Pyrene and other polycyclic aromatic hydrocarbons (PAHs) are likely the most widely employed substrates for excimer (and exciplexes) formation. Among other desirable features, being fluorescent molecules, they possess relatively low non-radiative decay rates and long fluorescence lifetimes, up to hundreds of nanoseconds. In compliance with the standard mechanistic scheme proposed (see Figure ), they have a higher chance to survive in the excited state while diffusing in solution, eventually meeting a ground state species to associate with. Indeed, excimer formation is most commonly regarded as a diffusion-controlled bimolecular reaction between an excited and a ground-state molecule. The association-dissociation equilibrium is competing with the radiative and non-radiative relaxation channels both on the monomer and on the excimer side. However, a detailed analysis of the fluorescence emission also suggested some degree of groundstate association between pyrene molecules; hence, excimer formation may also come from the internal rearrangement of this already bound species. Excimers deriving from freely diffusing monomers are sometimes referred to as dynamic excimers, as opposed to static excimers resulting from the photoexcitation of pre-associated ground-state dimers. The latter process can be debated based on other relevant experimental evidence, such as the induction phase in the fluorescence decay profiles, and the absence of any appreciable changes in the linear absorption spectra. Indeed, one can expect that excitonic interactions (mixing of localized excitations), which characterize the excimer state along with charge transfer (CT) contributions, give rise to the typical Davydov splitting in the absorption of the coupled dyes. Therefore, while some degree of pre-association may be at work in concentrated solutions, it seems that the encounter of two freely diffusing monomers, which is also favored in concentrated solutions, still plays a central role in driving the overall process and emission response.