Mechanism for the Nonadiabatic Photooxidation of Benzene to Phenol: Orientation‐Dependent Proton‐Coupled Electron Transfer

An efficient catalytic one‐step conversion of benzene to phenol was achieved recently by selective photooxidation under mild conditions with 2,3‐dichloro‐5,6‐dicyano‐p‐benzoquinone (DDQ) as the photocatalyst. Herein, high‐level electronic structure calculations in the gas phase and in acetonitrile solution are reported to explore the underlying mechanism. The initially populated ¹ππ* state of DDQ can relax efficiently through a nearby dark ¹nπ* doorway state to the ³ππ* state of DDQ, which is found to be the precursor state involved in the initial intermolecular electron transfer from benzene to DDQ. The subsequent triplet‐state reaction between DDQ radical anions, benzene radical cations, and water is computed to be facile. The formed DDQH and benzene‐OH radicals can undergo T₁→S₀ intersystem crossing and concomitant proton‐coupled electron transfer (PCET) to generate the products DDQH₂ and phenol. Two of the four considered nonadiabatic pathways involve an orientation‐dependent triplet PCET process, followed by intersystem crossing to the ground state (S₀). The other two first undergo a nonadiabatic T₁→S₀ transition to produce a zwitterionic S₀ complex, followed by a barrierless proton transfer. The present theoretical study identifies novel types of nonadiabatic PCET processes and provides detailed mechanistic insight into DDQ‐catalyzed photooxidation.     

SET and HAT/PCET acid-mediated oxidation processes in helical shaped fused bis-phenothiazines

Helical shaped fused bis-phenothiazines 1 – 9 have been prepared and their red-ox behaviour quantitatively studied. Helicene radical cations (Hel^.+) can be obtained either by UV-irradiation in the presence of PhCl or by chemical oxidation. The latter process is extremely sensitive to the presence of acids in the medium with molecular oxygen becoming a good single electron transfer (SET) oxidant. The reaction of hydroxy substituted helicenes 5 – 9 with peroxyl radicals (ROO^.) occurs with a ‘classical’ HAT process giving HelO^. radicals with kinetics depending upon the substitution pattern of the aromatic rings. In the presence of acetic acid, a fast medium-promoted proton-coupled electron transfer (PCET) process takes place with formation of HelO^. radicals possibly also via a helicene radical cation intermediate. Remarkably, also helicenes 1 – 4 , lacking phenoxyl groups, in the presence of acetic acid react with peroxyl radicals through a medium-promoted PCET mechanism with formation of the radical cations Hel^.+. Along with the synthesis, EPR studies of radicals and radical cations, BDE of Hel-OH group (BDE_OH), and kinetic constants (k_inh) of the reactions with ROO^. species of helicenes 1 – 9 have been measured and calculated to afford a complete rationalization of the redox behaviour of these appealing chiral compounds.     

Snapshots of a Migrating H‐Atom: Characterization of a Reactive Iron(III) Indenide Hydride and its Nearly Isoenergetic Ring‐Protonated Iron(I) Isomer

We report the characterization of an S= iron π‐complex, [Fe(η⁶‐IndH)(depe)]⁺ (Ind=Indenide (C₉H₇^?), depe=1,2‐bis(diethylphosphino)ethane), which results via C?H elimination from a transient Fe^III hydride, [Fe(η³:η²‐Ind)(depe)H]⁺. Owing to weak M?H/C?H bonds, these species appear to undergo proton‐coupled electron transfer (PCET) to release H₂ through bimolecular recombination. Mechanistic information, gained from stoichiometric as well as computational studies, reveal the open‐shell π‐arene complex to have a BDFE_C‐H value of ≈50 kcal mol^?1, roughly equal to the BDFE_Fe‐H of its Fe^III?H precursor (ΔG°≈0 between them). Markedly, this reactivity differs from related Fe(η⁵‐Cp/Cp*) compounds, for which terminal Fe^III?H cations are isolable and have been structurally characterized, highlighting the effect of a benzannulated ring (indene). Overall, this study provides a structural, thermochemical, and mechanistic foundation for the characterization of indenide/indene PCET precursors and outlines a valuable approach for the differentiation of a ring‐ versus a metal‐bound H‐atom by way of continuous‐wave (CW) and pulse EPR (HYSCORE) spectroscopic measurements.     

