Electrochemical and optical studies of new symmetrical and unsymmetrical imines with thiazole and thiophene moieties

In this study new symmetrical and unsymmetrical imines based on 4-[4-(4-fluorophenyl)1,3-thiazol-2-yl]aniline and 2,2′,5′,2”-terthiophene-5,5”-dicarboxaldehyde or 2,2’:5′, 2’’-terthiophene-5-carboxaldehyde, respectively, were synthesized by melt condensation with the yield of 86% and 40%. The 5% weight loss of the imines was found in temperature range from 294 to 390 °C in nitrogen, depending on the symmetry of aldehyde used in synthesis. The band gap of symmetrical imine (PV-BLJ-SC13) is lower by about ∼0.33 eV than that of unsymmetrical PV-BLJ-SC14. For both imines the electron density of HOMO level is mainly located on thiophene moieties and is extended to imine bonds in thiazole rings and imine moieties. The theoretical calculations of their HOMO-LUMO levels by DFT methods were also provided. The UV–Vis absorption spectra and cyclic voltammograms of imines both in the form of thin films cast on the electrode as well as dissolved in 1,2-dichloromethane were studied to determine the electrochemical and optical band gap energies as well as correlate the positions of HOMO and LUMO levels obtained by DFT and electrochemical methods. The resistance of imines was approximately 113.8 Ω and 102.4 Ω for PV-BLJ-SC13 and PV-BLJ-SC14, respectively as determined by IR thermal images and have a tendency to decrease by about 17–24 Ω when mixed with PTB7 and PC71BM. Both imines showed very bright luminescence. Moreover, optical absorption, photoluminescence and IR thermal images were measured for imine:PTB7 or imine:PTB7:PC71BM composite. Finally, simple organic solar cells based on (PTB7:imine):PC71BM in weight ratio (4:1):8 and (8:1):13 w/w were constructed and characterized. The highest power conversion efficiency, which was equal to 0.42% was determined for the ITO/PEDOT:PSS/PTB7:PV-BLJ-SC13:PC71BM (4:1):8/Al device. In addition, the power conversion efficiency was strongly dependent on the weight ratio of imine in active layer.