Reassessing carotenoid photophysics: shedding light on dark states

Abstract

Carotenoid molecules are critical in photosynthesis, performing functions at the heart of both light-harvesting and photoprotection. As both these processes involve excitation energy transfer, fully understanding them requires a precise description of the electronic states involved. The excited state manifold of carotenoids is not yet fully characterized, and includes several dark electronic states that remain elusive. Using femtosecond stimulated resonance Raman spectroscopy, where the vibrational contributions of each excited state can be observed selectively as a function of the Raman excitation, we resolve vibrational signatures consistent with three additional dark-state contributions and propose assignments for them. These results address long-standing controversies in carotenoid research and provide a spectroscopic framework relevant to the multiple roles of these molecules.