, LH,OH emits in the light green region (Figure 6 and Figure 7). The broad unstructured luminescence band of LH,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. LH,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 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.
LOH,OH demonstrates luminescence in the orange region (Figure 6 and Figure 7). As for LH,OH, the emission spectrum is dominated by a broad band at 450–800 nm centered at 568 nm. In contrast to LH,OH, an additional low-energy shoulder at ca. 670 nm appears in the case of LOH,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 LOH,OH is multiexponentional and more complex than for LH,OH: the long part of the photoluminescence decay reveals one lifetime in the microsecond range, τ = 1.05 μs (similar to LH,OH), whereas the short part reveals two lifetimes in the nanosecond range, τ = 2 ns and τ = 21 ns (Supplementary Materials, Figure S15). Thus, LOH,OH shows two emission mechanisms, i.e., phosphorescence and fluorescence. The photoluminescence quantum yield for LH,OH and LOH,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 LH,OH and LOH,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 LH,OH and 568 nm for LOH,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 LOH,OH may be responsible for the short lifetimes and can therefore be attributed to fluorescence. The absence of this shoulder for LH,OH may indicate that the fluorescence mechanism observed for LOH,OH cannot be realized for LH,OH. We hypothesize that this fluorescence mechanism is somehow related to the OImid–H···NPy proton transfer site, which is absent for LH,OH.

