An analysis of heavy-atom perturbation of intersystem crossing as a mechanistic tool in photochemistry : Probing for singlet and triplet dominated reactions

Abstract A kinetic analysis of heavy-atom enhanced intersystem crossing is presented, with the object of using this perturbation method for determining whether the majority of a photoproduct originates from an excited singlet or an excited triplet precursor. Expressions are derived which relate the quantum efficiencies of product formation (φ r ) and S 1 →T 1 intersystem crossing (φ isc ) with potential quenching or enhancement by the heavy-atom. These allow limits to be placed on the possible quenching or enhancement that may be observed in particular cases (see Fig. 4 and 6) and thereby make an interpretation of the data less ambiguous than has been suggested in recent discussions. A particularly useful observation is that for those molecules (for example, aromatics) having triplets which are relatively unaffected by heavy-atoms, quenching of a product by more than 50% demonstrates that a majority of this product must be derived from the excited singlet state.