Near zero singlet-triplet gap through nonfullerene core modification with phenalene derivative building blocks

Recent advances in data-driven machine learning have highlighted the critical importance of the singlet-triplet gap (ΔE_ST = E_S - E_T) in non-fullerene acceptor (NFA) molecules as a useful figure of merit to predict the efficiency of organic photovoltaic devices. By reducing ΔE_ST, the photovoltaic performance can be improved through the suppression of triplet state channels for non-geminate charge recombination. Encouraged by this strategy, we propose and theoretically explore the properties (particularly relative to ΔE_ST) of a new class of NFAs derived from modifications of the central core of the Y6 molecule (C₈₂H₈₆F₄N₈O₂S₅). The idea is to replace the benzothiadiazole chemical group by building blocks of phenalene derivatives, recognized for their unique inverted ΔE_ST. Using computational analysis that incorporates a double-hybrid exchange-correlation functional as a benchmark method, we anticipate a remarkable reduction of ΔE_ST upon phenalene derivative substitution, with some molecules achieving a near zero singlet-triplet gap. This is the first report that calls attention to new chemical strategies to synthesize NFA molecules with very low (eventually zero) ΔE_ST. Moreover, some modified molecules exhibited higher E_T compared to Y6, which is interesting to mitigate non-geminate recombination. The molecular modifications also lead to a decrease in intramolecular reorganization energy, thereby lowering the energy barrier for electron transfer. Additionally, a significant increase in the quadrupole moment component along the π-π stacking direction was observed-an essential property for strengthening quadrupole-quadrupole intermolecular interactions, which play a crucial role in molecular packing and charge transport. Overall, our research yields valuable insights into optimizing NFAs, opening the possibility of alternative molecular architectures to further the development of high-efficiency organic photovoltaic devices.