Remarkable Effects of Terminal Groups and Solvents on Helical Folding of o-Phenylene Oligomers

Although o-phenylene oligomers (OPnR) made of dimethoxyphenylene units are thought to be intrinsically dynamic due to π-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 π-conjugation and show a cyclic voltammogram featuring reversible multistep oxidation waves.