Optical properties of thin films of polyazomethine with flexible side chains

• Flexible side chains improve solubility and affect polyazomethine PPI properties.
• PPI with the BOO flexible chains turned out to be better conjugated and planar.
• Absorption spectra indicate that flexible side chains affect delocalized states.
• Absorption edge parameters confirm better conjugation and chain order of BOO-PPI.
• Experimental results are in accord with the geometry optimization calculations.

Synthesis of a soluble derivative of a basic aromatic polyazomethine, i.e. poly(1,4-(2,5-bisoctyloxy-phenylene-methylenenitrilo)-1,4-phenylene-nitrilomethylidyne) BOO-PPI, enabled us to prepare thin films of this polymer by spin coating technique. Transmission and fundamental reflectivity measurements, within the spectral range of 0.49–6.20 eV, have been performed to obtain the absorption coefficient spectra of both BOO-PPI and un-substituted, i.e. poly(1,4-phenylene-methylenenitrilo-1,4-phenylene-nitrilomethylidyne) PPI thin films. An amorphous character of the obtained films allowed us to determine the absorption edge parameters, in the way typical for amorphous semiconductors. Both the Urbach energy (EU) and the Tauc energy gap (EG) were found to decrease from 130 to 93 meV and from 2.30 to 2.19 eV, respectively, when the flexible side chains are linked to the polyazomethine chain. A better conjugation and planarization of the BOO-PPI polymer chain, as compared to PPI, have been confirmed by the results of attenuated total reflection mode of the Fourier transform infrared (ATR-FTIR) spectroscopy measurements and geometry optimization calculations. The wide-angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM) studies of BOO-PPI and PPI films complemented the results of the UV-Vis-NIR spectroscopy investigations.