J. Phys. Chem. C 2009, 113, 1709–1718
1709
New Nanoscale Insights into the Internal Structure of Tetrakis(4-sulfonatophenyl) Porphyrin Nanorods Benjamin A. Friesen, Krista R. A. Nishida, Jeanne L. McHale, and Ursula Mazur* Department of Chemistry and Materials Science Program, Washington State UniVersity, Pullman, Washington 99164-4630 ReceiVed: September 16, 2008; ReVised Manuscript ReceiVed: December 1, 2008
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. Introduction Porphyrin aggregation is a well-known phenomenon that has fascinated chemists for over half a century because of the interesting optical and electronic properties exhibited by these self-assembled nanostructures.1-3 In photosynthetic organisms, chlorophyll derivatives self-organize into circular4-6 and rodshaped7 supramolecular structures. Synthetic porphyrin aggregates have also been observed to form excitonically coupled ring- and rod-shaped nanostructures,8-14 which are of interest for their potential applications in optoelectronic devices and solar cells.8,9 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS4-) is probably one of the most studied synthetic porphyrin complexes; see Figure 1. Under acidic aqueous conditions (usually pH
