Unexpected Shortening of the Excited State Lifetime of Oxybenzone Radical Cation upon Excitation of the Band Origin

Abstract

The global rise in skin cancer incidence has intensified research into the photophysical mechanisms of sunscreen agents such as oxybenzone (OxB), a widely used UV filter. In its neutral form, OxB absorbs UV radiation and relaxes through ultrafast non-radiative pathways, although the detailed mechanisms remain controversial. This work focuses on the oxybenzone radical cation (OxB•+ ), a largely unexplored species that may form in environments where OxB coexists with metal ions, such as swimming pools treated with Ag⁺. Using a combination of photodissociation spectroscopy and theoretical calculations, the D 0 →D 5 electronic transition was identified and excited-state lifetimes on the femtosecond timescale were estimated. Unexpectedly, the vibronic band origin exhibits a shorter lifetime than higher vibronic levels, attributed to an ultrafast excited-state proton transfer. In contrast, out-of-plane vibrational excitation disrupts hydrogen-bond directionality and suppresses this process, leading to longer excited-state lifetimes.