An initial review (PCET1) on proton-coupled electron transfer (PCET) by Huynh and Meyer appeared in Chemical Reviews in 2007. 1 This is a perennial review, a follow up on the original. It was intended for the special Chemical Reviews edition on Proton Coupled Electron Transfer that appeared in December, 2010 (Volume 110, Issue 12 Pages 6937-710). The reader is referred to it with articles on electrochemical approaches to studying PCET by Costentin and coworkers, 2 theory of electron proton transfer reactions by Hammes-Schiffer and coworkers, 3 proton-coupled electron flow in proteins and enzymes by Gray and coworkers, 4 and the thermochemistry of proton-coupled electron transfer by Mayer and coworkers. 5 Coverage for the current review is intended to be broad, covering all aspects of the topic comprehensively with literature coverage overlapping with the later references in PCET1 until late 2010. There is a growing understanding of the importance of PCET in chemistry and biology and its implications for catalysis and energy conversion. This has led to a series of informative reviews that have appeared since 2007. They include: "The possible role of Proton-coupled electron Transfer (PCET) in Water oxidation by Photosystem II" by Meyer and coworkers in 2007, 6 "Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics" by Hammes-Schiffer and coworkers in 2008, 7 "Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer" by Costentin in 2008, 8 "Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems" by Nocera and Reece in 2009, 9 and "Integrating Proton-Coupled Electron Transfer and Excited States" by Meyer and coworkers in 2010. 10

Two-dimensional redox-active covalent organic frameworks (COFs) are ideal materials for energy storage applications due to their high surface area, extended π conjugated structure, tunable pore size and adjustable functionalities.[1-3] Herein, we report the synthesis and supercapacitor application of two redox active COFs [TpPa-(OH)2 and TpBD-(OH)2] along with the role of their redox active functional groups for the enrichment of specific capacitance.3 Of these COFs, TpPa-(OH)2 exhibited the highest specific capacitance of 416 F g−1 at 0.5 A g−1 current density in three electrode configuration while the highest specific capacitance was 214 F g−1 at 0.2 A g−1 current density in two electrode configuration. Superior specific capacitance was due to emergence of excellent pseudocapacitance by virtue of precise molecular level control over redox functionalities present in the COF backbone. This COF also demonstrated 66% capacitance retention after 10000 cycles along with 43% accessibility of the redox-active hydroquinone (H2Q) moieties in three electrode configuration while the capacitance retention was 88% after 10000 cycles in two electrode configuration. Exceptionally high specific capacitance of TpPa-(OH)2 was due to the reversible proton-coupled electron transfer (2H+/2e−) of hydroquinone/benzoquinone (H2Q/Q) moieties wherein H2Q and Q had comparable chemical stabilities during redox cycling that originated from H-bonding, which was supported by calculated structures.

The complex [Ru(Mebimpy)(bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2'-bipyridine] and its 4,4'-(PO(3)H(2)CH(2))(2)bpy derivative on oxide electrodes are water oxidation catalysts in propylene carbonate and 2,2,2-trifluoroethanol (TFE) to which water has been added as a limiting reagent. The rate of water oxidation is greatly enhanced relative to that with water as the solvent and occurs by a pathway that is first-order in H(2)O; an additional pathway that is first-order in acetate appears when TFE is used as the solvent.

Self-assembled monolayers of single-stranded (ss) peptide nucleic acids (PNAs) containing seven nucleotides (TTTXTTT), a C-terminus cysteine, and an N-terminus ferrocene redox group were formed on gold electrodes. The PNA monomer group (X) was selected to be either cytosine (C), thymine (T), adenine (A), guanine (G), or a methyl group (Bk). The charge transfer rate through the oligonucleotides was found to correlate with the oxidation potential of X. Kinetic measurements and computational studies of the ss-PNA fragments show that a nucleobase mediated charge transport mechanism in the deep tunneling superexchange regime can explain the observed dependence of the kinetics of charge transfer on the PNA sequence. Theoretical analysis suggests that the charge transport is dominantly hole-mediated and takes place through the filled bridge orbitals. The strongest contribution to conductance comes from the highest filled orbitals (HOMO, HOMO-1, and HOMO-2) of individual bases, with a rapid drop off in contributions from lower lying filled orbitals. Our studies further suggest that the linear correlation observed between the experimental charge transfer rates and the oxidation potential of base X arises from weak average interbase couplings and similar stacking geometries for the four TTTXTTT systems.