Delocalization versus Coherence under Vibrational and Environmental Disorder in Photoexcited Supramolecular Aggregates

Exciton and charge dynamics in photoexcited molecular materials depend critically on how delocalization competes with structural, vibrational, and environmental disorder. Yet, the origin and extent of coherent versus incoherent distribution of excitons and charges in such "noisy" supramolecular systems remain poorly understood. Here, we integrate all-atom classical dynamics with fully quantum vibronic dynamics to dissect these competing effects in aqueous self-assembled perylenediimide stacks. Our simulations quantitatively reproduce experimental absorption spectra and reveal that strong excitonic coupling and hybridization with charge-transfer states dictate the optical response. Following photoexcitation, the electronic populations rapidly spread across multiple molecules within tens of femtoseconds, yielding delocalized but largely incoherent exciton, hole, and electron wave function distributions. We find that high-frequency vibrational modes, and, to a lesser extent, the slow environmental and vibrational dynamics, intrinsic to the nature of such solvated supramolecular systems, set a fundamental limit to achieving fully coherent electronic delocalization. These results identify vibrational disorder as a universal constraint on coherent exciton dynamics and indicate that practical design principles for efficient organic optoelectronic and photocatalytic materials should focus on robust equi-distribution of the excited-state population.