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 (ΔEST = ES1 - ET1) in non-fullerene acceptor (NFA) molecules as a useful figure of merit to predict the efficiency of organic photovoltaic devices. By reducing ΔEST, 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 ΔEST) 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 ΔEST. Using computational analysis that incorporates a double-hybrid exchange-correlation functional as a benchmark method, we anticipate a remarkable reduction of ΔEST 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) ΔEST. Moreover, some modified molecules exhibited higher ET1 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.