激发态分子内质子转移分子2-羟基-1-萘甲醛半碳酰腙的光谱特性

通过考察2-羟基-1-萘甲醛半碳酰腙(HNLSC)在不同极性溶剂中的吸收光谱和荧光光谱,详细研究了HNLSC分子在不同溶剂及酸、碱条件下的不同构型,证实了HNLSC具有典型的ESIPT特性。在非极性溶剂中分子主要以分子内氢键的闭式构型存在,这种闭式构型使分子具有ESIPT特性,在环己烷溶剂和高酸度极性溶剂中分子均表现出～415nm的正常荧光和～435nm处的反常ESIPT荧光。在极性质子溶剂中,因溶质和溶剂之间形成了分子间的氢键以及进一步去质子化,HNLSC形成了基态的溶剂化开式构型和离子构型,在吸收光谱中表现出～395nm的离子构型特征吸收。开式构型和离子构型阻断了分子内质子转移途径,因而在荧光光谱中仅表现出一个特征峰。实验进一步通过三乙胺和稀硫酸调节溶液体系的极性和酸度环境,证明在不同溶剂极性和酸度环境下,HNLSC分子不仅存在萘环上羟基变化引起的多种互变异构体间的转化平衡,同时存在—CHN—NH—CO—NH2结构域的烯醇式和酮式结构的相互转化。     

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 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.     

Excited state intramolecular proton transfer in amino 2-(2′-hydroxyphenyl)benzazole derivatives: Effects of the solvent and the amino group position

The excited-state intramolecular proton transfer (ESIPT) mechanism in six amino 2-(2′-hydroxyphenyl)benzazole derivatives were investigated in different solvents by means of UV-vis absorption and steady-state fluorescence. The amino benzazoles are fluorescent in the blue-orange region under UV radiation. Changes in the absorption, emission and excitation spectra were analyzed and correlated to the position of the amino group and the solvent polarity. The equilibrium between the conformers in solution in the ground state, confirmed by the solvatochromic effect, reflects the dual fluorescence emission presented by these dyes.     

激发态质子转移分子2-（2′-羟基苯基）苯并噻唑在不同溶剂中的光谱特性

观测了2-（2′-羟基苯基）苯并噻唑（HBT）在不同极性溶剂中的吸收光谱和荧光光谱,详细研究了溶剂极性对HBT发生激发态分子内质子转移（ESWT）影响的机制。吸收光谱表明在常态条件下,HBT在各种溶剂中都以烯醇式构型和酮式构型共同存在,但以烯醇式构型占绝大多数。荧光光谱表明在纯环己烷溶剂中,HBT被紫外光激发时,绝大多数烯醇式构型发生ESIPPT转变为酮式构型,分子的ESIPT效率最大。在含有乙醇的极性溶剂中,HBT烯醇式会形成溶剂化的烯醇式构型,阻碍分子发生ESIPT反应。溶剂中乙醇含量愈多极性愈强,溶剂化烯醇式的成份就愈多,HBT的ESIPT效率就愈低。以400nm光激发HBT溶液时,在510nm处发现酮式构型荧光,从而确认了400nm处的弱吸收是酮式构型的吸收;且在436和456nm处还有新的荧光峰,分析其可能来源于酮式构型去质子化阴离子的发射。     

激发态质子转移分子2-(2'-羟基苯基)苯并噻唑在不同溶剂中的光谱特性

观测了2-(2’-羟基苯基)苯并噻唑(HBT)在不同极性溶剂中的吸收光谱和荧光光谱,详细研究了溶剂极性对HBT发生激发态分子内质子转移(ESIPT)影响的机制。吸收光谱表明在常态条件下,HBT在各种溶剂中都以烯醇式构型和酮式构型共同存在,但以烯醇式构型占绝大多数。荧光光谱表明在纯环己烷溶剂中,HBT被紫外光激发时,绝大多数烯醇式构型发生ESIPT转变为酮式构型,分子的ESIPT效率最大。在含有乙醇的极性溶剂中,HBT烯醇式会形成溶剂化的烯醇式构型,阻碍分子发生ESIPT反应。溶剂中乙醇含量愈多极性愈强,溶剂化烯醇式的成份就愈多,HBT的ESIPT效率就愈低。以400 nm光激发HBT溶液时,在510 nm处发现酮式构型荧光,从而确认了400 nm处的弱吸收是酮式构型的吸收;且在436和456nm处还有新的荧光峰,分析其可能来源于酮式构型去质子化阴离子的发射。     

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.     

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.     