Evidence for a Proton-Coupled Electron Transfer Mechanism in a Biomimetic System for Monoamine Oxidase B Catalysis

Mechanistic studies with 5-ethyl-3-methyllumiflavinium (Fl⁺) perchlorate, a biomimetic model for flavoenzyme monoamine oxidase B (MAO-B) catalysis, and the tertiary, allyl amine 1-methyl-4-(1-methyl-1 H-pyrrol-2-yl)-1,2,3,6-tetrahydropyridine (MMTP) reveal that proton-coupled electron transfer (PCET) may be an important pathway for MAO catalysis. The first step involves a single-electron transfer (SET) leading to the free radicals Fl^. and MMTP^., the latter produced by deprotonation of the initially formed and highly acidic MMTP^.+. Molecular oxygen (O₂) is found to play a hitherto unrecognized role in the early steps of the oxidation. MMTP and several structurally similar tertiary amines are the only tertiary amines oxidized by MAO, and their structural/electronic properties provide the key to understanding this behavior. A general hypothesis about the role of SET in MAO catalysis, and the recognition that PCET occurs with appropriately substituted substrates is presented.     

Phenol Oxidation by a Nickel(III)–Fluoride Complex: Exploring the Influence of the Proton Accepting Ligand in PCET Oxidation

In order to gain insight into the influence of the H⁺-accepting terminal ligand in high-valent oxidant mediated proton coupled electron transfer (PCET) reactions, the reactivity of a high valent nickel–fluoride complex [Ni^III(F)(L)] ( 2 , L=N,N’-(2,6-dimethylphenyl)-2,6-pyridinecarboxamidate) with substituted phenols was explored. Analysis of kinetic data from these reactions (Evans–Polanyi, Hammett, and Marcus plots, and KIE measurements) and the formed products show that 2 reacted with electron rich phenols through a hydrogen atom transfer (HAT, or concerted PCET) mechanism and with electron poor phenols through a stepwise proton transfer/electron transfer (PT/ET) reaction mechanism. The analogous complexes [Ni^III(Z)(L)] (Z=Cl, OCO₂H, O₂CCH₃, ONO₂) reacted with all phenols through a HAT mechanism. We explore the reason for a change in mechanism with the highly basic fluoride ligand in 2 . Complex 2 was also found to react one to two orders of magnitude faster than the corresponding analogous [Ni^III(Z)(L)] complexes. This was ascribed to a high bond dissociation free energy value associated with H−F (135 kcal mol⁻¹), which is postulated to be the product formed from PCET oxidation by 2 and is believed to be the driving force for the reaction. Our findings show that high-valent metal–fluoride complexes represent a class of highly reactive PCET oxidants.     

Olefin Isomerization via Radical‐Ion Pairs in Triplet States Studied by Chemically Induced Dynamic Nuclear Polarization (CIDNP)

