Optical properties of phenylene–thiophene-based polyazomethine thin films

Abstract

Thin films of new aromatic polyazomethines, containing thiophene rings in their polymer chain structures, were obtained by the chemical vapor deposition method, via a polycondensation process from thiophene-2,5-dicarbaldehyde and one of four different aromatic diamines, as initial monomers. Spectroscopic ellipsometry, attenuated total reflectance mode of Fourier transform infrared spectroscopy, atomic force microscopy, and wide-angle X-ray diffraction techniques have been used to characterize these films. However, the optical transmission and reflectivity measurements, within the wide spectral UV-Vis-NIR range (0.5–6.2 eV), were the basic method of presented investigations. On the basis of optical measurements, the absorption coefficient spectra of these thin films were obtained and the absorption edge parameters, that is, the energy gap ( EG) and Urbach energy ( EU), were calculated in a way typical for amorphous semiconductors. These parameters allowed one to evaluate the length of conjugation and the extension of localized, defect states inside the energy gap, respectively, and to discuss the electronic transitions, being dependent on the chemical structure and conformation of polymer chains. Optical properties of these new phenylene–thiophene-based polyazomethine films were compared with their entirely phenylene counterparts. In general, the energy gaps of investigated films were clearly smaller (about 0.2 eV) than those of their phenylene counterparts, confirming that the presence of thiophene rings in polyazomethine structures improves conjugation.