Remarkable Effects of Terminal Groups and Solvents on Helical Folding of o-Phenylene Oligomers
摘要:
Although o-phenylene oligomers (OPnR) made of dimethoxyphenylene units are thought to be intrinsically dynamic due to pi-electronic repulsion, we show that they fold into a regular helical geometry in CH3CN when they carry terminal groups such as CH3, CH2OH, Br, CO2Bn, and NO2. We evaluated their helical inversion kinetics via optical resolution of long-chain oligomers (e.g. 16- and 24-mers) by chiral HPLC. OP24Br at 298 K shows a half-life for the optical activity of 5.5 h in CH3OH/water (7/3 v/v) and requires 34 h for complete racemization. The perfectly folded helical conformers of OPnR, unlike their imperfectly folded ones, are devoid of extended pi-conjugation and show a cyclic voltammogram featuring reversible multistep oxidation waves.
Large area homeotropic columnar ordering of pi-conjugated discotic liquid crystals (LCs) is crucial for certain device applications but generally hard to achieve. Here we report polymeric p-phenylene octamer poly-1 and its monomer 1 as the first surface modifiers for homeotropic columnar order of a variety of discotic LCs up to a macroscopic length scale. Their octameric o-phenylene parts are known to fold helically into a cylinder that is reminiscent of a pi-stacked column of discotic LCs. Through-view X-ray diffraction patterns of 1 suggested that this molecule adheres to the glass substrate and directs its cylindrical axis perpendicular to the glass surface. This face on orientation likely nucleates the homeotropic columnar order of discotic LC materials.