Unusual relaxation pathway from the two-photon excited first singlet state of carotenoids

Transient infrared and visible absorption measurements along with density functional theory (DFT) calculations on carotenoids 8'-apo-ß-caroten-8'-al (I) and 7',7'-dicyano-7'-apo-ß-carotene (II) were used to explore the nature of a long-lived species observed in transient infrared absorption measurements following two-photon excitation (Pang et al. J. Phys. Chem. B 2009, 113, 13806). The long-lived species of I has a very strong infrared absorption around 1510 cm-1 and a visible transient absorption band centered at 760 nm. The long-lived species appears on two different time scales of ~ 16 and 140-270 ps. The longer rise component is absent in nonpolar solvents. DFT calculations using the B3LYP functional and the 6-31G(d) basis set were used to investigate the ground-state potential-energy surface of I and II including its conformational isomers, a p-diradical “kinked” structure, and cation and neutral radicals. From the simulated infrared spectra of all the structures considered, we found a close match in the cation radical spectrum to the experimental infrared spectrum of the long-lived species. However, the visible absorption band does not match that of the monomeric cation radical. On the basis of our experimental and theoretical results, we propose a charge-transfer complex between a carotenoid and a solvent molecule for the origin of the long-lived species formed from the direct two-photon excitation of the S1 state.