Using Raman Spectroscopy to Probe the Internal Structure and Excitonic Properties of Light-Harvesting Aggregates of Tetra(sulfonatophenyl)porphyrin
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Borrowing ideas from light-harvesting aggregates in nature for use in photovoltaics or solar fuels to improve light collection and solar energy efficiency is an attractive prospect. However an incomplete understanding of the aggregate internal structure and its relation to excitonic states hinders the progress in this field. In this work, aggregates of a synthetic porphyrin called tetra(sulfonatophenyl)porphyrin (TSPP) are used as a model system to probe this correlation using resonance Raman spectroscopy. The hypothesized structure for these nanotubular aggregates is that of a hierarchical assembly composed of circular aggregates of TSPP which are held together by electrostatic forces. The formation of the observed nanotube structure then derives from water-mediated hydrogen bonding. Ensemble, condensed phase resonance Raman spectroscopy, single aggregate resonance Raman spectroscopy/microscopy, and surface-enhanced resonance Raman spectroscopy (SERRS) prove to be powerful tools for testing this proposed model of the aggregate structure and understanding its excitonic properties. Analysis of Raman intensities and polarized Raman spectra, as well as models for the electronic absorption spectra of these assemblies, provide intriguing insights on the nature of the excitonic states and the influence of the local environment on the effective coherence of the aggregate.