Conformational Switching Controls Multiexciton State Generation in a Cofacial Perylene Dimer

Controlling excited-state pathways in supramolecular chromophore assemblies is central to advancing optoelectronic and energy conversion technologies. Here, we report the photoinduced dynamics of a perylene diimide (PDI) dimer, valPDI2, which undergoes reversible solvent-controlled conformational switching between open and closed geometries with each form possessing distinct photophysics, as confirmed by exciton coupling calculations. In moderately polar chloroform, valPDI2 adopts a weakly coupled, open conformation that supports ps multiexciton generation. In polar DMSO/water, valPDI2 adopts a closed cofacial stacked geometry, exhibiting sub-ps formation of the multiexciton state, followed by population of the triplet manifold. Two-dimensional electronic spectroscopy uncovers distinct vibrational coherences that correlate with conformation and coherent multiexciton generation in the stacked conformer. These results demonstrate how polarity affects dimer geometry thus modulating multiexciton formation rates, suggesting design rules which can be generalised to PDI-based organic materials. Moreover, environmental-driven conformational switching serves as a tuning strategy to modulate multiexciton dynamics, with implications in both solution and solid-state materials. Our findings establish conformational switching as a design strategy for modulating excited-state pathways in organic chromophore arrays.