Spectroscopic features of thin films of thiophene/phenylene co-oligomers with vertical molecular alignment

Spectroscopic features of various thiophene/phenylene co-oligomers with various molecular sizes and shapes have been investigated in the form of thin films. Of these compounds, shorter oligomers assume the vertical molecular alignment more readily than longer oligomers, The latter oligomers, however, tend to exhibit the vertical alignment when deposited under a high vacuum and at a small deposition rate. The thin films of this alignment were examined by the theta -2 theta X-ray diffraction and incident-angle-dependent polarized absorption spectroscopy. A sharpened spectral line markedly grows around 300 nm in the polarized absorption spectra with increasing incident angles. We assign this line to the upper Davydov level and estimate the magnitude of the Davydov splitting on the basis of energetic consideration of the spectra in combination with the results of quantum chemical calculations.