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

Abstract

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 (< or = 10(-3)) and the lifetimes are extremely short (< or = 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 x 10(10) and 2.6 x 10(10) 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 (DeltaV(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 (DeltaV(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 H(2)O and D(2)O 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.

Solvent‐Dependent Fluorescence Lifetimes of Estrone, 17β‐Estradiol and 17α‐Ethinylestradiol

“…Kaneko et al . also observe similar solvent trends in phenol lifetimes: the fluorescence lifetime in water is longer than in cyclohexane, but shorter than in acetonitrile .…”

“…The latter is the same lifetime as observed in pH 7 buffer. Such a fast emission lifetime for anionic E2 agrees with the emission lifetime of the phenolate anion in water being less than 100 ps , whereas neutral phenol's lifetime in water is 3.1 ns . The 2% relative amplitude is reasonable, given the amount of neutral E2 expected to be present at pH 12.…”

“…The fluorescence lifetimes of E1 at 304 nm were significantly shorter than the lifetimes of E2 and EE2 at 304 nm (Table ), and indeed are shorter than many phenolic compounds in solution . The excited state lifetime at 304 nm was shorter than the lifetime at 412 nm, agreeing with the observation that energy is very efficiently transferred from the phenol to the carbonyl group.…”

“…The longer lifetime, 3.4 ns, agrees well with the 3.2 ns lifetime observed for phenol in water by Kaneko et al . . However, they did not observe a second lifetime component for phenol.…”

“…). When the same substitution is made in phenol, its lifetime in water goes from 3.1 to 5.0 ns . Kaneko et al .…”