Investigation on the effect of water on the spectroscopic behaviors of TPPS in acidic imidazolium-based ionic liquids

Abstract

The effect of water on the spectroscopic behaviors of meso-tetrakis-(4-sulfonatophenyl) porphyrin (TPPS) in acidic 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) is investigated by spectroscopic methods. In acidic ionic liquids without water, the Soret bands of the protonated form (H4TPPS2-) and J-aggregates of TPPS are centered at 449 and 469 nm, respectively, and the Q-band is centered at 679 nm. When increasing the content of water, the Soret band of H4TPPS2- is blue-shifted to 430 nm. However, the Soret and Q-bands of J-aggregates are red-shifted to 487 and 701 nm, respectively. The NMR and pyrene-scale polarity experiments reveal that the change of the bulk environment is responsible for the red-shift of J-aggregates and the blue-shift of H4TPPS2-. According to the analysis of the fluorescence spectra, the content of H4TPPS2- increases gradually with the addition of water, while the content of H2TPPS4- takes the opposite trend. Simultaneously, the content of J-aggregates decreases substantially when the addition of water is 0 ~ ca. 12 μL (or 13 μL) but it decreases slowly with further addition of water. Remarkably, the fluorescence intensity of J-aggregates is relatively strong. In addition, the fluorescence of J-aggregates can be detected when λex is set at both λex for TPPS monomers and λex for J-aggregates. While the fluorescence of TPPS monomers can be detected only when λex is set at λex for monomers. Moreover, with the addition of water, the fluorescence lifetimes of H2TPPS4- and H4TPPS2- increase from 3.202 and 0.830 ns to 9.623 and 2.964 ns, respectively.