Photochemistry of “Super” Photoacids. 2. Excited-State Proton Transfer in Methanol/Water Mixtures Kyril M. Solntsev,* ,²,‡ Dan Huppert, ² Noam Agmon, ‡ and Laren M. Tolbert § Raymond and BeVerly Sackler Faculty of Exact Sciences, School of Chemistry, Tel AViV UniVersity, Tel AViV, 69978, Israel, Department of Physical Chemistry and Fritz Haber Research Center, The Hebrew UniVersity, Jerusalem, 91904, Israel, and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, 30332-0400 ReceiVed: December 23, 1999; In Final Form: March 6, 2000 Excited-state proton transfer to solvent (PTTS) of 5-cyano-2-naphthol was investigated in methanol/water mixtures. We have found that the time-resolved fluorescence data fit the solution of the Debye-Smoluchowski equation for the reversible geminate recombination of ions over the whole range of methanol/water concentration ratios. The rate constants of the elementary protolytic photochemical processes and their isotope effects were determined by a simultaneous analysis of the time-resolved fluorescence of the photoacid and its conjugated anion. The competition between adiabatic protonation and quenching of the excited naphtholate anion by the geminate proton was observed to diminish sharply near pure methanol. The dissociation rate coefficient near pure methanol depends on a power of the water concentration, which appears to decrease from 2 (for “ordinary” photoacids) to below 1 for “super” photoacids. I. Introduction Proton transfer, in both ground and excited states, is a fundamental process in chemistry and biology. The subject of our current research is the investigation of the mechanism of ultrafast excited-state proton transfer reactions of exceptionally strong photoacids in methanol/water solutions. Protolytic pho- todissociation (excited-state proton transfer to solvent or PTTS) has been studied intensively in the past 50 years. 1 The acidity of various hydroxyaromatic compounds (ROH) increases sig- nificantly upon excitation, and therefore molecules of this type are widely used as excited-state acid-base fluorescent probes in homogeneous solutions and microheterogeneous systems. The influence of water structure on the PTTS kinetics is most conspicuous in the investigation of proton transfer in mixed water/organic solvents. Protolytic photodissociation of various hydroxyaromatic compounds was studied in series of mixtures of water with alcohols 2-18 and other solvents. 14,19,20 In all cases PTTS rates were found to decrease with decreasing molar fraction of water in the mixture. This was accompanied by an increase of excited ROH (R*OH) fluorescence decay times and quantum yields, and a reduction of R*O - emission. For 2-naphthol (2OH, pK a * ) 2.8) the protolytic dissociation can be observed only up to 50 vol % methanol. 7 At higher methanol contents, the typical dissociation time becomes considerably longer than the excited-state lifetime (typically, 5-10 ns). For a much stronger photoacid like 1-naphthol (1OH, pK a * ) 0.4), dissociation is still observed at 98 wt % MeOH. 9 It was suggested 5-7 that a proton transfers only to water clusters of a certain size. In contrast to water solutions where such clusters already exist in the ROH solvation shell, in mixed solvents an additional step of their formation takes place. Robinson et al. 7 described it as a sequence of reversible processes corresponding to the substitution of alcohol molecules in the ROH solvation shell with water. This sequence terminates when the cluster size reaches a critical value. Experimental PTTS rate constants in methanol/water mixtures of different compositions are in best agreement with this model at critical cluster size of 4 ( 1. This cluster size was identical for both fast (1OH) and slow (2OH) photoacids. 14 Schulman et al. 13,19 studied PTTS of 2OH, 2,6-naphtholsul- fonate, and 8-hydroxy-1,3,6-pyrenetrisulfonate (HPTS) in etha- nol/water and DMSO/water mixtures. It was found that the overall dissociation rate constant, k off , had a linear dependence on the water activity, R(H 2 O), of the corresponding solution: where k off ° is the rate constant in pure water (R) 1) and τ 0 is the lifetime of the R*OH in its lowest excited state in water in the absence of proton transfer. It was found for all compounds that F) 12, and this value was interpreted as the number of water molecules in the proton acceptor cluster. 13,19 The signifi- cant difference with the value of 4 ( 1 obtained by Robinson et al. 5-7,14 was explained by the possible involvement of water molecules of outer coordination shells in the reaction. Klein et al. 11c suggested that protolytic photodissociation of 1-hydroxy-4-naphthalenesulfonate (4S1OH) (pK a * ≈ -0.1 in water) in propanol/water mixtures requires a critical water concentration (4.1 ( 0.3 M at 25 o C). Above this critical value, only two water molecules are needed to sustain the proton- accepting cluster. Agmon et al. 9 applied the theory of geminate diffusion- influenced reactions 21 to the PTTS of HPTS in methanol/water mixtures in the water-rich region. At all water concentrations the kinetics obeyed the transient Smoluchowski equation for diffusion of the geminate proton in a Coulomb potential with “back-reaction” boundary conditions. This was manifested in * Address correspondence to this author at The Hebrew University. E-mail: solntsev@fh.huji.ac.il. ² Tel Aviv University. ‡ The Hebrew University. § Georgia Institute of Technology. ln(k off τ 0 ) ) ln(k off °τ 0 ) +F ln R(H 2 O) (1) 4658 J. Phys. Chem. A 2000, 104, 4658-4669 10.1021/jp994454y CCC: $19.00 © 2000 American Chemical Society Published on Web 04/25/2000