Triplet quantum chain process in the photoisomerization of 9-cis retinal as revealed by nanosecond time-resolved infrared spectroscopy

The mechanism of the photoisomerization of 9-cis retinal has been studied by nanosecond time-resolved infrared spectroscopy. A cyclohexane solution of 9-cis retinal was photoexcited at 349 nm and the subsequent photodynamics were traced. A singular value decomposition (SVD) analysis of the time-resolved infrared data shows that there are two distinct isomerization pathways. One is the triplet pathway that takes place in the picosecond time regime from 9-cis to all-trans. The other involves the energy transfer between the all-trans triplet state and the 9-cis ground state with the resultant 9-cis triplet state subsequently reproducing the all-trans by fast isomerization on the triplet potential surface. This quantum chain process occurs in the microsecond time regime.