Probing the excited state relaxation dynamics of pyrimidine nucleosides in chloroform solution

Ultrafast transient electronic and vibrational absorption spectroscopy (TEAS and TVAS) of 2'-deoxy-cytidine (dC) and 2'-deoxy-thymidine (dT) dissolved in chloroform examines their excited-state dynamics and the recovery of ground electronic state molecules following absorption of ultraviolet light. The chloroform serves as a weakly interacting solvent, allowing comparisons to be drawn with prior experimental studies of the photodynamics of these nucleosides in the gas phase and in polar solvents such as water. The pyrimidine base nucleosides have some propensity to dimerize in aprotic solvents, but the monomer photochemistry can be resolved clearly and is the focus of this study. UV absorption at a wavelength of 260 nm excites a ¹ππ* ← S₀ transition, but prompt crossing of a significant fraction (50% in dC, 17% in dT) of the ¹ππ* population into a nearby ¹nπ* state is too fast for the experiments to resolve. The remaining flux on the ¹ππ* state leaves the vertical Franck-Condon region and encounters a conical intersection with the ground electronic state of ethylenic twist character. In dC, the ¹ππ* state decays to the ground state with a time constant of 1.1 ± 0.1 ps. The lifetime of the ¹nπ* state is much longer in the canonical forms of both molecules: recovery of the ground state population from these states occurs with time constants of 18.6 ± 1.1 ps in amino-oxo dC and ∼114 ps in dT, indicating potential energy barriers to the ¹nπ*/S₀ conical intersections. The small fraction of the imino-oxo tautomer of dC present in solution has a longer-lived ¹nπ* state with a lifetime for ground state recovery of 193 ± 55 ps. No evidence is found for photo-induced tautomerization of amino-oxo dC to the imino-oxo form, or for population of low lying triplet states of this nucleoside. In contrast, ∼8% of the UV-excited dT molecules access the long-lived T₁ (³ππ*) state through the ¹nπ* state. The primary influence of the solvent appears to be the degree to which it destabilizes the states of ¹nπ* character, with consequences for the lifetimes of these states as well as the triplet state yields.