Energy migration, charge transfer, and charge dissociation in self-assembling nonfullerene acceptor aggregates with zincporphyrin-nonfullerene acceptor dyads

Abstract

The synergy between self-assembling donor–acceptor–donor type nonfullerene acceptors (TACIC-Br) and zincporphyrin-nonfullerene acceptor linked molecules (ZnP-TACIC) provides a compelling model for examining key multi-step processes, including energy migration, charge transfer (CT), and charge dissociation (CD) in photosynthesis and organic photovoltaics (OPVs). Remarkably, TACIC-Br molecules exhibited a strong tendency to aggregate, even in the good solvent CHCl₃. However, when the proportion of the poor solvent (MeOH) exceeded 40% in a CHCl₃/MeOH mixture (v/v), these aggregates displayed an unusually prolonged excited singlet-state lifetime, comparable to TACICs in thin films. Solid-state NMR spectroscopy and theoretical calculations revealed that within the TACIC aggregates, a slipped or T-shaped dimeric π–π packing arrangement is favored, positioning the thienoazacoronene donor unit and the 1,1-dicyanomethylene-3-indanone acceptor unit in close proximity. This supramolecular packing effectively suppresses both nonradiative and radiative decay processes in CHCl₃/MeOH mixtures and thin films, contrasting sharply with typical self-quenching observed in conventional dye aggregates. Time-resolved transient absorption measurements showed efficient energy migration, CT, and CD within these composite aggregates. With an extremely long singlet excited-state diffusion length (L_D) of 45.6 nm, facilitated by the prolonged excited singlet-state lifetime, TACICs are well-suited for efficient energy migration. Notably, after quantitative CT at the ZnP-TACIC molecule, 35% of the CT states in the aggregates dissociated to form free ion pairs. This integrated supramolecular approach adeptly emulates both light-harvesting and CT and CD processes in photosynthesis and OPVs, thereby offering potential applications in solar energy conversion.