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.

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.

High Vertical Carrier Mobilities of Organic Semiconductors Due to a Deposited Laid-Down Herringbone Structure Induced by a Reduced Graphene Oxide Template.

High vertical carrier mobilities in organic semiconductor films are a challenging issue for fundamentally improving the performance of vertical devices. To achieve improvement in the vertical direction, a reduced graphene oxide (rGO) template is used with pentacene and DNTT having a herringbone structure enabling two-dimensional (2D) transport in comparison with CuPc having a slipped-stack structure. A thin-film structure and the optoelectrical properties of the oriented films are investigated with respect to molecular structures and packing modes. The rGO template induces a "laid-down" herringbone structure for pentacene and DNTT with a face-on orientation. Our results reveal that intermolecular dispersion energy is an additional important factor to form face-on states of molecules and influences face-on ratios in the films on rGO. Vertical charge mobilities of the films are significantly enhanced by the rGO template. Particularly, the DNTT film with a laid-down herringbone structure produces a vertical mobility as high as 0.27 cm2 V-1 s-1, one of the highest values for ordinary thin films with several hundred nanometer thickness. These findings suggest that 2D transport is advantageous for vertical carrier transport also.

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).

Effect of reabsorption of fluorescence on transient absorption measurements.

Elimination of fluorescence reabsorption effects is necessary to obtain reliable kinetic data in fluorescence spectroscopy. This effect must also be considered in transient absorption spectroscopy. We devised two methods to achieve this goal. The first was use of a thin optical cell (<10 µm) rather than a much thicker conventional cuvette in the experimental setup. The second was use of an equation to correct data obtained using a conventional cuvette when there were fluorescence reabsorption effects. These methods were successfully used in sub-nanosecond transient absorption spectroscopy to obtain the kinetics of excimer formation by perylene in toluene.

Synthesis of fluorescent polycarbonates with highly twisted N,N-bis(dialkylamino)anthracene AIE luminogens in the main chain.

A synthetic route to embed aggregation-induced-emission-(AIE)-active luminophores in polycarbonates (PCs) in various ratios is reported. The AIE-active monomer is based on the structure of 9,10-bis(piperidyl)anthracene. The obtained PCs display good film-forming properties, similar to those observed in poly(bisphenol A carbonate) (Ba-PC). The fluorescence quantum yield (Φ) of the PC with 5 mol% AIE-active monomer was 0.04 in solution and 0.53 in solid state. Moreover, this PC is also miscible with commercially available Ba-PC at any blending ratio. A combined analysis by scanning electron microscopy and differential scanning calorimetry did not indicate any clear phase separation. These results thus suggest that even engineering plastics like polycarbonates can be functionalized with AIE luminogens without adverse effects on their physical properties.

Solution-processable reduced graphene oxide template layer for molecular orientation control of organic semiconductors.

A reduced graphene oxide (rGO) film is first applied to a surface template layer to control the molecular orientation of crystalline organic semiconductors. The ultrathin and ultrasmooth rGO layer was successfully prepared on a substrate without a transfer process by spin-coating a carefully purified GO aqueous dispersion. This rGO layer exhibited a strong templating effect rivaling monolayer graphene, inducing a face-on orientation of copper phthalocyanine molecules leading to significant improvement of vertical carrier mobilities. The highly-conductive and transparent rGO film does not hamper charge transport at the interface and photoabsorption unlike conventional templating materials. This method can be widely used for vertical organic devices that require high carrier mobilities and strong photoabsorption/emission.

Heterogeneous catalase-like activity of gold(i)-cobalt(iii) metallosupramolecular ionic crystals.

Unique heterogeneous catalase-like activity was observed for metallosupramolecular ionic crystals [AuI4CoIII2(dppe)2(d-pen)4]X n ([1]X n ; dppe = 1,2-bis(diphenylphosphino)ethane; d-pen = d-penicillaminate; X n = (Cl-)2, (ClO4-)2, (NO3-)2 or SO42-) consisting of AuI4CoIII2 complex cations, [1]2+, and inorganic anions, X- or X2-. Treatment of the ionic crystals with an aqueous H2O2 solution led to considerable O2 evolution with a high turnover frequency of 1.4 × 105 h-1 for the heterogeneous cobalt complexes, which was dependent on their size and shape as well as the arrangement of cationic and anionic species. These dependencies were rationalized by the presence of cobalt(ii) centers on the crystal surface and their efficient exposure on the (111) plane rather than the (100) plane based on morphological and theoretical studies.

