Inorganometallic Photocatalyst for CO Reduction.

ConspectusDuring the last few decades, the design of catalytic systems for CO reduction has been extensively researched and generally involves (1) traditional approaches using molecular organic/organometallic materials and heterogeneous inorganic semiconductors and (2) combinatory approaches wherein these materials are combined as needed. Recently, we have devised a number of new TiO-mediated multicomponent hybrid systems that synergistically integrate the intrinsic merits of various materials, namely, molecular photosensitizers/catalysts and -type TiO semiconductors, and lower the energetic and kinetic barriers between components. We have termed such multicomponent hybrid systems assembled from the hybridization of various organic/inorganic/organometallic units in a single platform .

Rapid Exciton Migration and Amplified Funneling Effects of Multi-Porphyrin Arrays in a Re(I)/Porphyrinic MOF Hybrid for Photocatalytic CO Reduction.

A porphyrinic metal-organic framework () known as PCN-222(Zn) was chemically doped with a molecular Re(I) catalyst-bearing carboxylate anchoring group to form a new type of metal-organic framework (MOF)-Re(I) hybrid photocatalyst. The porphyrinic MOF-sensitized hybrid () was prepared with an archetypical CO reduction catalyst, (L)Re(CO)Cl (Re(I); L = 4,4'-dicarboxylic-2,2'-bipyridine), in the presence of 3 vol % water produced CO with no leveling-off tendency for 59 h to give a turnover number of ≥1893 [1070 ± 80 μmol h (g MOF)]. The high catalytic activity arises mainly from efficient exciton migration and funneling from photoexcited porphyrin linkers to the peripheral Re(I) catalytic sites, which is in accordance with the observed fast exciton (energy) migration (≈1 ps) in highly ordered porphyrin photoreceptors and the effective funneling into Re(I) catalytic centers in the Re(I)-doped sample.

Ancillary Ligand Effects on Heteroleptic Ir Dye in Dye-Sensitized Photocatalytic CO Reduction: Photoaccumulation of Charges on Arylated Bipyridine Ligand and Its Control on Catalytic Performance.

Herein, we report the synthesis, and photochemical and -physical properties, as well as the catalytic performance, of a series of heteroleptic Ir photosensitizers (IrPSs), [Ir(C^N) (N^N )] , possessing ancillary ligands that are varied with aryl-substituents on bipyridyl unit [C^N=(2-pyridyl)benzo[b]thiophen-3-yl (btp); N^N =4,4'-Y -bpy (Y=-Ph or -PhSi(Ph) ]. We found that the π-extension of bipyridyl ligand by aryl-substitution put bipyridyl ligand in use as an electron relay unit that performed charge accumulation before delivering to the catalytic center, greatly improving the overall CO -to-CO conversion activities. In a typical run, the aryl-substituted IrPS ( IrP-Ph )-sensitized homogeneous systems (IrPS+Re catalyst) gave a turnover number of 1340 (Φ =24.

Utility of Squaraine Dyes for Dye-Sensitized Photocatalysis on Water or Carbon Dioxide Reduction.

Red light-sensitized squaraine () dyes were developed and incorporated into dye-sensitized catalysts (DSCs) with the formula of /TiO/Cat, and their efficacies were evaluated in terms of performance on either water or carbon dioxide reduction. Pt nanoparticles or -[Re(4,4'-bis-(diethoxyphosphorylmethyl)-2,2'-bipyridine)(CO)Cl] were used as each catalytic center within the DSC frame of /TiO/Pt (Type I) or /TiO/Re(I) (Type II). In order to convey the potential utility of in low energy sensitization, the following catalytic reductions were carried out under selective lower energy irradiation (>500 nm).

Organometallic Iridium(III) Complex Sensitized Ternary Hybrid Photocatalyst for CO to CO Conversion.

A series of heteroleptic iridium(III) complexes functionalized with two phosphonic acid (-PO H ) groups ( IrP, IrP, IrP, and IrP) were prepared and anchored onto rhenium(I) catalyst (ReP)-loaded TiO particles (TiO /ReP) to build up a new IrP-sensitized TiO photocatalyst system (IrP/TiO /ReP). The photosensitizing behavior of the IrP series was examined within the IrP/TiO /ReP platform for the photocatalytic conversion of CO into CO. The four IrP-based ternary hybrids showed increased conversion activity and durability than that of the corresponding homo- (IrP+ReP) and heterogeneous (IrP+TiO /ReP) mixed systems.

Photosensitization Behavior of Ir(III) Complexes in Selective Reduction of CO by Re(I)-Complex-Anchored TiO Hybrid Catalyst.

