Photoisomerization of Silyl-Substituted Cyclobutadiene Induced by σ → π* Excitation: A Computational Study

Photoinduced chemical processes upon Franck–Condon (FC) excitation in tetrakis­(trimethylsilyl)-cyclobutadiene (TMS-CBD) have been investigated through the exploration of potential energy surface crossings among several low-lying excited states using the complete active space self-consistent field (CASSCF) method. Vertical excitation energies are also computed with the equation-of-motion coupled-cluster model with single and double excitations (EOM-CCSD) as well as the multireference Møller–Plesset (MRMP) methods. Upon finding an excellent coincidence between the computational results and experimental observations, it is suggested that the Franck–Condon excited state does not correspond to the first π–π* single excitation state (<i>S</i>₁, 1¹<i>B</i>₁ state in terms of <i>D</i>₂ symmetry), but to the second ¹<i>B</i>₁ state (<i>S</i>₃), which is characterized as a σ–π* single excitation state. Starting from the Franck–Condon region, a series of conical intersections (CIs) are located along one isomerization channel and one dissociation channel. Through the isomerization channel, TMS-CBD is transformed to tetrakis­(trimethylsilyl)-tetrahedrane (TMS-THD), and this isomerization process could take place by passing through a “tetra form” conical intersection. On the other hand, the dissociation channel yielding two bis­(trimethylsilyl)-acetylene (TMS-Ac) molecules through further stretching of the longer C–C bonds might be more competitive than the isomerization channel after excitation into <i>S</i>₃ state. This mechanistic picture is in good agreement with recently reported experimental observations