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.     

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.     

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.     

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.     

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.     

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.     

A theoretical assignment on excited‐state intramolecular proton transfer mechanism for quercetin

In the present work, we investigate a new chromophore (ie, quercetin) (Simkovitch et al J Phys Chem B 119 [2015] 10244) about its complex excited‐state intramolecular proton transfer (ESIPT) process based on density functional theory and time‐dependent density functional theory methods. On the basis of the calculation of electron density ρ( r ) and Laplacian ?²ρ( r ) at the bond critical point using atoms‐in‐molecule theory, the intramolecular hydrogen bonds (O₁‐H₂?O₅ and O₃‐H₄?O₅) have been supported to be formed in the S₀ state. Comparing the prime structural variations of quercetin involved in its 2 intramolecular hydrogen bonds, we find that these 2 hydrogen bonds should be strengthened in the S₁ state, which is a fundamental precondition for facilitating the ESIPT process. Concomitantly, infrared vibrational spectra analysis further verifies this viewpoint. In good agreement with previous experimental spectra results, we find that quercetin reveals 2 kinds of excited‐state structures (quercetin* and quercetin‐PT1*) in the S₁ state. Frontier molecular orbitals depict the nature of electronically excited state and support the ESIPT reaction. Our scanned potential energy curves according to variational O₁‐H₂ and O₃‐H₄ coordinates demonstrate that the proton transfer process should be more likely to occur in the S₁ state via hydrogen bond wire O₁‐H₂?O₅ rather than O₃‐H₄?O₅ because of the lower potential energy barrier 2.3 kcal/mol. Our present work explains previous experimental result and makes up the deficiency of mechanism in previous experiment. In the end, we make a reasonable assignment for ESIPT process of quercetin.     

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.     

黄芩素激发态分子内质子转移耦合电荷转移反应的实验和理论研究

通过稳态光谱实验和量子化学计算相结合,研究了黄芩素激发态质子转移耦合电荷转移的反应. 实验和计算中S₁态吸收峰的缺失表明S₁态是暗态. S₁暗态导致在实验中观察不到黄芩素在乙醇溶液中的荧光峰,且固体的荧光峰很弱. 黄芩素分子的前线分子轨道和电荷差异密度表明S₁态是电荷转移态,然而S₂态是局域激发态. 计算的黄芩素分子的势能曲线在激发态只有一个稳定点,这表明了黄芩素激发态分子内质子转移的过程是一个无     

Ab initio investigation of excited state dual hydrogen bonding interactions and proton transfer mechanism for novel oxazoline compound

Owing to the importance of excited state dynamical relaxation, the excited state intramolecular proton transfer(ESIPT) mechanism for a novel compound containing dual hydrogen bond(abbreviated as "1-enol") is studied in this work.Using density functional theory(DFT) and time-dependent density functional theory(TDDFT) method, the experimental electronic spectra can be reproduced for 1-enol compound. We first verify the formation of dual intramolecular hydrogen bonds, and then confirm that the dual hydrogen bond should be strengthened in the first excited state. The photo-excitation process is analyzed by using frontier molecular orbital(HOMO and LUMO) for 1-enol compound. The obvious intramolecular charge transfer(ICT) provides the driving force to effectively facilitate the ESIPT process in the S1 state. Exploration of the constructed S0-state and S1-state potential energy surface(PES) reveals that only the excited state intramolecular single proton transfer occurs for 1-enol system, which makes up for the deficiencies in previous experiment.     

3发色团激发态分子内质子转移机理的研究

采用密度泛函(DFT)和含时密度泛函理论(TDDFT)方法对一种新合成的发色团(3)在非质子性溶剂DMSO中的激发态分子内质子转移机制进行了理论研究.基于3发色团的基态和激发态优化结构, 计算得到了该发色团中与氢键相关的键长和键角的大小, 以及与氢键相连接的 O-H键红外振动光谱, 发现分子内氢键在激发态下有增强的趋势. 理论计算得到的吸收谱和荧光谱的峰值与实验测得的结果吻合得很好, 证明了所采用的理论方法的正确性与合理性. 最终, 通过对该发色团的分子内电荷转移与电荷分布的分析, 证实了激发态分子内质子转移发生的可能性, 并说明了其转移过程的发生机制.     

