Synthesis and study on the light absorbing, emitting, redox and electrochromic properties of azines and polyazines with thiophene units

Low molecular weight compounds and polymers consisting of 1 and 2 thiophenes and double azomethine bonds prepared from hydrazine and thiophene aldehydes are presented. The effect of the number of thiophene rings on thermal, optical and electrochemical properties was examined. Polyazine with bithiophene structure exhibited slightly higher both glass transition temperature (Tg = 121 °C) and the decomposition temperature (T10 = 345 °C) than polymer with one thiophene ring in repeating unit (Tg = 115 °C, T10 = 321 °C). A higher degree of conjugation due to presence of bithiophene structure was confirmed by bathochromic shift of the absorbance and photoluminescence. Doping with HCl and FeCl3 resulted in increase of fluorescence intensity. All the obtained compounds emitted blue light with the highest intensity both in solution and in solid state as a blend with PMMA. The investigated compounds exhibited electrochemical energy gap (Eg) in the range of 1.94–3.07 eV. Introduction of a second thiophene ring resulted in a decrease of Eg by about 0.9 eV in the case of azines and 0.67 eV in polyazines. Additionally, energy band gap (Eg) was calculated theoretically at B3LYP/6-31G(d,p) level of theory.