Effect of Ionic Liquid on J-Aggregation of meso-Tetrakis (4-sulfonatophenyl)porphyrin within Aqueous Mixtures of Poly(ethylene glycol)

Aqueous mixtures of poly(ethylene glycol) (PEG) of different compositions offer widely varying physicochemical properties that may support porphyrin aggregation. Aggregation behavior of a common water-soluble porphyrin, meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS), is investigated within aqueous PEG mixtures constituted of PEGs of average molecular weights 200 (PEG200), 400 (PEG400), 600 (PEG600), and 1000 (PEG:1000) using UV-vis molecular absorbance, steady-state fluorescence, and resonance light scattering techniques. No aggregation of TPPS is observed in neat PEGs; addition of 10 wt % water to PEG at pH 1.0 is found to trigger TPPS into significant J-aggregation. The J-aggregation is observed to be most efficient within an aqueous mixture of 90 wt % PEG1000 at pH 1.0. The effect of ionic liquids, 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), as additives on the J-aggregation efficiency of TPPS within aqueous mixtures of PEG400 at pH 1.0 is investigated and compared with the effect of salts NaCl, NaPF6, and NaBF4 as additives on the J-aggregation of TPPS under the same conditions. In an aqueous mixture of 10 wt % PEG400 at pH 1.0, ionic liquids are observed to increase the J-aggregation efficiency more than the salts at lower concentrations. The efficiency of J-aggregation decreases upon further addition of [bmim][BF4] due to reduced dissociation of this ionic liquid in the mixture. While the three salts show limited solubility, the two ionic liquids are completely miscible in a 90 wt % PEG400 mixture in water at pH 1.0. The J-aggregation efficiency of TPPS increases rapidly and reaches a maximum before decreasing gradually as more and more ionic liquid is added to the mixture. The results draw attention to the unique dual role of ionic liquids as additives in affecting the J-aggregation of TPPS within aqueous mixtures of PEG as well as to their proficiency over common salts in J-aggregation.