Helical molecular duplex strands: multiple hydrogen-bond-mediated assembly of self-complementary oligomeric hydrazide derivatives.

Careful examination of the X-ray structure of a ditopic hydrazide derivative 7 led to the concept that with malonyl groups as interhydrazide linkers hydrogen-bonding-mediated molecular duplex strands might be obtained. Complexation studies between 7, 8, and 9 confirmed this hypothesis. Two quadruple hydrogen-bonded heterodimers formed, in which spectator repulsive secondary electrostatic interaction was found to play an important role in determining the stability of the complexes. Extensive studies on 1-4 indicated that the hydrogen-bonding mode could persist in longer oligomeric hydrazide derivatives with chain extension from monomer to tetramer. Molecular duplex strands via two to fourteen interstrand hydrogen bonds were obtained. In addition to affecting the stability of the duplex strands, spectator repulsive secondary electrostatic interaction also played an important role in determining dynamic behavior of the duplex strands as exemplified by variable temperature (1)H NMR experiments. IR studies confirmed stronger hydrogen bonding in the longer oligomers. The assemblies of 1-4 on HOPG were also studied by STM technology. Molecular mechanical calculations further revealed double-helical structures for the longer oligomers. The results provide new opportunities for development of polymeric helical duplexes with well-defined structures.