Hydrogen bonding-driven highly stable homoduplexes formed by benzene/naphthalene amide oligomers

Abstract

This paper describes the dimerization of aromatic amide oligomers in chloroform. The oligomers were prepared by step-by-step coupling of benzene-m-dicarboxylic acid and naphthalene-2,7-diamine. Triethylene glycol chains were introduced into the 5-position of the benzene rings to provide solubility in organic solvents. ¹H NMR investigations revealed that, although shorter oligomers did not exhibit good dimerization ability, longer oligomers with five or more amide units could form stable homoduplexes which were stabilized by intermolecular multiple N–H⋯OC hydrogen bonds. The association constants (K_a) of the 3-, 4- and 5-mer oligomers were determined using ¹H NMR dilution experiments, while the K_a values of a 5-mer oligomer and a 7-mer oligomer, which were appended with two or one pyrene unit, were also evaluated using fluorescent dilution experiments. The results showed that the stability of the homoduplexes of the aromatic amide oligomers is nearly linearly related to the number of their amide units, and a K_a of 3.0 × 10⁷ M⁻¹ was determined for the 7-mer oligomer.