A series of cationic Ir(III) complexes ([Ir(btp)(bpy-X)] (Ir-X: btp = (2-pyridyl)benzo[b]thiophen-3-yl; bpy-X = 4,4'-X-2,2'-bipyridine (X = OMe, Bu, Me, H, and CN)) were applied as visible-light photosensitizer to the CO reduction to CO using a hybrid catalyst (TiO/ReP) prepared by anchoring of Re(4,4'-Y-bpy)(CO)Cl (ReP; Y = CHPO(OH)) on TiO particles. Irradiation of a solution containing Ir-X, TiO/ReP particles, and an electron donor (1,3-dimethyl-2-phenyl-1,3-dihydrobenzimidazole) in N,N-dimethylformamide at greater than 400 nm resulted in the reduction of CO to CO with efficiencies in the order X = OMe > Bu ≈ Me > H; Ir-CN has no photosensitization effect. A notable observation is that Ir-Bu and Ir-Me are less efficient than Ir-OMe at an early stage of the reaction but reveal persistent photosensitization behavior for a much longer period of time unlike the latter.

Photophysics and Excited-State Properties of Cyclometalated Iridium(III)-Platinum(II) and Iridium(III)-Iridium(III) Bimetallic Complexes Bridged by Dipyridylpyrazine.

We investigated the electrochemical and excited-state properties of 2,3-bis(2-pyridyl)pyrazine (dpp)-bridged bimetallic complexes, (L)Ir-dpp-PtCl [1, L = 2-(4',6'-difluorophenyl)pyridinato-N,C (dfppy); 2, L = 2-phenylpyridinato-N,C (ppy)] and [(L)Ir](dpp) [3, L = dfppy; 4, L = ppy] compared to monometallic complexes, (L)Ir-dpp (5, L = dfppy; 6, L = ppy) and dpp-PtCl (dpp-PtCl; 7). The single-crystal X-ray crystallographic structures of 1, 3, 5, and 6 showed that 1 and 3 have approximately coplanar structures of the dpp unit, while the noncoordinated pyridine ring of dpp in 5 and 6 is largely twisted with respect to the pyrazine ring. We found that the properties of the bimetallic complex significantly depended on the electronic and geometrical modulations of each fragment: (1) electronic structure of the main L (C^N) ligand in an iridium chromophore (L = dfppy or ppy) and (2) planarity of the bridging ligand (dpp).

Organic-inorganic hybrid photocatalyst for carbon dioxide reduction.

Efficient hybrid photocatalysts for carbon dioxide reduction were developed from dye-sensitized TiO nanoparticles and their catalytic performance was optimized by ternary organic/inorganic components. Thus, the hybrid system consists of (E)-2-cyano-3-(5'-(5''-(p-(diphenylamino)phenyl)thiophen-2''-yl)thiophen-2'-yl)-acrylic acid as a sensitizer and fac-[Re(4,4'-bis(diethoxyphosphorylmethyl)-2,2'-bipyridine)(CO)Cl] as a reduction catalyst (ReP), both of which have been fixed onto TiO semiconductors (s-TiO, h-TiO, d-TiO). Mott-Schottky analysis on flat-band potential (E) of TiO mesoporous films has verified that E can be finely modulated by volume variation of water (0 to 20 vol%).

Widely Controllable Syngas Production by a Dye-Sensitized TiO Hybrid System with Re and Co Catalysts under Visible-Light Irradiation.

Visible-light irradiation of a ternary hybrid catalyst prepared by grafting a dye, an H evolving Co catalyst and a CO-producing Re catalyst on TiO have been found to produce both H and CO (syngas) in CO -saturated N,N-dimethyl formamide (DMF)/water solution containing a 0.1 m sacrificial electron donor. The H /CO ratios are effectively controlled by changing either the water content of the solvent or the molar ratio of the Re and Co catalysts ranging from 1:2 to 15:1.

Stable Blue Phosphorescence Iridium(III) Cyclometalated Complexes Prompted by Intramolecular Hydrogen Bond in Ancillary Ligand.

Improvement of the stability of blue phosphorescent dopant material is one of the key factors for real application of organic light-emitting diodes (OLEDs). In this study, we found that the intramolecular hydrogen bonding in an ancillary ligand from a heteroleptic Ir(III) complex can play an important role in the stability of blue phosphorescence. To rationalize the role of intramolecular hydrogen bonding, a series of Ir(III) complexes is designed and prepared: Ir(dfppy)2(pic-OH) (1a), Ir(dfppy)2(pic-OMe) (1b), Ir(ppy)2(pic-OH) (2a), and Ir(ppy)2(pic-OMe) (2b).

Highly Robust Hybrid Photocatalyst for Carbon Dioxide Reduction: Tuning and Optimization of Catalytic Activities of Dye/TiO2/Re(I) Organic-Inorganic Ternary Systems.

Herein we report a detailed investigation of a highly robust hybrid system (sensitizer/TiO2/catalyst) for the visible-light reduction of CO2 to CO; the system comprises 5'-(4-[bis(4-methoxymethylphenyl)amino]phenyl-2,2'-dithiophen-5-yl)cyanoacrylic acid as the sensitizer and (4,4'-bis(methylphosphonic acid)-2,2'-bipyridine)Re(I)(CO)3Cl as the catalyst, both of which have been anchored on three different types of TiO2 particles (s-TiO2, h-TiO2, d-TiO2). It was found that remarkable enhancements in the CO2 conversion activity of the hybrid photocatalytic system can be achieved by addition of water or such other additives as Li(+), Na(+), and TEOA. The photocatalytic CO2 reduction efficiency was enhanced by approximately 300% upon addition of 3% (v/v) H2O, giving a turnover number of ≥570 for 30 h.

