Relaxation Dynamics of Hydrated Thymine, Thymidine, and Thymidine Monophosphate Probed by Liquid Jet Time-Resolved Photoelectron Spectroscopy

The relaxation dynamics of thymine and its derivatives thymidine and thymidine monophosphate were studied using time-resolved photoelectron spectroscopy applied to a water microjet. Two absorption bands were studied, the first is a bright ππ* state which was populated using tunable-ultraviolet light in the range of 4.74 – 5.17 eV and probed using a 6.20 eV probe pulse. By reversing the order of these pulses, a band containing multiple ππ* states was populated by the 6.20 eV pulse and the lower energy pulse served as the probe. The lower lying ππ* state was found to decay in ~400 fs in both thymine and thymidine independent of pump photon energy while thymidine monophosphate decays varied from 670-840 fs with some pump energy dependence. The application of a computational QM/MM scheme at the XMS-CASPT2//CASSCF/AMBER level of theory suggests that conformational differences existing between thymidine and thymidine monophosphate in solution accounts for this difference. The higher lying ππ* band was found to decay in ~600 fs in all three cases, but was only able to be characterized when using the 5.17 eV probe pulse. Notably, no long-lived signal from an np* state could be identified in either experiment on any of the three molecules.