Tilted vs. parallel assembly caused birefringent reversal in columnar phases of oligothiophene as well as applications for LEDs and hole transport materials

Abstract

Polycatenar columnar liquid crystals nT (n = 1–6) of π-conjugated oligothiophenes end-capped with bulky N-tris(hexadecyloxy)benzyl carbazole groups exhibit unusual molecular tilted and parallel assemblies, realizing the reversal of birefringence within the columnar liquid crystalline (LC) phases depending on core length and temperature. In the parallel arrangement of short core compounds 2T and 3T, molecules align perpendicular to the column axis. The stacking of these molecules involves a rotational component, resulting in local helical assembly, and this specific packing motif collectively stabilizes the Col_hex/p6mm phase with negative birefringence. In the tilted arrangement of long core compounds 5T and 6T, the long rigid core-driven tilted assemblies induce a structural transition to the Col_rec/c2mm phase, accompanied by a characteristic reversal of birefringence from negative to positive. In particular, compound 4T forms a Col_hex/p6mm phase at lower temperature, but transforms into a Col_rec/c2mm phase at higher temperature, accompanied by an inversion of optical birefringence. These special LC nanostructures provide important insights into the correlation between the molecular structure and assembly orientation in π-conjugated oligothiophenes. Furthermore, with an increase in π-conjugation length, these compounds exhibit bright blue-to-red emission in both solution and thin-film states, covering the entire visible spectrum and enabling their use in the preparation of full-color light-emitting diodes (LEDs), including white light. Most interestingly, solution-processable nano-films derived from columnar polycatenar oligothiophenes exhibit excellent hole mobility. Thus, this work proposes a strategy to develop polycatenar LC semiconductors that exhibit full-color emission.