Pyrene is an exemplary conjugated organic chromophore with a strong propensity for self-association through an excited-state process known as excimer formation. Pyrene “excimers” and molecular “excimers” more generally are strongly avoided in some applications, such as in light harvesting, yet have found widespread use in others, such as in sensing and structure determination. Despite this disparate view and despite their widespread use, a fundamental understanding of the structure and dynamics of these collective excitations remains outstanding. In this work, we shed key insights into the nature of excimer formation in crystalline pyrene. We developed a flash precipitation procedure incorporating a polymer additive that enabled us to prepare aqueous suspensions of crystalline pyrene nanoparticles. We provide evidence that the molecular-level packing in the nanoparticles is equivalent to the equilibrium packing of the single crystal and show that excimer formation is the primary excited-state decay pathway. We find that excimer formation in the crystalline pyrene nanoparticles occurs in two stages on a picosecond time scale and suggest that intermolecular structural dynamics are largely responsible for the observed two-stage dynamics. We discuss an exciton theory description of molecular “excimers” and provide insights into their mechanism of formation, which we argue is best viewed simply as the relaxation of a singlet exciton into an excimer geometry.

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Nanoscale plasmonic structures have been primarily characterized through scattering studies, but electroluminescence offers an exciting alternative from a technological standpoint by removing the need for optical excitation. In sub-nanometer biased junctions, electronic tunneling can serve as the excitation source for plasmon-coupled electroluminescence, but the gap size dependence to this plasmonic enhancement has not been characterized. Here, we simultaneously probe the electroluminescence and conductance of Au tunnel junctions. We find that plasmonic enhancement increases as the gap size is reduced for junctions biased between 1.4 and 1.8 V, consistent with the behavior of charge transfer plasmons. At biases above 1.9 V, we see decreasing plasmonic enhancement with the decreasing gap, showing quenching due to tunneling in remarkable agreement with the trends observed for high energy plasmons in scattering experiments. Critically, we find that plasmonic enhancement of electroluminescence is gap size-dependent and, furthermore, is in agreement with the nature of modes excited by scattering.