Formation of Long-Lived Dark States during Electronic Relaxation of Pyrimidine Nucleobases Studied Using Extreme Ultraviolet Time-Resolved Photoelectron Spectroscopy

Ultrafast electronic relaxation of nucleobases from 1ππ∗ states to the ground state (S0) is considered essential for the photostability of DNA. However, transient absorption spectroscopy (TAS) has indicated that some nucleobases in aqueous solutions create long-lived 1nπ*/3ππ∗ dark states from the 1ππ∗ states with a high quantum yield of 0.4-0.5. We investigated electronic relaxation in pyrimidine nucleobases in both aqueous solutions and the gas phase using extreme ultraviolet (EUV) time-resolved photoelectron spectroscopy. Femtosecond EUV probe pulses cause ionization from all electronic states involved in the relaxation process, providing a clear overview of the electronic dynamics. The 1nπ∗ quantum yields for aqueous cytidine and uracil (Ura) derivatives were found to be considerably lower (<0.07) than previous estimates reported by TAS. On the other hand, aqueous thymine (Thy) and thymidine exhibited a longer 1ππ∗ lifetime and a higher quantum yield (0.12-0.22) for the 1nπ∗ state. A similar trend was found for isolated Thy and Ura in the gas phase: the 1ππ∗ lifetimes are 39 and 17 fs and the quantum yield for 1nπ∗ are 1.0 and 0.45 for Thy and Ura, respectively. The result indicates that single methylation to the C5 position hinders the out-of-plane deformation that drives the system to the conical intersection region between 1ππ∗ and S0, providing a large impact on the photophysics/photochemistry of a pyrimidine nucleobase. The significant reduction of 1nπ∗ yield in aqueous solution is ascribed to the destabilization of the 1nπ∗ state induced by hydrogen bonding.