Theoretical characterizations of electronically excited silaazulene skeletons

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 S1–S0 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 S1 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 S1 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 S1 and S0 (S1/S0-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 S1/S0-CIX of 2SIAZ. Based on these computational findings, we characterize the SIAZ skeletons from a viewpoint of the excited state.