Reflection absorption infrared spectra of thin solid films. Molecular orientation and film structure

The calculated and observed vibrational spectra of a new series of perylene tetracarboxylic derivatives (PTCDs) with different alkyl lateral chain lengths (5, 10, 15, and 20 C) are reported. With the use of characteristic vibrational modes, the question about the degree of molecular organization and packing in these PTCD evaporated films has been resolved by using transmission and reflection-absorption infrared spectroscopy (RAIRS) techniques. From vibrational analysis, it was extracted that the molecules are preferentially oriented with the PTCD moiety head on the surface substrate. The molecular organization detected (by using RAIRS) in the evaporated films is not changed when the films are exposed to different solvents, such as water, methanol, and propanol, or when they are annealed up to 155 degreesC under normal atmospheric conditions or in vacuum. Evidence of molecular packing in the evaporated films is also provided by the observation of strong excimer emission.