A theoretical study on the role of conical intersection in the photioisomerization of conjugated polyenes

π共轭多烯的光异构化在化学、物理、生物和材料科学等领域引起了广泛的关注。然而，合理的画面离现实还很远。其中最重要的成分之一是关于发生松弛的圆锥相交（CIX）的信息。本课题采用可靠的从头算分子轨道（MO）计算方法，计算了π共轭多烯的cix和其他重要构象，从而给出了π共轭多烯光化学的新的、合理的图象。本项目实际处理的分子有苯乙炔（PA）、二苯乙炔（DPA）和二甲氨基苯腈（DMABN）。由于缺少S_2/S_1-CIX，这些图片不合理。PA: S_2/S_1-CIX具有两个特征。即苯环失去芳香性，变为类醌结构，乙基部分变为类丙烯结构。S_2/S_1-CIX的特征可以合理地描绘复杂的吸收光谱。DPA: S_1（S_1-geometry）的全局稳定结构为C_<2h>对称的反弯形式。S_/S_1-CIX位于D_<2h>结构和s_1 -几何的中间。通过势能面分析，我们给出了一个合理的时间行为图。DMABN：在S_2/S_1-CIX处，DMABN具有醌类结构，二甲氨基部分与其余部分垂直扭曲。S_2/S_1-CIX是电荷转移和π-π^*局域激发态之间的交叉区。为了解决DMABN光化学中TICT和PICT模型之间的严重争议，我们进一步分析了势能面。

The photoisomerizations of π-conjugated polyenes have been of great interest in a wide variety of chemistry, physics, biology and material science. However, the reasonable pictures are still far from reality. One of the most important ingredients is the information on the conical intersection (CIX) where the relaxation takes place. In this project, we calculated CIXs as well as other important conformations by means of reliable levels of ab initio molecular orbital (MO) calculations so as to give new and reasonable pictures of photochemistry of π-conjugated polyenes. Actual molecules treated in this project are phenylacetylene (PA), diphenylacetylene (DPA) and dimethylaminobenzonitrile (DMABN). These have no reasonable pictures because of lack of S_2/S_1-CIX.PA : S_2/S_1-CIX is characterized by two factors. That is, the benzene ring loses its aromaticity and turns to be a quinoid structure and the ethynyl part turns to be allene-like. The feature of S_2/S_1-CIX is possible to give a reasonable picture of complicated absorption spectrum.DPA : The globally stable structure in S_1(S_1-geometry) takes a trans-bent form with C_<2h> symmetry. The S_/S_1-CIX is located in the half way from the D_<2h> structure and S_1-geometry. We gave a reasonable picture on the temporal behavior by an analysis of potential energy surfaces.DMABN : At S_2/S_1-CIX, DMABN has a quinoid structure where the dimethylamino part is perpendicularly twisted against the remaining part. S_2/S_1-CIX is a crossing region between charge transfer and π-π^* locally excited states. We further analyzed the potential energy surfaces so as to settle down serious controversy between TICT and PICT models for the DMABN photochemistry.