Photoisomerization of simple merocyanines: a theoretical and experimental comparison with polyenes and symmetric cyanines
作者:I. Baraldi、F. Momicchioli、G. Ponterini、A. S. Tatikolov、D. Vanossi
DOI:10.1039/b208829k
日期:2003.2.27
The structure and electronic spectra of merocyanines are intermediate between those of acyclic polyenes and cationic symmetric cyanines of comparable sizes, shifting from one to the other as a result of a solvent polarity change. In this paper we address, both theoretically and experimentally, the question of how the photochemical behaviour of simple merocyanines compares with that of polyenes and symmetric cyanines. After a brief theoretical re-appraisal of the absorption maxima dependence on the chain length in the three classes of chromogens, we analyse the calculated potential-energy curves for two photoisomerization coordinates of a polyene, a symmetric cyanine and a merocyanine of comparable sizes. The results concerning both the energy curves and the nature of the relevant electronic states, particularly the TICT (twisted intramolecular charge transfer) character of the S1 state at the perpendicular geometry, reveal the existence of a near relationship between merocyanines and symmetric ionic polymethines, traceable back to their being odd-alternant systems. The presence of a low-lying 1(nπ*) state at the trans geometry and the associated solvent-modulated vibronic coupling with the 1(πHπL*) state dominate the experimentally accessible photochemical behaviour of two simple merocyanines and make it peculiar with respect to those of both polyenes and symmetric ionic cyanines.
美拉德花青素的结构和电子光谱介于无环多烯类和大小相当的阳离子对称氰类之间,在溶剂极性改变时会从其中一种转变为另一种。在本文中,我们从理论和实验两方面探讨了简单梅花氰的光化学行为与多烯和对称氰的光化学行为之间的比较。在对三类致色剂的吸收最大值与链长的关系进行简要的理论重新评估后,我们分析了计算出的多烯、对称氰和大小相当的美拉德菁的两个光异构化坐标的势能曲线。有关能量曲线和相关电子态性质的结果,特别是垂直几何形状下 S1 态的 TICT(扭曲分子内电荷转移)特性,揭示了美罗氰与对称离子聚甲醛之间存在着一种近似关系,可追溯到它们是奇数对立体系。反式几何中存在的低洼 1(nπ*)态以及与 1(πHπL*)态相关的溶剂调制振子耦合,主导了两种简单美拉德花青素的实验光化学行为,并使其与多烯类和对称离子氰类的光化学行为截然不同。