Optical absorbance of oriented thin films

We report that the optical absorbance of oriented thin films of para-hexaphenyl (PHP), produced by vacuum deposition, shows strong anisotropy effects. An explanation for the orientation dependence of optical absorption of PHP is proposed, using first principles dielectric tensor calculations. The lack of the π-π* absorption band in the normal UV-Vis absorption spectrum is consistent with theoretical predictions for an orthorhombic thin PHP film with oligomer chains oriented near normal to the substrate surface.