Geometric isomerizations of olefins following photoinduced electron transfer (PET) are classified according to the relative energetic positions of the radical‐ion pairs and the reactant triplets. Each class exhibits characteristic CIDNP (chemically induced dynamic nuclear polarization) effects, for which typical examples are presented. Time‐resolved CIDNP experiments on the system triphenylamine/fumarodinitrile (= (2E)‐but‐2‐enedinitrile), where formation of the olefin triplet is impossible, show that there is also no isomerization of the olefin radical anion. With triisopropylamine or fumarodinitrile as the reaction partner for 4,4′‐dimethoxystilbene (= 1,1′‐[(1E)‐ethane‐1,2‐diyl]bis[4‐methoxybenzene]), both oxidative and reductive quenching give almost mirror‐image CIDNP spectra because of the pairing theorem; reverse electron transfer of the triplet radical‐ion pairs populates the stilbene triplet only, which then isomerizes. With anethole (= 1‐methoxy‐4‐(prop‐1‐enyl)benzene; M), the competition between electron return of triplet pairs to give either M + ³X or ³M + X was studied by using a second isomerizable olefin (diethyl fumarate (= diethyl (2E)‐but‐2‐enedioate) or cinnamonitrile (= (2E)‐3‐phenylprop‐2‐enenitrile)) as the reaction partner X. Classes can be changed by employing PET sensitization. With ACN (anthracene‐9‐carbonitrile) as the sensitizer, anethole does not produce any directly observable polarizations, but a substitution of ACN^.? by the radical anion of 1,4‐benzoquinone (= cyclohexa‐2,5‐diene‐1,4‐dione) or fumarodinitrile within the lifetime of the spin‐correlated radical‐ion pairs leads to very strong CIDNP signals that reflect the effects of both pairs.     

Protonation and Proton‐Coupled Electron Transfer at S‐Ligated [4Fe‐4S] Clusters

Biological [Fe‐S] clusters are increasingly recognized to undergo proton‐coupled electron transfer (PCET), but the site of protonation, mechanism, and role for PCET remains largely unknown. Here we explore this reactivity with synthetic model clusters. Protonation of the arylthiolate‐ligated [4Fe‐4S] cluster [Fe₄S₄(SAr)₄]^2? ( 1 , SAr=S‐2,4‐6‐(iPr)₃C₆H₂) leads to thiol dissociation, reversibly forming [Fe₄S₄(SAr)₃L]^1? ( 2 ) and ArSH (L=solvent, and/or conjugate base). Solutions of 2 +ArSH react with the nitroxyl radical TEMPO to give [Fe₄S₄(SAr)₄]^1? ( 1_ox ) and TEMPOH. This reaction involves PCET coupled to thiolate association and may proceed via the unobserved protonated cluster [Fe₄S₄(SAr)₃(HSAr)]^1? ( 1‐H ). Similar reactions with this and related clusters proceed comparably. An understanding of the PCET thermochemistry of this cluster system has been developed, encompassing three different redox levels and two protonation states.     

A Paramagnetic Copper(III) Complex Containing an Octahedral CuIIIS6 Coordination Polyhedron

Only the second octahedral, paramagnetic copper(III ) complex (S=1) has now been synthesized and characterized. Six thiolato bridging ligands in the heterotrinuclear species [LCo^IIICu^IIICo^IIIL](ClO₄)₃⋅2 Me₂CO (L=1,4,7‐tris(4‐tert‐butyl‐2‐sulfidobenzyl)‐1,4,7‐triazacyclononane) stabilize this rare electron configuration. A section of the structure of the reduced form (Cu^II, S=½) is shown. XAS, EXAFS, and EPR spectroscopy prove unambiguously that the one‐electron oxidation to the copper(III ) is metal‐ rather than ligand‐centered.     

Exceptionally Long‐Lived Luminescence Emitted from TbIII Ion Caged in an AgI–TbIII–Thiacalix[4]arene Supramolecular Complex in Water