2.6. Elucidation of the Fluorescence and Phosphorescence Mechanisms for LH,OH and LOH,OH

Geometry optimizations of the excited states were performed in order to establish the photoluminescence mechanisms for LH,OH and LOH,OH and to verify our predictions from the previous paragraph. The PEC of the first triplet excited state of LH,OH reveals two minima, T1N and T1T (Figure 5). The T1N optimized geometry is characterized by a slightly enlarged OPh–H distance (1.006 Å for T1N vs. 0.980 Å for S0N) and a shortened OPh···NImid hydrogen bond length (2.550 Å for T1N vs. 2.627 Å for S0N) compared with the S0N relaxed geometry. The calculated T1N → S0N 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, T1N → S0N 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 T1N → S0N phosphorescence is associated with charge transfer from the pyridine moiety to the hydroxyphenyl moiety (this is directly opposite to the S0N → S1N absorption mechanism discussed above). Although the second minimum on the PEC of the T1 state, T1T (OPh–H 1.841 Å, Figure 5), is thermodynamically more stable than T1N by ca. 16 kJ/mol, the energy barrier separating T1N and T1T is as high as ca. 14 kJ/mol, which impedes efficient ESIPT in the triplet manifold. Furthermore, the computed T1T → S0T phosphorescence wavelength (1095 nm) is largely overestimated compared with the position of the phosphorescence band. Thus, we attribute the observed phosphorescence of LH,OH with τ = 1.05 μs to the T1N → S0N transition of the N-LH,OH form, which is not related to the ESIPT process.
Having established the phosphorescence mechanism (T1N → S0N) for LH,OH, the following question arises: how can the molecules of LH,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 S0N → S1N vertical absorption is computed at 336 nm for LH,OH (Figure 3). At the same time, the phosphorescence band of LH,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 LH,OH there is a direct population of the triplet manifold from the ground state, S0N → T1N, 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 (S0N → S1N → T1N) 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 LH,OH. After S0N → S1N 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 CPh–CImid bond in our case). Subsequent ultrafast internal conversion via S0/S1 CI results in the non-radiative deactivation of the excited twisted phototautomer. Since LH,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 LH,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).
LOH,OH has two proton transfer sites and therefore provides more possible emission mechanisms than LH,OH. The PES of the T1 state shows four energy minima: T1N,N (OPh–H 1.008 Å, OImid–H 1.065 Å), T1T,N (OPh–H 0.994 Å, OImid–H 1.931 Å), T1N,T (OPh–H 1.829 Å, OImid–H 1.051 Å) and T1T,T (OPh–H 1.807 Å, OImid–H 1.787 Å, Figure 4). The mechanisms of the population of the T1 state for LOH,OH are similar to those for LH,OH. Upon excitation with high energies (S0N,N → Sn N,N, where n ≥ 1), the S1N,N state can be reached, and the T1N,N state can be populated from S1N,N via S1N,N → T1N,N intersystem crossing. Upon excitation with lower energies, the S1N,N state cannot be reached, and the T1N,N state can be populated only via direct S0N,N → T1N,N excitation. In comparison with S0N,N, both hydrogen bonds become stronger in the T1N,N energy minimum (OPh–H···NImid: 2.628 Å for S0N,N vs. 2.546 Å for T1N,N; OImid–H···NPy: 2.616 Å for S0N,N vs. 2.509 Å for T1N,N). The computed T1N,N → S0N,N phosphorescence wavelength (586 nm) is in good agreement with the experimental emission maximum (546 nm). It corresponds to LUMO (π*) → HOMO (π) transition of the N,N-LOH,OH form, which is not related to ESIPT and has both protons at the oxygen atoms. Same as for LH,OH, this transition represents charge transfer from the pyridine heterocycle to the hydroxyphenyl moiety (Figure 8).
Three other minima on the T1 state PES of LOH,OH, i.e., T1T,N, T1N,T and T1T,T, are energetically more favorable than T1N,N by ca. 58, 14 and 43 kJ/mol, respectively (Figure 4). However, these three minima do not lead to emission for the following reasons. Firstly, the population of the T1N,T minimum after S0N,N → T1N,N excitation is kinetically restricted due to the high energy barrier between the T1N,N and T1N,T minima (ca. 14 kJ/mol). Secondly, although the energy barriers for the T1N,N → T1T,N and T1N,N → T1T,T ESIPT processes are significantly lower (ca. 1 kJ/mol), the calculated T1T,N → S0T,N and T1T,T → S0T,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 T1T,N and T1T,T minima most likely deactivate non-radiatively, for example via S0/T1 conical intersections. Thus, among four possible radiative deactivation channels in the triplet manifold associated with four energy minima, only one (T1N,N → S0N,N) takes place according to the experimental data.
We did not plot the PES of the S1 state for LOH,OH because geometry optimizations of the S1 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 S1 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 S0-S1 energy gap during the geometry optimization is shown in Figure 10. Starting from the planar geometry with the OPh–H distance of 0.95 Å, this distance tends to increase during each optimization cycle. In parallel with the energy stabilization, the S0-S1 energy gap decreases during the optimization process. At the OPh–H distance of 1.75 Å, the dihedral angle θ starts to increase drastically and reaches 55° at the near-CI geometry with the S0-S1 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 LOH,OH. The geometry of the T,N-LOH,OH form can be successfully optimized in the S1 state without falling into S0/S1 CI. The corresponding S1T,N → S0T,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 LOH,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 LOH,OH (τ = 2 ns and τ = 21 ns) are due to the S1T,N → S0T,N fluorescence. Now it becomes obvious that the same low-energy shoulder does not appear for LH,OH due to the lack of the OImid–H···NPy proton transfer site. Summing up, two major emission channels have been established for LOH,OH: (i) T1N,N → S0N,N phosphorescence of the N,N-LOH,OH form related to the most intensive emission band at 500–800 nm; and (ii) S1T,N → S0T,N fluorescence of the T,N-LOH,OH form related to the low-energy shoulder at ca. 670 nm (Figure 11).

3. Materials and Methods

3.1. 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.1H and 13C 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 (CDCl3) at 7.24 ppm for 1H and 76.9 ppm for 13C with respect to TMS as the internal standard. Corrected photoluminescence spectra were recorded on a Fluorolog 3 spectrometer (Horiba Jobin Yvon).

3.2. 1-(2-Benzoyloxyphenyl)-2-(Hydroxyimino)Propan-1-One (B)

