Illuminating Excitonic Structure in Ion-Dependent Porphyrin Aggregates with Solution Phase and Single-Particle Resonance Raman Spectroscopy

Abstract

Self-assembled, excitonically coupled aggregates of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) prepared in aqueous LiCl, NaCl, KCl, CsCl, and HCl were studied with solution phase and single-particle resonance Raman (RR) spectroscopy. Solution phase excitation profiles are sharply peaked at 488.0 nm excitation for samples in HCl, LiCl, and CsCl, which show narrow J-bands in the corresponding absorption spectra, but more gently peaked for those induced by NaCl or KCl, which show broader absorption J-bands. The former three samples also exhibit larger low to high frequency mode intensity ratios with excitation near the J-band peak and smaller depolarization ratios compared to the latter two samples. Polarized spectra of individual aggregates correlate with the solution phase results, exhibiting an increase in intensities involving either incident or scattered light polarized transversely to the aggregate long axes in conditions shown previously to induce bundling of nanotubes or greater disorder. These results, along with previous absorption, resonance light scattering (RLS), and imaging data, suggest that excitonic coupling across nanotubular components in bundled aggregates leads to spectral broadening. This is attributed to increased spectral density of allowed excitonic transitions, particularly those polarized transversely to the aggregate length. Disorder leads to deviation of excitonic transition polarizations from pure axial and transverse directions, resulting in greater transverse relative to axial polarization but smaller excitonic coherence as measured by RLS intensity. These results suggest that environment-induced morphological variations can affect the energies, polarizations, and spatial structure of excitons in dye aggregates.