Syntheses and Properties of Five-Ring Fused Azo- and Thio-Aromatic Compounds Containing Imide Substituent

Five-ring fused azo- and thio-aromatic compounds 1 and 2 containing imide substituent were designed and synthesized. 3,4-Dibromo-1-(2-ethylhexyl)-1H-pyrrole-2,5-dione reacted with lithium indyl and benzothiophene-3-boronic acid respectively, affording intermediates 3 and 4. Compound 3 was intramolecular cyclized in the presence of PdCl2 to give target compound 1. And compound 2 was prepared through intramolecular cyclization of intermediate 4 by means of photochemical ringclosure reaction and oxidation. The physicochemical properties of compounds 1 and 2 were thoroughly investigated with TGA, UV-vis absorption spectra and cyclic voltammetry. Experimental results showed the introduction of imide substituent not only increased the solubility of compounds 1 and 2, but also decreased their energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The HOMO/LUMO energy levels of compounds 1 and 2 are -5.58/-2.25 eV and -6.04/-3.51 eV respectively. Single crystals of compound 1 were grown through solvent evaporation method in the mixture of dichloromethane and petroleum ether. Single crystal structure revealed compound 1 has a planar conjugated core and forms dimmer in the crystal. Strong pi-pi intermolecular interactions exist in the dimmer, and hydrogen bonds (NH center dot center dot center dot O=C) are observed among dimmers. The charge carrier mobilities of compounds 1 and 2 were investigated through thin film transistors. The transistors were fabricated with top-contact/bottom-gate device configurations. And thin films were deposited in vacuum on octadecyltrichlorosilane (OTS)-modified Si/SiO2 substrates. Transistors performance of compound 2 displays obvious p-type performance with a mobility of 2.75 x 10(-3) cm(2).V-1.s(-1). However, compound 1 exhibited no organic field-effect transistor (OFET) behavior. In order to understand the different device performances of compounds 1 and 2, their thin films were investigated by atomic force microscopy (AFM) and X-ray diffraction (XRD). AFM images showed that compound 1 formed continuous thin film with small size of microstructures, the existence of grain boundaries hindered the transport of charge carriers in the film. XRD curves revealed that compound 2 formed crystalline thin films. Though the continuity of 2 films was worse than that of 1, the larger size of microstructures and the crystalline property of the films facilitated the transport of charge carriers.