Effect of Deuteration on Relaxation Dynamics of the Perylene Excimer Studied by Subnanosecond Transient Absorption Spectroscopy.

We studied the effect of deuteration on the relaxation dynamics of the excimer of perylene in solution using subnanosecond time-resolved transient absorption spectroscopy based on the randomly interleaved pulse-train method. We found that the deuterated perylene excimer in solution had a longer lifetime than the undeuterated excimer but that deuteration had a little effect on the ground-state and transient absorption spectra of the excimer, suggesting that deuteration altered the relaxation dynamics by inducing small changes in vibrational properties. To confirm the origin of the deuteration effect, we quantitatively analyzed the kinetics of transient absorption decay, including the decay of triplet-triplet absorption. In addition, we evaluated the effects of temperature on the lifetime of the excimers. On the basis of these results, we concluded that the rate of internal conversion was suppressed by deuteration. By comparing our results with previously reported results on the effect of deuteration on the fluorescence properties of crystalline perylene, we proposed a model that may explain the effect of deuteration on the lifetime of the perylene excimer in solution.

Reaction of Oxygen with the Singlet Excited State of [n]Cycloparaphenylenes (n = 9, 12, and 15): A Time-Resolved Transient Absorption Study Seamlessly Covering Time Ranges from Subnanoseconds to Microseconds by the Randomly-Interleaved-Pulse-Train Method.

Reaction of 3O2 with singlet excited state (S1) of highly luminescent cycloparaphenylenes (CPPs), i.e., [n]CPP where n = 9, 12, and 15 in solution has been studied by transient absorption (TA) measurements seamless for the time range from subnanosecond to microsecond based on the randomly-interleaved-pulse-train (RIPT) method recently developed by our group. We found efficient quenching of S1 by 3O2 through observation of Sn ← S1 transient absorption and the steady state fluorescence measurements. Concomitantly, we have become aware of the acceleration of the rate of intersystem crossing (ISC) from S1 to the triplet excited state (T1) through the observation of the evident enhancement of Tn ← T1 absorption intensity. We have established the analysis procedure to evaluate the rate constant of ISC (kISC0) in the absence of O2 and the bimolecular rate constant of ISC induced by 3O2 (kISCO2) only by using TA decay data in the presence of O2. On the basis of these analyses, we further succeeded in determining the quantum yield of T1 (ΦT) with and without O2. In addition, the absorption coefficient of T1 (εT1) and S1 (εS1) could be estimated with reference to that of T1 of C60. These photophysical parameters are largely dependent on the ring size, where the lifetime of S1 (τS) in the absence and presence of O2 dominates ΦT as well as the quantum yield of fluorescence (ΦF).

Bridged Stilbenes: AIEgens Designed via a Simple Strategy to Control the Non-radiative Decay Pathway.

To broaden the application of aggregation-induced emission (AIE) luminogens (AIEgens), the design of novel small-molecular dyes that exhibit high fluorescence quantum yield (Φfl ) in the solid state is required. Considering that the mechanism of AIE can be rationalized based on steric avoidance of non-radiative decay pathways, a series of bridged stilbenes was designed, and their non-radiative decay pathways were investigated theoretically. Bridged stilbenes with short alkyl chains exhibited a strong fluorescence emission in solution and in the solid state, while bridged stilbenes with long alkyl chains exhibited AIE. Based on this theoretical prediction, we developed the bridged stilbenes BPST[7] and DPB[7], which demonstrate excellent AIE behavior.

Catalytic Formation of Hydrogen Peroxide from Coenzyme NADH and Dioxygen with a Water-Soluble Iridium Complex and a Ubiquinone Coenzyme Analogue.

A ubiquinone coenzyme analogue (Q0: 2,3-dimethoxy-5-methyl-1,4-benzoquinone) was reduced by coenzyme NADH to yield the corresponding reduced form of Q0 (Q0H2) in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex (1: [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H2O)]2SO4) in water at ambient temperature as observed in the respiratory chain complex I (Complex I). In the catalytic cycle, the reduction of 1 by NADH produces the corresponding iridium hydride complex that in turn reduces Q0 to produce Q0H2. Q0H2 reduced dioxygen to yield hydrogen peroxide (H2O2) under slightly basic conditions. Catalytic generation of H2O2 was made possible in the reaction of O2 with NADH as the functional expression of NADH oxidase in white blood cells utilizing the redox cycle of Q0 as well as 1 for the first time in a nonenzymatic homogeneous reaction system.

