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反应在化学、物理、生物学和材料科学中引起了广泛的兴趣。然而，合理的图景仍与现实相去甚远。最重要的成分之一是关于松弛发生的锥形交点(CIX)的信息。在这个项目中，我们通过可靠的从头算分子轨道(MO)水平计算了CIX以及其他重要的构象，从而给出了新的合理的π共轭多烯的光化学图像。该项目中实际处理的分子是苯乙炔(PA)、二苯乙炔(DPA)和二甲氨基苯甲腈(DMABN)。这些都没有合理的图片，因为缺乏S_2/S_1-CIX。PA：S_2/S_1-CIX由两个因素表征。也就是说，苯环失去了芳香性，变成了奎宁结构，乙炔部分变成了类似联烯的结构。S_2/S_1-CIX的特征可能给出复杂吸收光谱的合理图像。DPA：S_1(S_1-几何)的全局稳定结构呈反折形式，具有C_&lt；2；h&gt；对称。S/S_1-CIX位于D_2小时构造和S_1几何结构的中途。DMABN：在S_2/S_1-CIX处，DMABN的二甲氨基部分与其余部分垂直扭曲，具有奎尼型结构。S_2/S_1-CIX是电荷转移和π-π^*局域激发态的交叉区。我们进一步分析了势能面，以解决TICT和PICT模型之间关于DMABN光化学的严重争议。

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.