posted on 2022-02-02, 21:43authored byDilhan Manawadu, Darren J. Valentine, Max Marcus, William Barford
Internal
conversion from the photoexcited state to a correlated
singlet triplet-pair state is believed to be the precursor of singlet
fission in carotenoids. We present numerical simulations of this process
using a π-electron model that fully accounts for electron–electron
interactions and electron–nuclear coupling. The time-evolution
of the electrons is determined rigorously using the time-dependent
density matrix renormalization group method, while the nuclei are
evolved via the Ehrenfest equations of motion. We
apply this to zeaxanthin, a carotenoid chain with 18 fully conjugated
carbon atoms. We show that the internal conversion of the primary
photoexcited state, S2, to the singlet
triplet-pair state occurs adiabatically via an avoided
crossing within ∼50 fs with a yield of ∼60%. We further
discuss whether this singlet triplet-pair state will undergo exothermic
versus endothermic intra- or interchain singlet fission.