Photocatalytic Hydroxylation of Benzene by Dioxygen to Phenol with a Cyano-Bridged Complex Containing Fe(II) and Ru(II) Incorporated in Mesoporous Silica-Alumina.

Photocatalytic hydroxylation of benzene to phenol was achieved by using O2 as an oxidant as well as an oxygen source with a cyano-bridged polynuclear metal complex containing Fe(II) and Ru(II) incorporated in mesoporous silica-alumina ([Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41). An apparent turnover number (TON) of phenol production per the monomer unit of [Fe(H2O)3]2[Ru(CN)6] was 41 for 59 h. The cyano-bridged polynuclear metal complex, [Fe(H2O)3]2[Ru(CN)6], exhibited catalytic activity for thermal hydroxylation of benzene by H2O2 in acetonitrile (MeCN), where the apparent TON of phenol production reached 393 for 60 h. The apparent TON increased to 2500 for 114 h by incorporating [Fe(H2O)3]2[Ru(CN)6] in sAl-MCM-41. Additionally, [Fe(H2O)3]2[Ru(CN)6] acts as a water oxidation catalyst by using [Ru(bpy)3](2+) (bpy = 2,2'-bipyridine) and Na2S2O8 as a photosensitizer and a sacrificial electron acceptor as evidenced by (18)O-isotope labeling experiments. Photoirradiation of an O2-saturated MeCN solution containing [Fe(H2O)3]2[Ru(CN)6]@sAl-MCM-41 and scandium ion provided H2O2 formation, where photoexcited [Ru(CN)6](4-) moiety reduces O2 as indicated by laser flash photolysis measurements. Thus, hydroxylation of benzene to phenol using molecular oxygen photocatalyzed by [Fe(H2O)3]2[Ru(CN)6] occurred via a two-step route; (1) molecular oxygen was photocatalytically reduced to peroxide by using water as an electron donor, and then (2) peroxide thus formed is used as an oxidant for hydroxylation of benzene.

Laser-Induced Dynamics of Peroxodicopper(II) Complexes Vary with the Ligand Architecture. One-Photon Two-Electron O2 Ejection and Formation of Mixed-Valent Cu(I)Cu(II)-Superoxide Intermediates.

Photoexcitation of end-on trans-µ-1,2-peroxodicopper(II) complex [(tmpa)2Cu(II)2(O2)](2+) (1) (λmax = 525 and 600 nm) and side-on µ-η(2):η(2)-peroxodicopper(II) complexes [(N5)Cu(II)2(O2)](2+) (2) and [(N3)Cu(II)2(O2)](2+) (3) at -80 °C in acetone led to one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O2(2-) + hν → O2 + 2e(-)). Interestingly, light excitation of 2 and 3 (having side-on µ-η(2):η(2)-peroxo ligation) led to release of dioxygen, while photoexcitation of 1 (having an end-on trans-1,2-peroxo geometry) did not, even though spectroscopic studies revealed that both reactions proceeded through previously unknown mixed-valent superoxide species: [Cu(II)(O2(•-))Cu(I)](2+) (λmax = 685-740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer to yield an "O2-caged" dicopper(I) adduct, Cu(I)2-O2, and a barrierless stepwise back electron transfer to regenerate 1 occurred. Femtosecond laser excitation of 2 and 3 under the same conditions still led to [Cu(II)(O2(•-))Cu(I)](2+) intermediates that, instead, underwent O2 release with a quantum yield of 0.14 ± 0.1 for 3. Such remarkable differences in reaction pathways likely result from the well-known ligand-derived stability of 2 and 3 vs 1 indicated by ligand-Cu(II/I) redox potentials; (N5)Cu(I) and (N3)Cu(I) complexes are far more stable than (tmpa)Cu(I) species. The fast Cu(I)2/O2 rebinding kinetics was also measured after photoexcitation of 2 and 3, with the results closely tracking those known for the dicopper proteins hemocyanin and tyrosinase, for which the synthetic dicopper(I) precursors [(N5)Cu(I)2](2+) and [(N3)Cu(I)2](2+) and their dioxygen adducts serve as models. The biological relevance of the present findings is discussed, including the potential impact on the solar water splitting process.

