New Nanoscale Insights into the Internal Structure of Tetrakis(4-sulfonatophenyl) Porphyrin Nanorods

Nanorods produced from the sodium salt of tetrakis(4-sulfonatophenyl) porphyrin, dissolved in acidic aqueous solutions, were deposited onto Au(111) substrates and imaged by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). The AFM and STM images revealed individual rods with a diameters of 25-40 nm and lengths of hundreds of nanometers. Bundles of individual rods fashioned larger structures. We report for the first time high resolution STM images of TSPP on Au(111) which reveal that the rods are composed of disk-like building blocks approximately 6.0 nm in diameter. We speculate that the disks are formed by a circular J-aggregation of 14-16 overlapping electronically coupled porphyrin chromophores and that this circular porphyrin organization is driven by nonplanar distortions of the porphyrin diacid. The resonance Raman spectra of the solution phase aggregate and the surface-enhanced resonance Raman spectra of the aggregate on gold films were obtained at an excitation wavelength coincident with the exchange-narrowed J-band and found to be similar in peak frequencies and relative intensities. The UV-visible absorption spectrum of the solution phase aggregate was also found to be similar to that of the aggregate deposited on quartz. These comparisons confirm similar ground and excited electronic state structures of the excitonically coupled chromophores which comprise the aggregate in solution and on gold. Our results shed light on a number of previous experimental observations that could not be rationalized within the typical presumed staircase model of J-aggregation.