Water and oxygen electrochemistry lies at the heart of interfacial processes controlling energy transformations in fuel cells, electrolyzers, and batteries. Here, by comparing results for the ORR obtained in alkaline aqueous media to those obtained in ultra-dry organic electrolytes with known amounts of H 2 O added intentionally, we propose a new rationale in which water itself plays an important role in determining the reaction kinetics. This effect derives from the formation of HO ad ···H 2 O (aqueous solutions) and LiO 2 ···H 2 O (organic solvents) complexes that place water in a configurationally favorable position for proton transfer to weakly adsorbed intermediates. We also find that even at low concentrations (<10 ppm), water acts simultaneously as a promoter and as a catalyst in the production of Li 2 O 2 , regenerating itself through a sequence of steps that include the formation and recombination of H + and OH -. We conclude that although the binding energy between metal surfaces and oxygen intermediates is an important descriptor in electrocatalysis, understanding the role of water as a proton-donor reactant may explain many anomalous features in electrocatalysis at metal-liquid interfaces.

We describe an approach to regular triangular arrays of dipolar molecular rotors based on insertion of dipolar rotator carrying shafts as guests into channels of a host, tris(o-phenylenedioxy)cyclotriphosphazene (TPP). The rotor guests can either enter the bulk of the host or stay at or near the surface, if a suitable stopper is installed at the end of the shaft. Differential scanning calorimetry, solid-state NMR, and powder X-ray diffraction were used to examine the insertion of a dipolar rotor synthesized for the purpose, 1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, and it was found that it forms a surface inclusion compound. Rotational barriers from 1.2 to 9 kcal/mol were found by dielectric spectroscopy and were attributed to rotors inserted into the surface to different degrees, some rubbing the surface as they turn.

Photoexcitation of the electron donor (D) within a linear, covalent donor-acceptor-acceptor molecule (D-A(1)-A(2)) in which A(1) = A(2) results in sub-nanosecond formation of a spin-coherent singlet radical ion pair state, (1)(D(+•)-A(1)(-•)-A(2)), for which the spin-spin exchange interaction is large: 2J = 79 ± 1 mT. Subsequent laser excitation of A(1)(-•) during the lifetime of (1)(D(+•)-A(1)(-•)-A(2)) rapidly produces (1)(D(+•)-A(1)-A(2)(-•)), which abruptly decreases 2J 3600-fold. Subsequent coherent spin evolution mixes (1)(D(+•)-A(1)-A(2)(-•)) with (3)(D(+•)-A(1)-A(2)(-•)), resulting in mixed states which display transient spin-polarized EPR transitions characteristic of a spin-correlated radical ion pair. These photodriven J-jump experiments show that it is possible to use fast laser pulses to transfer electron spin coherence between organic radical ion pairs and observe the results using an essentially background-free time-resolved EPR experiment.

We examine the insertion of two dipolar molecular rotors as guests into a host, tris(o-phenylenedioxy)cyclotriphosphazine (TPP, 1), using differential scanning calorimetry, solid-state NMR, powder X-ray diffraction, and dielectric spectroscopy. The rotors are 1-(4'-n-pentylbiphenyl-4-yl)-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane and 1,12-bis(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane. Both enter the bulk even though their nominal diameter exceeds the nominal channel diameter and although a closely related rotor, 1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, is known to produce a surface inclusion compound. Rotational barriers of 5.4-9.3 kcal/mol were found for the dichlorophenyl rotator contained within the TPP channel. Clearly, van der Waals diameters in themselves do not suffice to predict TPP channel entry. It is suggested that the efficacy of the p-carborane stopper is reduced by the presence of the two relatively bulky adjacent benzene rings, which help to stretch the channel, and by the axial direction of its axis, which prevents the attached rotator from contributing to the stopping action.

Selective photoexcitation of the donor in an electron donor-acceptor 1 -acceptor 2 (D-A 1 -A 2 ) molecule, in which D = perylene and both A 1 and A 2 = naphthalene-1,8:4,5-bis(dicarboximide), results in sub-nanosecond formation of a spin-correlated singlet radical pair 1 (D + -A 1 À -A 2 ) having a large electron spin-spin exchange interaction, 2J, which precludes its observation by transient EPR spectroscopy. Subsequent selective photoexcitation of A 1 À rapidly produces 1 (D + -A 1 -A 2 À ), resulting in a dramatic decrease in 2J, which allows coherent spin evolution to mix the singlet (S) radical pair state 1 (D + -A 1 -A 2 À ) with the T 0 triplet sublevel of 3 (D + -A 1 -A 2 À ) in an applied magnetic field, where B c 2J. A spin-polarized transient EPR spectrum characteristic of the spin-correlated radical pair D + -A 1 -A 2 À is then observed. The time delay between the two laser pulses was incremented to measure the rate of decoherence in 1 (D + -A 1 À -A 2 ) in toluene at 295 K, which was found to be 8.1 Â 10 7 s À1 . Deuteration of the perylene donor or the toluene solvent decreases the decoherence rate constant of 1 (D + -A 1 À -A 2 ) to 4.3 Â 10 7 s À1 and 4.6 Â 10 7 s À1 , respectively, while deuteration of both the perylene donor and the toluene solvent reduced the decoherence rate constant by more than half to 3.4 Â 10 7 s À1 . The data show that decreasing electron-nuclear hyperfine interactions significantly increases the zero quantum coherence lifetime of the spin-correlated radical pair. † Electronic supplementary information (ESI) available: Synthesis of donoracceptor 1 -acceptor 2 molecule. See