Excited State Dynamics and Charge Transfer in Quinoidal Cyclopenta- dithiophene Derivative

Singlet fission (SF) holds great promise for advancing optoelectronics, with ultrafast timescales and high exciton yield per absorbed photon. Despite extensive experimental and theoretical investigations, understanding the nature of charge-transfer and triplet-pair states remains challenging. Our previous study on cyclopentadithiophene-based quinoidal-biradical resonance structures Phys. Chem. Chem. Phys. 25, 29698 (2023) revealed their excellent tunability in chemical structure for SF. While the ultrafast dynamics in the excited state have not been fully explored, experimental data indicate a singlet excited state lifetime of approximately 2.1 ps. In our current study, we employ excited-state dynamics simulations to theoretically investigate the formation of charge-transfer (CT) configuration during the SF process. Our simulations reveal correlated rotational angles of fluorenes, ranging from −40° to 60°, and detect primary charge transfer from the two fluorene moieties to the dithiophene fragment. Interestingly, the direction of charge transfer may alter within the simulation timeframe of hundreds of femtoseconds. Our theoretical simulations provide an informative reference for future design in SF.