A solution of isopropyl nitrite (0.74 g, 8.3 mmol) in methanol (5 mL) and then conc. HCl acid (1.4 mL) were added dropwise to a solution of 2-(benzyloxy)propiophenone (A) (synthesized according to the procedure reported in ref. [88]) (1.27 g, 5 mmol) in methanol (25 mL) under heating at 40 °C. The reaction mixture was stirred at 40–45 °C for 8 h, cooled and neutralized with a solution of NaHCO3. After evaporation to remove methanol, the aqueous layer was extracted with CHCl3 and dried over MgSO4. After solvent removal under reduced pressure, the residue was purified by column chromatography (silica gel, CHCl3) and then triturated with hexane to give the title product. Yield: 0.78 g (55%), m.p. 102–103 °C (100–101 °C [88]). Anal. Calc. for C16H13NO4: C, 67.84; H, 4.62; N, 4.95. Found: C, 67.97; H, 4.62; N, 5.02%. 1H NMR (400.13 MHz, CDCl3) δ (ppm): 8.30 (s, 1H, OH), 8.08 (d, 2H, J = 7.4 Hz, HAr), 7.61 (t, 1H, J = 7.4 Hz, HAr), 7.57–7.52 (m, 2H, HAr), 7.47 (m, 2H, HAr), 7.33–7.27 (m, 2H, HAr), 1.97 (s, 3H, Me). 13C NMR (125.76 MHz, CDCl3) δ (ppm): 189.30, 164.61, 156.71, 148.41, 133.68, 132.04, 130.88, 130.05, 129.98, 128.68, 128.46, 125.53, 122.93, 8.95. IR (KBr, ν cm−1): 3311, 1713 (C=O), 1670 (C=O), 1603, 1279, 1269, 1203, 1178, 1115, 1086, 1018, 906, 702, 656.

3.3. 1-Hydroxy-4-(2-Hydroxyphenyl)-5-Methyl-2-(Pyridin-2-yl)-1H-Imidazole (LOH,OH)

Conc. NH4OH (19 mL) and pyridine-2-carboxaldehyde (0.44 g, 4.1 mmol) were added to a solution of 1-(2-benzoyloxyphenyl)-2-(hydroxyimino)propan-1-one (1.13 g, 4 mmol) in a mixture of 1,4-dioxane (16 mL) and EtOH (4 mL). The reaction mixture was stirred at room temperature for 3 days. After removing of the solvent, the residue was purified by column chromatography (silica gel, CHCl3) and recrystallized from the hexane-ethylacetate mixture (10:1) to afford the title product. Yield: 0.91 g (85%), m.p. 121–122 °C. Anal. Calc. for C15H13N3O2: C, 67.40; H, 4.90; N, 15.72. Found: C, 67.45; H, 5.02; N, 15.81%. 1H NMR (400.13 MHz, CDCl3) δ (ppm): 13.02 (br. s, 1H, OH), 8.43 (ddd, 1H, J = 5.1, 1.5, 0.5 Hz, HAr), 7.99 (dt, 1H, J = 7.8, 1.3 Hz, HAr), 7.88 (dt, 1H, J = 7.8, 1.3 Hz, HAr), 7.50 (dd, 1H, J = 7.8, 1.3 Hz, HAr), 7.28 (ddd, 1H, J = 7.5, 5.1, 1.1 Hz, HAr), 7.16 (ddd, 1H, J = 8.1, 7.5, 1.5 Hz, HAr), 7.01 (dd, 1H, J = 8.1, 1.1 Hz, HAr), 6.88 (dt, 1H, J = 7.8, 1.3 Hz, HAr), 2.57 (s, 3H, Me). 13C NMR (125.76 MHz, CDCl3) δ (ppm): 156.40, 148.37, 145.61, 138.47, 132.56, 128.60, 127.77, 125.42, 122.48, 121.61, 119.42, 118.71, 117.79, 117.08, 9.19. IR (KBr, ν cm−1): 1603, 1566, 1489, 1439, 1389, 1288, 1255, 1178, 1153, 1126, 1014, 773, 741, 652.

3.4. 4-(2-Hydroxyphenyl)-5-Methyl-2-(Pyridin-2-yl)-1H-Imidazole (LH,OH)

