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 1irir* 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 1nir*/3irir* dark states from the 1irir* 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 1nir* 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 1irir* lifetime and a higher quantum yield (0.12-0.22) for the 1nir* state. A similar trend was found for isolated Thy and Ura in the gas phase: the 1irir* lifetimes are 39 and 17 fs and the quantum yield for 1nir* 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 1irir* and S0, providing a large impact on the photophysics/photochemistry of a pyrimidine nucleobase. The significant reduction of 1nir* yield in aqueous solution is ascribed to the destabilization of the 1nir* state induced by hydrogen bonding. (GRAPH)

Automated summary

Ultrafast electronic relaxation plays a crucial role in the photostability of DNA. However, some nucleobases in aqueous solutions can form long-lived dark states from the excited states with a high quantum yield. By using extreme ultraviolet time-resolved photoelectron spectroscopy, we investigated the electronic relaxation in pyrimidine nucleobases in both aqueous solutions and the gas phase. We found that the quantum yields of the long-lived dark states were considerably lower for cytidine and uracil derivatives in aqueous solutions, while thymine and thymidine showed a longer excited state lifetime and a higher quantum yield. The reduction of the quantum yield in aqueous solutions is attributed to the destabilization of the dark states induced by hydrogen bonding.