Luminescent and photoconductive liquid crystalline lamellar and helical network phases of achiral polycatenars

Abstract

π-Conjugated molecules are of special interest for designing new materials with potential optoelectronic applications. Herein, we report the synthesis of new functional non-symmetric substituted triple-chain π-conjugated 5,5′-diphenyl-2,2′-bithiophenes. They have a long rod-like aromatic core unit with a 3,5-diheptyloxy substituted-pattern at one end and a single thioalkyl or alkoxy chain with variable length at the other end. Their molecular self-assembly was studied by polarized optical microscopy, differential scanning calorimetry, X-ray diffraction, luminescence and photoconductivity techniques. The shortest homologue forms an isotropic liquid phase with a broken mirror symmetry (Iso₁^[*^]) beside smectic and 3D phases. Longer derivatives have no Iso₁^[*^] or smectic components, and only exhibit 3D phases. The 3D phases are achiral double-gyroid bicontinuous cubic liquid-crystalline (LC) phases with a double helical network structure and Iad symmetry. Both the smectic and cubic phases exhibit photoconductivity, photoluminescence (PL), phosphorescence and electroluminescence at temperatures up to 150 °C. The smectic liquid crystals show weaker PL, but stronger photoconductivity; they could be polarized by an applied electric field, which turned a photoresistive device into both a photodiode and a light-emitting diode. Hence this work provides new functional bithiophene-based materials, which could be applied in optoelectronic devices. A prototype of such a device, namely a wavelength-measuring photodetector, is demonstrated.