Investigation on solvation and protonation of meso-tetrakis(p-sulfonatophenyl)porphyrin in imidazolium-based ionic liquids by spectroscopic methods

The solvation and protonation of the meso-tetrakis(p-sulfonatophenyl)porphyrin (TPPS) were investigated by spectroscopic methods in pure or mixed imidazolium-based ionic liquids: 1-butyl-3-methyl-imidazolium terafluoroborate ([MBIM]BF4), 1-butylimidazolium terafluoroborate ([HBIM]BF4), 1-butyl-imidazolium dodecylalkylbenzenesulfonate ([HBIM]DS), 1-butyl-imidazolium p-toluenesulfonate ([HBIM]TS) and 1-butyl-imidazolium methylsulfonate ([HBIM]MS). Compared with absorption properties of TPPS in aqueous solution, Soret band of TPPS monomer was obviously red-shifted in the ionic liquids, while special absorption of TPPS J-aggregates was located at higher energy level, 483 nm and 702 nm, in protonic ionic liquids (PILs) [HBIM]BF4. Next, the protonation of TPPS in aprotonic ionic liquids (AILs, i.e., [MBIM]BF4) is dependent not only on the concentration of protonic ionic liquids as proton sources, but also on the characteristic of anion and viscosity of PILs. The proton transfer constants between TPPS and four protonic ionic liquids are (2.32 ± 0.23) × 102 for [HBIM]BF4, (1.52 ± 0.08) × 102 for [HBIM]MS, (1.12 ± 0.21) × 102 for [HBIM]DS and (0.84 ± 0.45) × 102 for [HBIM]TS, respectively.