Computational studies on amino-type excited-state intramolecular proton transfer and subsequent cis–trans isomerisation reactions of three 2-(2'-aminophenyl)benzothiazole derivatives

Excited-state intramolecular proton transfer (ESIPT) reactions of three amino-type 2-(2'-aminophenyl)benzothiazole (PBT-NH₂) derivatives, that is, 2-(2'-methylaminophenyl)benzothiazole (PBT-NHMe), 2-(2'-acetylaminophenyl)benzothiazole (PBT-NHAc) and 2-(2'-tosylaminophenyl)benzothiazole (PBT-NHTs), have been explored by the time-dependent density functional theory (TD-DFT) method with the B3LYP density functional. In addition, their absorption and fluorescence spectra were also simulated at the same theoretical level. The present studies reveal that the energy barriers of the first singlet excited state of the three titled compounds along the ESIPT reactions are predicted at 0.39, 0.30 and 0.12 eV, respectively, suggesting that the inclusion of a strong electron-withdrawing tosyl group can remarkably facilitate the occurrence of the ESIPT reaction, while the involvement of an electron-donating methyl group has no effect on the ESIPT process of the amino-type hydrogen-bonding system. Following the ESIPT, both PBT-NHAc and PBT-NHTs molecules can also undergo the cis–trans isomerisation reactions along the C₂–C₃ bond between benzothiazole and phenyl moieties, in which the energy barriers of the trans-tautomer → cis-tautomer isomerisations in both ground states are calculated at 0.33 and 0.27 eV, respectively. This implies that there may exist a long-lived trans-tautomer species in the ground states for PBT-NHAc and PBT-NHTs, as observed in the spectroscopic experiments of PBT-NHTs.     

Theoretical insight into the excited‐state behavior of a novel Compound 1: A TDDFT investigation

In the present work, using density functional theory and time‐dependent density functional theory methods, we investigated and presented the excited‐state intramolecular proton transfer (ESIPT) mechanisms of a novel Compound 1 theoretically. Analyses of electrostatic potential surfaces and reduced density gradient (RDG) versus sign(λ₂)ρ, we confirm the existence of intramolecular hydrogen bond O1‐H2···N3 for Compound 1 in the S₀ state. Comparing the primary structural variations of Compound 1 involved in the intramolecular hydrogen bond, we find that O1‐H2···N3 should be strengthened in the S₁ state, which may facilitate the ESIPT process. Concomitantly, infrared (IR) vibrational spectra analyses further verify the stability of hydrogen bond. In addition, the role of charge transfer interaction has been addressed under the frontier molecular orbitals, which depicts the nature of electronical excited state and supports the ESIPT reaction. The theoretically scanned and optimized potential energy curves according to variational O1‐H2 coordinate demonstrate that the proton transfer process should occur spontaneously in the S₁ state. It further explains why the emission peak of Compound 1‐enol was not reported in previous experiment. This work not only presents the ESIPT mechanism of Compound 1 but also promotes the understanding of this kind of molecules for further applications in future.     

A TDDFT study on the hydrogen-bonding effect on ESIPT mechanism for [2,2′-bipyridyl]-3,3′-diol-(H2O) n (n = 0, 1, 2) clusters: single or double?

The excited-state intramolecular proton transfer (ESIPT) mechanisms of [2,2′-bipyridyl]-3,3′-diol (BP(OH)₂) in gas are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The intramolecular hydrogen bond (H-bond) is strengthened in the first excited-state in view of the structural parameters and infrared (IR) vibrational frequencies. The enhanced intramolecular H-bond is favourable for ESIPT process. The effect of the extra intermolecular H-bond between BP(OH)₂ and water on ESIPT is considered. The potential energy surfaces, molecular electrostatic potential, topological analysis, frontier molecular orbitals, absorption and fluorescence spectra are investigated. Our calculated results show that the intermolecular H-bond enhances the intramolecular H-bond, changes the mechanism and decreases the barrier height of ESIPT process.     

