Assemblies of TPPS4 porphyrin investigated by TEM, SPM and UV-vis spectroscopy

The aggregates of the tetrakis(4-sulfonatophenyl)porphyrin (TPPS4) growth from solution on flat solid crystalline hydrophilic (mica, glass, Si) and hydrophobic (graphite) substrates were investigated. The spatial structure of TPPS4 deposited on the surface was imaged using transmission electron microscopy (TEM), scanning atomic force microscopy (AFM). Scanning tunneling microscopy (STM) visualized porphyrin rings structure with molecular resolution. The aggregation of TPPS4 was investigated by UV-vis spectroscopy. The investigation revealed a three-dimensional rod and wheel-like molecular order of the TPPS4 porphyrin assemblies, while a corresponding shorter and monodisperse oligomer exhibited 2D ribbon-like structure. The TPPS4 porphyrin molecules self-assemble into micrometer-long nanorods and fibers on substrate surface. Their cross-section was determined quantitatively by scanning force microscopy; this revealed a typical thickness of two or four molecular layers and a width of tenths of molecule diameter. This indicates that the molecules are fully extended in the ribbons oriented with the conjugated backbone with the 54.7° to the long axis of the rod. We propose here TPPS4 ribbons as polymolecular architectures, which are building blocks of TPPS4 rods and built from the homoassociated porphyrin molecules. The TPPS4 nanoribbons could be used in a molecular-scale electronic device.