Photoisomerization
of Silyl-Substituted Cyclobutadiene
Induced by σ → π* Excitation: A Computational
Study
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Abstract
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><sub>1</sub>, 1<sup>1</sup><i>B</i><sub>1</sub> state in terms of <i>D</i><sub>2</sub> symmetry), but to the second <sup>1</sup><i>B</i><sub>1</sub> state (<i>S</i><sub>3</sub>), 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><sub>3</sub> state. This mechanistic picture is in good agreement
with recently reported experimental observations