Carotenoid radical formation after multi-photon excitation of 8′-apo-β-carotenal

Abstract

Carotenoids containing a conjugated CO group exhibit complex excited-state dynamics that are influenced by solvent polarity due to the involvement of an intramolecular charge transfer (ICT) state. Our study explores the excited-state behavior of 8′-apo-β-carotenal under multi-photon excitation conditions. Using near-infrared (1300 nm) multi-photon excitation, we observe the formation of a cation radical of 8′-apo-β-carotenal, a process distinct from those following one-photon visible or UV excitation. Our findings suggest that this radical formation results from multi-photon excitation involving a higher-lying dark state, supported by intensity-dependent experiments. This work demonstrates that radical generation is a characteristic of this higher excited state and is not produced during relaxation from the S₁/ICT state. The results open new pathways for understanding carotenoid radical formation mechanisms under intense multi-photon excitation.