Improved GAFF Force Field for Fused Ring Non-Fullerene Acceptors With 1,1-Dicyanomethylene-3-Indanone Based End Groups.

The introduction of non-fullerene acceptors (NFAs) in bulk heterojunction organic solar cells has advanced their power conversion efficiencies beyond 20%. NFAs consisting of a fused ring electron-donating core, capped with electron-withdrawing end groups, are being intensely investigated both experimentally and computationally, since they demonstrate superior performance. 1,1-dicyanomethylene-3-indanone (IC) is a widely used end group due to its strong electron-withdrawing ability and ease of chemical modification. The functionality of this end group can be further enhanced by employing two common strategies. Halogenation, that adjusts energy levels, and addition of π-extensions that red-shifts optical absorption. In the present work we improve the General Amber Force Field to model fused ring NFAs with a thiophene unit at the edge of the core. To this end, 10 different NFA structures that incorporate the IC end group along with its halogenated and π-extended variants are employed. They are classified into three popular families according to their structural characteristics. Using ab initio computations, the dihedral angles between the fused ring core and the end group are reparametrized, while partial atomic charges are derived using the standard RESP procedure, thus creating complete force field models. Using experimentally resolved crystal structures available in the literature we validate the new parameterization and demonstrate its suitability for modeling a variety of fused ring NFAs.