The compositions and photophysical properties of luminescent ternary complexes of thiacalix[4]arene‐p‐sulfonate (TCAS), Tb^III, and Ag^I ions were determined. At pH 6, Ag^I₂?Tb^III₂?TCAS₂ formed. Moreover, at pH 10, in the presence of a 20‐fold excess of Ag^I and a 50‐fold excess of TCAS with respect to Tb^III, Ag^I₂?Tb^III?TCAS₂ formed as the main luminescent species. The structure of these complexes was proposed: two TCAS ligands are linked by two S–Ag^I–S linkages to adopt a double‐cone supramolecular structure. Furthermore, each Tb^III ion in the former complex accepts O^?, S, O^? donation, whereas in the latter, the Tb^III center accepts eightfold O^? donation. The luminescence quantum yield (Φ) of Ag^I₂?Tb^III₂?TCAS₂ (0.16) was almost equal to that of Tb^III?TCAS, but the luminescence lifetime τ of the former (=1.09 ms) was larger than that of the latter. For Ag^I₂?Tb^III?TCAS₂, the yield Φ (=0.11) was small, which is attributed to the low efficiency of photosensitization (η=0.11). However, the τ value (4.61 ms) was exceptionally large and almost equal to the natural luminescence lifetime of Tb^III (4.7 ms), which is due to the absence of coordinating water molecules (q=0.1). This is compatible with the proposed structure in which the Tb^III ion is shielded by a supramolecular cage that expels coordinated water molecules responsible for luminescence quenching.     

New Long‐Chain Esters and Adenine Analogs from the Leaves of Formosan Bridelia balansae

Six new compounds, including the two long‐chain esters balansenate I (=6,8,11‐trimethyldodecanoic acid (2E)‐3‐methylhexadec‐2‐enyl ester; 1 ) and balansenate II (=10,12,15‐trimethylhexadedecanoic acid (2E)‐3‐methylhexadec‐2‐enyl ester; 2 ), the eburicane‐like triterpenoid bridelone (=hexadecahydro‐4,4,10,13,14‐pentamethyl‐17‐(5‐methyl‐1,4‐dimethylenehexyl)‐3H‐cyclopenta[a]phenanthren‐3‐one; 3 ), the ‘deimino‐xanthine', bridelonine (=5‐(3‐methylbut‐2‐enyl)pyrrolo[3,4‐d]imidazole‐4,6(1H,5H)‐dione; 6 ), and the two adenine analogs 9‐(3‐methylbut‐2‐enyl)adenine ( 7 ) and 1‐(3‐methylbut‐2‐enyl)adenine ( 8 ), besides three known compounds, i.e., N⁶‐(3‐methylbut‐2‐enyl)adenine ( 4 ), 3‐(3‐methylbut‐2‐enyl)adenine ( 5 ), and adenine ( 9 ), were isolated from the leaves of Formosan Bridelia balansae. The novel skeleton of 6 consists of a fused pyrrolidine‐2,5‐dione and imidazole moiety. The already known adenines 7 and 8 were isolated for the first time from a plant. The structures of the isolated compounds were elucidated by spectroscopic analyses.     

Long‐Lived Charge Separation in Novel Axial Donor–Porphyrin–Acceptor Triads Based on Tetrathiafulvalene,Aluminum(III) Porphyrin and Naphthalenediimide

Two self‐assembled supramolecular donor–acceptor triads consisting of Al^III porphyrin (AlPor) with axially bound naphthalenediimide (NDI) as an acceptor and tetrathiafulvalene (TTF) as a secondary donor are reported. In the triads, the NDI and TTF units are attached to Al^III on opposite faces of the porphyrin, through covalent and coordination bonds, respectively. Fluorescence studies show that the lowest excited singlet state of the porphyrin is quenched through electron transfer to NDI and hole transfer to TTF. In dichloromethane hole transfer to TTF dominates, whereas in benzonitrile (BN) electron transfer to NDI is the main quenching pathway. In the nematic phase of the liquid crystalline solvent 4‐(n‐pentyl)‐4′‐cyanobiphenyl (5CB), a spin‐polarized transient EPR spectrum that is readily assigned to the weakly coupled radical pair TTF^.+NDI^.? is obtained. The initial polarization pattern indicates that the charge separation occurs through the singlet channel and that singlet–triplet mixing occurs in the primary radical pair. At later time the polarization pattern inverts as a result of depopulation of the states with singlet character by recombination to the ground state. The singlet lifetime of TTF^.+NDI^.? is estimated to be 200–300 ns, whereas the triplet lifetime in the approximately 350 mT magnetic field of the X‐band EPR spectrometer is about 10 μs. In contrast, in dichloromethane and BN the lifetime of the charge separation is <10 ns.     

