Singlet Triplet-Pair Production and Possible Singlet-Fission in Carotenoids

TL;DR

This study uses advanced simulations to investigate the ultrafast internal conversion process in carotenoids, specifically zeaxanthin, revealing a rapid adiabatic transition to a singlet triplet-pair state with significant yield, and discusses subsequent fission pathways.

Contribution

It provides a detailed numerical simulation of singlet-triplet pair formation in carotenoids, incorporating electron-electron and electron-nuclear interactions with a rigorous time-evolution approach.

Findings

Internal conversion occurs within 100 fs via an avoided crossing.

Predicted singlet triplet-pair yield is approximately 50%.

Discusses potential pathways for singlet fission in carotenoids.

Abstract

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 pi-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 method, while the nuclei are evolved via the Ehrenfest equations of motion. We apply this to zeaxanthin, a carotenoid chain with 22 conjugated carbon atoms (i.e., 11 double bonds). We show that the internal conversion of the photoexcited state to the singlet triplet-pair state occurs adiabatically via an avoided crossing within 100 fs and we predict a yield of ~ 50% We further discuss whether this singlet triplet-pair state will undergo exothermic versus endothermic intra or inter chain singlet fission.