Bridging the Gap between the Gas Phase and Solution Phase: Solvent Specific Photochemistry in 4-tert-Butylcatechol.

Abstract

Eumelanin is a naturally synthesized ultraviolet light absorbing biomolecule, possessing both photoprotective and phototoxic properties. We infer insight into these properties of eumelanin using a bottom-up approach, by investigating an ultraviolet absorbing motif of eumelanin, 4-tert-butylcatechol. Utilizing a combination of femtosecond transient electronic absorption spectroscopy and time-resolved velocity map ion imaging, our results suggest an environmental-dependent relaxation pathway, following irradiation at 267 nm to populate the S1 ((1)ππ*) state. Gas-phase and nonpolar solution-phase measurements reveal that the S1 state decays primarily through coupling onto the S2 ((1)πσ*) state which is dissociative along the nonintramolecular hydrogen bonded "free" O-H bond. This process occurs in 4.9 ± 0.6 ps in the gas-phase and 18 ± 1 ps in the nonpolar cyclohexane solution. Comparative studies on the deuterated isotopologue of 4-tert-butylcatechol in both the gas- and solution-phase (cyclohexane) reveal kinetic isotope effects of ∼19 and ∼4, respectively, supportive of O-H dissociation along a barriered pathway, and potentially mediated by quantum tunneling. In contrast, in the polar solvent acetonitrile, the S1 state decays on a much longer time scale of 1.7 ± 0.1 ns. We propose that the S1 decay is now multicomponent, driven by internal conversion, intersystem crossing, and fluorescence, as well as O-H dissociation. The attribution of conformer-driven excited state dynamics to explain how the S1 state decays in the gas- and nonpolar solution-phase versus the polar solution-phase, demonstrates the influence the environment can have on the ensuing excited state dynamics.