Theoretical characterizations of electronically excited silaazulene skeletons

Abstract

Silaazulene (SIAZ) skeletons where one of the C atoms in the azulene skeleton is replaced by Si atom have been characterized from a viewpoint of the excited state by means of ab initio complete active space self-consistent field (CASSCF) calculations. SIAZs studied in the present study are 2-silaazulene (2SIAZ) and 6-silaazulene (6SIAZ). The model reactions for the characterizations of 2SIAZ and 6SIAZ are an S 1–S 0 internal conversion (IC) process, which has been extensively studied on that of parent azulene. The initial processes of 2SIAZ and 6SIAZ upon electronic excitation into S 1 are similar to that of azulene, i.e. change from an aromatic geometry into a non-aromatic planar one. Contrary to the case of azulene, however, 2SIAZ and 6SIAZ in S 1 are further stabilized so as to take a non-planar geometry where the local geometry around the Si atom is non-planar. At the conical intersection between S 1 and S 0 (S 1/S 0-CIX) of 6SIAZ, only the seven-membered ring takes a non-planar geometry. On the other hand, both the five-membered and the seven-membered rings are non-planar at the S 1/S 0-CIX of 2SIAZ. Based on these computational findings, we characterize the SIAZ skeletons from a viewpoint of the excited state.