Resonance Raman Spectra of Individual Excitonically Coupled Chromophore Aggregates

We report resonance Raman spectra of individual porphyrin nanotubular aggregates of meso-tetra(4-sulfonatophenyl)porphyrin (TSPP) deposited on glass. Using a novel internal/external standard method, we show that absolute Raman cross sections of low-frequency vibrational modes are greatly enhanced by J-band excitation. We report single-aggregate resonance Raman spectra obtained without surface enhancement. Variations in the relative intensities of low- and high-frequency Raman modes of different aggregates and images of the resonance light scattering in epiillumination reveal variations in aggregate structure and allow the possible correlation between Raman intensity and coherence to be explored. Polarized Raman spectra of individual aggregates confirm that the J-band is a composite of two closely spaced vibronically coupled transitions polarized parallel and perpendicular to the long axis of the aggregate, in accordance with our structural model of a hierarchical helical nanotube. The evolution of the Raman spectrum of a single aggregate during laser heating reveals the role of water in the assembly of structural subunits. Our experimental results provide insight into the concept of aggregation-enhanced Raman scattering.