Optical Spectroscopy and XRD Study of Molecular Orientation, Polymorphism, and Phase Transitions in Fluorinated Vanadyl Phthalocyanine Thin Films

The molecular arrangement and phase transitions in the vanadyl hexadecafluorophthalocyanine (VOPcF16) thin films grown by physical vapor deposition have been studied using in situ X-ray diffraction, atomic force microscopy, and optical spectroscopy techniques (UV, IR, and Raman). The complete transition from the low-temperature linear cofacial structure to the slipped dimeric one occurs in the temperature range 16-220 degrees C. This conversion was found to be irreversible upon cooling the VOPcF16 film back to 20 degrees C. The structural transformation leads to decrease of the in-plane conductivity of the film by 2 orders of magnitude. According to the polarized Raman spectroscopy measurements, the mean tilt angles between the VOPcF16 species and the substrate surface were 59 +/- 5 degrees and 30 +/- 5 degrees in the as-deposited and annealed films, respectively. For the sake of comparison, the structure of the thin films of vanadyl phthalocyanine (VOPc) was also studied. The mean tilt angle between the VOPc species and the substrate surface was found to be 77 +/- 5 degrees, in good agreement with existing experimental data (similar to 70 degrees). All intense bands in the experimental IR and Raman spectra of VOPcF16 and VOPc were assigned using DFT calculations (B3LYP) and the N-15 isotopic shifts in the vibrational spectra of VOPc.