Activationless Multiple-Site Concerted Proton–Electron Tunneling

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.