An Investigation of Porphyrin Aggregation Using Spectroscopic and Microscopic Methods
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Aggregates of diacid tetrasulfonatophenylporphine (H2(H4TSPP)) exhibit light harvesting and electron transport capabilities and are therefore promising candidates as device components. Before these aggregates can be used to construct devices their structural and electronic properties must be understood. Solution UV-visible and RLS studies confirmed the formation of H2(H4TSPP) aggregates with increasing solution ionic strength. Aggregation of H2(H4TSPP) was indicated by the presence of new absorbance bands in UV-visible spectra and a marked increase in RLS concurrent with the appearance of the new UV-visible absorbance bands. Both UV-visible and Raman spectroscopy confirmed the intact deposition of H2(H4TSPP) aggregates on solid substrates. The deposited aggregates were imaged by AFM and STM. The AFM and STM images revealed individual rods with diameters of ~ 30 nm and lengths of hundreds of nanometers. We report for the first time high resolution STM images of H2(H4TSPP) on Au(111) and HOPG. In addition to aggregates an ordered monolayer of H2(H4TSPP) monomers was found on HOPG. The well-defined monolayer islands of H2(H4TSPP) self- assembled on HOPG were studied in ultrahigh vacuum using STM, OMTS, UPS, and XPS. Unlike meso-tetrakis(4-carboxyphenyl)porphine (Hx(H4TCPP)), the carboxylate analog, H2(H4TSPP) monolayers are stable on HOPG and can be studied at room temperature without the addition of a second stabilizing compound. Protonation of the porphyrin nitrogens in the surface species is confirmed by XPS. High-resolution STM images of single molecule layers show a well-defined deformation of the porphyrin ring, as expected with complete protonation of the central nitrogen atoms. OMTS and UPS were used to identify the HOMO and LUMO of the H2(H4TSPP) monolayer species, and results are contrasted to those of nickel(II)tetraphenylporphyrin (NiTPP). Current vs. Voltage (I(V)) curves of single and stacked rods taken by STM are consistent with conduction in a band formed from the LUMO of H2(H4TSPP). Aggregate I(V) curves were consistent with N-type semiconductors and showed increasing current rectification with increasing aggregate thickness. These findings show that H2(H4TSPP) aggregates can be used as organic semiconductors with tunable current versus voltage characteristics.