Photoprotodesilylation of 9-trimethylsilylanthracene in alcohols

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)) approximate to 2.5 x 10(11 )s(-1), Ea((X)) approximate to 19.5 kJ mol(-1)) and for the global reaction (A((R)) approximate to 4 x 10(10) s(-1) and Ea((R)) approximate to 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 (a approximate to 0.18) and the starting material. The isotopic effect for the quenching by methanol was found to be k(x)H/k(X)(D) approximate to 1.9, in keeping with the proposed protonation in the excited singlet state (S-1). The reactivity ratio between the S-1 state and the ground state, k(R)(S-1)/k(R) (S-0) approximate to 10(12) is in line with those observed by other authors. This unusual photoreaction can proceed for 2 with other alcohols. ipso Protodesilylation of aromatic silanes are known to occur in the ground state but in acidic media.