Multiple-site concerted proton-electron transfer reactions of hydrogen-bonded phenols are nonadiabatic and well described by semiclassical Marcus theory

Abstract

Photo-oxidations of hydrogen-bonded phenols using excited-state polyarenes are described to derive fundamental understanding of multiple-site concerted proton-electron transfer reactions (MS-CPET). Experiments have examined phenol bases having -CPh(2)NH(2), -Py, and -CH(2)Py groups ortho to the phenol hydroxyl group and tert-butyl groups in the 4,6-positions for stability (HOAr-NH(2), HOAr-Py, and HOAr-CH(2)Py, respectively; Py = pyridyl; Ph = phenyl). The photo-oxidations proceed by intramolecular proton transfer from the phenol to the pendent base concerted with electron transfer to the excited polyarene. For comparison, 2,4,6-(t)Bu(3)C(6)H(2)OH, a phenol without a pendent base and tert-butyl groups in the 2,4,6-positions, has also been examined. Many of these bimolecular reactions are fast, with rate constants near the diffusion limit. Combining the photochemical k(CPET) values with those from prior thermal stopped-flow kinetic studies gives data sets for the oxidations of HOAr-NH(2) and HOAr-CH(2)Py that span over 10(7) in k(CPET) and nearly 0.9 eV in driving force (ΔG(o)'). Plots of log(k(CPET)) vs ΔG(o)', including both excited-state anthracenes and ground state aminium radical cations, define a single Marcus parabola in each case. These two data sets are thus well described by semiclassical Marcus theory, providing a strong validation of the use of this theory for MS-CPET. The parabolas give λ(CPET) ≅ 1.15-1.2 eV and H(ab) ≅ 20-30 cm(-1). These experiments represent the most direct measurements of H(ab) for MS-CPET reactions to date. Although rate constants are available only up to the diffusion limit, the parabolas clearly peak well below the adiabatic limit of ca. 6 × 10(12) s(-1). Thus, this is a very clear demonstration that the reactions are nonadiabatic. The nonadiabatic character slows the reactions by a factor of ~45. Results for the oxidation of HOAr-Py, in which the phenol and base are conjugated, and for oxidation of 2,4,6-(t)Bu(3)C(6)H(2)OH, which lacks a base, show that both have substantially lower λ and larger pre-exponential terms. The implications of these results for MS-CPET reactions are discussed.

Figure 1

Examples of HOAr-CH₂Py quenching the fluorescence of (A) 9,10-dicyanoanthracene and (B) perylene. The instrument response function (IRF), which indicates the temporal width of the excitation pulse, in shown in both cases. (C) Stern-Volmer plots for these reactions, which allow calculation of the quenching rate constant (k_q) from the slope (m).

Figure 2

Plots of photochemical k_CPET values vs. ΔG^o' for (A) HOAr-NH₂ and (B) HOAr-CH₂Py. The points for the reaction with TPP⁺ (the most exergonic of the photo-oxidations) occur at the diffusion limit and have been removed to allow for an expanded x-axis. The remaining points at the diffusion limit are indicated as lower limits. A curve that represents a good fit of eq 1 to the substituted anthracene data (black circles) is provided in each case.

Figure 3

k_CPET plotted as a function of ΔG^o' for (A) HOAr-NH₂ and (B) HOAr-CH₂Py for oxidation by both excited-state polyarenes (black circles and red squares) and aminiums (orange triangles). Note the log scale on the y-axis. Various Marcus parabolas with variable λ have been provided with pre-exponential terms spanning 10¹⁰–10¹² s⁻¹. The arrows in the plots indicate that these points, which were observed at the diffusion limit, are lower limits for k_CPET. The other polyarenes (red squares) are not considered in making the fits.

Scheme 1

(A) Kinetic scheme for phenol-base MS-CPET. (B) Phenols, Photo-oxidants, and Oxidants.