Excimer-mediated multiexciton generation in covalently linked cross foldamers of thiophene-fused perylene bisimides

Abstract

Singlet fission (SF) is a promising strategy to surpass the Shockley–Queisser limit in organic photovoltaics, but the structural and electronic determinants controlling excimer-mediated multiexciton (ME) generation remain insufficiently understood. Herein, we designed two covalently linked cross foldamers, CF(4) and CF(7), based on thiophene-fused perylene bisimide units, with controlled intramolecular chromophore rotational angles (88° and 55°) via alkyl chain engineering, while maintaining a constant π–π stacking distance. Steady-state and ultrafast spectroscopy studies revealed that both foldamers undergo excimer-mediated ME generation but with distinct dynamics: weakly coupled CF(4) (88°) achieves rapid ME formation in 27 ps, whereas strongly coupled CF(7) (55°) stabilizes the excimer, slowing ME formation to 114 ps. These results establish a clear structure–dynamics relationship, demonstrating that rotational angle modulation enables tuning of excimer energetics and SF efficiency, providing a general design strategy for π-conjugated assemblies.