Electronic and Vibrational Spectroscopy of 1‐Methylthymine and its Water Clusters: The Dark State Survives Hydration

Electronic and vibrational gas phase spectra of 1‐methylthymine (1MT) and 1‐methyluracil (1MU) and their clusters with water are presented. Mass selective IR/UV double resonance spectra confirm the formation of pyrimidine‐water clusters and are compared to calculated vibrational spectra obtained from ab initio calculations. In contrast to Y. He, C. Wu, W. Kong; J. Phys. Chem. A, 2004 , 108, 94 we are able to detect 1MT/1MU and their water clusters via resonant two‐photon delayed ionization under careful control of the applied water‐vapor pressure. The long‐living dark electronic state of 1MT and 1MU detected by delayed ionization, survives hydration and the photostability of 1MT/1MU cannot be attributed solely to hydration. Oxygen coexpansions and crossed‐beam experiments indicate that the triplet state population is probably small compared to the ¹nπ* and/or hot electronic ground state population. Ab initio theory shows that solvation of 1MT by water does not lead to a substantial modification of the electronic relaxation and quenching of the ¹nπ* state. Relaxation pathways via ¹ππ*¹–nπ*¹ and ¹ππ *–S₀ conical intersections and barriers have been identified, but are not significantly altered by hydration.