Photochemistry of a Trisilane Substituted by a Pendant p-Cyanostilbene Electron Acceptor Chromophore.

Abstract

Photolysis of (E)-4-cyano-4'-(2-heptamethyltrisilanyl)stilbene (E)-3 in a medium polarity solvent (CH(2)Cl(2)) results in efficient intramolecular electron transfer, which converts the initial pi,pi excited state ((1)LE state) to the charge transfer (CT) excited state. The CT excited state fluorescesces, undergoes E,Z photoisomerization, and reacts with MeOH to produce hydrosilane 4 via Si-Si bond cleavage and protonation of the central silicon, as shown by deuterium labeling. The CT excited state assignments are consistent with the observed quadratic plots of 1/Phi(f) versus [MeOH] and 1/Phi(EZ)() versus [MeOH], which indicate that both the initial (1)LE state and the CT excited state are quenched by MeOH, with the CT excited state serving as the emissive state and the state primarily responsible for the E,Z photoisomerization. Although biacetyl triplet sensitized photolysis results in efficient E,Z isomerization, quenching of direct photolyses by azulene shows (Z)-3 is not a product of the lowest energy triplet excited state, populated from the CT state by back electron transfer. The azulene quenching also shows that hydrosilane 4 is formed from the same excited state that gives (Z)-3. The intermediacy of a complex of the CT excited state with MeOH accounts for the observed upward curvature in the plot of 1/Phi(SiH) vs 1/[MeOH]. The linear behavior of Phi(EZ)()/Phi(SiH) x [MeOH] versus 1/[MeOH] further suggests that this complex reacts with uncomplexed MeOH to give hydrosilane 4.