Protonation-coupled redox reactions in planar antiaromatic meso-pentafluorophenyl-substituted o-phenylene-bridged annulated rosarins.

Proton-coupled electron transfer (PCET) processes are among the most important phenomena that control a variety of chemical and biological transformations. Although extensively studied in a variety of natural systems and discrete metal complexes, PCET mechanisms are less well codified in the case of purely organic compounds. Here we report that a planar β,β'-phenylene-bridged hexaphyrin (1.

Quartet formation of a guanine derivative with an isopropyl group: crystal structures of "naked" G-quartets and thermodynamics of G-quartet formation.

The formation of guanine quartets with 9-isopropylguanine ((i)PG) is discussed in organic solvents. Crystal structures of the (i)PG quartets were determined by X-ray crystallography with template cations (Na(+) and Ca(2+)) and the structure without a template cation was also obtained by virtue of the stabilization by intermolecular hydrogen bonding with water molecules of crystallization. The difference in the quartet formation of (i)PG in the presence and absence of a template cation was clearly demonstrated by (1)H NMR measurements in CDCl(3)-CH(3)OH mixed solvents.

A triangular prismatic hexanuclear iridium(III) complex bridged by flavin analogues showing reversible redox processes.

[Ir(6)(μ-alloCl(2)(2-))(3)(Cp*)(6)(OH)(3)](PF(6))(3) (1) having 7,8-dichloroalloxazine dianion (alloCl(2)(2-)) as bridging ligands was synthesized and characterized by X-ray crystallography, spectroscopic and electrochemical measurements. The alloxazine ligands showed unprecedented coordination modes to link the six Ir(III) centres. The complex exhibited remarkable stability and reversible six-electron redox processes at the bridging alloxazine ligands in organic solvents.

Copper complexes of the non-innocent β-diketiminate ligand containing phenol groups.

A new type of non-innocent β-diketiminate ligand having redox active phenol groups (LH(3), fully protonated form) has been developed, and the structure, physical properties and reactivity of the supported copper(II) complex [Cu(II)(L(3-))](-) (L(3-), fully deprotonated tri-anionic form) as well as the one-electron and two-electron oxidised complexes, [Cu(II)(L˙(2-))] and [Cu(II)(L(-))](+), have been examined in detail. The two-electron oxidised form [Cu(II)(L(-))](+) exhibited hydrogen atom abstraction ability from 1,4-cyclohexadiene (CHD), whereas the one-electron oxidised form [Cu(II)(L˙(2-))] was found to disproportionate into [Cu(II)(L(3-))](-) and [Cu(II)(L(-))](+) during the course of the reaction with CHD.

Catalysis of nickel ferrite for photocatalytic water oxidation using [Ru(bpy)3]2+ and S2O8(2-).

Single or mixed oxides of iron and nickel have been examined as catalysts in photocatalytic water oxidation using [Ru(bpy)(3)](2+) as a photosensitizer and S(2)O(8)(2-) as a sacrificial oxidant. The catalytic activity of nickel ferrite (NiFe(2)O(4)) is comparable to that of a catalyst containing Ir, Ru, or Co in terms of O(2) yield and O(2) evolution rate under ambient reaction conditions. NiFe(2)O(4) also possesses robustness and ferromagnetic properties, which are beneficial for easy recovery from the solution after reaction.

Photoelectrocatalysis to improve cycloreversion quantum yields of photochromic dithienylethene compounds.

An open and shut case: photoirradiation of the 9-mesityl-10-methylacridinium ion, which acts as a photoredox catalyst, evoked catalytic cycloreversion of the photochromic 1,2-dithienylethene (DTE) compounds with one order of magnitude enhancement in quantum yields. Mechanistic studies revealed that the back electron transfer and electron transfer from the neutral closed form of DTE compounds to the open-form radical cation are key steps.

Catalytic electron-transfer oxygenation of substrates with water as an oxygen source using manganese porphyrins.

Manganese(V)-oxo-porphyrins are produced by the electron-transfer oxidation of manganese-porphyrins with tris(2,2'-bipyridine)ruthenium(III) ([Ru(bpy)(3)](3+); 2 equiv) in acetonitrile (CH(3)CN) containing water. The rate constants of the electron-transfer oxidation of manganese-porphyrins have been determined and evaluated in light of the Marcus theory of electron transfer. Addition of [Ru(bpy)(3)](3+) to a solution of olefins (styrene and cyclohexene) in CH(3)CN containing water in the presence of a catalytic amount of manganese-porphyrins afforded epoxides, diols, and aldehydes efficiently.

Ultrafast charge separation in supramolecular tetrapyrrole-graphene hybrids.

Supramolecular donor-acceptor hybrids composed of few-layer graphene as an electron acceptor and phthalocyanine or porphyrin bearing four pyrene entities as photosensitizer donors have been synthesized, and occurrence of ultrafast charge separation in the order of 10(11)-10(12) s(-1) due to close proximity of the donor and acceptor entities is demonstrated.

Synthesis, photophysical and electrochemistry of near-IR absorbing bacteriochlorins related to bacteriochlorophyll a.

