A new homologous series of semi-conducting liquid crystals based on phenyl-anthracene: synthesis and effect of the alkyloxy terminal chain on charge transport and photoconductive properties

Abstract

π-Conjugated systems are the key to the charge transport properties of organic semiconductors. Four multifunctional mesogenic materials were designed and synthesised from 2-amino anthracene and para-hydroxybenzaldehyde. Each material contains a central, rigid phenyl-anthracene core and one flexible alkyloxy chain with different lengths based on the concept of combining liquid crystal (LC) materials featuring facile processability, highly ordered alignment, and effective charge transport. (E)-N-(Anthracen-2-yl)-1-(4-(alkyloxy)phenyl) methanimine (n-OPIA) with n = 8, 10, 12, and 16 was chemically characterized by nuclear magnetic resonance spectroscopy and high-resolution mass spectrometry. Their thermal behaviours were performed by means of differential scanning calorimetry and polarised optical microscopy and showed differences in mesomorphic behaviours with respect to the alkyloxy chain length. Concerning their frontier molecular energy levels, they were studied by optical spectroscopy and cyclic voltammetry and compared to values obtained via ab initio density-functional theory (DFT) calculations. LC field-effect transistors (LC-FETs) using a solvent vapour enhanced drop casting based n-OPIA material showed efficient electrical parameters and interesting photoconductive properties, indicating their potential applications in multifunctional integrated devices. As the alkyloxy chain length of the n-OPIA compounds increased, the hole mobility, on/off current ratio, and responsivity decreased. The 8-OPIA homologue exhibits a mobility one order of magnitude larger than the previously studied homologue, namely 10-OPIA.