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     

2-(2''''-羟基苯基)间氮杂氧茚放大的自发辐射效应的实验和理论研究

报道了在氮分子激光泵浦下，激发态分子内质子转移分子2－（2＇－羟基苯基）间氮杂氧茚（HBO）环乙烷溶液放大的自发辐射（ASE）实验和理论研究。在环己烷溶液中，HBO的增益系数α（510um）约为1．2cm-1。在建立了HBO激发态分子内质子转移（ESIPT）的放大的自发辐射动态模型基础上，通过数值模拟得到了HBO的增益光谱和放大的自发辐射光谱，计算结果与实验很好相符，证实了HBO的酮式异构体的基态寿命更接近于260ns而非亚纳秒级。     

Interaction of the excited state intramolecular proton transfer probe 3-hydroxy-2-naphthoic acid with poly N-vinyl-2-pyrrolidone polymer in water: An insight into the water structure in the binding region

The enhancement of the excited state intramolecular proton transfer (ESIPT) emission of 3-hydroxy-2-naphthoic acid (3HNA) in presence of poly N-vinyl-2-pyrrolidone (PVP) has been investigated by time resolved fluorescence and anisotropy measurement. In other water soluble polymers and monomers there is no enhancement of the ESIPT emission of 3HNA. The microenvironment of the probe in PVP-water mixture is investigated by comparing the ESIPT emission of 3HNA in mixed solvents of acetonitrile-toluene. The anisotropy, rotational correlation time and the blue shift observed in the ESIPT emission in PVP-water mixture have been ascribed to both the polarity and the motional restriction of the probe imposed by the bound water region of PVP.     

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.     

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.     

Intramolecular energy and charge transfer in 5-(9-anthryl)-3-(4-nitrophenyl)-1-phenyl-2-pyrazoline

The absorption and photoluminescence of the newly synthesized 5-(9-anthryl)-3-(4-nitrophenyl)-1-phenyl-2-pyrazoline (AN PP) were investigated. The absorption is the absorption of anthryl moiety at about 325-400 nm, superimposed on the broader absorption of 3-(4-nitrophenyl)-1-phenyl-2-pyrazoline moiety peaked at 420 nm. On excitation at 420 nm, the fluorescence spectrum has only one emission band from the pyrazoline moiety. This emission band exhibits a larger red shift with an increase in the polarity of solvents. But on excitation at 365 nm, the fluorescence spectrum has two emission bands coming from the anthryl and pyrazoline moieties, respectively. The intensity ratio of the two bands is different in solvents of different polarity. It is concluded that photo-induced intramolecular energy transfer from the anthryl to pyrazoline moiety exists simultaneously with the charge transfer from N (1) to C (3) in the pyrazoline moiety in the excited state and both compete with each other.     

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 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.     

Photo-induced charge transfer emission in (E)-3-(4-dimethylamino-naphthalen-1-yl)-acrylic acid and its use as metal ions fluorosensor

In this study, spectroscopic properties of (E)-3-(4-dimethylamino-naphthalen-1-yl)-acrylic acid (DMANAA) have been explored using absorption and emission spectroscopy in combination with quantum chemical calculations. In addition to the local emission, the molecule shows a red-shifted emission from the charge transfer state. The charge transfer emission band is found to be dependent on polarity and hydrogen-bonding ability of the solvents. This photo-induced charge transfer process has been explored theoretically at Density Functional Theory level using twisted intramolecular charge transfer model. Structural calculations and potential energy surfaces along the donor twist coordinate done by Density Functional Theory correlate well with the spectroscopic observations. Addition of metal ions to DMANAA solutions in acetonitrile solvent shows progressive quenching of the charge transfer band and growth of a blue-shifted band, making way for its use as a good fluorosensor for various metal ions.     

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.     

1-Hydroxy-2-naphthaldehyde: A prospective excited-state intramolecular proton transfer (ESIPT) probe with multi-faceted applications

The present account aims at amassing and recounting on our series of spectroscopic studies with a potential excited state intramolecular proton transfer (ESIPT) probe 1-hydroxy-2-naphthaldehyde (HN12). After a detailed investigation from experimental as well as theoretical viewpoints, a deeper insight into the photophysics of the selected molecular system is provided from thorough spectral deciphering of the effects of solvent, medium pH and temperature. In the following sections, the ESIPT emission of HN12 has been documented to be a potential avenue wherefrom characterization of a wide variety of biological, biomimetic and supramolecular assemblies has been executed to commendable fruition. Efforts are also invested to delineate the location, distribution and strength of interaction of the probe with various microheterogeneous environments.     

Spectral behavior and laser activity of 3-(4′-dimethylaminophenyl)-1-(1H-pyrrol-2-yl) prop-2-en-1-one (DMAPrP). A new laser dye

The photophysical properties of DMAPrP have been investigated in different solvents. DMAPrP dye exhibits a large change in dipole-moment upon excitation due to an intramolecular charge transfer interaction. A crystalline solid of DMAPrP give an excimer like emission at 546 nm. The ground and excited state protonation constants of DMAPrP are calculated. DMAPrP acts as good laser dye upon pumping with nitrogen laser in some organic solvents. The laser parameters such as the tuning range, gain coefficient (α), emission cross section (σ_e) and half-life energy (E_1/2) are also calculated. The photoreactivity and net photochemical quantum yield of DMAPrP in chloromethane solvents are also studied.