2,6-Diphenyl- and -distyryl-capped 3,7-dialkoxybenzo[1,2-b:4,5-b′]dithiophenes and their dithieno-annulated higher homologs: structural phase transition with enhanced charge carrier mobility
Synthesis of the title benzo[1,2-b:4,5-b′]dithiophenes was achieved using 2-ethylhexyl 3,7-dihydroxybenzo[1,2-b:4,5-b′]dithiophene-2,6-dicarboxylate as the common starting material. The effect of the introduction of phenyl and styryl groups as well as thieno-annulation to the benzo[1,2-b:4,5-b′]dithiophene core on π-conjugation was estimated by means of absorption and emission spectrometry and cyclic voltammetry. The phase behaviours of the compounds were also observed by differential scanning calorimetry and the dithieno-annulated higher homologs were found to show a solid–solid (crystalline–crystalline) phase transition. Then, intrinsic charge carrier mobilities in the π-systems were measured by the flash-photolysis time-resolved microwave conductivity (FP-TRMC) method and the values were in the range of 0.04–0.17 cm2 V−1 s−1. Remarkably, the thieno-annulated and phenyl-capped derivative showed a temperature/phase-dependent hole mobility profile with 3-fold increment in the second crystalline phase above 100 °C.