Characterization, liquid crystalline behavior, optical and electrochemical study of new aliphatic–aromatic polyimide with naphthalene and perylene subunits

A new aliphatic–aromatic diamine with naphthalene diimide (DA) moiety and a polyimide with naphthalene and perylene diimide subunits (PI) were synthesized and their thermotropic liquid crystalline behavior was examined by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Both compounds exhibited liquid crystalline properties. The temperatures of 5% weight loss (T5%) of the diamine and polyimide range from 350 to 373 °C in nitrogen. The optical properties, that is, UV–vis absorption and photoluminescence (PL) of obtained monomer and polyimide were studied in solution and in solid state as a blend with inert poly(methyl methacrylate) (PMMA). Influence of solvent polarity, excitation wavelength and concentration on DA and PI emission was found. The investigated compounds emitted green light in chloroform solution. Relative PL intensity of the DA and PI was investigated in NMP in relation to rhodamine-B used as a standard. The electrochemical behavior of diamine and polyimide was studied (in solution) by differential pulse voltammetry (DPV). The HOMO and LUMO levels of these compounds were in the range of −5.44 to −6.03 eV and −3.81 eV, respectively. Energy gap (Eg) values were calculated using cyclic voltammetry (CV) and UV–vis measurements. Introduction of a perylene moiety (PI) resulted in increase of Eg (2.22 eV) compared to DA which exhibited a Eg value of approximately 1.63 eV. The electrical properties of DA and PI were investigated by current–voltage (I–V) measurements on ITO/compound/Al devices.

► We obtained new thermotropic diamine and a six-membered polyimide.
► We analyse mesomorphic behaviour in the level of structural changes.
► We examine new application of polyimide by exploiting their photoluminescence, and electrochemical experiments paving the way toward stable organic optoelectronic devices.