High-turnover visible-light photoreduction of CO2 by a Re(I) complex stabilized on dye-sensitized TiO2.

Hybrid systems prepared by fixing a Re(i) complex and a dye on three types of TiO2 nanoparticles in two different ways commonly revealed persistent photocatalysis of the CO2 reduction to CO with no levelling-off tendency under visible-light irradiation in DMF, giving a turnover number of ≥435.

Hydrophilicity control of visible-light hydrogen evolution and dynamics of the charge-separated state in dye/TiO2/Pt hybrid systems.

Visible-light-driven H(2) evolution based on Dye/TiO(2)/Pt hybrid photocatalysts was investigated for a series of (E)-3-(5'-{4-[bis(4-R(1)-phenyl)amino]phenyl}-4,4'-(R(2))(2)-2,2'-bithiophen-5-yl)-2-cyanoacrylic acid dyes. Efficiencies of hydrogen evolution from aqueous suspensions in the presence of ethylenediaminetetraacetic acid as electron donor under illumination at λ>420 nm were found to considerably depend on the hydrophilic character of R(1), varying in the order MOD (R(1)=CH(3)OCH(2), R(2)=H)≈MO4D (R(1)=R(2)=CH(3)OCH(2))>HD (R(1)=R(2)=H)>PD (R(1)=C(3)H(7), R(2)=H). In the case of MOD/TiO(2)/Pt, the apparent quantum yield for photocatalyzed H(2) generation at 436 nm was 0.

Effects of intermolecular interaction on the energy distribution of valance electronic states of a carbazole-based material in amorphous thin films.

Effects of intermolecular interactions on the occupied electronic structure of amorphous solid of a carbazole-based material were investigated under an assumption that the organic solid consists of randomly oriented assemblies of dimers. The electronic energy states were calculated on the ensemble of large number of random dimers, of which geometries are relaxed using semiempirical van der Waals density functional theory. Intermolecular interactions result in splitting of energy level, and further disorders occur by aggregation of randomly orientated molecules.

Carborane photochemistry triggered by aryl substitution: carborane-based dyads with phenyl carbazoles.

A bright combination: a new type of donor-acceptor dyad, carbazolylaryl-substituted ortho-carboranes, which are conveniently prepared from the corresponding acetylenes and decaborane pathways, showed unique excited-state behavior associated with electron transfer unlike the meta- and para-counterparts.

Electronic optimization of heteroleptic Ru(II) bipyridine complexes by remote substituents: synthesis, characterization, and application to dye-sensitized solar cells.

We prepared a series of new heteroleptic ruthenium(II) complexes, Ru(NCS)(2)LL' (3a-3e), where L is 4,4'-di(hydroxycarbonyl)-2,2'-bipyridine and L' is 4,4'-di(p-X-phenyl)-2,2'-pyridine (X = CN (a), F (b), H (c), OMe (d), and NMe(2) (e)), in an attempt to explore the structure-activity relationships in their photophysical and electrochemical behavior and in their performance in dye-sensitized solar cells (DSSCs). When substituent X is changed from electron-donating NMe(2) to electron-withdrawing CN, the absorption and emission maxima reveal systematic bathochromic shifts. The redox potentials of these dyes are also significantly influenced by X.

Significance of hydrophilic characters of organic dyes in visible-light hydrogen generation based on TiO2.

A series of dyes were synthesized to examine the roles of the hydrophilic characteristics of R in sensitized hydrogen generation by dye-grafted Pt/TiO(2) under visible light irradiation. The hydrogen-generation efficiencies and optimum amounts of the dyes grafted to Pt/TiO(2) were affected substantially by the hydrophilic and steric effects of R; moderately hydrophilic DEO1 and DEO2 showed higher sensitization activity at a lower loading than hydrophobic D-H.

Catalytic Activities of CuSO(4)/Al(2)O(3) in Dehydrogenation of Arenes by Dioxygen.

The oxidation reactions of hydroquinones, 2-naphthols, or 2,6-di-tert-butylphenol efficiently occurred by catalysis with alumina-supported copper(II) sulfate to give the corresponding benzoquinones, 1,1'-bi-2-naphthols, and 4,4'-diphenoquinone, respectively, in good yields. The synthetic potentiality of the catalytic reactions was demonstrated by easy isolation of the final products using only filtration and solvent evaporation as well as by application to large-scale syntheses of the benzoquinones and binaphthols. The catalysis with alumina-supported copper(II) sulfate was also applied to the oxidative intramolecular coupling of 5,5'-diacenaphthene to the corresponding perylene compound.