A Comprehensive Therotical Investigation of Intramolecular Proton Transfer in the Excited States for Some Newly-designed Diphenylethylene Derivatives Bearing 2-(2-Hydroxy-Phenyl)-Benzotriazole Part

This article presents a comprehensive therotical investigation of excited state intramolecular proton transfer (ESIPT) for some newly-designed diphenylethylene derivatives containing 2-(2-hydroxy-phenyl)-benzotriazole moiety with various substituted groups. The calculation shows the structural parameters and Mulliken charges of phototautomers enol (E) and keto (K) of these compounds exhibit no or tiny changes from S₀ to S₁. The calculated results suggest that HOMO and LUMO + 1 of the compounds displays excellent overlapping nature, and thus the absorption and emission could be from the electron transition of HOMO→LUMO + 1. The electron density distribution in the frontier orbital of E and K are influenced remarkably by various substituted groups in S₀ and S₁ states. Electron density distribution deficiency in 2-(2-hydroxy-phenyl)-benzotriazole part is observed in L + 1 for these derivatives. The calculation also suggests the potential energy curves of ESIPT are shown to be a strong relationship with electron donor-acceptor groups. The absorption spectra, normal emission spectra and ESIPT spectra of the derivatives were also calculated.     

A DFT/TD-DFT study of effect of different substituent on ESIPT fluorescence features of 2-(2'-hydroxyphenyl)-4-chloro- methylthiazole derivatives

Based on density functional theory (DFT) and time-dependent density functional theory (TD-DFT), the effects of substituent on the excited-state intramolecular proton transfer (ESIPT) process and photophysical properties of 2-(2'-hydroxyphenyl)-4-chloromethylthiazole (HCT) are studied. The electron-donating group (CH₃, OH) and electron-withdrawing group (CF₃, CHO) are introduced to analyze the changes of intramolecular H-bond, the frontier molecular orbitals, the absorption/fluorescence spectra, and the energy barrier of ESIPT process. The calculation results indicate that electron-donating group strengthens the intramolecular H-bond in the S₁ state, and leads to an easier ESIPT process. The electron-withdrawing group weakens the corresponding H-bond and makes ESIPT process a little harder. Different substituents also affect the photophysical properties of HCT. The electron-withdrawing group (CF₃, CHO) has a little effect on electronic spectra. The electron-donating group (CH₃, OH) red-shifts both the absorption and fluorescence emission peaks of HCT, respectively, which causes the Stokes shift to increase.     

The ESIPT mechanism of dibenzimidazolo diimine sensor: a detailed TDDFT study

Spectroscopic investigations on excited state proton transfer of a new dibenzimidazolo diimine sensor (DDS) were reported by Goswami et al. recently. In our present work, based on the time‐dependent density functional theory (TDDFT), the excited‐state intramolecular proton transfer (ESIPT) mechanism of DDS is studied theoretically. Our calculated results reproduced absorption and fluorescence emission spectra of the previous experiment, which verifies that the TDDFT method we adopted is reasonable and effective. The calculated dominating bond lengths and bond angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared vibrational spectra. Further, hydrogen bonding strengthening manifests the tendency of ESIPT process. The calculated frontier molecular orbitals further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O–H coordinate, the potential energy barrier of about 5.02 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 0.195 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Theoretical investigation on the excited state intramolecular proton transfer in Me_2N substituted flavonoid by the time-dependent density functional theory method

Time-dependent density functional theory(TDDFT) method is used to investigate the details of the excited state intramolecular proton transfer(ESIPT) process and the mechanism for temperature effect on the Enol*/Keto*emission ratio for the Me_2N-substited flavonoid(MNF) compound. The geometric structures of the S_0 and S_1 states are denoted as the Enol, Enol*, and Keto*. In addition, the absorption and fluorescence peaks are also calculated. It is noted that the calculated large Stokes shift is in good agreement with the experimental result. Furthermore, our results confirm that the ESIPT process happens upon photoexcitation, which is distinctly monitored by the formation and disappearance of the characteristic peaks of infrared(IR) spectra involved in the proton transfer and in the potential energy curves. Besides, the calculations of highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) reveal that the electronegativity change of proton acceptor due to the intramolecular charge redistribution in the S_1 state induces the ESIPT. Moreover, the thermodynamic calculation for the MNF shows that the Enol*/Keto*emission ratio decreasing with temperature increasing arises from the barrier lowering of ESIPT.     