3发色团激发态分子内质子转移机理的研究

采用密度泛函(DFT)和含时密度泛函理论(TDDFT)方法对一种新合成的发色团(3)在非质子性溶剂DMSO中的激发态分子内质子转移机制进行了理论研究.基于3发色团的基态和激发态优化结构,计算得到了该发色团中与氢键相关的键长和键角的大小,以及与氢键相连接的O-H键红外振动光谱,发现分子内氢键在激发态下有增强的趋势.理论计算得到的吸收谱和荧光谱的峰值与实验测得的结果吻合得很好,证明了所采用的理论方法的正确性与合理性.最终,通过对该发色团的分子内电荷转移与电荷分布的分析,证实了激发态分子内质子转移发生的可能性,并说明了其转移过程的发生机制.     

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.     

Synthesis,Structure, Luminescent and Intramolecular Proton Transfer in Some Imidazole Derivatives

A group of novel 2-aryl imidazole derivatives were synthesized and characterized by NMR spectra, X-ray, mass and CHN analysis. An excited state intramolecular proton transfer (ESIPT) process in hydroxy imidazoles (dmip and dmtip) have been studied using emission spectroscopy and it was detected that the two distinct ground state rotamers of I and II are responsible for the normal and the tautomer emission respectively. In hydrocarbon solvent, the tautomer emission predominates over the normal emission for both dmip and dmtip. This reveal that rotamer II is responsible for the tautomer emission and it is stabler than rotamer I which causes the normal emission. In alcoholic solvent like ethanol, a dramatic enhancement of normal emission is observed which was due to increased solvation, the more polar rotamer I become stabler than rotamer II. In dioxane—water mixtures it is observed that the addition of water inhibits the ESIPT process due to the formation of the intermolecular hydrogen bonding involving water. DFT calculations on energy, dipole moment, charge distribution of the rotamers in the ground and excited states of the imidazole derivatives were performed and discussed. PES calculation indicates that the energy barrier for the interconversion of two rotamers is too high in the excited state than the ground state.     

Excited state intramolecular proton transfer of 1,2-dihydroxyanthraquinone by femtosecond transient absorption spectroscopy

1,2-Dihydroxyanthraquinone (alizarin) shows dual emission bands with a large Stokes shift from a “locally-excited (LE)” and “proton-transferred (PT)” tautomers in the excited state. Excited state intramolecular proton transfer (ESIPT) reaction of alizarin is tunable by changing concentration, solvent polarity, excitation wavelength, and etc. ESIPT reaction of alizarin in the excited state was investigated by steady-state absorption/emission spectroscopy and femtosecond transient absorption spectroscopy. In ethanol solution, the lifetime of PT tautomer of alizarin was measured as 87 ps, in addition to 0.35 and 8.3 ps vibrational cooling dynamics for the LE and PT tautomers of alizarin, respectively. In binary mixtures of ethanol and water, the excited state dynamics became more complicated; the LE and PT tautomers appeared to decay with 8.9 and 30.8 ps lifetimes, which is much shorter compared to the lifetime of the PT tautomer in ethanol. A long-lived nonradiative state in the excited states of alizarin was found as well, which was proposed as a “trapped” state with tightly hydrogen-bonded water molecules. The ESIPT reaction of alizarin was blocked in a 1:1 mixture of ethanol-water due to strong hydrogen bonding between water molecules and alizarin, which was further confirmed by the efficient coupling of alizarin to TiO₂ nanoparticles in the 1:1 binary mixture of ethanol-water.     

