Relaxation Dynamics in Dihydroxychalcones: Insights from Ultrafast Spectroscopy and Quantum Computations

Chalcones present a potentially promising form of natural photoprotection for inclusion in sunscreen formulations. Here, using femtosecond transient electronic absorption spectroscopy and high-level quantum computations, we explore the differing photophysics of two members of the chalcone family: 4,4'-dihydroxychalcone and 4,4'-dihydroxychalcone-α-methoxylate. From experiment, trapped excited-state population in 4,4'-dihydroxychalcone is alleviated by functionalization at the α carbon, affording vast acceleration in nonradiative deactivation. From theory, the ultrashort excited-state lifetime of the α-substituted analog is explained by a barrierless S₁/S₀ conical intersection, providing a route for ultrafast internal conversion, whereas a significant potential energy barrier prohibits the excited system from approaching this conical intersection in the nonsubstituted chalcone. These observations are supported by results from nonadiabatic dynamics simulations. Our investigations elucidate how targeted chemical modifications can perturb potential energy surfaces, resulting in distinct photophysical behaviors. We demonstrate that chalcones' deactivation mechanisms are sensitive to substitution at the aliphatic bridge connecting the two aromatic rings.