Thymine Dimerization in DNA Model Systems: Cyclobutane Photolesion Is Predominantly Formed via the Singlet Channel

UV-induced formation of cylcobutane pyrimidine dimers (CPD) in all thymine DNA models have been studied by femtosecond IR spectroscopy. CPDs are shown to form within similar to 1 ps during the decay of the initially excited (1)pi pi(star) state. The quantum yields phi(ps)(D) determined after the (1)pi pi(star) decay equal the final yield phi(cw)(D). This gives evidence for a predominance of the singtet channel in CPD formation.