Photodimerization of benzyl 9-anthroate in the presence of triethylamine. Role of exciplex and triplet formation

Abstract

The mechanism of the photodimerization of benzyl 9-anthroate in benzene has been investigated in the absence and presence of triethylamine. Whilst in the direct irradiation the dimer formation proceeds essentially via the singlet state, it could also be formed in a triplet sensitized reaction.

The irradiation of the dimer at 77 K regenerates the monomer and leads to the appearance of a new emission at 493 nm assigned to an excimer. The estimate of the binding enthalpy for the excimer formation as ΔH²⁹³_e=–2.9 kcal mol^–1 leads to the conclusion that the singlet excimer in this system is thermodynamically unfavourable.

The estimation of triplet yields and the determination of quantum yields in the presence of the amine support a mechanistic scheme in which the triplet population can be achieved not only by direct intersystem crossing but also by the intersystem crossing of the exciplex formed either via the singlet state and/or from excitation of the ground state complex.

The contribution of these effects to the total quantum yield of dimerization is discussed.