Sequence- and Chain-Length-Specific Complementary Double-Helix Formation

The artificial sequential strands consisting of two, three, or four m-terphenyl groups joined by diacetylene linkers with complementary binding sites, either the chiral amidine (A) or achiral carboxyl (C) group, were synthesized in a stepwise manner. Using circular dichroism and ¹H NMR spectroscopies along with liquid chromatography, we showed that, when three dimeric molecular strands (AA, CC, and AC) or six trimeric molecular strands (AAA, CCC, AAC, CCA, ACA, and CAC) were mixed in solution, the complementary strands were sequence-specifically hybridized to form one-handed double-helical dimers AA·CC and (AC)₂ or trimers AAA·CCC, AAC·CCA, and ACA·CAC, respectively, through complementary amidinium−carboxylate salt bridges. Upon the addition of CCA to a mixture of AAA, AAC, and ACA, the AAC·CCA double helix was selectively formed and then isolated from the mixture by chromatography. Moreover, the homo-oligomer mixtures of amidine or carboxylic acid from the monomers to tetramers (A, AA, AAAA, C, CC, and CCCC) assembled with a precise chain length specificity to form A·C, AA·CC, and AAAA·CCCC, which were separated by chromatography.