Intermolecular states in organic dye dispersions: excimers vs aggregates

Rapid excited-state quenching in the solid state is a widespread limitation for organic chromophores. Even when molecules are dispersed in neutral host matrices, photoluminescence quantum yields decrease sharply with increased concentration, pointing to efficient intermolecular non-radiative decay pathways that remain poorly understood. Here we study the nature of the intermolecular states formed in dispersions of the prototypical BODIPY dyes. Using temperature-dependent and time-resolved photoluminescence measurements, we describe the processes of energy transfer into excimer states and, in materials with suitable chemical structure, excitonically coupled dimers. These dimer states exhibit remarkable near-unity quantum yield.