扭曲单次甲基菁的价键非平衡溶剂化模型
Valence-bond Non-equilibrium Solvation Model for a Twisting Monomethine Cyanine
摘要：
提出并分析了一次甲基菁激发态扭转反应的非平衡溶剂化效应的二态价键模型。该反应的抑制被认为是这些染料中环境依赖性荧光产率增强的原因。荧光猝灭，因为扭曲伴随着通过形成暗扭曲分子内电荷转移（TICT）状态。对于单次甲基花菁，基态是具有不同键和电荷局域化的结构的叠加，在激发态中存在两种可能的具有不同电荷局域化的扭曲路径。对于对应于对称单次甲基的参数，该模型预测两个低能量扭曲的激发态表面上的通道，导致扭曲的分子内电荷转移（TICT）状态的歧管。对于典型的单次甲基，在激发态表面上的扭曲将在小势垒或没有势垒的情况下发生。溶剂化配置的变化可以不同地稳定对应于不同键的通道中的TICT状态，并且绝热状态之间的圆锥形交叉点的位置响应于溶剂化而移动以稳定一个通道或另一个通道。随着溶剂极性的增加，在激发态电势的底部有一个圆锥形的相交缝。对于具有中等大小的端基的单次甲基花菁，在中等极性溶液中，激发态势面的底部完全由圆锥形相交缝跨越。
英文摘要：
We propose and analyze a two-state valence-bond model of non-equilibrium solvation effects on the excited-state twisting reaction of monomethine cyanines. Suppression of this reaction is thought responsible for environment-dependent fluorescence yield enhancement in these dyes. Fluorescence is quenched because twisting is accompanied via the formation of dark twisted intramolecular charge-transfer (TICT) states. For monomethine cyanines, where the ground state is a superposition of structures with different bond and charge localization, there are two possible twisting pathways with different charge localization in the excited state. For parameters corresponding to symmetric monomethines, the model predicts two low-energy twisting channels on the excited-state surface that lead to a manifold of twisted intramolecular charge-transfer (TICT) states. For typical monomethines, twisting on the excited state surface will occur with a small barrier or no barrier. Changes in the solvation configuration can differentially stabilize TICT states in channels corresponding to different bonds, and that the position of a conical intersection between adiabatic states moves in response to solvation to stabilize either one channel or the other. There is a conical intersection seam that grows along the bottom of the excited-state potential with increasing solvent polarity. For monomethine cyanines with modest-sized terminal groups in moderately polar solution, the bottom of the excited-state potential surface is completely spanned by a conical intersection seam.
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