Influence of hydrogen bonds on the supramolecular order of hexa-peri-hexabenzocoronenes

Three different hexa-peri-hexabenzocoronene (HBC) derivatives with carboxylic acid functions have been synthesized to study the effect of hydrogen bonding on the already pronounced columnar it-stacking. This functionalization improves the degree of order in the bulk and influences the surface patterning at solid-liquid interfaces as well as the thermal properties of these materials. The length of the tether between the aromatic core and the hydrogen-bond- forming carboxy group appears as a key factor influencing the supramolecular self-assembly, since only the HBCs with shorter spacers showed a strong synergetic effect of the columnar π-stacking with the hydrogen-bonding motifs. Clear signatures for this are unusually high mesophase transition temperatures and non-tilted columnar stacking in the pseudocrystalline phase. As model systems, HBC dyads have been synthesized with the two HBC disks linked through a covalent spacer fitting the intercolumnar distance of the short-tethered, hydrogen-bridged dimer. This comparison emphasized the impact of the hydrogen bonds on the supramolecular properties of the materials. A broad range of analytical methods have been used, including differential scanning calorimetry, wide-angle X-ray diffractometry, solid-state NMR spectroscopy, and scanning tunneling microscopy.