Spectroscopic investigation on the effect of pairing anions in imidazolium-based ionic liquids on the J-aggregation of meso-tetrakis-(4-sulfonatophenyl)porphyrin in aqueous solution

Abstract

The effect of pairing anions in imidazolium-based ionic liquids (ILs) on the J-aggregation of meso-tetrakis-(4-sulfonatophenyl)porphyrin (TPPS) is investigated in aqueous solutions by the spectroscopic methods. The ILs used herein possess similar cations but different pairing anions, tetrafluoroborate ( BF 4 − ) and p-toluenesulfonate (TS −). The results from absorption and fluorescence spectra indicate that the J-aggregation is mainly regulated by anionic species. The J-aggregation of TPPS is initially promoted with the increase of 1-butyl-3-methylimidazolium tetrafluoroborate/1-ethyl-3-methylimidazolium tetrafluoroborate (BMIMBF 4/EMIMBF 4) until 0.10/0.40 mol/L and subsequently weakened slightly with further increase of BMIMBF 4/EMIMBF 4 concentration. However, the J-aggregation of TPPS hardly occurs in the tested concentration range of 1-butyl-3-methylimidazolium-tosylate/1-ethyl-3-methylimidazolium-tosylate (BMIMTS/EMIMTS). Furthermore, obvious redshifts in both absorption and fluorescence spectra of TPPS are observed with progressive addition of BMIMTS/EMIMTS. TPPS induces proton chemical shifts of BMIMBF 4/EMIMBF 4 and BMIMTS/EMIMTS to shift downfield and upfield, respectively. So it can be concluded that ILs with simple pairing anion BF 4 − promote the J-aggregation of TPPS by charge equilibrium, while ILs with pairing anion TS − inhibit the J-aggregation of TPPS by the π– π and hydrophobic interaction.