A theoretical study on the ESPT mechanism for a novel Bis‐HPBT fluorophore

In this work, based on the density functional theory and time‐dependent density functional theory methods, the properties of the 2 intramolecular hydrogen bonds (O1‐H2···N3 and O4‐H5···N6) of a new photochemical sensor 4‐(3‐(benzo[d]thiazol‐2‐yl)‐5‐tert‐butyl‐4‐hydroxybenzyl)‐2‐(benzo[d]thiazol‐2‐yl)‐6‐tert‐butyl phenol (Bis‐HPBT) have been investigated in detail. The calculated dominating bond lengths and bond angles about these 2 hydrogen bonds (O1‐H2···N3 and O4‐H5···N6) demonstrate that the intramolecular hydrogen bonds should be strengthened in the S₁ state. In addition, the variations of hydrogen bonds of Bis‐HPBT have been also testified based on infrared vibrational spectra. Our theoretical results reproduced absorption and emission spectra of the experiment, which verifies that the theoretical level we used is reasonable and effective in this work. Further, hydrogen bonding strengthening manifests the tendency of excited state intramolecular proton transfer (ESIPT) process. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. According to the calculated results of potential energy curves along stepwise and synergetic O1‐H2 and O4‐H5 coordinates, the potential energy barrier of approximately 1.399 kcal/mol is discovered in the S₁ state, which supports the single ESIPT process along with 1 hydrogen bond of Bis‐HPBT. In other words, the proton transfer reaction can be facilitated based on the electronic excitation effectively. In turn, through the process of radiative transition, the proton‐transfer Bis‐HPBT‐SPT form regresses to the ground state with the fluorescence of 539 nm.     

Time-dependent density functional theory study on excited state intramolecular proton transfer of 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one

The time-dependent density functional theory (TDDFT) method was carried out to investigate the excited state intramolecular proton transfer (ESIPT) process of 3-hydroxy-2-(pyridin-2-yl)-4H-chromen-4-one (1a). 1a has two tautomeric forms: one is 1a(O), which is induced by intramolecular hydrogen bond O-H?O=C, and the other one is 1a(N), which is caused by intramolecular hydrogen bond O-H?N. From excited state to tautomer excited state coming from ESIPT, the hydroxyl hydrogen breaks away and the dissociated hydrogen adsorbed on pyridinic nitrogen or carbonyl oxygen formed new intramolecular HB and the corresponding bond length and bond angle varied greatly. In comparison, a similar process of proton transfer for 1a(N)H⁺ protonated 1a(N) from ground state to excited state was obtained. This detailed proton transfer mechanism was provided by molecular orbitals analysis and it may be applied to molecular switch and organic Lewis acid/base. We investigated the excited state proton transfer mechanism of the four molecules through the theoretical method for the first time and gave unambiguous geometry of excited state.     

取代基对2-(2-羟基苯基)苯并噻唑分子内氢键及质子转移的影响:密度泛函理论研究

运用密度泛函(DFT)和含时密度泛函(TD DFT)理论方法研究了在2-(2-羟基苯基)苯并噻唑(HBT)苯环羟基的邻位或对位分别引入羟基和醛基后的衍生物分子内质子转移过程,考察了取代基的电子效应及取代位置对分子内氢键和质子转移反应的影响,模拟计算了各分子的IR振动光谱和电子光谱.研究发现,HBT及其衍生物分子可以形成分子内氢键,且激发态时氢键增强.基态时以醇式构型稳定存在,激发态时酮式结构为优势构象.分子的最大吸收峰和发射峰主要源于电子从前线分子轨道HOMO到LUMO之间的跃迁.基态分子内质子转移需要越过较高的能垒因而难以发生,而激发态时只需越过较低能垒就很容易发生激发态分子内质子转移.取代基的电子效应和取代位置对HBT分子氢键强度、互变异构体的相对稳定性、电子光谱及质子转移反应的能垒均有一定影响.     

Theoretical investigation on the excited state intramolecular proton coupled charge transfer phenomenon for a novel fluorophore

We theoretically investigate the excited state behaviors of the novel fluorophore tetraphenylethene‐2‐(2′‐hydroxyphenyl)benzothiazole (TPE‐HBT), which was designed based on the intersection of TPE and HBT, using density functional theory and time‐dependent density functional theory methods. Compared with previous experimental results about fluorescence peaks, our calculated results are in good agreement with experimental data, which further confirms that the theoretical level we used is reasonable. Furthermore, our results confirm that the excited state intramolecular proton transfer (ESIPT) process happens upon photoexcitation, which is distinctly monitored by the infrared spectra and the potential energy curves. In addition, the calculation of highest occupied molecular orbital and lowest unoccupied molecular orbital reveals that the electron density change of proton acceptor because of the intramolecular charge transfer (ICT) process in the S₁ state induces the ESIPT. Moreover, the transition density matrix is worked out to facilitate deeper insight into the ESIPT coupled ICT process. It is hoped that the present work not only elaborates the ESIPT coupled ICT phenomenon and corresponding mechanisms for the TPE‐HBT but also may be helpful to design and develop new materials and applications involved in TPE‐HBT systems in future.     