Excited State Intramolecular Proton Transfer in O-Tosylaminobenzaldehyde

Excited state intramolecular proton transfer (ESIPT) in o-tosylaminobenzaldehyde has been investigated. According to quantum-chemical calculations ESIPT in o-tosylaminobenzaldehyde is barrierless. Product of ESIPT undergoes efficient nonradiative deactivation caused by internal rotation of C(H)OH-group. The solvent orientational relaxation in anionic form of o-tosylaminobenzaldehyde was detected. The mechanism of anionic form fluorescence quenching at the addition of the base in a protic solvent is proposed. It consists in the intermolecular proton transfer from the protonated base to oxygen atom of aldehyde group followed by the internal rotation of C(H)OH-group.     

Exploring excited‐state proton transfer mechanism for 9,10‐dihydroxybenzo[h]quinolone

In this work, we mainly focus on the excited‐state intramolecular proton transfer mechanism of a new molecule 9,10‐dihydroxybenzo[h]quinoline (9‐10‐HBQ). Within the framework of density functional theory and time‐dependent density functional theory methods, we have theoretically investigated its excited‐state dynamical process and our theoretical results successfully reappeared previous experimental electronic spectra. The ultrafast excited‐state intramolecular proton transfer process occurs in the first excited state (S₁ state) forming 9‐10‐HBQ‐PT1 structure without potential energy barrier along with hydrogen bond (O₃–H₄···N₅). Then the second proton may transfer via another intramolecular hydrogen bonded wire (O₁–H₂···N₃) with a moderate potential energy barrier (about 7.69 kcal/mol) in the S₁ state forming 9‐10‐HBQ‐PT2 configuration. After completing excited‐state dynamical process, the molecule on the first excited electronic state would come back to the ground state. We not only clarify the excited‐state dynamical process for 9‐10‐HBQ but also put forward new predictions and successfully explain previous 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.     

Solvent controlling excited state proton transfer reaction in quinoline/isoquinoline‐pyrazole isomer QP‐I: A theoretical study

In this present work, we theoretically study the excited state intramolecular proton transfer (ESIPT) mechanism about a quinoline/isoquinoline‐pyrazole isomer QP‐I system. Compared with previous experimental results, our calculated results reappear previous data, which further confirm the theoretical level we used is reasonable. We mainly adopt 2 kinds of solvents (nonpolar cyclohexane and polar acetonitrile) to explore solvents effects on this system. Through reduced density gradient (RDG) function, the intramolecular hydrogen bond N1─H2···N3 has been confirmed existing in both S₀ and S₁ states, although the distance between H2 and N3 is not short. In addition, the strengthening N1─H2···N3 in the S₁ state provides possibility for ESIPT. Explorations about charge redistribution reveal the trend of ESIPT, and frontier orbital gap reflects the reactivity in polar and nonpolar solvents. The constructing potential energy curves reveal that potential energy barriers could be controlled and regulated by solvent polarity.     

Reaction pathway and H/D kinetic isotope effects of the triple proton transfer in a 7‐hydroxyquinoline‐methanol complex in the ground state: A computational approach

Multiple proton transfer (PT) is an essential process in long‐range proton transport such as proton relay in enzymes. 7‐Hydroxyquinoline (7HQ) undergoes alcohol‐mediated PT in both the excited and ground states, and this system has been investigated as a biomimetic model. In the present study, the reaction pathway of triple PT in a 7HQ‐methanol cluster, 7HQ·(MeOH)₂, in the ground state has been investigated using density functional theory calculations to clarify the reaction mechanism and the origin of the experimentally observed kinetic isotope effect (KIE). The PT takes place in an asynchronous concerted fashion, in which the oxygen atom in 7HQ first accepts a proton from the directly hydrogen‐bonded MeOH. The rate constants and primary H/D KIEs have been estimated with canonical variational transition state theory in combination with the small curvature tunneling approximation. The tunneling effect on the PT rate is significant, and the KIE is much greater than 1 at room temperature. The rule of the geometric mean for the KIEs breaks down because of the asynchronicity in the motions of 3 protons and tunneling effect. In addition, the rate constant is smaller, the KIE is larger, and the activation energy is higher compared with the experimental values in heptane solution, suggesting that the PT dynamics in solution is governed by not only the intrinsic PT process but also thermal fluctuation of the solute and solvent molecules, which plays an important role in the configurational change of the 7HQ·(MeOH)₂ complex.