Ditriphenylenothiophene butterfly-shape liquid crystals. The influence of polyarene core topology on self-organization, fluorescence and photoconductivity

Two series of regio-isomeric butterfly-shaped liquid crystalline systems, based on a ditriphenylenothiophene core (alpha/beta-DTPT) equipped with eight peripheral Long alkoxy chains, were synthesized in two steps by the Suzuki-Miyaura cross-coupling/Scholl cyclo-dehydrogenation reactions tandem from appropriate precursors. The influence of the core topology (i.e. positional isomerism) on the mesomorphism, gelling self-assembly, absorption and Luminescence, and charge-transport properties of both sets of isomers was investigated and compared. The six fused pi-conjugated molecules systematically display columnar hexagonal (Col(hex)) mesophases with beta-fused ditriphenylenothiophene compounds (DTPTBn) showing more extended mesophase ranges and higher phase transition temperatures than their isomeric alpha-counterparts (DTPTAn). All ditriphenylenothiophenes also show strong aggregation behavior in organic solvents, and, in addition, good organic gelling abilities were found for the beta-isomers. Interestingly too, high hole mobility rates greater than 10(-3) cm(2) v(-1) s(-1) were measured for the beta-isomers, which were about 10 times higher than those of the alpha-counterparts. Modeling of the molecular structures combined with DFT calculations showed that the beta-isomers possess flatter and more extended pi-conjugated cores than the slightly distorted oc-isomers, and consequently were more prone to optimize pi - pi intermolecular interactions, in agreement with the high-temperature mesophases and high charge carrier mobilities observed for these isomers. Finally, both sets of compounds emit blue-purple light in solution and the emission is substantially red-shifted when organized in films. DFT calculations of the HOMO-LUMO energy levels were also consistent with the optical measurements. This original molecular design and straightforward synthesis combination provides an efficient way to obtain novel pi-extended organic, potentially semiconductor materials for further electronic device investigations.