A series of new bacteriochlorins was synthesized using 13(2)-oxo-bacteriopyropheophorbide a (derived from bacteriochlorophyll a) as a starting material, which on reacting with o-phenylenediamine and 1,10-diaminonaphthalene afforded highly conjugated annulated bacteriochlorins with fused quinoxaline, benzimidazole, and perimidine rings, respectively. The absorption spectra of these novel bacteriochlorins demonstrated remarkably red-shifted intense Q(y) absorption bands observed in the range of 816-850 nm with high molar extinction coefficients (89,900-136,800). Treatment of 13(2)-oxo-bacteriopyropheophorbide a methyl ester with diazomethane resulted in the formation of bacterioverdins containing a fused six-membered methoxy-substituted cyclohexenone (verdin) as an isomeric mixture.

Hydrogen storage and evolution catalysed by metal hydride complexes.

The storage and evolution of hydrogen are catalysed by appropriate metal hydride complexes. Hydrogenation of carbon dioxide by hydrogen is catalysed by a [C,N] cyclometalated organoiridium complex, [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(OH(2))](2)SO(4) [Ir-OH(2)](2)SO(4), under atmospheric pressure of H(2) and CO(2) in weakly basic water (pH 7.5) at room temperature.

Electronic properties of trifluoromethylated corannulenes.

With great stacking comes great conductivity: trifluoromethylated corannulenes are unexpectedly strong electron acceptors in solution. Even a polycrystalline film of 1,2-(F(3)C)(2)-corannulene (top structure and curve in the plot) exhibits conductivity three orders of magnitude higher than that of pristine corannulene!

Electron-transfer properties of a nonheme manganese(IV)-oxo complex acting as a stronger one-electron oxidant than the iron(IV)-oxo analogue.

Electron-transfer properties of a nonheme Mn(IV)-oxo complex, [(Bn-TPEN)Mn(IV)(O)](2+), reveals that Mn(IV)-oxo complex acts as a stronger one-electron oxidant than the Fe(IV)-oxo analogue. As a result, an electron transfer process in N-dealkylation has been detected by a transient radical cation intermediate, para-Me-DMAË™(+), in the oxidation of para-Me-DMA by [(Bn-TPEN)Mn(IV)(O)](2+).

The remarkable effect of the manganese ion with dioxygen on the stability of π-conjugated radical cations.

In this paper, nanosecond laser flash photolysis has been used to investigate the influence of metal ions on the kinetics of radical cations of a range of carotenoids (astaxanthin (ASTA), canthaxanthin (CAN), and β-carotene (β-CAR)) and various electron donors (1,4-diphenyl-1,3-butadiene (14DPB), 1,6-diphenyl-1,3,5-hexatriene (16DPH), 4-methoxy-trans-stilbene (4 MeOSt), and trans-stilbene (trans-St)) in benzonitrile. Radical cations have been generated by means of photosensitized electron-transfer (ET) using 1,4-dicyanonaphthalene (14DCN) and biphenyl (BP). The kinetic decay of CAR(· +) shows a strong dependence on the identity of the examined metal ion.

Ultrafast photoinduced energy and electron transfer in multi-modular donor-acceptor conjugates.

New multi-modular donor-acceptor conjugates featuring zinc porphyrin (ZnP), catechol-chelated boron dipyrrin (BDP), triphenylamine (TPA) and fullerene (C(60)), or naphthalenediimide (NDI) have been newly designed and synthesized as photosynthetic antenna and reaction-center mimics. The X-ray structure of triphenylamine-BDP is also reported. The wide-band capturing polyad revealed ultrafast energy-transfer (k(ENT) =1.

Electroreduction and acid-base properties of dipyrrolylquinoxalines.

The electroreduction and acid-base properties of dipyrrolylquinoxalines of the form H(2)DPQ, H(2)DPQ(NO(2)), and H(2)DPQ(NO(2))(2) were investigated in benzonitrile (PhCN) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). This study focuses on elucidating the complete electrochemistry, spectroelectrochemistry, and acid-base properties of H(2)DPQ(NO(2))(n) (n = 0, 1, or 2) in PhCN before and after the addition of trifluoroacetic acid (TFA), tetra-n-butylammonium hydroxide (TBAOH), tetra-n-butylammonium fluoride (TBAF), or tetra-n-butylammonium acetate (TBAOAc) to solution.

Mechanistic borderline of one-step hydrogen atom transfer versus stepwise Sc(3+)-coupled electron transfer from benzyl alcohol derivatives to a non-heme iron(IV)-oxo complex.

The rate of oxidation of 2,5-dimethoxybenzyl alcohol (2,5-(MeO)(2)C(6)H(3)CH(2)OH) by [Fe(IV)(O)(N4Py)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) was enhanced significantly in the presence of Sc(OTf)(3) (OTf(-) = trifluoromethanesulfonate) in acetonitrile (e.g., 120-fold acceleration in the presence of Sc(3+)).

Enhanced photoinduced electron-transfer reduction of Li(+)@C60 in comparison with C60.

