Model for the α→β1 phase transition in phthalocyanine thin films

The α→β1 phase transition induced by annealing free-base phthalocyanine thin films grown by organic molecular-beam deposition has been studied by powder x-ray diffraction, electronic absorption spectroscopy, and Nomarski interference microscopy. The final β1 phase consists of two domains having a different orientation relative to the substrate surface. Starting with the α phase, by careful control of the annealing temperature, a series of intermediate states are observed consisting of coexisting α and β1 phases. At the beginning of the phase transition, only one domain is observed with the β1 crystal phase aligned with the (001) plane parallel to the substrate. As the transition proceeds, however, the amount of the other domain having the (201) plane parallel to the substrate increases rapidly. A mechanism is proposed for the transition that can account for all the experimental results.