Photocatalytic water oxidation by persulphate with a Ca2+ ion-incorporated polymeric cobalt cyanide complex affording O2 with 200% quantum efficiency.

Incorporation of a small amount of Ca2+ ions into a polymeric cobalt cyanide complex to form Cax[CoII(H2O)2]1.5-x[CoIII(CN)6] resulted in a significant enhancement of activity for photocatalytic water oxidation in a buffer solution (pH 7.0) containing [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) as a photocatalyst and Na2S2O8 as an electron acceptor to achieve a quantum efficiency of 200%.

Dual function photocatalysis of cyano-bridged heteronuclear metal complexes for water oxidation and two-electron reduction of dioxygen to produce hydrogen peroxide as a solar fuel.

The photocatalytic production of hydrogen peroxide from water and dioxygen under visible light irradiation was made possible by using polymeric cyano-bridged heteronuclear metal complexes (MII[RuII(CN)4(bpy)]; MII = NiII, FeII and MnII), where the photocatalytic two-electron reduction of O2 and water oxidation were catalysed by the Ru and MII moieties, respectively.

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.

Nanofabrication of a Solid-State, Mesoporous Nanoparticle Composite for Efficient Photocatalytic Hydrogen Generation.

Room-temperature self-assembly was used to fabricate a periodic array of uniformly sized Al3+ -doped SiO2 nanoparticles (Al-SiO2 NPs, 20-30 nm). The uniform mesoporous structure was suitable for uniformly incorporating and distributing Pt nanoparticles (PtNPs), which were used as hydrogen-evolution catalysts in artificial photosynthetic systems, without agglomeration during the catalytic reaction. When the surfaces of the Al-SiO2 NPs were covered with an organic photocatalyst (2-phenyl-4-(1-naphthyl)quinolinium ion, QuPh+ -NA), each PtNP was surrounded by multiple QuPh+ -NA ions. The structure allowed the PtNP to receive multiple electrons from QuPh. -NA molecules, which were generated by reduction of the photoexcited state of QuPh+ -NA ions (QuPh. -NA. + ) with ß-dihydronicotinamide adenine dinucleotide (NADH), thereby resulting in efficient photocatalytic H2 evolution.

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.

Influence of pH on the decay of ß-carotene radical cation in aqueous Triton X-100: A laser flash photolysis study.

The identification of the spectral information of carotenoid neutral radicals is essential for studying their reactivities towards O2 and thereby evaluating their role in the antioxidant-prooxidant properties of the corresponding carotenoid. Recently, it was reported that ß-carotene neutral radical (ß-CAR) has an absorption maximum at 750 nm. This contradicts the results of many reports that show carotenoid neutral radicals (CAR) absorb in the same or near to the spectral region as their parent carotenoids. In this manuscript, the influence of pH on the decay of ß-carotene radical cation (ß-CAR-H(+)), generated in an aqueous solution of 2% Triton X-100 (TX-100), was investigated, employing laser flash photolysis (LFP) coupled with kinetic absorption spectroscopy, to identify the absorption bands of the ß-carotene neutral radicals. By increasing the pH value of the solution, the decay of ß-CAR-H(+) is enhanced and this enhancement is not associated with the formation of any positive absorption bands over the range 550-900 nm. By comparing these results with the literature, it can be concluded that ß-carotene neutral radicals most probably absorb within the same spectral range as that of ß-carotene. The reaction pathways of the reaction of ß-CAR-H(+) with (-)OH have been discussed.

Catalytic hydrogen production from paraformaldehyde and water using an organoiridium complex.

Paraformaldehyde was decomposed using an organoiridium complex (1, [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H2O)]2SO4) as a catalyst in water to produce H2 and CO2 in a 2 : 1 molar ratio at room temperature. The catalytic cycle is composed of the reduction of 1 by paraformaldehyde under basic conditions to produce formic acid and the hydride complex, which reacts with protons to produce H2. Formic acid further decomposed to H2 and CO2 with 1.

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.

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.

The long-lived electron transfer state of the 2-phenyl-4-(1-naphthyl)quinolinium ion incorporated into nanosized mesoporous silica-alumina acting as a robust photocatalyst in water.

A simple electron donor-acceptor linked dyad, the 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh(+)-NA), was incorporated into nanosized mesoporous silica-alumina to form a composite, which is highly dispersed in water and acts as an efficient and robust photocatalyst for the reduction of O2 by oxalate to produce hydrogen peroxide with a quantum yield of 10%.