On the other hand, if they were significant contributors, the corresponding spectral features should exhibit concentration dependence, as higher concentrations would populate all available clusters, including cluster 9. In other words, the relative intensity of the excimer band compared to the vibronically resolved, short-wavelength components should reach a plateau. However, this hypothesis is contradicted by the available experimental observations. This rather confirms the idea that cluster 9 represents an inefficient form of intermolecular pairing in solution, where rapid dissociation into an excited and a ground-state monomer-and possibly reassociation-effectively competes with fluorescence emission. Analogous conformations, where stacking is absent, are likely to play a more critical role in the organization of pyrene in the solid state due to the different surrounding environments and the effect of crystal packing forces. Indeed, crystalline aggregates of pyrene show a combination of stacked and T-shaped motifs, as in the case of benzene. , copyright (2018) American Chemical Society.