Long-lived charge separation and applications in artificial photosynthesis.

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.

Formation of a long-lived electron-transfer state in mesoporous silica-alumina composites enhances photocatalytic oxygenation reactivity.

A simple donor-acceptor linked dyad, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes) was incorporated into nanosized mesoporous silica-alumina to form a composite, which in acetonitrile is highly dispersed. In this medium, upon visible light irradiation, the formation of an extremely long-lived electron-transfer state (Acr(•)-Mes(•+)) was confirmed by EPR and laser flash photolysis spectroscopic methods. The composite of Acr(+)-Mes-incorporated mesoporous silica-alumina with an added copper complex [(tmpa)Cu(II)] (ClO(4)(-))2 (tmpa = tris(2-pyridylmethyl)amine) acts as an efficient and robust photocatalyst for the selective oxygenation of p-xylene by molecular oxygen to produce p-tolualdehyde and hydrogen peroxide. Thus, incorporation of Acr(+)-Mes into nanosized mesoporous silica-alumina combined with an O(2)-reduction catalyst ([(tmpa)Cu(II)](2+)) provides a promising method in the development of efficient and robust organic photocatalysts for substrate oxygenation by dioxygen, the ultimate environmentally benign oxidant.

Direct synthesis of hydrogen peroxide from hydrogen and oxygen by using a water-soluble iridium complex and flavin mononucleotide.

H2 , O2 to H2 O2 : The direct synthesis of hydrogen peroxide from hydrogen and oxygen in water has been made possible by using an iridium(III) complex, [Ir(III) (Cp*)(4-(1H-pyrazol-1-yl-κN(2) )benzoic acid-κC(3) )(H2 O)]2 SO4 , and flavin mononucleotide. This method gives hydrogen peroxide with a high turnover number (847) and yield (19.2 %) under normal pressure and at room temperature.

Assembly and stepwise oxidation of interpenetrated coordination cages based on phenothiazine.

A breath of fresh air is sufficient for the eightfold S-monooxygenation of an interpenetrated double cage based on eight phenothiazine ligands and four square-planar-coordinated Pd(II) cations. Besides these two cages, which were both characterized by X-ray crystallography, an eightfold S-dioxygenated double-cage was obtained under harsher oxidation conditions.

Efficient catalytic interconversion between NADH and NAD+ accompanied by generation and consumption of hydrogen with a water-soluble iridium complex at ambient pressure and temperature.

Regioselective hydrogenation of the oxidized form of ß-nicotinamide adenine dinucleotide (NAD(+)) to the reduced form (NADH) with hydrogen (H(2)) has successfully been achieved in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H(2)O)](2) SO(4) [1](2)·SO(4) under an atmospheric pressure of H(2) at room temperature in weakly basic water. The structure of the corresponding benzoate complex Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))-benzoate-κC(3))(H(2)O) 2 has been revealed by X-ray single-crystal structure analysis. The corresponding iridium hydride complex formed under an atmospheric pressure of H(2) undergoes the 1,4-selective hydrogenation of NAD(+) to form 1,4-NADH. On the other hand, in weakly acidic water the complex 1 was found to catalyze the hydrogen evolution from NADH to produce NAD(+) without photoirradiation at room temperature. NAD(+) exhibited an inhibitory behavior in both catalytic hydrogenation of NAD(+) with H(2) and H(2) evolution from NADH due to the binding of NAD(+) to the catalyst. The overall catalytic mechanism of interconversion between NADH and NAD(+) accompanied by generation and consumption of H(2) was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates.

Hydrogen evolution from aliphatic alcohols and 1,4-selective hydrogenation of NAD+ catalyzed by a [C,N] and a [C,C] cyclometalated organoiridium complex at room temperature in water.