基于ESIPT机理检测Al3+荧光探针分子结构和电子光谱的理论研究

本文应用密度泛函理论（DFT）和含时密度泛函理论（TD-DFT）方法，研究了具有激发态分子内质子转移（ESIPT）特性的3-羟基黄酮（3HF）及其两种氰基和氨基取代衍生物（3HF-CY和3HF-AM）作为水溶液中Al3+离子检测的荧光探针分子结构和电子光谱性质. 计算得到了与ESIPT过程相关的键长、键角以及势能曲线，模拟计算了单独分子和分子@Al3+复合物的吸收和荧光光谱. 结果表明，氰基或氨基的引入均会抑制3HF的质子在基态（S0）或激发态（S1）的转移. 而从得到的吸收光谱可以看出，在3HF中引入吸电子基团氰基可以引起其吸收光谱的红移，而给电子基团氨基的引入则出现相反现象. 此外，与3HF-AM的荧光光谱相比，3HF-AM@Al3+复合物发生了75.88 nm的蓝移，由此推测3HF-AM与水中的Al3+反应后，在光激发下溶液荧光会由绿色转变为紫色，表明3HF-AM分子可以作为有效检测水中Al3+的荧光探针.     

Exploring the effect of aggregation-induced emission on the excited state intramolecular proton transfer for a bis-imine derivative by quantum mechanics and our own n-layered integrated molecular orbital and molecular mechanics calculations

We theoretically investigate the excited state intramolecular proton transfer(ESIPT) behavior of the novel fluorophore bis-imine derivative molecule HNP which was designed based on the intersection of 1-(hydrazonomethyl)-naphthalene-2-ol and 1-pyrenecarboxaldehyde. Especially, the density functional theory(DFT) and time-dependent density functional theory(TDDFT) methods for HNP monomer are introduced. Moreover, the "our own n-layered integrated molecular orbital and molecular mechanics"(ONIOM) method(TDDFT:universal force field(UFF)) is used to reveal the aggregation-induced emission(AIE) effect on the ESIPT process for HNP in crystal. Our results confirm that the ESIPT process happens upon the photoexcitation for the HNP monomer and HNP in crystal, which is distinctly monitored by the optimized geometric structures and the potential energy curves. In addition, the results of potential energy curves reveal that the ESIPT process in HNP will be promoted by the AIE effect. Furthermore, the highest occupied molecular orbital(HOMO) and lowest unoccupied molecular orbital(LUMO) for the HNP monomer and HNP in crystal have been calculated. The calculation demonstrates that the electron density decrease of proton donor caused by excitation promotes the ESIPT process. In addition, we find that the variation of atomic dipole moment corrected Hirshfeld population(ADCH) charge for proton acceptor induced by the AIE effect facilitates the ESIPT process. The results will be expected to deepen the understanding of ESIPT dynamics for luminophore under the AIE effect and provide insight into future design of high-efficient AIE compounds.     

Intra- and intermolecular proton transfer in methyl-2-hydroxynicotinate

Spectral characteristics of methyl 2-hydroxynicotinate (MEHNA) have been studied using absorption, fluorescence excitation and fluorescence spectroscopy, as well as, using single photon counting nanosecond spectrofluorimeter. MEHNA is present as enol in less polar solvents and keto in polar media. In non-polar solvents, large Stokes shifted fluorescence band is assigned to phototautomer, formed by excited state intramolecular proton transfer (ESIPT), whereas fluorescence is only observed from keto form in polar solvents. In aqueous and polar solvents monocation (MC) is formed by protonating the exo carbonyl oxygen atom in the ground state (S₀) and in the first excited singlet state (S₁), MC is obtained by protonating carbonyl oxygen atom of the ester. It is formed by ESIPT from exo carbonyl proton to carbonyl oxygen atom of the ester. Dication is formed by protonating both the oxygen atoms. Two kinds of monoanions formed by deprotonating phenolic proton or >N-H proton of keto suggest the presence of enol and keto in aqueous solution. In cyclohexane MC is formed by protonating carbonyl oxygen in both S₀ and S₁ states. The electronic structure calculations were performed on each species using semi-empirical quantum mechanical AM1 method and density functional theory B3LYP with 6-31G^** basis set using Gaussian 98 program, along with potential energy mapping, to characterize the particular species.     

