Strickler-Berg analysis of excited singlet state dynamics in DNA and RNA nucleosides

The excited singlet state lifetime of the ribonucleoside uridine was found to be 210+/-30 fs by femtosecond transient absorption spectroscopy. This value is considerably shorter than all previous time-domain measurements. This result and our previous lifetime measurements [see J.-M. L. Pecourt, J. Peon and B. Kohler, J. Am. Chem. Soc., 2001, 123, 10 370] for the other common nucleosides are compared with lifetimes calculated from available photophysical data using the Strickler-Berg (SB) equation. The calculated lifetimes for pyrimidine nucleosides are 10-25% lower than the lifetimes measured in femtosecond transient absorption experiments. For the purine nucleosides, guanosine and adenosine, consideration of just the lowest 1pi --> pi* transition led to predicted lifetimes that are three times greater than experimental ones. On the other hand, inclusion of both of the lowest energy 1pi --> pi* absorption bands in the SB equation resulted in much better agreement with the experimental values. This suggests that both 1pi pi* states of the purine nucleosides contribute to their emission. Decay by the bright 1pi pi* state (or states, in the case of the purines) is believed to be responsible for the experimentally observed lifetimes.