A [C,N] cyclometalated Ir complex, [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H(2)O)](2)SO(4) [1](2)·SO(4), was reduced by aliphatic alcohols to produce the corresponding hydride complex [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))-benzoate-κC(3))H](-)4 at room temperature in a basic aqueous solution (pH 13.6). Formation of the hydride complex 4 was confirmed by (1)H and (13)C NMR, ESI MS, and UV-vis spectra. The [C,N] cyclometalated Ir-hydride complex 4 reacts with proton to generate a stoichiometric amount of hydrogen when the pH was decreased to pH 0.8 by the addition of diluted sulfuric acid. Photoirradiation (λ > 330 nm) of an aqueous solution of the [C,N] cyclometalated Ir-hydride complex 4 resulted in the quantitative conversion to a unique [C,C] cyclometalated Ir-hydride complex 5 with no byproduct. The complex 5 catalyzed hydrogen evolution from ethanol in a basic aqueous solution (pH 11.9) under ambient conditions. The 1,4-selective catalytic hydrogenation of ß-nicotinamide adenine dinucleotide (NAD(+)) by ethanol was also made possible by the complex 1 to produce 1,4-dihydro-ß-nicotinamide adenine dinucleotide (1,4-NADH) at room temperature. The overall catalytic mechanism of hydrogenation of NAD(+), accompanied by the oxidation of ethanol, was revealed on the basis of the kinetic analysis and detection of the reaction intermediates.

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+)). Such a remarkable enhancement of the reactivity of [Fe(IV)(O)(N4Py)](2+) in the presence of Sc(3+) was accompanied by the disappearance of a kinetic deuterium isotope effect. The radical cation of 2,5-(MeO)(2)C(6)H(3)CH(2)OH was detected in the course of the reaction in the presence of Sc(3+). The dimerized alcohol and aldehyde were also produced in addition to the monomer aldehyde in the presence of Sc(3+). These results indicate that the reaction mechanism is changed from one-step hydrogen atom transfer (HAT) from 2,5-(MeO)(2)C(6)H(3)CH(2)OH to [Fe(IV)(O)(N4Py)](2+) in the absence of Sc(3+) to stepwise Sc(3+)-coupled electron transfer, followed by proton transfer in the presence of Sc(3+). In contrast, neither acceleration of the rate nor the disappearance of the kinetic deuterium isotope effect was observed in the oxidation of benzyl alcohol (C(6)H(5)CH(2)OH) by [Fe(IV)(O)(N4Py)](2+) in the presence of Sc(OTf)(3). Moreover, the rate constants determined in the oxidation of various benzyl alcohol derivatives by [Fe(IV)(O)(N4Py)](2+) in the presence of Sc(OTf)(3) (10 mM) were compared with those of Sc(3+)-coupled electron transfer from one-electron reductants to [Fe(IV)(O)(N4Py)](2+) at the same driving force of electron transfer. This comparison revealed that the borderline of the change in the mechanism from HAT to stepwise Sc(3+)-coupled electron transfer and proton transfer is dependent on the one-electron oxidation potential of benzyl alcohol derivatives (ca. 1.7 V vs SCE).

Photocatalytic generation of a non-heme oxoiron(IV) complex with water as an oxygen source.

The photocatalytic formation of a non-heme oxoiron(IV) complex, [(N4Py)Fe(IV)(O)](2+) [N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine], efficiently proceeds via electron transfer from the excited state of a ruthenium complex, [Ru(II)(bpy)(3)](2+)* (bpy = 2,2'-bipyridine) to [Co(III)(NH(3))(5)Cl](2+) and stepwise electron-transfer oxidation of [(N4Py)Fe(II)](2+) with 2 equiv of [Ru(III)(bpy)(3)](3+) and H(2)O as an oxygen source. The oxoiron(IV) complex was independently generated by both chemical oxidation of [(N4Py)Fe(II)](2+) with [Ru(III)(bpy)(3)](3+) and electrochemical oxidation of [(N4Py)Fe(II)](2+).

