Quantum chemical study of the Si–C bond photodissociation in benzylsilane derivatives: a specific ‘excited-state’ silicon effect

Abstract

Structure and properties of the ground ( S 0) and lowest excited triplet ( T 1) states of benzylsilane derivatives bearing the benzophenone chromophore group, p-PhCO-C 6H 4-CR″R‴-SiR′ 3 (R′, R″, R‴=H, H, H; Me, H, H; Me, H, Me; Me, Me, Me; Me, H, Ph; Me, Me, Ph; Me, Ph, Ph), were calculated using the semiempirical PM3 method. The bond dissociation energy (BDE) of the Si–C bond in the S 0 state and the heat of the bond dissociation reaction (Δ H r) in the T 1 state were found to decrease with increasing substitution at silicon and/or benzylic carbon atom. There exists a straight-line dependence between the BDE and Δ H r values in the series of the compounds studied. Minimal energy paths for the ‘heterobenzylic’ Si–C bond dissociation in benzylsilane PhCH 2-SiH 3 and for the benzylic C–C bond dissociation in the carbon analogue ethylbenzene PhCH 2-CH 3 in T 1 states were calculated using PM3 and B3LYP/6-31G**. The activation energies obtained are 13.3 and 36.7 kcal/mol (PM3), and 6.3 and 19.6 kcal/mol (B3LYP/6-31G** with ZPE correction) for benzylsilane and ethylbenzene, respectively. Both methods predict much lower activation barrier for benzylsilane compared to ethylbenzene. The difference in activation energies explains the experimentally observed high quantum yields (up to 0.9) of the Si–C bond photodissociation in silicon derivatives of benzophenone as compared to the C–C bond photodissociation in the corresponding carbon analogues (quantum yields <0.17).