Aggregation behavior of aqueous tetrakis(4-sulfonatophenyl)porphyrin in the presence of 1-butyl-3-methylimidazolium tetrafluoroborate

Abstract

Due to their unusual properties the role of ionic liquids (ILs) in affecting aqueous aggregation has become a topic of immense interest. Depending on the pH and the ionic strength of the solution many common porphyrins form J-aggregates in the presence of appropriate external additives. Effect of addition of a ‘hydrophilic’ IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF 4]) on aqueous aggregation of a common porphyrin, tetrakis(4-sulfonatophenyl)porphyrin (TSPP), is investigated at different pH in the presence of varying concentration of [bmim][BF 4] using molecular absorbance and steady-state fluorescence. The kinetics of the interconverting species in the presence of [bmim][BF 4] within aqueous TSPP at different pH is also studied. It is observed that the low amount of [bmim][BF 4] ( ca. 0.221–0.442 M or ca. 4.5–9.0 vol.%) favors the formation of J-aggregates at low solution pH; further increasing [bmim][BF 4] concentration leads to the destabilization of J-aggregates due to the deprotonation of TSPP from H 4TSPP 2− to H 2TSPP 4−, which, in turn, gets involved in formation of TSPP–[bmim][BF 4] complex. J-aggregation at low pH in the presence of [bmim][BF 4] is demonstrated to be significantly more efficient as compared to that in the presence of NaBF 4. Presence of [bmim][BF 4] at low pH facilitates the conversion of monomeric species to the J-aggregates which follows pseudo first-order kinetics with rate constants in agreement with experimental observations. Unique and effective role of IL [bmim][BF 4] in modifying aggregation behavior of aqueous TSPP is clearly demonstrated.