Catalytic mechanism of water oxidation with single-site ruthenium-heteropolytungstate complexes.

Catalytic water oxidation to generate oxygen was achieved using all-inorganic mononuclear ruthenium complexes bearing Keggin-type lacunary heteropolytungstate, [Ru(III)(H(2)O)SiW(11)O(39)](5-) (1) and [Ru(III)(H(2)O)GeW(11)O(39)](5-) (2), as catalysts with (NH(4))(2)[Ce(IV)(NO(3))(6)] (CAN) as a one-electron oxidant in water. The oxygen atoms of evolved oxygen come from water as confirmed by isotope-labeled experiments. Cyclic voltammetric measurements of 1 and 2 at various pH's indicate that both complexes 1 and 2 exhibit three one-electron redox couples based on ruthenium center. The Pourbaix diagrams (plots of E(1/2) vs pH) support that the Ru(III) complexes are oxidized to the Ru(V)-oxo complexes with CAN. The Ru(V)-oxo complex derived from 1 was detected by UV-visible absorption, EPR, and resonance Raman measurements in situ as an active species during the water oxidation reaction. This indicates that the Ru(V)-oxo complex is involved in the rate-determining step of the catalytic cycle of water oxidation. The overall catalytic mechanism of water oxidation was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates. Complex 2 exhibited a higher catalytic reactivity for the water oxidation with CAN than did complex 1.

Cupric superoxo-mediated intermolecular C-H activation chemistry.

The new cupric superoxo complex [LCu(II)(O(2)(•-))](+), which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH). Kinetic studies indicated a first-order dependence on both the Cu complex and BNAH with a deuterium kinetic isotope effect (KIE) of 12.1, similar to that observed for certain copper monooxygenases.

Combination of visible-light responsive heterogeneous and homogeneous photocatalysts for water oxidation.

Bismuth vanadate (BiVO(4)), which is a visible-light responsive heterogeneous photocatalyst, was combined with homogeneous ruthenium complexes to increase the overall photocatalytic reactivity for water oxidation with a one-electron oxidant, [Co(III)(NH(3))(5)Cl](2+). Photoinduced electron transfer from the excited state of ruthenium(II) complexes to [Co(III)(NH(3))(5)Cl](2+) affords ruthenium(III) complexes which can oxidize water to oxygen with BiVO(4) under visible light irradiation.

Formic acid acting as an efficient oxygen scavenger in four-electron reduction of oxygen catalyzed by a heterodinuclear iridium-ruthenium complex in water.

A heterodinuclear iridium-ruthenium complex [Ir(III)(Cp*)(H(2)O)(bpm)Ru(II)(bpy)(2)](SO(4))(2) (Cp* = eta(5)-pentamethyl-cyclopentadienyl, bpm = 2,2'-bipyrimidine, bpy = 2,2'-bipyridine) acts as an effective catalyst for removal of dissolved O(2) by the four-electron reduction of O(2) with formic acid in water at an ambient temperature.

Unusually large tunneling effect on highly efficient generation of hydrogen and hydrogen isotopes in pH-selective decomposition of formic acid catalyzed by a heterodinuclear iridium-ruthenium complex in water.

A heterodinuclear iridium-ruthenium complex [Ir(III)(Cp*)(H(2)O)(bpm)Ru(II)(bpy)(2)](SO(4))(2) {1(SO(4))(2), Cp* = eta(5)-pentamethylcyclopentadienyl, bpm = 2,2'-bipyrimidine, bpy = 2,2'-bipyridine} acts as the most effective catalyst for selective production of hydrogen from formic acid in an aqueous solution at ambient temperature among catalysts reported so far. An unusually large tunneling effect was observed for the first time for the catalytic hydrogen production in H(2)O vs D(2)O.

Mononuclear copper complex-catalyzed four-electron reduction of oxygen.

A mononuclear Cu(II) complex acts as an efficient catalyst for four-electron reduction of O(2) to H(2)O. Its reduction by a ferrocene derivative (Fc*) and reaction with O(2) leads to the formation of a peroxodicopper(II) complex; this is subsequently reduced by Fc* in the presence of protons to regenerate the Cu(II) complex.