Distance Dependence of Bidirectional Concerted Proton–Electron Transfer in Phenol‐Ru(2,2′‐bipyridine)32+ Dyads

Proton‐coupled electron transfer (PCET) was investigated in three covalent donor–bridge–acceptor molecules with different bridge lengths. Upon photoexcitation of their Ru(bpy)₃²⁺ (bpy=2,2′‐bipyridine) photosensitizer in acetonitrile, intramolecular long‐range electron transfer from a phenolic unit to Ru(bpy)₃²⁺ occurs in concert with release of the phenolic proton to pyrrolidine base. The kinetics of this bidirectional concerted proton–electron transfer (CPET) reaction were studied as a function of phenol–Ru(bpy)₃²⁺ distance by increasing the number of bridging p‐xylene units. A distance decay constant (β) of 0.67±0.23 Å^?1 was determined. The distance dependence of the rates for CPET is thus not significantly steeper than that for ordinary (i.e., not proton coupled) electron transfer across the same bridges, despite the concerted motion of oppositely charged particles into different directions. Long‐range bidirectional CPET is an important reaction in many proteins and plays a key role in photosynthesis; our results are relevant in the context of photoinduced separation of protons and electrons as a means of light‐to‐chemical energy conversion. This is the first determination of β for a bidirectional CPET reaction.     

An Unusually Delocalized Mixed‐Valence State of a Cyanidometal‐Bridged Compound Induced by Thermal Electron Transfer

The heterometallic complexes trans ‐[Cp(dppe)FeNCRu(o ‐bpy)CNFe(dppe)Cp][PF₆]_n ( 1 [PF₆]_n , n =2, 3, 4; o ‐bpy=1,2‐bis(2,2′‐bipyridyl‐6‐yl)ethane, dppe=1,2‐bis(diphenylphosphino)ethane, Cp=1,3‐cyclopentadiene) in three distinct states have been synthesized and fully characterized. 1 ³⁺[PF₆]₃ and 1 ⁴⁺[PF₆]₄ are the one‐ and two‐electron oxidation products of 1 ²⁺[PF₆]₂, respectively. The investigated results suggest that 1 [PF₆]₃ is a Class II mixed valence compound. 1 [PF₆]₄ after a thermal treatment at 400 K shows an unusually delocalized mixed valence state of [Fe^III‐NC‐Ru^III‐CN‐Fe^II], which is induced by electron transfer from the central Ru^II to the terminal Fe^III in 1 [PF₆]₄, which was confirmed by IR spectroscopy, magnetic data, and EPR and Mössbauer spectroscopy.     

Organometallic mediated radical polymerization of vinyl acetate with Fe(acac)2

The radical polymerization of vinyl acetate (VAc) is moderated by iron(II) acetylacetonate (Fe(acac)₂) by the organometallic route (OMRP), as well as by degenerative transfer polymerization (DTP) when in the presence of excess radicals, through the formation of thermally labile organometallic Fe^III dormant species. The poly(vinyl acetate) (PVAc)‐Fe^III(acac)₂ dormant species has been isolated in the form of an oligomer and characterized by ¹H NMR, EPR, and IR methods, and then used as a single‐component initiator for the OMRP of VAc. The degree of polymerization of this isolated oligomeric species demonstrates the limited ability of Fe(acac)₂, relative to the Co(acac)₂ congener, to rapidly trap the growing PVAc radical chain. Control under OMRP conditions is improved by the presence of Lewis bases, especially PMe₂Ph. On the other hand, iron(II) phthalocyanine inhibits the radical polymerization of VAc completely. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3494–3504     

Synthesis and Characterization of Acetylene‐Linked Bisphenalenyl and Metallic‐Like Behavior in Its Charge‐Transfer Complex

