Femtosecond time- and wavelength-resolved fluorescence and absorption spectroscopic study of the excited states of adenosine and an adenine oligomer

By employing broadband femtosecond Kerr-gated time-resolved fluorescence (KTRF) and transient absorption (TA) techniques, we report the first (to our knowledge) femtosecond combined time- and wavelength-resolved study on an ultraviolet-excited nucleoside and a single-stranded oligonucleotide (namely adenosine (Ado) and single-stranded adenine oligomer (dA)20) in aqueous solution. With the advantages of the ultrafast time resolution, the broad spectral and temporal probe window, and a high sensitivity, our KTRF and TA results enable the real time monitoring and spectral characterization of the excited-state relaxation processes of the Ado nucleoside and (dA)20 oligonucleotide investigated. The temporal evolution of the 267 nm excited Ado KTRF spectra indicates there are two emitting components with lifetimes of ∼0.13 ps and ∼0.45 ps associated with the La and L b ππ* excited states, respectively. These Ado results reveal no obvious evidence for the involvement of the nπ* state along the irradiative internal conversion pathway. A distinct mechanism involving only the two ππ* states has been proposed for the ultrafast Ado deactivation dynamics in aqueous solution. The time dependence of the 267 nm excited (dA)20 KTRF and TA spectra reveals temporal evolution from an ultrafast "A-like" state (with a ∼0.39 ps decay time) to a relatively long-lived E1 "excimer" (∼4.3 ps decay time) and an E2 "excimer-like" (∼182 ps decay time) state. The "A-like" state has a spectral character closely resembling the excited state of Ado. Comparison of the spectral evolution between the results for Ado and (dA)20 provides unequivocal evidence for the local excitation character of the initially photoexcited (dA)20. The rapid transformation of the locally excited (dA)20 component into the delocalized E1 "excimer" state which then further evolves into the E2 "excimer-like" state indicates that base stacking has a high ability to modify the excited-state deactivation pathway. This modification appears to occur by suppressing the internal conversion pathway of an individually excited base component where the stacking interaction mediates efficient interbase energy transfer and promotes formation of the collective excited states. This feature of the local excitation that is subsequently followed by rapid energy delocalization into nearby bases may occur in many base multimer systems. Our results provide an important new contribution to better understanding DNA photophysics.