Polymorphic transformations observed on molecular organic thin films: p-nitrophenyl nitronyl nitroxide radical

We report on the experimental observation of the polymorphic transformation from the monoclinic alpha- to the orthorhombic beta-phase of thin films of the molecular organic radical p-nitrophenyl nitronyl nitroxide (p-NPNN). T his solid-solid phase transition to the thermodynamically most stable beta- phase encompasses clearly differentiated changes in structure and morpholog y and may be inhibited if the thickness of the films of the initial a-phase lies below a certain critical value. In our case, the alpha-phase of p-NPN N (alpha-p-NPNN) has been stabilized as thin film obtained by thermal evapo ration in high vacuum (thickness less than or equal to 1 mu m).