Near Zero Singlet-Triplet Gap Through Nonfullerene Core Modification with Phenalene Derivatives Building Blocks

Recent advances in data-driven machine learning have highlighted the critical importance of the singlet-triplet gap (ΔE_ST=E_S1-E_T1) in non-fullerene acceptors (NFAs) molecules as an 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 (specially 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 a computational analysis that incorporates double-hybrid exchange-correlation functional as a benchmark method, we anticipate a remarkable reduction of ΔE_ST upon phenalene derivatives substitution, with some molecules achieving near zero singlet-triplet gap. This is the first report that call attention to new chemical strategies to synthesize NFA molecules with very low (eventually zero) ΔE_ST. Moreover, some modified molecules exhibited higher E_T1 compared to Y6, which is interesting to mitigate non-geminate recombination. The molecular modifications also improve the electron transfer properties, such as the intramolecular reorganization energy and the quadrupole moment, leading to more rigid and polarized molecules. 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 development of high-efficiency organic photovoltaic devices.