Mapping the UV Photophysics of Platinum Metal Complexes Bound to Nucleobases: Laser Spectroscopy of Isolated Uracil·Pt(CN)₄^2– and Uracil·Pt(CN)₆^2– Complexes

Abstract

We report the first UV laser spectroscopic study of isolated gas-phase complexes of platinum metal complex anions bound to a nucleobase as model systems for exploring at the molecular level the key photophysical processes involved in photodynamic therapy. Spectra of the Pt^IV(CN)₆^2–·Ur and Pt^II(CN)₄^2–·Ur complexes were acquired across the 220–320 nm range using mass-selective photodepletion and photofragment action spectroscopy. The spectra of both complexes reveal prominent UV absorption bands (λ_max = 4.90 and 4.70 eV) that we assign primarily to excitation of the Ur π–π* localized chromophore. Distinctive UV photofragmentation products are observed for the complexes, with Pt^IV(CN)₆^2–·Ur photoexcitation resulting in complex fission, while Pt^II(CN)₄^2–·Ur photoexcitation initiates a nucleobase proton-transfer reaction across 4.4–5.2 eV and electron detachment above 5.2 eV. The observed photofragments are consistent with ultrafast decay of a Ur localized excited state back to the electronic ground state followed by intramolecular vibrational relaxation and ergodic complex fragmentation