Excited state proton transfer in ionic liquid mixed micelles

Abstract

Excited state proton transfer (ESPT) of pyranine (8-hydroxypyranine-1,3,6-trisulfonate, HPTS) in room temperature ionic liquid (RTIL) mixed micelles is studied by femtosecond up-conversion. The mixed micelle consists of a triblock copolymer, (PEO)₂₀-(PPO)₇₀-(PEO)₂₀ (Pluronic P123), and one of the two RTILs, 1-pentyl-3-methyl-imidazolium bromide ([pmim][B_r]) and 1-pentyl-3-methyl-imidazolium tetra-fluoroborate ([pmim][BF₄]). The size and structure of the mixed micelle vary with the relative amount of the RTIL. For [pmim][B_r], the hydrodynamic diameter of the mixed micelle is 26 nm in 0.3 M RTIL and 3500 nm in 3.0 M RTIL. The time constant of initial proton transfer (τ_PT) in P123 micelle (65 ps) is 10 times slower than that (5 ps) in water, while the time constants of recombination (t_rec) and dissociation (τ_diss) are 2-3 times slower in P123 micelle. On addition of the RTIL, the rate of ESPT is markedly modified. In 0.3 M RTIL-P123 mixed micelle, τ_PT is shorter than that in P123 micelle. In the mixed micelle, τ_PT increases with an increase in the concentration of the RTIL (230 ps in 3 M [pmim][Br] and 55 ps in 0.9 M [pmim][BF₄]). This is attributed to large scale penetration of the P123 micelle by RTIL replacing water molecules. The time constants of proton transfer (τ_PT, τ_rec, and τ_diss) are faster than the slowest component (200-500 ps) of solvation dynamics. It seems that the ultrafast component of solvation (<0.3 ps and <5 ps) is enough for inducing proton transfer. The time constant of the proton transfer (τ_PT) in [pmim][BF₄]-P123 mixed micelle is longer (~20%) than that in [pmim][Br]-P123 mixed micelle for the same concentration of RTIL. The counterion dependence of ESPT is attributed to the difference in the structure and greater hydrophobicity of the [pmim][BF₄].