一个新的氨基型氢键分子的激发态分子内质子转移的理论研究

Theoretical Insight into Excited-State Intramolecular Proton Transfer of a New Amino-Type Hydrogen Bonding Molecule

运用密度泛函理论(DFT)方法,优化得到了2-(5′-氰基-2′-甲基氨基苯基)苯并噻唑(CN-PBT-NHMe)的N式构型的基态稳定结构.采用含时密度泛函理论(TD-DFT)方法,优化得到了CN-PBT-NHMe的N式和T式构型的第一激发态的稳定结构,并在相同理论水平下,计算了它们的吸收和发射光谱.此外,用TD-DFT方法还探讨了CN-PBT-NHMe发生激发态分子内质子转移(ESIPT)的反应机制.研究表明,CN-PBT-NHMe分子的最大吸收峰位于3.26eV,与可用的实验值(3.15eV)符合得很好.它的N式和T式结构的发射峰分别位于2.94和2.25eV,与实验上测得的双重发射峰(2.84和1.99eV)吻合得也很好.在紫外光的照射下,CN-PBT-NHMe分子被光激发到Franck-Condon区域后,它在第一激发态上会发生一个缓慢的质子转移反应,其反应的能垒高达0.33eV.这种缓慢的ESIPT反应是在实验上观测得到CN-PBT-NHMe分子发生双重荧光的主要原因.

Density functional theory（DFT）approach has been used to optimize the ground-state（S0）equilibrium structure of the normal form of 2-（5′-cyano-2′-methylaminophenyl）benzothiazole（CN-PBT-NHMe）.Timedependent density functional theory（TD-DFT）method has been used for optimizations of the excited singlet state（S1）involving in the normal（N）and tautomer（T）forms.The B3 LYP density functional and the 6-311＋＋G（d,p）basis set have been chosen,and the polarizable continuum model（PCM）using dichloromethane as solvent has been considered throughout the whole theoretical studies unless otherwise specified.In addition,the absorption and emission spectra of CN-PBT-NHMe have also been calculated at the TD-B3LYP/6-311＋＋G（d,p）level of theory.To explore the excited-state intramolecular proton transfer（ESIPT）reaction mechanism,the constrained energy profiles（CEPs）along the proton transfer coordinate have been computed at the same theoretical level.These studies indicate that the absorption peak of CN-PBT-NHMe is calculated to locate 3.26 eV,which agrees very well with the available experimental value of 3.15 eV.The emission bands of its N and T forms are predicted to be at 2.94and2.25 eV,respectively,also in good agreement with the emission peaks at 2.84 and 1.99 eV observed experimentally,suggesting that the fluorescence emission of CN-PBT-NHMe should arise from the contribution of both N and T forms.Upon UV irradiation,when CN-PBT-NHMe molecule is populated into the Franck-Condon region,a slow proton transfer is found in the S1 state,and the reaction barrier is as high as 0.33 eV.Such a remarkably slow ESIPT reaction is considered to be responsible for the dual fluorescence of CN-PBT-NHMe observed in experiment.