Competition and compromise between discotic and calamitic mesogens in triphenylene and azobenzene based shape-amphiphilic liquid crystals

Abstract

Clarifying the relationship and interplay between disc-like (discotic) and rod-like (calamitic) mesogens is highly appealing. In this article, a series of novel disc-rod shaped amphiphilic liquid crystalline dimers and side-chain polymers based on typical discotic triphenylene (TP) and calamitic azobenzene (AZO) mesogens have been well synthesized and systematically investigated. It was found that lamello-columnar mesophases were characteristic for disc-rod hybrids and bridge the lamellar and columnar phases typical for calamitic or discotic mesogens alone. For the TP6 based hybrid dimers with peripheral hexyloxy substituents, the phase structure evolves from a sequence of lamello-columnar mesophases Col_r/L and slim Col^S_r/L for dimers TP6–AZO6 and TP6–AZO8 with shorter hexyloxy or octyloxy tails attached on AZO, to a single Col_r/L mesophase persisting to lower temperature for TP6–AZO12 and TP6–AZO16 with longer dodecyloxy or hexadecyloxy tails. With the extension of TP peripheral substituents to decyloxy, although TP10–AZO12 exhibits only a stable lamello-columnar mesophase Col_r/L, the TP10–AZO6 represents an ideal sample of majority discotic moieties and just the right calamitic fraction, which demonstrates a fascinating phase transformation from a simple lamellar phase to a disordered oblique columnar phase Col_ob-d through an intermediate lamello-columnar mesophase Col_r/L. Although the methacrylate monomer derived from the hybrid TP6–AZO10 exhibits similar lamello-columnar mesophase Col_r/L, the corresponding polymer displays only a simple lamellar phase due to the constraint of the polymethacrylate backbone. The elucidation of complex self-assembly behavior and in-depth understanding of the competition and compromise between different shape aromatic moieties in disc-rod shaped amphiphiles is of crucial importance for clarifying the aromatic interactions involved in complex biological systems and guiding molecular design of advanced organic materials for various optoelectronic applications.