Photoprotodesilylation of 9-trimethylsilylanthracene in alcohols

Abstract

It was found that the anomalous fluorescence quenching in methanol (as compared to other solvents) of 9-trimethylsilylanthracene (2) is accompanied by a chemical reaction leading to anthracene. This ipso substitution is due to a hydrogen bonding formation in the excited singlet state preceding the carbon–silicon bond cleavage. The mechanism was studied using stationary and dynamic fluorescence, laser flash photolysis and reactivity kinetics as a function of temperature. The kinetic parameters were determined for the formation of an intermediate “X” (A_(X) ≈2.5 × 10¹¹ s⁻¹, Ea_(X) ≈ 19.5 kJ mol⁻¹) and for the global reaction (A_(R) ≈ 4 × 10¹⁰ s⁻¹ and Ea_(R) ≈ 19.2 kJ mol⁻¹), indicating that, after the formation of X, the reaction does not involve any step with an activation energy larger than 19 kJ mol⁻¹. The intermediate X undergoes partitioning between the product (α ≈0.18) and the starting material. The isotopic effect for the quenching by methanol was found to be k_X^H/k_X^D ≈ 1.9, in keeping with the proposed protonation in the excited singlet state (S₁). The reactivity ratio between the S₁ state and the ground state, k_R(S₁)/k_R(S₀) ≈ 10¹² is in line with those observed by other authors. This unusual photoreaction can proceed for 2 with other alcohols. ipsoProtodesilylation of aromatic silanes are known to occur in the ground state but in acidic media.