J. Phys. Chem. 1990, 94, 6368-6371
6368
Fast and Slow Dynamic Probes in Solvent Structural Investigations T. G. Fillingim, Ningyi Luo, J. Lee, Picosecond and Quantum Radiation Laboratory, Texas Tech University, P.O. Box 4260, Lubbock, Texas 79409
and G . Wilse Robinson* Department of Physical Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia (Received: November 10, 1989; In Final Form: March I , 1990) The ability of water molecules to hydrate a proton from a photon-initiated weak acid depends on the microscopic concentration and structure of water in the aqueous solvent. Such acids thus can serve as dynamic probes of the local solvent environment in aqueous solvents perturbed by salts, surfaces, and dilution effects. In this paper, the microscopic local structures of pure liquid water and of two-component mixed aqueous solvents were examined by means of this technique. Aqueous solutions of methanol, ethanol, acetonitrile, dioxane, and glycerol served as solvents. Two different probes were used: I-naphthol (a "fast" probe) and 2-naphthol (a "slow" probe). A single-exponential decay was observed from these two probes in each of the neat solvents and also in low-viscosity mixed solvents. I n these cases local solvent homogeneity is attained during the probe lifetime. The single-exponential result was also found for 2-naphthol in mixed aqueous solvents of dioxane and glycerol. However, when I-naphthol was dissolved in these mixed solvents, and for water concentrations higher than 40%, a single exponential no longer provided a good fit to the decays. This observation indicates that solvent inhomogeneity is being probed during the short fluorescent lifetime of the 1-naphthol probe.
I. Introduction Our previous studies1-" have shown that proton hydration and electron hydration take place in exactly parallel ways, thus suggesting that similar hydration structures take part in the dynamics. When the water concentration in a methyl or ethyl alcohol/water solution is increased, the lifetime of the neutral fluorescent species decreases by virtue of the competitive occurrence of the proton or electron dissociation/hydration process. Some time ago, results nearly identical with those using ethanol had been obtained for acetonitrile/water mixed solvents," indicating a near equality of the behaviors of acetonitrile (ACN) and ethyl alcohol in this type of dynamics. A fit of these single-exponential decays, or of their related quantum yields, as a function of water concentration, showed that the best fit occurs for a theoretical curve generated with the requirement of a neighboring cluster containing no fewer than four water molecules.1~2~"'0A decreased rate and, during the short lifetime of the excited state, a cutoff of the proton or electron-transfer process take place when this number of water molecules is not available. These experiments thus indicate a sharp distinction between a "liquid-water-like" local environment, with four or more water molecules, and a "non-liquid-water-like" local environment, with fewer than four water molecules. One purpose of the present paper is to investigate a wider range of binary mixed aqueous solvent systems in order to examine the effect of changes in viscosity and other solvent properties in these dynamical studies. A second purpose is to see what happens to the experimental dynamics when the time scale of a chemical reaction becomes comparable to or faster than that of "solvent interchange motions" in a mixed solvent. To carry out these investigations, we use two different molecules as "probes" for the solvent structure and dynamics, 1-naphthol (I-ROH) and 2naphthol (2-ROH), that in their excited states can transfer a proton to a liquid water environment. The rates of these processes ( I ) Lee, J.; Griffin, R. D.; Robinson, G.W. J. Cfiem. Pfiys. 1985,82,4920. (2) Robinson, G . W.; Thistlethwaite, P. J.; Lee, J. J . Pfiys. Cfiem. 1986, 90, 4224. (3) Hameka, H. F.; Robinson, G. W.; Marsden, C. J. J . Phys. Chem. 1987, 91, 3150. (4) Lee, J . ; Robinson, G. W.; Webb, S. P.; Philips, L. A,: Clark, J. H. J . Am. Chem. SOC.1986, 108, 6538. ( 5 ) Robinson, G. W.; Robbins. R. J.; Fleming, G. R.; Morris, J . M.; Knight, A. E. W.; Morrison, R. J. S . J . Am. Cfiem. SOC.1978, 100, 7145. (6) Robinson, G . W.; Lee, J.; Moore, R. A. In Ultrafast Phenomena IV; Auston, D. H., Eisenthal, K. B., Eds.; Springer-Verlag: Berlin, 1984. (7) Lee, J.: Robinson, G.W. J . Chem. Pfiys. 1984, 81, 1203. (8) Lee, J.; Robinson, G . W. J . Pfiys. Chem. 1985, 89, 1872. (9) Moore, R. A.; Lee, J.; Robinson, G. W. J . Pfiys. Cfiem. 1985,89, 3648. (IO) Lee, J.; Robinson, G. W. J . Am. Cfiem. Soc. 1985, 107. 6153. ( I I ) Lee, J . ; Robinson, G . W. Unpublished work.
0022-3654/90/2094-6368$02.50/0
TABLE I: Lifetime of the Fluorescent Probes 1-Naphthol and 2-Naohthol in Various Neat Solvents 7,
solvent water ethanol
methanol ACN
dioxane glycerol
n," CP
eo
1.002
78.54
1.200 0.597 0.345 2.063 945.0
24.30 32.63 36.20 2.21 42.50
I-naDhtho1 0.032 5.75
5.82 5.01 10.34 10.11
ns 2-na~hthol 4.81b 5.89 5.88 5.98 7.66 8.1 1
aData at room temperature from refs 24 and 25. b20 O C ; all other lifetimes are insensitive to the temperature. in this environment span more than 2 orders of magnitude, 2.5 X IO'O and 1 . I X IO8 s-' for 1-ROH and 2-ROH, 11. Experimental Section HPLC grade deionized water and Kodak Chemical Co. I-ROH and 2-ROH were used without further purification. The nonaqueous solvents methanol, ethanol, ACN, dioxane, and glycerol were purified by the methods described in ref 12. If, under the conditions of excitation used in the experiments, no detectable emission was observed from the neat solvents, the solvents were considered pure. The emission spectrum of the neat Gold Label glycerol showed a slight trace of impurity emission when excited at 305 nm. However, its contribution to the spectrum of M naphthol solutions is negligible,
