Radiative defects and optical response in oriented para-hexaphenyl films

The optical properties of an ordered para-hexaphenyl film, grown with molecules lying parallel to the substrate, are investigated. The absorption spectrum exhibits two peaks at 3.1 (L) and 4.35 (H) eV, featuring a different degree of polarization. The photoluminescence emission and the absorption at 3.1 eV are highly polarized. The presence of the L peak is strongly related to the growth conditions, and is favored at high deposition rates and low substrate temperatures. Upon 4.66 eV ultraviolet irradiation in air, the photoluminescence intensity and the absorption at 3.1 eV are reduced. Conversely, transitions at higher energies are almost unaffected by irradiation. Experimental data show that the L peak and emission arise from a crystal defect.