Remarkable effects of terminal groups and solvents on helical folding of o-phenylene oligomers.
Although o-phenylene oligomers (OP(n)R) 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 CH(3)CN when they carry terminal groups such as CH(3), CH(2)OH, Br, CO(2)Bn, and NO(2). We evaluated their helical inversion kinetics via optical resolution of long-chain oligomers (e.g. 16- and 24-mers) by chiral HPLC. OP(24)Br at 298 K shows a half-life for the optical activity of 5.5 h in CH(3)OH/water (7/3 v/v) and requires 34 h for complete racemization. The perfectly folded helical conformers of OP(n)R, unlike their imperfectly folded ones, are devoid of extended π-conjugation and show a cyclic voltammogram featuring reversible multistep oxidation waves.
RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
Shinji
S
Ando
Eisuke
E
Ohta
Atsuko
A
Kosaka
Daisuke
D
Hashizume
Hiroyuki
H
Koshino
Takanori
T
Fukushima
Takuzo
T
Aida