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited state of lithium ion-encapsulated C60 (Li(+)@C60) was investigated in comparison with the corresponding kinetics of the photoinduced electron transfer to the triplet excited state of pristine C60. Femtosecond laser flash photolysis measurements of Li(+)@C60 revealed that singlet excited state of Li(+)@C60 (λmax = 960 nm) underwent intersystem crossing to the triplet excited state [(3)(Li(+)@C60)*: λmax = 750 nm] with a rate constant of 8.9 × 10(8) s(-1) in deaerated benzonitrile (PhCN).

Active site models for the Cu(A) site of peptidylglycine α-hydroxylating monooxygenase and dopamine β-monooxygenase.

A mononuclear copper(II) superoxo species has been invoked as the key reactive intermediate in aliphatic substrate hydroxylation by copper monooxygenases such as peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and tyramine β-monooxygenase (TβM). We have recently developed a mononuclear copper(II) end-on superoxo complex using a N-[2-(2-pyridyl)ethyl]-1,5-diazacyclooctane tridentate ligand, the structure of which is similar to the four-coordinate distorted tetrahedral geometry of the copper-dioxygen adduct found in the oxy-form of PHM (Prigge, S. T.

Photocatalytic production of hydrogen peroxide by two-electron reduction of dioxygen with carbon-neutral oxalate using a 2-phenyl-4-(1-naphthyl)quinolinium ion as a robust photocatalyst.

Efficient photocatalytic production of hydrogen peroxide (H(2)O(2)) from O(2) and oxalate has been made possible by using a 2-phenyl-4-(1-naphthyl)quinolinium ion as a robust photocatalyst in an oxygen-saturated mixed solution of a buffer and acetonitrile with a high quantum yield of 14% (maximum 50% for the two-electron process) at λ = 334 nm and a high H(2)O(2) yield of 93% at λ > 340 nm.

Supramolecular electron transfer by anion binding.

Anion binding has emerged as an attractive strategy to construct supramolecular electron donor-acceptor complexes. In recent years, the level of sophistication in the design of these systems has advanced to the point where it is possible to create ensembles that mimic key aspects of the photoinduced electron-transfer events operative in the photosynthetic reaction centre. Although anion binding is a reversible process, kinetic studies on anion binding and dissociation processes, as well as photoinduced electron-transfer and back electron-transfer reactions in supramolecular electron donor-acceptor complexes formed by anion binding, have revealed that photoinduced electron transfer and back electron transfer occur at time scales much faster than those associated with anion binding and dissociation.

Tetrathiafulvalene-fused porphyrins via quinoxaline linkers: symmetric and asymmetric donor-acceptor systems.

A tetrathiafulvalene (TTF) donor is annulated to porphyrins (P) via quinoxaline linkers to form novel symmetric P-TTF-P triads 1 a-c and asymmetric P-TTF dyads 2 a,b in good yields. These planar and extended π-conjugated molecules absorb light over a wide region of the UV/Vis spectrum as a result of additional charge-transfer excitations within the donor-acceptor assemblies. Quantum-chemical calculations elucidate the nature of the electronically excited states.

Photocatalytic hydrogen evolution from carbon-neutral oxalate with 2-phenyl-4-(1-naphthyl)quinolinium ion and metal nanoparticles.

Photocatalytic hydrogen evolution has been made possible by using oxalate as a carbon-neutral electron source, metal nanoparticles as hydrogen-evolution catalysts and the 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA), which forms the long-lived electron-transfer state upon photoexcitation, as a photocatalyst. The hydrogen evolution was conducted in a deaerated mixed solution of an aqueous buffer and acetonitrile (MeCN) [1:1 (v/v)] by photoirradiation (λ > 340 nm). The gas evolved during the photocatalytic reaction contained H(2) and CO(2) in a molar ratio of 1:2, indicating that oxalate acts as a two-electron donor.

Improvement of durability of an organic photocatalyst in p-xylene oxygenation by addition of a Cu(II) complex.

The catalytic durability of an organic photocatalyst, 9-mesityl-10-methyl acridinium ion (Acr(+)-Mes), has been dramatically improved by the addition of [{tris(2-pyridylmethyl)amine}Cu(II)](ClO(4))(2) ([(tmpa)Cu(II)](2+)) in the photocatalytic oxygenation of p-xylene by molecular oxygen in acetonitrile. Such an improvement is not observed by the addition of Cu(ClO(4))(2) in the absence of organic ligands. The addition of [(tmpa)Cu](2+) in the reaction solution resulted in more than an 11 times higher turnover number (TON) compared with the TON obtained without [(tmpa)Cu(II)](2+).

Photochemical properties of a myoglobin-CdTe quantum dot conjugate.

A myoglobin-cadmium telluride quantum dot conjugate was constructed using an artificial heme modified with a thiol moiety as a linker. Irradiation of the myoglobin-cadmium telluride conjugate generated the photoreduced ferrous myoglobin via an electron transfer from the photoexcited quantum dot, leading to the formation of CO-bound myoglobin under a CO atmosphere.