Synthesis of alkoxy-substituted hexa-peri-hexabenzocoronenes as molecular hosts

Abstract

Supramolecular chemistry has attracted a lot of attention due to a diversity of applications based on the aggregation of molecules through non-covalent interactions. One molecule enveloping another molecule through these non-covalent interactions is defined as host-guest chemistry, an important topic in supramolecular chemistry. Electron-rich polycyclic aromatic hydrocarbons (PAHs) provide attractive hosts to electron-deficient guests such as buckminsterfullerene (C60). Hexabenzocoronenes (HBCs), a type of PAH consisting of 42 carbons in thirteen fused benzenoid rings, can be synthesized using a number of now well-developed synthetic approaches. However, highly substituted, electron-rich and non-planar HBCs have been less studied.

Dodecamethoxy-HBC, HBC(OMe)12 has been synthesized by cyclotrimerization of bis(3,5-dimethoxyphenyl)acetylene, followed by a cyclodehydrogenation, but shows low solubility to common organic solvents. Introduction of hexyl chains on the dodecamethoxy-HBC, HBC(OMe2)12(C6H13)6, dramatically improves solubility. It exhibits a highly distorted core conformation with bay region torsion angles of 31.85 - 35.16˚. In the solid state, HBC(OMe2)12(C6H13)6 exhibits a herringbone-type packing with no close face-to-face contacts. Slow evaporation of the solution of HBC(OMe2)12(C6H13)6 and C60 (1:1) in toluene gives black, block crystals which show a columnar packing with alternating HBC and C60 units. The closest intracolumn approach between HBC(OMe2)12(C6H13)6 and C60 is 2.94 Å and the column separation is 16.7 Å.

An alternative approach to “double concave” HBC hosts is by the introduction of pendant aryl groups with 2,6-substituents. The six-fold symmetric HBC precursor hexa[4-(2ʹ,6ʹ-dimethoxyphenyl)phenyl]benzene was found to be poorly soluble but the hexaalkoxy-analogue, hexa[4-(2ʹ,6ʹ-(dihexyloxyphenyl)phenyl]benzene, is readily dissolved. Attempted planarisation of this later HPB did not proceed cleanly, nor was the product highly soluble. A related trimethylsilyl-functionalized hexaaryl-HPB was also prepared, and provides entry to hexaryl-HBCs with a plethora of functional groups.