Aggregates of tetra-(4-sulfonatophenyl)porphyrin (TSPP) are model systems for excitonic solar energy generation analogous to chlorophyll assemblies in photosynthetic bacteria. The effects of a wide variety of solution and surface environment conditions on these aggregates have previously been examined. However, cohesive models of these effects in terms of fundamental intramolecular interactions remain elusive. We approach this challenge systematically, starting with monatomic counterion species in aqueous solution. TSPP aggregates were prepared in aqueous alkali halide solutions of variable concentration. Visible absorption, resonance light scattering (RLS), and resonance Raman spectroscopy reveal counterion- and concentration-dependent trends in the excitonic J-band structure, indicating dependence of aggregate geometry and excitonic coherence on cation identity and concentration.Microscale geometries of aggregates formed in these solutions were examined by optical microscopy, and distributions of excitonic coherence are revealed by RLS imaging (Fig.1: clockwise from upper left, normalized RLS intensity overlaid on white light images of aggregates deposited from 0.75 M LiCl, NaCl, KCl, and CsCl). Nanoscale details of structure are revealed by TEM images. These results provide insight into interactions in solution driving self-assembly of ionic porphyrins into ordered nanostructures, and inform strategies for tuning chromophore aggregates to function efficiently as solar light-harvesting antennae. Molecular dynamics calculations on TSPP in these salt solutions are currently underway to further illuminate relationships between solvation environment and aggregation processes.

Effects of alkali halides on structural and excitonic properties of light-harvesting porphyrin aggregates

248th National Meeting (2014)

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