Light-Harvesting Properties and Morphology of Porphyrin Nanostructures Depend on Ionic Species Inducing Aggregation

Abstract

Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) in aqueous solutions of HCl and of the alkali chloride salts LiCl, NaCl, KCl, and CsCl were studied by absorption and resonance light scattering (RLS) spectroscopy. Aggregates deposited from these solutions were imaged by transmission electron microscopy and atomic force microscopy. In NaCl and KCl, the J band in the absorption spectrum was broadened, while in CsCl it was narrowed, as compared to aggregates in HCl. In LiCl and NaCl, the relative excitonic coherence as determined from RLS was increased as compared to aggregates in HCl, while in KCl and CsCl it was reduced. The discrepancy between measures of coherence based on exchange narrowing in the absorption spectrum and those on RLS intensity points to morphology-dependent composite J band structures. Delocalization of excitons between nanotubular subunits of bundled aggregates appears to increase excitonic coherence. Loosening of intermonomer forces, observed as bending in the images, and possibly the inclusion of alkali and/or chloride ions within composite structures, appear to limit excitonic coherence by increasing disorder. The dependence of morphologies on counterion species can be explained, in part, by the cations’ differing kosmotropic versus chaotropic ion-pairing properties and effects on water structure. The results may inform methods to tune the spatial and spectral properties of excitons in these systems.