How solution environment shapes light-harvesting properties in porphyrin aggregates through influences on morphology

Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis-(4-sulfonatophenyl) porphyrin (TSPP) are model systems analogous to the light-harvesting antenna complexes of green sulfur bacteria. TSPP aggregates were studied in several solution conditions including aqueous acids, aqueous alkali halide and acidified alcohol solutions. Characterization of the optical properties of these aggregates by absorption, resonance light scattering, and resonance Raman spectroscopy revealed variations in the excitonic properties which depended sensitively on solution conditions. Imaging by atomic force, scanning-tunneling, and transmission electron microscopies revealed morphological variations that also depended on solution conditions. In alkali halide solutions, these variations included variable propensities of nanotubular aggregate units form close-packed bundles and different apparent degrees of flexibility. In alcohols, differing acidity led to formation of differently sized nanosheet aggregates. We correlated these morphological variations qualitatively with the spectroscopically observed optical properties, finding that nanotubular aggregates tended to exhibit greater spatial extent of coherently coupled excited states, and bundling tended to lead to delocalization of excited states over multiple nanotubular subunits. These results support the idea that the morphology determines the spatial extent and energy level structure of excitonic states. The findings provide insights into simple, solution-based methods to tune dye aggregates for potential applications in solar energy conversion and other optoelectronic devices.