The primary deep-UV photochemistry of aqueous fumarate and maleate

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 primary 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 quantum yields of maximum Φ = 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 returns to its ground state in less than 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 subsequent bimolecular reactions - can be strongly impacted.