氢键加强引起的链式激发态多质子转移的动力学研究进展

Excited-state of multiple proton transfer via molecular wire facilitated by hydrogen bond strengthening in excited state

作为自然界中最广泛存在的基本反应过程之一,质子转移(proton transfer,PT)一直是动力学研究中的一个重要话题.而沿着一系列氢键进行的多质子转移过程,更是由于绿色荧光蛋白的发现及广泛应用,成为了近年来的热门课题.我们使用密度泛函理论(DFT)对新兴的链式激发态多质子转移过程进行了系统的理论研究.我们以赵广久与韩克利研究员建立的激发态氢键加强理论为基础,对6-hydroxyquinoline(6HQ)光酸分子及其阳离子形式6HQc的激发态质子转移反应,给出了新的解释及机理:(1)在一个体系中,氢键加强与氢键减弱可能会同时存在,归结其原因,主要取决于激发态分子的电荷再分布;(2)单个或者少数的水分子是很难从酸性分子上夺得质子的,只有当水分子形成更大的团簇(即氢键更多)时,其夺质子能力会大大加强;(3)链式激发态多质子转移(即多质子转移机理)并不单纯的是分步机理或者是协同机理,也有可能是分步与协同相结合的进行,是一种复杂多变的反应.

As the most common reactions in nature,Proton transfer（PT） is one of the most important subjects in dynamic research.In particular,the novel excited-state multiple proton transfer（ESMPT） occurred along hydrogen-bonded wire illustrates a good prospect of application in the aspects of green fluorescent protein.In this work,we systematically study the dynamic behavior of ESMPT by using density functional theory（DFT） and time-dependent density functional theory（TDDFT） methods.Based on the excited-state hydrogen bond strengthening theory established by Zhao Guang-Jiu and Han Ke-Li,we obtained some novel mechanism for the excited-state proton transfer（ESPT） reaction of 6-hydroxyquinoline（6HQ） and its cationic form 6HQc：（1） Excited-state hydrogen bond strengthening and weakening could exist simultaneously in one hydrogen-bonded complex,which dependent on the charge transfer upon photoexcitation.（2） Only the water cluster containing more than 3 water molecules could capture the proton in photoacid,while the single water molecule is incapable for this PT reaction.（3） The mechanism of ESMPT reaction is a complicated process,which might contain both the step-wise and concerted fashions.