New theoretical insights on the nonradiative relaxation mechanism of the core structure of mycosporines: The amino-cyclohexenone central template.

We present a comprehensive computational study describing the excited state dynamics and consequent photostability of amino-cyclohexenone (ACyO), the central template of mycosporine systems, widely recognized for their photoprotection of aquatic species. Photoexcitation to the first excited electronic state (S₁, ¹nπ*) of ACyO is considered an optically dark transition, while photoexcitation to the second excited electronic state (S₂¹ππ*) is an optically bright ¹ππ* transition and largely responsible for UV absorption properties of this molecule. We show that following initial photoexcitation to S₂, ACyO relaxes via two competing deactivation mechanisms, each mediated by an S₁/S₀ conical intersection, which directs the excited state population to the electronic ground state (S₀). Our ab initio computational results are supported with nonadiabatic dynamics simulation results, yielding an excited state lifetime of ∼280 fs for this system in vacuo. These results explain the inherent photostability of this core structure, commonplace in a wide range of microorganisms. [ABSTRACT FROM AUTHOR]