Coverage dependent supramolecular structures:: C60:ACA monolayers on Ag(111)

The dependence of supramolecular structure on fractional molecular coverage has been investigated for acridine-9-carboxylic acid ( ACA) and the C-60:ACA binary molecular system. The coverage-dependent phase diagram for ACA is first determined from room-temperature STM imaging. At low molecular coverages ( theta < 0.4 ML, ML) monolayer), ACA forms a 2-D gas phase. Ordered ACA structures appear with increasing coverage: first a chain structure composed of ACA molecules linked by consecutive O-(HN)-N-... hydrogen bonds ( theta > 0.4 ML), then a dimer structure composed of ACA dimers linked by paired carboxyl-carboxyl hydrogen bonds ( theta approximate to 1.0 ML). Structures of the C-60: ACA binary system depend on the coverage of predeposited ACA. At intermediate ( 0.4 ML similar to 0.8 ML) ACA coverages, C-60 deposition results in a hexagonal cooperative structure with the C-60 periodicity nearly 3 times that of the normal C-60 2-D packing of 1 nm and exists in enantiopure domains. At higher ACA coverages, a C-60 quasi-chain structure is formed in which parallel C-60 chains are spaced by ACA dimer domains. The mechanistic role of the initial ACA phase in the formation of C-60: ACA supramolecular structures is described. Chemically intuitive molecular packing models are presented based on the observed STM images.