The splitting of water through artificial photosynthesis (AP) is a key transformation toward the conversion of solar energy into stored chemical potential in the form of fuel and oxidizer.[1] For water splitting, the reductive side of the reaction involves the light-driven conversion of aqueous protons into H 2 . To perform this half-reaction, photocatalytic systems typically consist of a catalyst, photosensitizer (PS), and sacrificial electron donor.[2] Recent studies on noblemetal-based [3] and noble-metal-free [4] homogeneous systems for light-driven hydrogen production have shown high activity. However, significant problems in the noble-metalfree molecularly based systems include relatively low catalyst turnover numbers (TON < 500 mole H 2 /mole catalyst) for hydrogen formation, and photodecomposition of the systems within a few hours. For most organic dye based systems that have recently been reported, the photochemical quenching step of the excited-state dye (PS*) is reductive, thus leading to unstable PS À radical anions that undergo decomposition.[4b]Thus, the development of more active catalysts, specifically ones that quench PS* oxidatively, would be of great value for obtaining long-lived homogeneous AP systems. Herein, we describe a new homogenous catalyst for H 2 production that has both high activity and the ability to oxidatively quench PS*, thus leading to a much longer system lifetime. Nickel cathodes are used in commercial electrolyzers, suggesting that nickel may be a worthwhile basis for homogeneous catalysts as well.[5] Nickel thiolate complexes have received special attention in recent years because sulfurligated nickel complexes mimic the [Fe-Ni]-hydrogenase active site, [6] and dimeric metal complexes based on nickel thiolate hydrides have been shown to be catalytically active for proton reduction.[7] DuBois and co-workers have also shown that mononuclear nickel(II) bis(diphosphine) complexes are effective catalysts for electrochemical hydrogen generation.[8] While photocatalytic hydrogen generation from the nickel-phosphine complexes is long-lived, the activity of the photocatalytic system with the nickel phosphine catalyst is low, with a turnover frequency (TOF) of approximately 20 equivalents of H 2 per hour.[9] Related nickel(II) complexes containing pyridine-2-thiolate ligands have been known for over two decades, [10] but their catalytic properties for proton reduction have not been reported. In the present study, the complex [Ni(pyS) 3 ] À (1 À ; pyS = pyridine-2-thiolate) is found to have impressive catalytic activity for the photocatalytic production of H 2 in a homogeneous system with fluorescein (Fl) as the PS and triethylamine (TEA) as the sacrificial electron donor.Photolysis of a solution of Fl and 1 À in EtOH/H 2 O (1:1) using a green-light-emitting diode (LED) (l = 520 nm, 0.12 W) at 15 8C results in H 2 generation which was monitored in real time by the pressure change in the reaction vessel, and quantified at the end of the photolysis by GC analysis of the headspace gase...

New dyads consisting of a strongly absorbing Bodipy (dipyrromethene-BF 2 ) dye and a platinum diimine dithiolate (PtN 2 S 2 ) charge transfer (CT) chromophore have been synthesized and studied in the context of the light-driven generation of H 2 from aqueous protons. In these dyads, the Bodipy dye is bonded directly to the benzenedithiolate ligand of the PtN 2 S 2 CT chromophore. Each of the new dyads contains either a bipyridine (bpy) or phenanthroline (phen) diimine with an attached functional group that is used for binding directly to TiO 2 nanoparticles, allowing rapid electron photoinjection into the semiconductor. The absorption spectra and cyclic voltammograms of the dyads show that the spectroscopic and electrochemical properties of the dyads are the sum of the individual chromophores (Bodipy and the PtN 2 S 2 moieties), indicating little electronic coupling between them. Connection to TiO 2 nanoparticles is carried out by sonication leading to in situ attachment to TiO 2 without prior hydrolysis of the ester linking groups to acids. For H 2 generation studies, the TiO 2 particles are platinized (Pt-TiO 2 ) so that the light absorber (the dyad), the electron conduit (TiO 2 ), and the catalyst (attached colloidal Pt) are fully integrated. It is found that upon 530 nm irradiation in a H 2 O solution (pH 4) with ascorbic acid as an electron donor, the dyad linked to Pt-TiO 2 via a phosphonate or carboxylate attachment shows excellent light-driven H 2 production with substantial longevity, in which one particular dyad [4(bpyP)] exhibits the highest activity, generating ∼40,000 turnover numbers of H 2 over 12 d (with respect to dye).photochemistry | solar energy conversion | hydrogen | spectroscopy | synthesis W ater splitting into hydrogen and oxygen is the key energystoring reaction of artificial photosynthesis (AP) and one of the most promising long-term strategies for carbon-free energy on a potentially global scale (1). As a redox reaction, water splitting has been studied primarily in terms of its two halfreactions, the reduction of aqueous protons to H 2 and the oxidation of water to O 2 (2-13). Whereas some of these studies date back more than 30 y (14-22), recent progress on each halfreaction has been notable, particularly with regard to catalyst development and mechanistic understanding of each transformation (6,(23)(24)(25)(26)(27)(28)(29). In this paper, we focus on efforts dealing with the lightdriven generation of H 2 , which in its simplest form requires a light absorber or photosensitizer (PS) for electron-hole creation, a means or pathway for charge separation and electron transfer, an aqueous proton source, a catalyst for collecting electrons and protons and promoting their conversion to H 2 , and an ultimate source of electrons in the form of an electron donor.Dating from the earliest work on the light-driven generation of H 2 , the photosensitizer has most often been a Ru(II) complex with 2,2′-bipyridine (bpy) and/or related heterocyclic ligands having a long-lived triplet metal-to-ligand ...

Newly prepared hydrido iridium(III) complexes [Ir(ppy)(PPh3)2(H)L](0,+) (ppy = bidentate 2-phenylpyridinato anionic ligand; L = MeCN (1b), CO (1c), CN(-) (1d); H being trans to the nitrogen of ppy ligand) emit blue light at the emission lambda(max) (452-457, 483-487 nm) significantly shorter than those (468, 495 nm) of the chloro complex Ir(ppy)(PPh3)2(H)(Cl) (1a). Replacing ppy of 1a-d with F2ppy (2,4-difluoro-2-phenylpyridinato anion) and F2Meppy (2,4-difluoro-2-phenyl-m-methylpyridinato anion) brings further blue-shifts down to the emission lambda(max) at 439-441 and 465-467 nm with CIE color coordinates being x = 0.16 and y = 0.18-0.20 to display a deep-blue photoemission. No significant blue shift is observed by replacing PPh3 of 1a with PPh2Me to produce Ir(ppy)(PPh2Me)2(H)(Cl) (1aPPh 2Me), which displays emission lambda max at 467 and 494 nm. The chloro complexes, [Ir(ppy)(PPh3)2(Cl)(L)](0,+) (L = MeCN (2b), CO (2c), CN(-) (2d)) having a chlorine ligand trans to the nitrogen of ppy also emit deep-blue light at emission lambda(max) 452-457 and 482-487 nm.