Self-assembly of a strapped linear porphyrin oligomer on HOPG

Abstract

Polymeric structures based on porphyrin units exhibit a range of complex properties, such as nanoscale charge transport and quantum interference effects, and have the potential to act as biomimetic materials for light-harvesting and catalysis. These functionalities are based upon the characteristics of the porphyrin monomers, but are also emergent properties of the extended polymer system. Incorporation of these properties within solid-state devices requires transfer of the polymers to a supporting substrate, and may require a high-degree of lateral order. Here we show that highly ordered self-assembled structures can be formed via a simple solution deposition protocol; for a strapped linear porphyrin oligomer adsorbed on a highly oriented pyrolytic graphite (HOPG) substrate. Two distinct molecule-molecule interactions are observed to drive the formation of two molecular phases (‘Interdigitated’ and ‘Bridge-stabilised’) characterised by scanning tunnelling microscopy, providing information on the unit cell dimensions and self-assembled structure. The concentration dependence of these phases is investigated, and we conclude that a bridge-stabilised phase is the thermodynamically stable structure at room temperature.