Rate Control Of Helix Oscillation Of Poly(Arylacetylene)S Achieved By Design Of Side-Group Structures

Poly(2-ethynylnaphthalene)s with different alkylester moieties at the 6-position (PNRs) were synthesized to observe the first helix oscillation (HELIOS) of helical poly(arylacetylene)s, a transition between stretched and contracted helices in solution unique to helical structures. Variable-temperature (VT) H-1 NMR spectra of PNRs were measured to analyze the microstructures of the two helices. The size of the aromatic ring and the structures of the alkyl chain in the side group on the PNRs strongly affected the rate of the HELIOS mode. The temperature dependencies of the relative ratio of populations between stretched and contracted helices were estimated. The differences in microstructures between stretched and contracted helices were also discussed based on the results of molecular mechanics calculations and H-1-C-13 heteronuclear single-quantum correlation spectroscopy (H-1-C-13 HSQC). Consequently, we successfully changed the rate of HELIOS of PNRs and obtained detailed information on the molecular conformation of the stretched and contracted helices.