Energy levels of metal porphyrins upon molecular alignment during layer-by-layer electrostatic assembly: scanning tunneling spectroscopy vis-à-vis optical spectroscopy

Abstract

We report ultrathin-film formation of metal-porphyrins with their molecular plane aligned parallel to the substrate. Such an alignment has been achieved through application of an external magnetic field to a monolayer of the porphyrin derivatives followed by immobilization of the molecules with a layer of a polyion. The orientation of metal-porphyrin molecules in a monolayer responded to the magnetic field due to their anisotropic magnetic moment arising out of unpaired d-electrons of the central atom. In this work, we compared characteristics of different metal-porphyrin molecules in their aligned and unaligned forms. Scanning tunneling spectroscopy (STS) of a monolayer of the porphyrins were recorded that in turn yielded density of states (DOS) from which highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) could be located. We observed that the energies responded upon alignment of the molecules; transport gap of metal porphyrins representing both Soret and Q-bands decreased in the aligned films due to an interaction of the molecular planes with the electrode. We compared the transport gap from STS and optical gap representing Soret and Q-bands in films of aligned and unaligned molecules.