We prepared and isolated a phenalenyl‐based neutral hydrocarbon ( 1 b ) with a biradical index of 14 %, as well as its charge‐transfer (CT) complex 1 b –F₄‐TCNQ. The crystal structure and the small HOMO–LUMO gap assessed by electrochemical and optical methods support the singlet‐biradical contribution to the ground state of the neutral 1 b . This biradical character suggests that 1 b has the electronic structure of phenalenyl radicals coupled weakly through an acetylene linker, that is, some independence of the two phenalenyl moieties. The monocationic species 1 b^{. +} was obtained by reaction with the organic electron acceptor F₄‐TCNQ. The cationic species has a small disproportionation energy ΔE for the reaction 2× 1 b^{. +}? 1 b + 1 b ²⁺, which presumably originates from the independence of the phenalenyl moieties. The small ΔE led to a small on‐site Coulombic repulsion U_eff=0.61 eV in the CT complex. Moreover, a very effective orbital overlap of the phenalenyl rings between molecules afforded a relatively large transfer integral t=0.09 eV. The small U_eff/4t ratio (=1.7) resulted in a metallic‐like conductive behavior at around room temperature. Below 280 K, the CT complex showed a transition into a semiconductive state as a result of bond formation between phenalenyl and F₄‐TCNQ carbon atoms.     

Three‐Axis Anisotropic Exchange Coupling in the Single‐Molecule Magnets NEt4[MnIII2(5‐Brsalen)2(MeOH)2MIII(CN)6] (M=Ru,Os)

We have investigated the single‐molecule magnets [Mn^III₂(5‐Brsalen)₂(MeOH)₂M^III(CN)₆]NEt₄ (M=Os ( 1 ) and Ru ( 2 ); 5‐Brsalen=N,N′‐ethylenebis(5‐bromosalicylidene)iminate) by frequency‐domain Fourier‐transform terahertz electron paramagnetic resonance (THz‐EPR), inelastic neutron scattering, and superconducting quantum interference device (SQUID) magnetometry. The combination of all three techniques allows for the unambiguous experimental determination of the three‐axis anisotropic magnetic exchange coupling between Mn^III and Ru^III or Os^III ions, respectively. Analysis by means of a spin‐Hamiltonian parameterization yields excellent agreement with all experimental data. Furthermore, analytical calculations show that the observed exchange anisotropy is due to the bent geometry encountered in both 1 and 2 , whereas a linear geometry would lead to an Ising‐type exchange coupling.     

Electrophilic Arene Hydroxylation and Phenol O?H Oxidations Performed by an Unsymmetric μ‐η1:η1‐O2‐Peroxo Dicopper(II) Complex

Reactions of the unsymmetric dicopper(II) peroxide complex [Cu^II₂(μ‐η¹:η¹‐O₂)(m‐XYL^N3N4)]²⁺ ( 1 O₂ , where m‐XYL is a heptadentate N‐based ligand), with phenolates and phenols are described. Complex 1 O₂ reacts with p‐X‐PhONa (X=MeO, Cl, H, or Me) at ?90 °C performing tyrosinase‐like ortho‐hydroxylation of the aromatic ring to afford the corresponding catechol products. Mechanistic studies demonstrate that reactions occur through initial reversible formation of metastable association complexes [Cu^II₂(μ‐η¹:η¹‐O₂)(p‐X‐PhO)(m‐XYL^N3N4)]⁺ ( 1 O₂ ?X‐PhO) that then undergo ortho‐hydroxylation of the aromatic ring by the peroxide moiety. Complex 1 O₂ also reacts with 4‐X‐substituted phenols p‐X‐PhOH (X=MeO, Me, F, H, or Cl) and with 2,4‐di‐tert‐butylphenol at ?90 °C causing rapid decay of 1 O₂ and affording biphenol coupling products, which is indicative that reactions occur through formation of phenoxyl radicals that then undergo radical C? C coupling. Spectroscopic UV/Vis monitoring and kinetic analysis show that reactions take place through reversible formation of ground‐state association complexes [Cu^II₂(μ‐η¹:η¹‐O₂)(X‐PhOH)(m‐XYL^N3N4)]²⁺ ( 1 O₂ ?X‐PhOH) that then evolve through an irreversible rate‐determining step. Mechanistic studies indicate that 1 O₂ reacts with phenols through initial phenol binding to the Cu₂O₂ core, followed by a proton‐coupled electron transfer (PCET) at the rate‐determining step. Results disclosed in this work provide experimental evidence that the unsymmetric 1 O₂ complex can mediate electrophilic arene hydroxylation and PCET reactions commonly associated with electrophilic Cu₂O₂ cores, and strongly suggest that the ability to form substrate?Cu₂O₂ association complexes may provide paths to overcome the inherent reactivity of the O₂‐binding mode. This work provides experimental evidence that the presence of a H⁺ completely determines the fate of the association complex [Cu^II₂(μ‐η¹:η¹‐O₂)(X‐PhO(H))(m‐XYL^N3N4)]ⁿ⁺ between a PCET and an arene hydroxylation reaction, and may provide clues to help understand enzymatic reactions at dicopper sites.     

