Molecular engineering of coaxial donor-acceptor heterojunction by coassembly of two different hexabenzocoronenes: Graphitic nanotubes with enhanced photoconducting properties

Coassembly of two different hexa-peri-hexabenzocoronenes (HBC), one possessing an electron-accepting trinitrofluorenone (TNF) unit (1) and the other without TNF (2), takes place to give graphitic nanotubes with varying acceptor concentrations on their surface. While the diameters of the homotropic nanotubes of 1 and 2 differ from one another, the coassembled nanotubes change their dimensions from one to the other as a function of the molar ratio of 1 to 2. The photoconductivity of the coassembled nanotubes shows a bell-shaped dependency on the molar ratio, where the maximum photoconductivity arises at around a 75% mole fraction of 1. Because of a possible excitation energy migration in the graphitic layer of pi-stacked HBC, an efficient HBC-to-TNF electron transfer, leading to charge separation, is realized even when the surface concentration of TNF is low. On the other hand, a high concentration of TNF on the nanotube surface hardly results in promoting the anticipated charge recombination, thanks to the properly designed gap between the donor and acceptor layers in the coaxial geometry.