A mixture of 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LOH,OH) (0.19 g, 0.71 mmol), chloroacetone (0.066 g, 0.71 mmol) and dried K2CO3 (0.11 g, 0.8 mmol) in dried dimethylformamide (6 mL) was stirred at room temperature for 1 h and then at 40–45 °C for 4 h. After cooling the reaction mixture was diluted with water, the residue formed was filtered, washed with water, dried and purified by column chromatography (silica gel, CHCl3). The recrystallization of the residue from EtOH afforded LH,OH. Yield: 0.16 g (89%), m.p. 202–203 °C. Anal. Calc. for C15H13N3O: C, 71.70; H, 5.21; N, 16.72. Found: C, 71.62; H, 5.34; N, 16.65%. 1H NMR (400.13 MHz, CDCl3) δ (ppm): 12.38 (s, 1H, NH), 11.04 (br. s, 1H, OH), 8.51 (ddd, 1H, J = 5.1, 1.3, 0.5 Hz, HAr), 8.07 (dt, 1H, J = 8.0, 1.2 Hz, HAr), 7.79 (dt, 1H, J = 7.8, 1.2 Hz, HAr), 7.47 (dd, 1H, J = 7.8, 1.2 Hz, HAr), 7.25 (ddd, 1H, J = 7.5, 5.1, 1.2 Hz, HAr), 7.17 (ddd, 1H, J = 8.0, 7.5, 1.4 Hz, HAr), 7.03 (dd, 1H, J = 7.8, 1.4 Hz, HAr), 6.88 (dt, 1H, J = 7.5, 1.3 Hz, HAr), 2.53 (s, 3H, Me). 13C NMR (100.61 MHz, CDCl3) δ (ppm): 156.38, 148.64, 147.49, 141.49, 137.62, 136.87, 127.97, 125.68, 124.42, 123.37, 120.15, 118.91, 118.30, 117.25, 12.47. IR (KBr, ν cm−1): 3311, 1597, 1578, 1443, 1400, 1286, 1244, 1134, 999, 825, 783, 756, 742, 700.

3.5. X-ray Crystallography

Diffraction data for single-crystal LOH,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).

3.6. 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 LH,OH and LOH,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 (S1) as well as S1-S0 fluorescence energies were also determined using the TDDFT approach. The optimizations of the lowest triplet excited state (T1) geometries of LH,OH and LOH,OH were carried out by an unrestricted DFT (uDFT) method. Subsequent single-point TDADFT computations on T1 optimized geometries revealed T1-S0 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 T1 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 (S0, S1, T1) 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].

4. Conclusions

In this work we presented the synthesis of imidazole-based ESIPT-capable compounds, 1-hydroxy-4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LOH,OH) and 4-(2-hydroxyphenyl)-5-methyl-2-(pyridin-2-yl)-1H-imidazole (LH,OH). In the LOH,OH trinuclear molecule, the central moiety, i.e., the 1-hydroxy-1H-imidazole one, is decorated with the proton-donating and proton-accepting peripheral groups and, therefore, under photoexcitation can act both as a proton acceptor and a proton donor in the ESIPT reactions. Importantly, we found a convenient synthetic pathway for the conversion of 1-hydroxy-4-(2-hydroxyphenyl)-1H-imidazoles to 4-(2-hydroxyphenyl)-1H-imidazoles. This synthetic pathway is based on the reaction of the 1-hydroxy-4-(2-hydroxyphenyl)-1H-imidazole derivative with chloroacetone. Despite chloroacetone being known to interact with phenolic hydroxy groups, in our case the reaction proceeded selectively with the imidazolic hydroxy group only, leaving the phenolic hydroxy group unaffected. Thus, this reaction has high synthetic potential for selective reduction of 1-hydroxy-1H-imidazoles decorated with hydroxyphenyl groups to corresponding 1H-imidazoles.
A slight structural difference between these two compounds leads to significant changes in their photoluminescence response. LH,OH emits in the light green region, while LOH,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 (T1N → S0N for the N-LH,OH form and T1N,N → S0N,N for the N,N-LOH,OH form), which is not related to ESIPT. After the ESIPT process, both compounds can decay non-radiatively through S0/S1 and S0/T1 conical intersections, which explains their low photoluminescence quantum yield. The phosphorescence band is the most intensive for both compounds. However, LOH,OH also exhibits fluorescence of the T,N-LOH,OH form, S1T,N → S0T,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 LOH,OH. Thus, owing to the presence of two proton transfer sites, LOH,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.

