Molecular Symmetry Determines Excited State Lifetime in Aqueous Fumarate and Maleate

Abstract

Molecular symmetry can influence the photochemical fate of molecules by controlling excited-state lifetimes and, consequently, the time window available for secondary reactions. We have investigated the photodynamics of aqueous fumarate and maleate, the trans-and cis-isomers of 2-butenedioate, using femtosecond transient infrared absorption spectroscopy. Following π → π* excitation at 200 nm, both isomers undergo rapid decarboxylation to form CO₂ and acrylate with identical yields of 30 ± 10%, independent of symmetry. Strikingly, excited-state lifetimes differ by more than an order of magnitude: fumarate remains excited for 5.9 ps, while maleate relaxes in under 0.5 ps. We attribute this disparity to the higher symmetry of fumarate, which may restrict nonradiative decay pathways. These results demonstrate that even when primary photoproducts are unaffected by symmetry, excited-state lifetimes -and thus the potential for bimolecular reactions -can be strongly impacted. In atmospheric environments such as aerosols, sea spray, and industrial emissions, the longer-lived excitation of fumarate could enable a richer photochemistry than is possible for maleate.