基于ESIPT机理的极性敏感荧光探针的理论研究

Theoretical Study of an Excited-State Intramolecular Proton Transfer(ESIPT)Based and Polarity-Sensitive Fluorescent Probe

激发态分子内质子转移(ESIPT)型荧光探针因具有双发射和斯托克斯位移大的特性,近年来受到广泛的关注。实验发现,随着溶剂极性的增大,此类荧光探针M-HA的发射光谱红移且发光强度增强,但这一奇异现象的光物理机制并不清楚。从第一性原理出发,我们研究了M-HA在五种不同极性溶剂中的辐射跃迁和无辐射衰减过程。研究发现,随着溶剂极性的增大,M-HA的吸收和发射波长先红移后趋于不变,激发态表现出杂化的局域-电荷转移(HLCT)特征。烯醇式在无辐射衰减过程中的振动弛豫程度较大,因而发光较弱。酮式的振动弛豫程度随溶剂极性的增大逐渐减小,因而发光逐渐增强。结构弛豫分析表明,酮式的烯基氯及与其相连的部位受溶剂极性影响较大,乙基吲哚部位却影响不大。

Fluorescent probes based on excited-state intramolecular proton transfer(ESIPT)have aroused wide attention due to their characteristics of dual emission and large Stokes shift.M-HA of this type is experimentally observed to exhibit red-shifted and enhanced emission with the rise of solvent polarity.However,the photophysics behind such intriguing phenomenon remains unclear.Herein,we investigated the radiative and non-radiative decay processes of M-HA in five solvents with different polarity.It is found that the absorption and emission wavelength of M-HA are firstly red-shifted then tend to be constant with increasing solvent polarity,and the excited states present hybridized local and charge-transfer(HLCT)features.The enol form provides considerable vibrational relaxations during the non-radiative decay process and thereby displays weak fluorescence in all solvents.The keto form gives gradually decreased vibrationalrelaxations,and therefore its emission becomes brighter with the elevated solvent polarity.Geometric relaxation analysis indicates that vinyl chlorine and its adjacent regions are susceptible to the solvent polarity,but the ethyl indoline part is hardly affected.