Author Contributions

Conceptualization, A.Y.T., N.A.S. and M.B.B.; methodology, E.B.N., A.Y.T. and N.A.S.; formal analysis, E.B.N., N.A.S. and D.G.S.; investigation, E.B.N., N.A.S., D.G.S. and A.A.R.; writing—original draft preparation, N.A.S., E.B.N., A.Y.T. and M.B.B.; writing—review and editing, N.A.S., E.B.N., A.Y.T. and M.B.B.; visualization, N.A.S.; supervision, M.B.B.; project administration, M.B.B.; funding acquisition, M.B.B. All authors have read and agreed to the published version of the manuscript.

Funding

The synthesis of the ESIPT-capable compounds LOH,OH and LH,OH, the study of the ESIPT reactions and the emission properties of LOH,OH and LH,OH and the interpretation of the experimental and theoretical results were funded by the Russian Science Foundation (grant № 21-13-00216, https://rscf.ru/en/project/21-13-00216/ (accessed on 26 January 2023) and performed at the Nikolaev Institute of Inorganic Chemistry.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data will be made available on request.

Acknowledgments

The elemental analysis data, X-ray single crystal data, the 1H NMR, 13C NMR, IR and UV-vis spectra were obtained using the equipment of the Multi-Access Chemical Research Centre SB RAS at the N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry and the Nikolaev Institute of Inorganic Chemistry with the financial support from the Ministry of Science and Higher Education of the Russian Federation. N.A.S. is grateful to the Foundation for the Advancement of Theoretical Physics and Mathematics “BASIS”. The Siberian Branch of the Russian Academy of Sciences (SB RAS) Siberian Supercomputer Centre is gratefully acknowledged for providing supercomputer facilities. M.B.B. acknowledges the access to the Computational Centre of Novosibirsk State University.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples of the compounds are available from the authors.

