生色团连接的苯骈三氮唑衍生物的激发态分子内质子转移

用从头算和密度泛函理论研究了对硝基二苯乙烯作为生色团连接的2-(2-羟基-苯基)-苯骈三氮唑的衍生物2-羟基-5-[对硝基-二苯乙烯基-氧亚甲基]-苯基-(2H-苯骈三氮唑)(C1)和4′-硝基-3,4-二[2-羟基-(2H-苯骈三氮唑)-苄氧基]-二苯乙烯(C2)发生激发态分子内质子转移(ESIPT)的可能性.系统研究了C1和C2发生ESIPT的互变异构体的基态与激发态的性质变化,包括相关的键长、键角等结构参数,Mulliken电荷和偶极矩,前线轨道以及势能曲线.计算结果表明,对于C1来讲,酮式(keto)的基态(K)不存在稳定结构,因此发生基态分子内质子转移(GSIPT)可能性很小.酮式的激发态(K*)的氢键强度要远强于烯醇式(enol)的激发态(E*)的氢键强度.分子在光致激发后,质子供体所带负电荷减小而质子受体所带负电荷增加.在K*,HOMO→LUMO的电子跃迁导致电子密度从"酚环"向质子化杂环转移.E*→K*跃迁只需要克服较小的能垒(约41 kJ.mol-1).计算结果表明C1发生ESIPT的可能性很大.C2由于具有高能量,其具有基态的单质子转移特征的异构体EK(同时含烯醇E与酮K结构)、具有基态的双质子转移特征的异构体2K(含有双酮结构),以及具有双酮结构特征的激发态2K*均无法获得它们的稳定结构,因此,基态分子内单或双质子转移和激发态分子内双重质子转移发生的可能性极小.然而,由于双烯醇式的激发态(2E*)和EK的激发态(EK*)存在稳定结构,且2E*→EK*跃迁具有低能垒,因此C2有可能发生激发态分子内单重质子转移.本文进一步计算了两个分子的紫外-可见吸收光谱与荧光发射光谱,获得了具有较大斯托克位移的ESIPT的荧光发射峰.

Excited State Intramolecular Proton Transfer of a Chromophore Linked to Benzotriazole Derivatives

This paper presents ab initio and density functional theory (DFT) calculations on the excited state intramolecular proton transfer (ESIPT) of a chromophore linked to 2-(2-hydroxyl-phenyl)-benzotriazole:{2-hydroxyl-5-[(p-nitro-styrene)yl-oxymethylene]-phenyl}-(2H-benzotrizole)(C1) and 4'-nitro-3,4-bis-[2-hydroxyl-(2H-benzotrizole)-benzyloxy]-stilbene(C2).We undertook a comprehensive investigation on the ground state and excited state changes of the ESIPT tautomers including their bond lengths,bond angles,Mulliken charges,dipole moments,frontier orbitals,and potenfial surface curves.A stable keto form (K) was not obtained for C1 in the ground state and a ground state intramolecular proton mansfer (GSIPT) was,therefore,impossible.The hydrogen bond strength of the keto form(K~*)was higher than that of the enol form (E~*) at their excited states.When excited,the negative charge of the hydrogen donor Was diminished while the negative charge of the hydrogen acceptor Was enhanced.For the excited state of keto from.the electron density moved from a"phenol cycle"to a protonated heterocycle because of the HOMO→LUMO electron transition.A small energy barrier from E~* to K~*(ca 41 kJ·mol~(-1)) was also observed.These resuIts indicated that the possibility of ESIPT occurring was high for C1.For C2,stable sffuctures for the EK (including keto and enol (E) forms simultaneously),2K (including two keto forms simultaneously),and 2K~* (the excited state of 2K) tautomers could not be obtained because of their high energies.As a consequence,the probability of intramolecular single or double proton transfer at the ground state or excited state intramolecular double proton transfer occurring Was negligible.The low transition energy for 2E~* (the excited state of 2E)→EK~* (the excited state of EK) suggested that an excited state intramolecular single proton transfer was very possible for C2.UV-Vis absorption and fluorescence spectra were calculated and an ESIPT fluorescence emission with a large Stoke's shift was observed.