Fig. 1 shows the absorption, emission and excitation spectra of a 50-nm-thick PP film on PMMA/quartz and the absorption and emission spectra of a PP dichloromethane solution (4 × 10^-5 M). The emission spectrum of the film is red-shifted as compared with the emission spectrum of the solution. The origin of this bathochromic shift is unclear. A similar red shift in the PL spectrum of poly(3-hexylthiophene) (P3HT) films have been attributed to intrachain planarization. It is noteworthy that P3HT films are arranged in an H-type packing arrangement, which should, in principle, result in a hypsochromic shift of the emission spectrum. Hence, the bathochromic shift in the PL spectrum of the PP film may be attributed to the planarization of the PP molecules, regardless of the type of packing arrangement. The absorption and excitation spectra of the PP film are blue-shifted as compared with the absorption spectrum of the PP solution. This indicates that the film adopts an H-type packing arrangement. In addition, an examination of the emission spectrum of the film reveals that the intensity ratio of the 0-0 band to the 0-1 band is smaller than 1, which is also a strong evidence of an H-type packing arrangement. The 0-0 transition is forbidden in H-aggregates, although the 0-0 emission band can have a non-zero intensity in the presence of structural or dynamic disorder. This explains why the emission spectrum of the PP film is dominated by the 0-1 emission band. This type of packing arrangement is also supported by the PP crystalline structure. Stefani et al. reported that crystalline PP 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. 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 than the former. 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). The long-energy tail in the absorption spectra of the film can be ascribed to mid-gap localized states