The Role of Solvent in the Hierarchal Structure of a Porphyrin Aggregate

Excitonically coupled J-aggregates of tetra(sulfonatophenyl)porphyrin (TSPP) are well-known to form in acidic aqueous solution and at high ionic strengths. Recently, we reported resonance Raman spectra of TSPP aggregates in solution and deposited on Au(111). Scanning tunneling microscopy (STM) images of the latter reveal a surprising hierarchal structure for the aggregate in which helical nanotubes about 50 nm in circumference are assembled from 6 nm disks. We postulate the latter to be circular 16-mers of protonated TSPP molecules similar to assemblies of bacteriochlorophylls in light-harvesting aggregates of purple photosynthetic bacteria. Nonplanar distortions of the porphyrin framework of TSPP on protonation play a key role in driving the assembly of the cyclic 16-mers. However, the absence of a discernable cavity in the STM images of the 6 nm disks raises the question of the role of water and proton counterions in the structure. In this study, resonance Raman and scanning probe microscopy are applied to study of TSPP aggregates from ethanol to probe the role of water in the aggregate structure.