Extracting physical structure information from thin organic films with reflection absorption infrared spectroscopy

Abstract

A RATIO method has been developed to extract physical structure information from anisotropic thin organic films on metal substrates using reflection absorption infrared spectroscopy (RAIR). The physical structure information referred to are the Euler angles, θ and ψ, specifying the molecular orientation relative to the surface normal and f, the fraction of material oriented in that manner. Such films are viewed as consisting of polycrystalline domains which have a natural tendency to orient preferentially with respect to the substrate normal direction. Two equations are developed involving these three physical structure parameters and two measured quantities obtained from the thin-film RAIR spectra. These measured quantities are derived from the ratios of band absorbances, the transition moments of which belong to different symmetry groups. The method is tested by applying it to vacuum-deposited metal-free phthalocyanine (H2Pc) films on air-oxidized aluminum. The molecular orientation derived in the α-H2Pc crystallites is in very good agreement with the stacking configurations published in the literature for the isomeric phthalocyanine derivatives, predicting a ‘‘standing b-axis’’ orientation with the normal to the molecular planes inclined 26.5° with respect to the b-axis. In addition, spectroscopic evidence is presented for the first time of the existence of more than one metastable α polymorph of H2Pc. The less stable α polymorph is thermally converted by vacuum annealing to the more stable α polymorph and the first order rate kinetics measured. The resulting activation enthalpy and entropy are very close to those reported by Sharp and Miller for the higher temperature α to β-H2Pc transition.