J-aggregation of ionic liquid solutions of meso-tetrakis(4-sulfonatophenyl)porphyrin

Abstract

The title porphyrin was dissolved in the hydrophilic ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF₄], and triggered to assemble into J-aggregates by the addition of incremental volumes of water containing various amounts of acid (0.1, 0.2, or 1.0 M HCl). In contrast to recent studies, the current investigation is unique in that it centers on media that contain a predominant ionic liquid component (2.9–5.4 M [bmim][BF₄]), as opposed to an aqueous electrolyte containing a small fraction of ionic liquid as dissociated solute. Complex aggregation and underlying photophysical behavior are revealed from absorption spectroscopy, steady-state fluorescence, and resonance light scattering studies. Upon addition of aqueous HCl, the efficient formation of H₄TPPS²⁻ J-aggregates from the diprotonated form of meso-tetrakis(4-sulfonatophenyl)porphyrin (H₂TPPS⁴⁻) occurs in [bmim][BF₄]-rich media in a manner highly dependent upon the acidity, TPPS concentration, and solvent composition. The unique features of TPPS aggregation in this ionic liquid were elucidated, including the surprising disassembly of J-aggregates at higher aqueous contents, and our results are described qualitatively in terms of the molecular exciton theory. Finally, the potential of this system for the optical sensing of water at a sensitivity below 0.5 wt% is demonstrated. Overall, our findings accentuate how little is known about functional self-assembly within ionic liquids and suggest a number of avenues for exploring this completely untouched research landscape.