Attractive acceptor-acceptor interactions in self-complementary quadruple hydrogen bonds for molecular self-assembly.

Molecular self-assembly provides the means for creating large supramolecular structures, extending beyond the capability of standard chemical synthesis. To harness the power of self-assembly, it is necessary to understand its driving forces. A potent method is to exploit self-complementary hydrogen bonding, where a molecule interacts with its own copy by suitable positions of hydrogen-bond donor (D) and acceptor (A) groups. With four hydrogen bonds, there are two possible self complementary patterns: the DDAA/AADD and the DADA/ADAD motifs. Of these, the DDAA pattern is usually more stable. The traditional explanation assumes that the secondary interactions between equal groups, that is, between donors (D⋯D) or acceptors (A⋯A), are repulsive. DDAA arrays would then have two, and DADA arrays six repulsive interactions. Here, using high-end quantum chemical analysis, we show that contrary to the traditional explanation, the secondary A⋯A interactions are, in fact, attractive. We revise the model of secondary interactions accordingly.