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Molecules 28 01793 sch001 550
Scheme 1. The ESIPT and GSIPT processes in molecules featuring short intramolecular hydrogen bonds.
Scheme 1. The ESIPT and GSIPT processes in molecules featuring short intramolecular hydrogen bonds.
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Molecules 28 01793 sch002 550
Scheme 2. Structural formulae of LOH,OH and LH,OH.
Scheme 2. Structural formulae of LOH,OH and LH,OH.
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Molecules 28 01793 sch003 550
Scheme 3. Synthesis of LOH,OH and LH,OH.
Scheme 3. Synthesis of LOH,OH and LH,OH.
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Molecules 28 01793 g001 550
Figure 1. Two crystallographically independent molecules in the structure of LOH,OH.
Figure 1. Two crystallographically independent molecules in the structure of LOH,OH.
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Figure 2. A supramolecular ribbon in the structure of LOH,OH.
Figure 2. A supramolecular ribbon in the structure of LOH,OH.
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Molecules 28 01793 sch004 550
Scheme 4. Tautomeric forms of LH,OH and LOH,OH.
Scheme 4. Tautomeric forms of LH,OH and LOH,OH.
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Molecules 28 01793 g003 550
Figure 3. Absorption spectra of LH,OH (red) and LOH,OH (blue) in MeCN. Vertical bars display the positions and oscillator strengths of the singlet-to-singlet electronic transitions for LH,OH (red) and LOH,OH (blue).
Figure 3. Absorption spectra of LH,OH (red) and LOH,OH (blue) in MeCN. Vertical bars display the positions and oscillator strengths of the singlet-to-singlet electronic transitions for LH,OH (red) and LOH,OH (blue).
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Molecules 28 01793 g004 550
Figure 4. The potential energy surfaces (PESes) of the S0 (a) and T1 (b) states of LOH,OH along the proton transfer paths OPh–H···NImid and OImid–H···NPy and their projections.
Figure 4. The potential energy surfaces (PESes) of the S0 (a) and T1 (b) states of LOH,OH along the proton transfer paths OPh–H···NImid and OImid–H···NPy and their projections.
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Molecules 28 01793 g005 550
Figure 5. The potential energy curves (PECs) of the S0, S1 and T1 (right) states of LH,OH along the proton transfer path OPh–H···NImid. The arrows show the energy minima on these PECs. The optimized geometries of the T1 and S1 states with the OPh–H distance of 2.0 Å are also depicted.
Figure 5. The potential energy curves (PECs) of the S0, S1 and T1 (right) states of LH,OH along the proton transfer path OPh–H···NImid. The arrows show the energy minima on these PECs. The optimized geometries of the T1 and S1 states with the OPh–H distance of 2.0 Å are also depicted.
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Figure 6. Excitation and emission spectra of LH,OH (a) and LOH,OH (b) in the solid state at room temperature.
Figure 6. Excitation and emission spectra of LH,OH (a) and LOH,OH (b) in the solid state at room temperature.
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Molecules 28 01793 g007 550
Figure 7. CIE 1931 diagram showing the chromaticity of the emission of LH,OH and LOH,OH in the solid state at λex = 320 nm.
Figure 7. CIE 1931 diagram showing the chromaticity of the emission of LH,OH and LOH,OH in the solid state at λex = 320 nm.
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Figure 8. Frontier molecular orbitals related to the emission processes observed for LH,OH (a) and LOH,OH (b).
Figure 8. Frontier molecular orbitals related to the emission processes observed for LH,OH (a) and LOH,OH (b).
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Figure 9. (a) Photophysical and photochemical properties of LH,OH in the solid state summarized in a simplified energy level diagram. ISC—intersystem crossing, phosph.—phosphorescence. (b) The geometry of LH,OH that is close to the conical intersection between the S0 and S1 states.
Figure 9. (a) Photophysical and photochemical properties of LH,OH in the solid state summarized in a simplified energy level diagram. ISC—intersystem crossing, phosph.—phosphorescence. (b) The geometry of LH,OH that is close to the conical intersection between the S0 and S1 states.
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Figure 10. Evolution of energy, dihedral angle θ and the S0-S1 energy gap during the geometry optimization of the S1 state for LOH,OH. The number of the optimization cycle is shown near the energy curve (black).
Figure 10. Evolution of energy, dihedral angle θ and the S0-S1 energy gap during the geometry optimization of the S1 state for LOH,OH. The number of the optimization cycle is shown near the energy curve (black).
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Figure 11. Photophysical and photochemical properties of LOH,OH in the solid state summarized in a simplified energy level diagram. ISC—intersystem crossing, phosph.—phosphorescence; fluor.—fluorescence.
Figure 11. Photophysical and photochemical properties of LOH,OH in the solid state summarized in a simplified energy level diagram. ISC—intersystem crossing, phosph.—phosphorescence; fluor.—fluorescence.
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Table
Table 1. The most relevant geometric parameters for the optimized ground and excited state geometries of LH,OH and LOH,OH.
Table 1. The most relevant geometric parameters for the optimized ground and excited state geometries of LH,OH and LOH,OH.
Cmpd.StateOPh–H, ÅOPh–H···NImid, ÅOImid–H, ÅOImid–H···NPy, Åθ1,° aθ2,° b
LH,OHS0N0.9802.627––14.970.38
T1N1.0062.550––5.790.20
T1T1.8412.603––4.140.25
near-CI c3.2643.411––84.771.92
LH,OHS0N,N0.9882.6281.0102.61614.780.92
S0T,N0.9922.5921.5952.5420.790.04
S1T,N0.9982.5841.7852.6740.000.23
T1N,N1.0082.5461.0652.509−0.020.00
T1N,T1.8292.5991.0512.531−0.010.00
T1T,N0.9942.5901.9312.728−0.020.00
T1T,T1.8072.5931.7872.648−0.02−0.01
near-CI c2.3472.9220.9642.73255.435.45
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.
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Shekhovtsov, N.A.; Nikolaenkova, E.B.; Ryadun, A.A.; Samsonenko, D.G.; Tikhonov, A.Y.; Bushuev, M.B. 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. Molecules 2023, 28, 1793. https://doi.org/10.3390/molecules28041793

AMA Style

Shekhovtsov NA, Nikolaenkova EB, Ryadun AA, Samsonenko DG, Tikhonov AY, Bushuev MB. 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. Molecules. 2023; 28(4):1793. https://doi.org/10.3390/molecules28041793

Chicago/Turabian Style

Shekhovtsov, Nikita A., Elena B. Nikolaenkova, Alexey A. Ryadun, Denis G. Samsonenko, Alexsei Ya. Tikhonov, and Mark B. Bushuev. 2023. "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" Molecules 28, no. 4: 1793. https://doi.org/10.3390/molecules28041793

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