Transition energies of benzoquinone anions are immune to symmetry breaking by a single water molecule

Abstract

p-Benzoquinone is the prototypical member of the quinone class of molecules with a basic functionality relevant for the primary reactions of photosynthesis. As electronically excited quinone anions are formed in near-resonant electron transfer, key issues are how the local environment affects excited-state energy levels and deexcitation times. The former we address here with action spectroscopy of mass-selected bare radical anions (pBQ⁻) and one-water pBQ⁻·H₂O complexes, isolated in vacuo. The complex represents a precursor for internal proton transfer to form the semiquinone free radical, the first chemical product in the light-driven electron transport chain. Both ions display bands in the visible and ultraviolet with, importantly, almost identical maxima. Despite localizing negative charge, thereby breaking the high orbital symmetries, water is surprisingly innocent. This finding implies that natural fluctuations in the quinone microenvironment cause only minor variations in excited-state energies and thus electron-transfer rates. Hence quinones are robust participants in electron transport.