Exploring the Vibrational Coherences in the Ultrafast Electronic Relaxation of Pyrimidine Nucleobases and Nucleosides

We studied the vibrational coherences during the ultrafast internal conversions (ICs) of pyrimidine nucleobases and -sides in aqueous solutions and the gas phase with an instrumental resolution of 14 fs. The coherence of the same ring-breathing vibrational mode with a frequency of 750 cm^–1 was observed. In the gas phase, the vibrational coherence was transferred during IC from the ¹ππ* to the ¹nπ* state, and it survived for approximately 1 ps. In an aqueous solution, the vibrational dynamics develop in the ¹ππ* state because the ¹nπ* state is energetically upshifted via hydrogen bonding with H₂O, and the vibrational coherence is lost on a similar time scale as that of IC to the ground electronic state.