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.