溶剂极性对1，5-二羟基蒽醌分子双质子转移过程的影响

具有激发态分子内双质子转移特性的分子在荧光传感器、激光材料、生物分子探针等领域具有广泛的应用. 羟基蒽醌作为蒽醌类化合物是自然界中广泛存在且具有质子转移特性的次级代谢物,其衍生物已被广泛研究并成功应用于染料、免疫增强和抗癌药物. 近年来,1,5-二羟基蒽醌(1,5-DHAQ)作为一种具有两个分子内氢键的羟基蒽醌衍生物受到了广泛的关注. 本文采用飞秒瞬态吸收光谱结合含时密度泛函理论方法研究了溶剂极性对1,5-DHAQ分子激发态分子内双质子转移过程的影响. 1,5-DHAQ分子在甲苯、四氢呋喃和乙腈溶剂中的稳态荧光光谱表明,溶剂极性的改变对1,5-DHAQ的荧光峰位置产生了影响. 瞬态吸收光谱表明,溶剂极性的增大加快了质子转移的速率. 超快动力学拟合结果表明,溶剂极性的增大有助于加快1,5-DHAQ分子中的激发态分子内双质子转移过程. 此外,通过理论计算得到的势能曲线分析表明质子转移的能垒随着溶剂极性的增加而逐渐减小,从而促进1,5-DHAQ分子激发态分子内双质子转移过程的发生,这进一步验证和解释了实验结果. 本工作有助于开发和合成更稳定、高效的羟基蒽醌衍生物.     

10-羟基苯并喹啉激发态分子内质子转移取代基效应的理论研究

运用密度泛函(DFT)和含时密度泛函(TDDFT)计算方法研究了10-羟基苯并喹啉(HBQ)及其衍生物分子内质子转移过程,探究了取代基效应对质子转移过程的影响,研究发现,HBQ及其衍生物可以形成分子内氢键,且激发态时氢键增强.基态时各分子以醇式构型稳定存在,激发态时酮式构型为优势构象.各化合物的最大吸收峰和发射峰主要是电子从HOMO到LUMO之间的跃迁引起的.基态分子内质子转移(醇式→酮式)需要跃过较高的能垒因而难以发生,而激发态时只需跃过较低能垒就很容易发生分子内质子转移,吸电子基的引入可以使该过程的能垒降低,因此吸电子基有利于激发态质子转移.取代基效应影响化合物的光谱性质.     

瞬态吸收光谱茜素超快动力学研究

激发态质子转移是光物理学、光化学和光生物过程中最基本的化学反应之一。激发态分子内质子转移(excited-state intramolecular proton transfer, ESIPT)通常是指有机分子受到激发,到达激发态后,质子在激发态势能面上从质子供体基团转移到质子受体基团并形成含有分子内氢键多元环的过程, 一般发生在亚皮秒量级。质子转移可应用于有机发光二极管、荧光探针等领域。茜素,即1,2-二羟基蒽醌,可从茜草根部提取,具有与醌类衍生物相似的结构,常用于染料、染色剂和药物等。近年来,发现茜素分子具有质子转移特性,可用来制备新型“绿色”染料敏化电池。利用稳态吸收、稳态荧光和飞秒瞬态吸收光谱技术以及第一性原理理论计算对溶于乙醇溶液的茜素分子的质子转移过程进行了研究和分析。稳态吸收和稳态荧光研究结果表明: 在基态时,茜素分子的正常构型9,10-酮处于稳定状态,容易发生跃迁;在激发态时,茜素分子的互变异构体构型1,10-酮处于稳定状态,容易产生荧光发射。飞秒瞬态吸收光谱测量使用的激光的激发波长为370 nm。测得的瞬态吸收光谱在430 nm附近存在茜素的基态漂白信号。通过使用全局拟合方法对瞬态吸收光谱进行分析研究发现：茜素正常构型9,10-酮的激发态分子内质子转移时间为110.5 fs,茜素互变异构体构型1,10-酮分子内振动弛豫时间为30.7 ps,茜素互变异构体构型1,10-酮荧光寿命为131.7 ps。通过使用单波长动力学拟合的方法对瞬态吸收光谱进行分析发现：发生质子转移的时间尺度与运用全局拟合方法得出的结果基本一致;茜素分子的正常构型9,10-酮分子在110.5 fs的时间尺度内处于快速减少的趋势,而茜素分子的互变异构体构型1,10-酮分子在这一时间尺度内处于快速上升的趋势。当延迟时间增大时,茜素分子的互变异构体构型1,10-酮分子又呈现缓慢衰减的趋势。     

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.     

