Strong intermolecular coupling protects delocalization and transport of organic exciton-polaritons against static...

Direct intermolecular interactions, being short-range and, hence, strongly dependent on local disorder, are commonly neglected in simulations of organic exciton-polaritons, and molecules are only indirectly linked through the cavity electromagnetic field, irrespective of intermolecular separations. Whereas accounting for direct intermolecular interactions has presumably little effect on the properties of organic exciton-polaritons in many experimental systems, disregarding these interactions might no longer be valid in systems with a certain degree of structural ordering, such as organic crystals, conjugated polymers, or molecular aggregates. In these systems, intermolecular couplings are comparable to the experimentally achieved collective light-matter coupling strengths and, therefore, may modify the properties of polaritons compared to those in weakly interacting molecular systems. To test this, we incorporate nearest-neighbor excitonic couplings into the multi-mode Tavis-Cummings Hamiltonian and perform numerical simulations of polariton delocalization and transport. The simulation results suggest that negative, or J-aggregate-type, coupling alters the energetics of lower polariton states such that these states become more robust against static excitation energy disorder. This results in better transport properties, in particular in less velocity renormalization and the transition from ballistic transport to diffusion occurring at larger exciton fractions than in the absence of excitonic coupling. In contrast, positive, or H-aggregate-type, coupling makes the lower polariton states more sensitive to disorder and deteriorates their propagation. These findings emphasize the importance of intermolecular interactions as a control parameter for achieving long-range excitation energy transfer in optical microcavities.