Synthesis and Characterization of Redox‐Active Charge‐Transfer Complexes with 2,3,5,6‐Tetracyanopyridine (TCNPy) for the Photogeneration of Pyridinium Radicals

The heteroaromatic polynitrile compound tetracyanopyridine (TCNPy) is introduced as a new electron acceptor for the formation of deeply colored charge‐transfer complexes. In MeCN, TCNPy is characterized by a quasireversible one‐electron‐reduction process at ?0.51 V (versus SCE). The tetracyanopyridine radical anion undergoes a secondary chemical reaction, which is assigned to a protonation step. TCNPy has been demonstrated to generate 1:1 complexes with various electron donors, including tetrathiafulvalene (TTF) and dihydroxybenzene derivatives, such as p‐hydroquinone and catechol. Visible‐ or NIR‐light‐induced excitation of the intense charge‐transfer bands of these compounds leads to a direct optical electron‐transfer process for the formation of the corresponding radical‐ion pairs. The presence of available electron donors that contain protic groups in close proximity to the TCNPy acceptor site opens up a new strategy for the photocontrolled generation of pyridinium radicals in a stepwise proton‐coupled electron‐transfer (PCET) sequence.     

A Novel Thermocell System Using Proton Solvation Entropy

The entropy change associated with proton-coupled electron transfer (PCET) reactions significantly enhance the Seebeck coefficient (S_e) of thermocells. A redox pair of [Ru(H_xim)₆]^2+/3+ (Him=imidazole, x=0≈1) releases three protons in their one-electron redox reactions in thermocells, which gave a remarkably high S_e of −3.7 mV K⁻¹ as confirmed by temperature-dependent square wave voltammetry. The value of S_e is proportional to the redox reaction entropy (ΔS_rc), which increased with the number of dissociating protons. This result demonstrates the utility of PCET reaction toward efficient thermoelectric conversion.     

Evaluation of the Oxo‐bridged Dinuclear Ruthenium Ammine Complex as Redox Mediator in an Electrochemical Biosensor

The mediation of electron‐transfer by oxo‐bridged dinuclear ruthenium ammine [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ for the oxidation of glucose was investigated by cyclic voltammetry. These ruthenium (III) complexes exhibit appropriate redox potentials of 0.131–0.09 V vs. SCE to act as electron‐transfer mediators. The plot of anodic current vs. the glucose concentration was linear in the concentration range between 2.52×10^?5 and 1.00×10^?4 mol L^?1. Moreover, the apparent Michaelis‐Menten kinetic (K_M^app) and the catalytic (K_cat) constants were 8.757×10^?6 mol L^?1 and 1,956 s^?1, respectively, demonstrating the efficiency of the ruthenium dinuclear oxo‐complex [(bpy)₂(NH₃)Ru^III(µ‐O)Ru^III(NH₃)(bpy)₂]⁴⁺ as mediator of redox electron‐transfer.