Excited State Proton Transfer in Ionic Liquid Mixed Micelles

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][Br]) 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][Br], 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 (tau(PT)) in P123 micelle (65 ps) is 10 times slower than that (5 ps) in water, while the time constants of recombination (tau(rec)) and dissociation (tau(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, tau(PT) is shorter than that in P123 micelle. In the mixed micelle, tau(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 (tau(PT), tau(rec), and tau(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 (tau(PT)) in [pmim][BF4]-P123 mixed micelle is longer (similar to 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].