Excited-State Proton Transfer to Solvent from Phenol and Cyanophenols in Water

The excited-state proton transfer (ESPT) to solvent from phenol (PhOH) and cyanophenols (CNOHs) in water was studied by means of time-resolved fluorescence and photoacoustic spectroscopy. A characteristic property of PhOH and CNOHs is that the fluorescence quantum yields of the deprotonated forms are remarkably small (≤10−3) and the lifetimes are extremely short (≤30 ps). Time-resolved fluorescence measurements for PhOH, CNOHs, and their methoxy analogues at 298 K indicate that o- and m-cyanophenols (o- and m-CNOH) undergo rapid ESPT to the solvent water with rate constants of 6.6 × 1010 and 2.6 × 1010 s−1, respectively, whereas the fluorescence properties of PhOH and p-CNOH does not exhibit clear evidence of the ESPT reaction. Photoacoustic measurements show that photoexcitation of o- and m-CNOH in water results in negative volume changes, supporting the occurrence of ESPT to produce a geminate ion pair. In contrast, the volume contractions for the PhOH and p-CNOH solutions are negligibly small, which indicates that, in these compounds, the yields of solvent-separated ion pairs resulting from the ESPT are very small. The volume change per absorbed Einstein (ΔV r) for o-CNOH is obtained to be −5.0 mL Einstein−1, which is much smaller than the estimated volume contraction per photoconverted mole (ΔV R). This suggests that the geminate recombination between the ejected proton and the cyanophenolate anion occurs after rapid deactivation of the excited ion pair. In the temperature range between 275 and 323 K, the proton dissociation rates of o- and m-CNOH in H2O and D2O are slower than the solvent relaxation rates evaluated from the Debye dielectric relaxation time, indicating that the overall rate constant is determined mainly by the proton motion along the reaction coordinate.