Triplet Energy Transfer versus Excited State Cyclization as the Controlling Step in Photosensitized Bipyrimidine...

Polymethylene-linked bipyrimidine models have been designed with different C5 substitutions and bridge lengths. Selective irradiation of 2'-methoxyacetophenone (2M) with the bipyrimidine models affords cyclobutane pyrimidine dimers, even in the presence of bulky substituents. Substitution at C5 affects both the relative triplet energies (E_T(rel)) of the pyrimidines (Pyr) and the steric hindrance toward intermolecular energy transfer and intramolecular triplet Pyr* quenching. Photophysical studies showed that alkyl substitution resulted in a significant decrease in the E_T(rel) value. Quenching of the triplet excited state of 2M by the Pyr derivatives was proven and established their quenching rate constants (k_q). As a general trend, the thymine-containing compounds showed k_q values higher than 10⁹ M^-1 s^-1, while in the uracil and tert-butyluracil analogues, k_q was markedly lower. These data are explained considering three different scenarios: (a) triplet energy transfer is the rate controlling step, (b) excited state cyclization is the rate controlling step, and (c) the rate controlling step switches along the reaction. Thus, by introducing variations in the substitution at C5, the length of the linking bridge, or the substrate concentration, it is possible to switch from a process governed by the intrinsic dimerization step to an energy transfer-controlled process.