Designing thiophene-based azomethine oligomers with tailored properties: Self-assembly and charge carrier mobility

This paper describes synthesis and characterization of two thiophene-based azomethines designed to optimize solubility, self-assembly, and charge carrier mobility. We found that incorporation of azomethine and amide moieties in the alpha,omega-position, and hexyl chains in the beta-position of the quaterthiophene, considerably improves the self-assembly properties without suppression of solubility. Self-assembly of azomethine oligomers with (QT-amide) and without amide moieties (QT-aniline) were monitored by UV-vis, XRD, and AFM. Although no changes in the UV-vis spectrum of QT-aniline is observed upon addition of hexane to the solution in THF, the addition of hexane to QT-amide solution induces a red shift of lambda(max) and appearance of fine structure believed to be vibronic in nature. The concentration dependence of the solvatochromism gives strong support for the intermolecular origin of this effect and clearly indicates that the planarization of the oligomer backbone is forced by the aggregation. Although, no clear signs of the molecular order for various QT-aniline films are observed by AFM, UV-vis, and XRD measurements, the QT-amide film after the annealing at 180 degreesC displays important molecular and macroscopic orientations. The sum of charge carrier mobilities as determined by pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) technique for QT-aniline was shown to be below the detectable limit; the mobility of QT-amide was determined to be 1 x 10(-2) cm(2) V-1 s(-1), which is comparable with the mobilities of the best organic semiconductors. All these significant differences in properties of related compounds can be attributed to the hydrogen bonding between QT-amide molecules responsible for the observed self-assembly.