Influence of Chemical Structure on Thermal, Optical and Electrochemical Properties of Conjugated Azomethines

A correlation between a chemical structure of π-conjugated azomethines and their physicochemical properties has been studied, towards their optoelectronic applications. Eight various homo-aryl moieties have been coupled with either phenylene or thiophene groups, via imine bond, resulting in 15 (9 novel) azomethines. Differential scanning calorimetry (DSC), thermogravimetric analysis (TG), UV-Vis absorption and photoluminescence spectroscopy, and cyclic voltammetry techniques have been used to investigate their thermal, optical and electrochemical properties. Moreover, these results have been supported by the density functional theory (DFT) calculations. Introduction of thiophene rings, into the structure of studied azomethines, proved to increase resistance towards hydrolysis, their thermal stability and reduction potential, together with an enhancement of their π-delocalization. Simultaneously, the presence of thiophene rings in azomethine structures has decreased their emissive properties, compared to their completely homo-aryl analogues. The highest thermal stability and the most enhanced conjugation has been observed for azomethines consisting of such central units moieties as fluorene, naphtalene and biphenylene. Surprisingly, the presence of conjugation breaking ether or methylene bridge provides interesting photoluminescence properties, when coupled with thiophene. Finally, the substitution with side methyl groups proved to induce a reduction of oxidation potential and an increase of melting temperatures. Thiophene, fluorene, naphtalene and biphenylene groups, due to enhancement of thermal stability and extension of absorption range, appear to be good building units for novel materials for photovoltaic structures, while the most promising properties in the context of application in emissive devices have been observed for phenylene-coupled compounds with such groups as phenylene or biphenylene.