Aryl/fluoroaryl-substituted triphenylene discotic liquid crystals: Synthesis, mesomorphism, and photophysical properties

Abstract

• Synthesis of novel aryl- and fluoroaryl-substituted triphenylenes from 2-bromo-3,6,7,10,11-pentakis(hexyloxy)triphenylene. • Suzuki-Miyaura cross-coupling. • Aromatic nucleophilic substitution of perfluoroarenes (S N Ar). • High thermal stability. • Self-organization into hexagonal columnar (Col hex ) mesophase. • Arene-fluorarene intermolecular interactions. • Blue-violet photoluminescence. • Fluorescence red-shift upon fluorination. Fluorine aromatic hydrocarbons are ubiquitous optoelectronic functional materials and their efficient synthetic methods are still the subject of rapid, ongoing development. Here, several aryl- and fluoroaryl-substituted triphenylenes ( Ar - TP / F Ar - TP ) were synthesized via either Suzuki-Miyaura cross-coupling or aromatic nucleophilic substitution (S N Ar) of perfluoroarenes, respectively, starting indifferently from 2-bromo-3,6,7,10,11-pentakis(hexyloxy)triphenylene (Br- TP ). These compounds possess very high thermal stability with initial decomposition temperature over 300 °C, as measured by thermal gravimetric analysis (TGA). Differential scanning calorimetry (DSC), polarized optical microscopy (POM), and small/wide angle X-ray scattering (S/WAXS) revealed that they all self-organize into hexagonal columnar (Col hex ) mesophase over broad temperature ranges, occurring at room temperature for some of them, with higher stability than the archetypical 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene parent compound. Under the combined influence of the conjugated π-system and core-side arm distortion, the clearing temperatures and mesophase ranges increase from phenyl- ( P - TP ), naphthyl- ( N - TP ), to biphenyl-triphenylene ( BP - TP ). The number of fluorine atoms on the side fluoroaryl group also impacts the columnar mesomorphism: the more fluorine atoms, the higher the clearing temperatures and the wider the Col hex phase ranges, concomitantly with increasing arene-fluorarene intermolecular interactions. In addition, these compounds show blue-violet photoluminescence in solution: the largest fluorescent absolute quantum yield is measured for BP - TP (45 %), while the smallest is for 7F N - TP (10 %). Perfluoroaryls tend to lower the LUMO, which results in fluorescence red-shift. The arene-perfluoroarene polar-π interactions improve the physical properties of these F Ar - TP discotic liquid crystals.

Topics & Concepts