Elaborating the excited state behavior of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine coupling with methanol solvent

We present a theoretical investigation about the excited state dynamical mechanism of 2‐(4′‐N,N‐dimethylaminophenyl)‐imidazo[4,5‐c]pyridine (DMAPIP‐c). Within the framework of density functional theory and time‐dependent density functional theory methods, we reasonably repeat the experimental electronic spectra, which further confirm the theoretical level used in this work is feasible. Given the best complex model, 3 methanol (MeOH) solvent molecules should be connected with DMAPIP‐c forming DMAPIP‐c‐MeOH complex in both ground state and excited state. Exploring the changes about bond lengths and bond angles involved in hydrogen bond wires, we find the O7‐H8···N9 one should be largely strengthened in the S₁ state, which plays an important role in facilitating the excited state intermolecular proton transfer (ESIPT) process. In addition, the analyses about infrared vibrational spectra also confirm this conclusion. The redistribution about charges distinguished via frontier molecular orbitals based on the photoexcitation, we do find tendency of ESIPT reaction due to the most charges located around N9 atom in the lowest unoccupied molecular orbital. Based on constructing the potential energy curves of both S₀ and S₁ states, we not only confirm that the ESIPT process should firstly occur along with hydrogen bond wire O7‐H8···N9, but also find a low potential energy barrier 8.898 kcal/mol supports the ESIPT reaction in the S₁ state forming DMAPIP‐c‐MeOH‐PT configuration. Subsequently, DMAPIP‐c‐MeOH‐PT could twist its dimethylamino moiety with a lower barrier 3.475 kcal/mol forming DMAPIP‐c‐MeOH‐PT‐TICT structure. Our work not only successfully explains previous experimental work but also paves the way for the further applications about DMAPIP‐c sensor in future.     

The effect of benzo‐annelation on intermolecular hydrogen bond and proton transfer of 2‐methyl‐3‐hydroxy‐4(1H)‐quinolone in methanol: A TD‐DFT study

Excited‐state intermolecular or intramolecular proton transfer (ESIPT) reaction has important potential applications in biological probes. In this paper, the effect of benzo‐annelation on intermolecular hydrogen bond and proton transfer reaction of the 2‐methyl‐3‐hydroxy‐4(1H)‐quinolone (MQ) dye in methanol solvent is investigated by the density functional theory and time‐dependent density functional theory approaches. Both the primary structure parameters and infrared vibrational spectra analysis of MQ and its benzo‐analogue 2‐methyl‐3‐hydroxy‐4(1H)‐benzo‐quinolone (MBQ) show that the intermolecular hydrogen bond O₁―H₂?O₃ significantly strengthens in the excited state, whereas another intermolecular hydrogen bond O₃―H₄?O₅ weakens slightly. Simulated electron absorption and fluorescence spectra are agreement with the experimental data. The noncovalent interaction analysis displays that the intermolecular hydrogen bonds of MQ are obviously stronger than that of MBQ. Additionally, the energy profile analysis via the proton transfer reaction pathway illustrates that the ESIPT reaction of MBQ is relatively harder than that of MQ. Therefore, the effect of benzo‐annelation of the MQ dye weakens the intermolecular hydrogen bond and relatively inhibits the proton transfer reaction.     

Theoretical investigation on the excited‐state intramolecular proton transfer mechanism of 2‐(2′‐benzofuryl)‐3‐hydroxychromone

Spectroscopic studies on excited‐state proton transfer of a new chromophore 2‐(2′‐benzofuryl)‐3‐hydroxychromone (BFHC) have been reported recently. In the present work, based on the time‐dependent density functional theory (TD‐DFT), the excited‐state intramolecular proton transfer (ESIPT) of BFHC is investigated theoretically. The calculated primary bond lengths and angles involved in hydrogen bond demonstrate that the intramolecular hydrogen bond is strengthened. In addition, the phenomenon of hydrogen bond reinforce has also been testified based on infrared (IR) vibrational spectra as well as the calculated hydrogen bonding energies. Further, hydrogen bonding strengthening manifests the tendency of excited state proton transfer. Our calculated results reproduced absorbance and fluorescence emission spectra of experiment, which verifies that the TD‐DFT theory we used is reasonable and effective. The calculated Frontier Molecular Orbitals (MOs) further demonstrate that the excited state proton transfer is likely to occur. According to the calculated results of potential energy curves along O―H coordinate, the potential energy barrier of about 14.5 kcal/mol is discovered in the S₀ state. However, a lower potential energy barrier of 5.4 kcal/mol is found in the S₁ state, which demonstrates that the proton transfer process is more likely to happen in the S₁ state than the S₀ state. In other words, the proton transfer reaction can be facilitated based on the photo‐excitation effectively. Moreover, the phenomenon of fluorescence quenching could be explained based on the ESIPT mechanism. Copyright © 2015 John Wiley & Sons, Ltd.     

