Fig. 1 shows the absorption, photoluminescence (PL) and excitation spectra of a 50-nm-thick Pyrene film on PMMA/quartz and the absorption and fluorescence spectra of a Pyrene dichloromethane solution (4 × 10^-5 M). The absorption and excitation spectra of the film are blue-shifted in comparison with the absorption spectrum of the dichloromethane solution, which suggests that the film is predominantly comprised of H-type aggregates. Furthermore, the inset in Fig. 1 shows that 0-1 PL transition is the most intense transition in the spectrum of the film. This provides further evidence in favour of the formation of H-type aggregates. An H-type packing arrangement is also supported by the Pyrene crystalline structure. X et al. reported that crystalline Pyrene adopts a 3D brick-wall arrangement in which the in-plane bonding is stabilized by hydrogen bonds. In this crystalline structure the planes are organized in a slipped p-p stacking arrangement with an inter-planar distance of 3.51 Å and a sliding along the molecular axis of 3.32 Å (65 degrees). This p-p stacking arrangement is entirely consistent with the geometrical disposition of molecules forming H-aggregates. The emission spectrum of the film is red-shifted as compared with the emission spectrum of the solution. Bathochromic shifts such as this are generally attributed to molecular planarization. It has been reported that molecular planarization can cause red shifts in the PL spectra of materials that are known to adopt an H-type packing arrangement, despite the fact that in principle an H-type packing arrangement should results in an hypsochromic shift of the PL spectrum. Hence, the bathochromic shift in the PL spectrum of the Pyrene film may be ascribed to molecular planarization regardless of the type of packing arrangement adopted by the film. In addition, it is evident from Fig. 1 that the excitation and absorption spectra of the film are different from each other, the latter being much broader. The differences between the absorption and excitation spectra of the film reveal the presence of non-emissive species, both at lower and higher energies than the main absorption peak (lambda max = 428 nm). It is also noteworthy that the absorption spectrum of the film displays a red tail that reaches wavelengths as long as 800 nm. This low-energy tail can be ascribed to mid-gap localized states.