Photoexcitation of iodide ion-pyrimidine clusters above the electron detachment threshold: Intracluster electron transfer versus nucleobase-centred excitations

Laser photodissociation spectroscopy of the I-center dot thymine (I-center dot T) and I-center dot cytosine (I-center dot C) nucleobase clusters has been conducted for the first time across the regions above the electron detachment thresholds to explore the excited states and photodissociation channels. Although photodepletion is strong, only weak ionic photofragment signals are observed, indicating that the clusters decay predominantly by electron detachment. The photodepletion spectra of the I-center dot T and I-center dot C clusters display a prominent dipole-bound excited state (I) in the vicinity of the vertical detachment energy(similar to 4.0 eV). Like the previously studied I-center dot uracil (I-center dot U) cluster [W. L. Li et al., J. Chem. Phys. 145, 044319 (2016)], the I-center dot T cluster also displays a second excited state (II) centred at 4.8 eV, which we similarly assign to a pi-pi* nucleobase-localized transition. However, no distinct higher-energy absorption bands are evident in the spectra of the I-center dot C. Time-dependent density functional theory (TDDFT) calculations are presented, showing that while each of the I-center dot T and I-center dot U clusters displays a single dominant pi-pi* nucleobase-localized transition, the corresponding pi-pi* nucleobase transitions for I-center dot C are split across three separate weaker electronic excitations. I- and deprotonated nucleobase anion photofragments are observed upon photoexcitation of both I-center dot U and I-center dot T, with the action spectra showing bands (at 4.0 and 4.8 eV) for both the I- and deprotonated nucleobase anion production. The photofragmentation behaviour of the I-center dot C cluster is distinctive as its I(-)photofragment displays a relatively flat profile above the expected vertical detachment energy. We discuss the observed photofragmentation profiles of the I-center dot pyrimidine clusters, in the context of the previous time-resolved measurements, and conclude that the observed photoexcitations are primarily consistent with intracluster electron transfer dominating in the near-threshold region, while nucleobase-centred excitations dominate close to 4.8 eV. TDDFT calculations suggest that charge-transfer transitions [Iodide n (5p(6)) -> Uracil sigma*] may contribute to the cluster absorption profile across the scanned spectral region, and the possible role of these states is also discussed. Published by AIP Publishing.