Activationless Multiple-Site Concerted Proton-Electron Tunneling

Abstract

The transfer of protons and electrons is key to energy conversion and storage, from photosynthesis to fuel cells. Increased understanding and control of these processes are needed. A new anthracene-phenol-pyridine molecular triad was designed to undergo fast photoinduced multiple-site concerted proton-electron transfer (MS-CPET), with the phenol moiety transferring an electron to the photoexcited anthracene and a proton to the pyridine. Fluorescence quenching and transient absorption experiments in solutions and glasses show rapid MS-CPET (3.2 × 10¹⁰ s^-1 at 298 K). From 5.5 to 90 K, the reaction rate and kinetic isotope effect (KIE) are independent of temperature, with zero Arrhenius activation energy. From 145 to 350 K, there are only slight changes with temperature. This MS-CPET reaction thus occurs by tunneling of both the proton and electron, in different directions. Since the reaction proceeds without significant thermal activation energy, the rate constant indicates the magnitude of the electron/proton double tunneling probability.

Figure 1.

(A) X-ray crystal structure of 1H. (B) Time evolution of fluorescence from 1H (red), 1D (blue), and 1Me (green) in acetonitrile at 298 K after excitation at 362 nm. Dotted line = instrument response function.

Figure 2.

(A) TA spectra of 1H in MeCN up to 100 ps after photoexcitation. (B) Component transient absorption spectra from global analysis of spectra from 0 to 100 ps: initial excited state (maroon), vibrationally cooled S₁-anth* state (red), and long-lived photoproduct (cyan). (C) Mole fractions of initial and cooled anth* states.

Figure 3.

(A) Time dependence of the fluorescence decay of 1H in 2-MeTHF from 5.5 to 300 K. (B) Arrhenius plots for 1H (red) and 1D (blue) in 2-MeTHF. (C) Arrhenius plot for 1H in a PMMA film. For (B) and (C), the experimental uncertainties in ln(k_MS-CPET) and 1/T are smaller than the size of the data points.

Scheme 1.

MS-CPET Mechanism for Excited State Quenching in 1H