Rationalising Exciton Interactions in Aggregates Based on the Transition Density

Aggregation effects of molecular chromophores play a crucial role in determining the spectroscopic properties of solid-state organic materials. Within this work, we focus on excitonic coupling and particularly the question of whether aggregation leads to H- or J-type coupling, that is, whether the lowest energy excited state of the aggregate is optically bright or not. Employing a supermolecular picture to represent the different terms giving rise to exciton splitting, we develop an intuitive and generally applicable phenomenological model for estimating the sign and magnitude of the exciton coupling. This model, which is based on the shape of the monomer transition density is shown to be suitable across the whole range of relevant wave function types from purely Coulomb-coupled Frenkel excitons to strongly charge-transfer admixed excimer states. The implications are illustrated in the stacked anthracene and perylene-diimide dimer systems. The presented model does not only explain the long-range behavior but provides a clear explanation of atom-scale oscillations in the couplings seen for these systems. We hope that this work will give a boost to modern molecular materials science by providing new insight into interactions in the solid state as well as by highlighting the power of going beyond a simple frontier orbital picture in the design of molecular materials.