Abstract: 

Photochemistry is useful because it enables more avenues of reactivity and energy conversion through processes involving electronic excited states as compared to chemistry done on the electronic ground state. There are three main roles of light in photochemistry: generating light as a product through a chemical reaction, using light to drive excited state reactions, and catalyzing a chemical reaction with light. This talk contains a computational study on each of these different cases of photochemistry: producing light from chemistry (bioluminescence), using light to drive a chemical system away from equilibrium (solar energy conversion), and using light as a catalyst for organic synthesis (photocatalysis).

Dinoflagellate luciferin bioluminescence differs from other bioluminescence since there is no decarboxylation but is poorly understood. We computationally investigate protomers, stereoisomers, and the dynamics of the intermediate that chemiexcites. We used semiempirical dynamics and time-dependent density functional theory calculations to find that chemiexcitation is due to the 4-member ring, a dioxetanol, that undergoes [2π+2π] cycloreversion and the biolumiphore is the cleaved structure.

Detailed-balance efficiency limits of energy conversion are the basis for the design rules of photochemical power conversion devices. We simulate efficiencies for photo-protonic power conversion in liquid water, which acts as the protonic semiconductor that is sensitized to visible-light absorption with photoacids or photobases. Our model includes proton-transfer processes based on the Förster cycle with rate constants that follow the empirical Brønsted relation. Based experimentally controllable variables such as photoacid/photobase concentration and pH as well as photophysical properties such as excited state lifetimes, simulations of steady-state concentrations of H+(aq) and OH– (aq) indicate that the maximum possible protonic chemical potentials result in a photovoltage of ~330 mV and a power conversion efficiency of ~ 10%.

We report that diphenyl diselenide undergoes homolytic photocleavage to catalyze the hydroselenation of simple olefins with high anti-Markovnikov selectivity. Computational mechanistic studies reveal that a selenide radical adds to the alkene to form a carbon-radical and a subsequent hydrogen atom transfer (HAT) generates the linear selenide with high selectivity. Density functional theory calculations of the HAT transition state show a delocalization of the singly occupied molecular orbital (SOMO) onto the β-selenide. This unique “β-selenide radical effect” leads to addition of the Se–H bond with high anti-selectivity.