title: Side chain effect on the double helix formation of ethynylhelicene oligomers.
abstract: Three series of ethynylhelicene oligomers with different side chains were synthesized: (P)-bD-n (n = 2-6) with branched alkyloxycarbonyl side chains; (P)-S-n (n = 2-7) with decylsulfanyl side chains; and (P)-DF-n (n = 4, 6, 8, 10) with alternating decyloxycarbonyl and perfluorooctyl side chains. The double helix formation of these side chain derivatives was compared to that of (P)-D-n with decyloxycarbonyl side chains. CD, UV-vis, and vapor pressure osmometry (VPO) studies showed that (P)-bD-n formed double helices as well as (P)-D-n. CD studies in trifluoromethylbenzene at different temperatures and concentrations indicated that the stability of the aggregate of (P)-bD-6 was similar to that of (P)-D-6. Bulkiness of side chains had little effect on aggregation, which indicated that π-π interactions of the aromatic moiety were essential for double helix formation. (P)-S-n were random coils in all solvents examined except in trifluoromethylbenzene. Whereas (P)-D-7 formed a double helix at 1 × 10(-3) M in toluene, (P)-S-7 was a random coil. This result indicated that the double helix forming ability of (P)-S-n was substantially lower than that of (P)-D-n. Based on the previous observation that (P)-F-n formed a more stable double helix than (P)-D-n, the order of stability may be summarized as follows: (P)-F-n > (P)-D-n and (P)-bD-n >(P)-S-n. The lower stability of (P)-S-n compared to that of (P)-F-n was ascribed to the softness and/or the electron-rich nature at the m-phenylene moiety. (P)-DF-n did not form a stable double helix. It was speculated that a regular alternating arrangement of soft/hard or electron-rich/deficient moieties is important for stable double helix formation. Side chains of ethynylhelicene oligomers can play significant roles in determining the stability of double helices.
authors.affiliation: Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai 980-8578, Japan.
authors.forename: Nozomi
authors.initials: N
authors.lastname: Saito
authors.forename: Ryo
authors.initials: R
authors.lastname: Terakawa
authors.forename: Masanori
authors.initials: M
authors.lastname: Shigeno
authors.forename: Ryo
authors.initials: R
authors.lastname: Amemiya
authors.forename: Masahiko
authors.initials: M
authors.lastname: Yamaguchi
mesh_terms: D002942:Circular Dichroism; D015394:Molecular Structure; D011083:Polycyclic Compounds / Q000138:chemical synthesis*
chemical_list: D011083:Polycyclic Compounds; C560958:ethynylhelicene; C031660:helicenes