Unraveling Charge and Energy Transfer in a Singlet Fission Donor–Acceptor Complex: An <i>Ab Initio</i> Quantum Dynamical Study

Singlet fission is a photophysical process in organic molecules that generates two triplet electronic states from an excited singlet electronic state. Molecules exhibiting singlet fission can multiply charge carriers and thus have the potential to enhance the performance of solar cells beyond the Shockley-Queisser limit by reducing thermalization losses. However, in order to implement singlet fission for applications in photovoltaics, it is essential to understand how charge or energy can be harvested from triplet excitons. In this work, we investigate these processes in a prototypical donor-acceptor complex consisting of a bis(diazadiborine)-based chromophore as a singlet fission-active donor and tetracyanoquinodimethane as an acceptor molecule. Using a combined approach of high-level <i>ab initio</i> multireference perturbation theory techniques and quantum dynamical simulations, we show the existence of intermolecular singlet fission, charge and energy transfer following intramolecular singlet fission, and energy loss decay channels to low-lying states as the three competing charge and energy transfer mechanisms from the donor to the acceptor molecule. We analyze the role of the different electronic states, specific vibrational modes, and vibronic couplings in these processes. The results provide insights into the rational design of donor-acceptor systems with efficient singlet fission-based charge and energy transfer.