1-羟基蒽醌的分子内质子转移动力学

利用飞秒瞬态吸收光谱结合量化计算研究了1-羟基蒽醌在溶剂中的激发态分子内质子转移动力学. 分子受到400 nm光激发后的瞬态吸收谱呈现出激发态吸收和受激辐射两个光谱带. 受激辐射信号较激发态吸收信号滞后出现,由此确定质子转移时间是32 fs. 量化计算表明分子在沿质子转移路径衰变时,分子轨道和能级次序发生了反转,形成锥形交叉. 在质子转移之后,经锥形交叉布居的电子态发生内转换和分子间能量弛豫,时间分别是200 fs和16 ps. 基态回复时间是300 ps. 通过实验和理论研究,证实存在两个质子转移路径,其中主要的衰变路径保持了原来轨道的性质.     

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.     

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     

苯硫酚分子内质子转移取代基效应的理论研究

在B3LYP/6-311+G(d, p)水平上研究了2-(1H-咪唑)苯硫酚(1d)、2-(噁唑)苯硫酚(2d) 、2-(噻唑)苯硫酚(3d)及其衍生物的基态质子转移过程, 探讨取代基电子效应对质子转移的影响. 结果表明: 吸电子引入后分子平面电子密度减小, N2-H1间距减小, 分子内氢键增强, 醇式到酮式质子转移能垒减小; 供电子基引入后分子平面电子密度增大, N2-H1间距增大, 分子内氢键减弱, 质子转移能垒增大. Localized orbital locator(LOL)分析表明: 取代基的引入对N1原子成键性质产生影响明显. 三者质子转移能垒大小为1d<3d<2d, 取代基引后能垒相对大小不变.     

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.     

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.     

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.     

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.     

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.     

Dynamics of Solvent Controlled ESIPT of π-Expanded Imidazole Derivatives - pH Effect

A set of π-expanded imidazole derivatives employing excited state intramolecular proton transfer (ESIPT) was designed and synthesized. The relationship between the structure and photophysical properties were thoroughly elucidated by comparing with the analogue blocked with ESIPT functionality. The compound possessing an acidic NH function as part of an intramolecular hydrogen bond system has much higher fluorescence quantum yield and Stokes shift and the π-expansion strongly influences the optical properties. The occurrence of ESIPT for imidazole tosylamide derivatives were less affected by the hydrogen-bonding ability of the solvents compared to the unprotected amine. The low pKa values for the monocation ? neutral equilibrium indicate the presence of intramolecular hydrogen bonding between the amino proton and tertiary nitrogen atom.     

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.     

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.     

Coumarin-Pyrazole Hybrid with Red Shifted ESIPT Emission and AIE Characteristics - a Comprehensive Study

The newly synthesized three coumarin pyrazole hybrid excited state intramolecular proton transfer (ESIPT) dyes show efficient charge transfer from the pyrazole ring and the coumarin towards the electron withdrawing dicyanovinylene group as revealed from the frontier molecular orbitals. Aggregation induced emission enhancement (AIEE) studies with 2-((3-(4-hydroxy-2-oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazol-4-yl)methylene) malononitrile showed 9 fold increase in the emission enhancement in 90% DMF-H₂O mixture. Lippert-Mataga theory explained the solvatochromic behavior of the dyes in various solvents. The charge transfer characteristics and non-linear optical (NLO) properties have been supported and correlated with bond length alternation, bond order alternation and vibrational spectrum. As values of bond order alternation (BOA) tend to be more towards negative and as the value of α increases β decreases while the values of γ depends on the values of α and β. The observed values of γ are positive which revealed that β contributes significantly. The dyes exhibit linear and NLO properties superior to urea. (E)-2-(3-(2-(3-(4-Hydroxy-2-oxo-2H-chromen-3-yl)-1-phenyl-1H-pyrazol-4-yl)vinyl)-5,5-dimethylcyclohex-2-en-1-ylidene) malononitrile shows enhanced linear and non-linear properties among the three dyes.