Bright and Dark Excited States of an Alternating AT Oligomer Characterized by Femtosecond Broadband Spectroscopy

The nature and dynamics of DNA excited states is of pivotal importance in determining both DNA ultraviolet photosiability and its vulnerability toward photodamage. The complexity regarding the involvement of bright and dark excited states, their molecular origin, and the roles played by these states in the course of electronic energy relaxation constitute an active and contentious area in current research of DNA excited states. As a case study, we report here a combined broadband femtosecond time-resolved fluorescence (TRF) and transient absorption (TA) study on a self-complementary d(AT)(10) oligomer and a reference system of an equal molar Mixture of the constituent bases represented by adenosine and thymidine (A+T). Comparison of the spectral character and temporal evolution of the TRF and TA data for 267 nm excited d(AT)(10) and A+T provides evidence for a base-localized excitation feature for an early (