Spectroscopic Studies of Fluorescent Dye <Emphasis Type="Italic">N</Emphasis>, <Emphasis Type="Italic">N</Emphasis>′- bis(2-hydroxy-5-methyl-benzylidene)-1,2-ethanediamine and its DNA Complex in Solution

Absorption and fluorescence emission properties of an N-salicylideneamine fluorescent dye molecule N, N-bis(2-hydroxy-5-methylbenzylidene)-1,2-ethanediamine (1) have been studied in three typical solvents—2-methylbutane, ethanol, dimethyl sulphoxide (DMSO), and its DNA complex in methanol/H₂O mixed solvent. The normal absorption band of 1 is observed in both aprotic and protic solvents and has been assigned to the l a transition in the enol form of 1. The long-wavelength absorption band of 1, which is caused by the formation of a cis-keto species in the ground state, is absent in aprotic solvents, but is observable in protic ones. Normal fluorescence emission from the excited enol state of 1 is obtained only when the normal absorption band is excited, while the excited-state intramolecular proton transfer (ESIPT) emissions from both cis- and trans-keto species are recorded in all cases, being acceptable for the variation of the relative emission intensities. A preliminary spectroscopic study of the 1–DNA complex indicates an intercalation-binding mode, the convincing supporting evidence being the enhanced ESIPT fluorescence intensity of 1 when complexed with DNA. Finally, a universal energy-state diagram is given to interpret the experimental results.     

Excited state intramolecular proton transfer mechanism of o-hydroxynaphthyl phenanthroimidazole

By utilizing the density functional theory(DFT) and the time-dependent density functional theory(TDDFT), the excited state intramolecular proton transfer(ESIPT) mechanism of o-hydroxynaphthyl phenanthroimidazole(HNPI) is studied in detail. Upon photo is excited, the intramolecular hydrogen bond is obviously enhanced in the S_1 state, which thus promotes the ESIPT process. Hydrogen bond is shown to be strengthened via comparing the molecular structures and the infrared vibration spectra of the S_0 and S_1 states. Through analyzing the frontier molecular orbitals, we can conclude that the excitation is a type of the intramolecular charge transfer excitation, which also indicates the trend of proton transfer in S_1 state. The vertical excitation based on TDDFT calculation can effectively repeat the absorption and fluorescence spectra of the experiment. However, the fluorescence spectrum of normal structure, which is similar to the spectrum of isomer structure is not detected in the experiment. It can be concluded that the fluorescence measured in the experiment is attributed to both structures. In addition, by analyzing the potential energy curves(PECs) calculated by the B3 LYP functional method, it can be derived that since the molecule to cross the potential barrier in the S_1 state is smaller than in the S_0 state and the reverse proton transfer process in the S_1 state is more difficult than in the S_0 state, the ESIPT occurs in the S_1 state.     

Synthesis,structural, and spectroscopic (FT-IR,NMR, and UV) Characterization of 1-(Cyclohexylmethyl)-2-(pyridin-2-yl)-1<Emphasis Type="Italic">H</Emphasis>-benzo[<Emphasis Type="Italic">d</Emphasis>]imidazole by experimental techniques and quantum chemical calculations

The title compound (II), 1-(cyclohexylmethyl)-2-(pyridin-2-yl)-1H-benzo[d]imidazole (C₁₉H₂₁N₃), was synthesized via N-alkylation of 2-(pyridin-2-yl)-1H-benzo[d]imidazole (I). Both compounds I and II were characterized by IR, NMR and UV-vis spectroscopy. Solid-state structure of compound II was determined by single-crystal X-ray diffraction technique. Furthermore, quantum chemical calculations employing density functional theory (DFT/B3LYP) method with the 6–311++G(d, p) basis set were performed for the theoretical characterization of the molecular and spectroscopic features of the compounds. Using the TD-DFT method, electronic absorption spectra of the compounds have been predicted at same level. When the obtained results were compared with the experimental findings, it is seen that theoretical results support the experimental data and a good agreement exists between them.