Reductive side of water splitting in artificial photosynthesis: new homogeneous photosystems of great activity and mechanistic insight

Rhodamine photosensitizers (PSs) substituting S or Se for O in the xanthene ring give turnover numbers (TONs) as high as 9000 for the generation of hydrogen via the reduction of water using [Co(III)(dmgH)(2)(py)Cl] (where dmgH = dimethylglyoximate and py = pyridine) as the catalyst and triethanolamine as the sacrificial electron donor. The turnover frequencies were 0, 1700, and 5500 mol H(2)/mol PS/h for O, S, and Se derivatives, respectively (Φ(H(2)) = 0%, 12.2%, and 32.8%, respectively), which correlates well with relative triplet yields estimated from quantum yields for singlet oxygen generation. Phosphorescence from the excited PS was quenched by the sacrificial electron donor. Fluorescence lifetimes were similar for the O- and S-containing rhodamines (～2.6 ns) and shorter for the Se analog (～0.1 ns). These data suggest a reaction pathway involving reductive quenching of the triplet excited state of the PS giving the reduced PS(-) that then transfers an electron to the Co catalyst. The longer-lived triplet state is necessary for effective bimolecular electron transfer. While the cobalt/rhodamine/triethanolamine system gives unprecedented yields of hydrogen for the photoreduction of water, mechanistic insights regarding the overall reaction pathway as well as system degradation offer significant guidance to developing even more stable and efficient photocatalytic systems.     

Cyclodextrin-based systems for photoinduced hydrogen evolution

Light-driven catalytic three component systems for the reduction of protons, consisting of a cyclodextrin-appended iridium complex as photosensitizer, a viologen-based electron relay, and cyclodextrin-modified platinum nanoparticles as the catalyst, were found to be capable of producing molecular hydrogen effectively in water, using a sacrificial electron donor. The modular approach introduced in this study allows the generation of several functional photo-active systems by self-assembly from a limited number of building blocks. We established that systems with polypyridine iridium complexes of general formula [Ir(ppy)(2)(pytl-R)]Cl (ppy, 2-phenylpyridine; pytl, 2-(1-substituted-1H-1,2,3-triazol-4-yl)pyridine) as photosensitizers are active in the production of H(2), with yields that under our experimental conditions are 20-35 times higher than those of the classical system with [Ru(bpy)(3)]Cl(2) (bpy, 2,2'-bipyridine), methyl viologen, and Pt. By investigating different photocatalytic systems, it was found that the amount of hydrogen produced was directly proportional to the emission quantum yield of the photosensitizer.     

Photocatalytic generation of hydrogen from water using a platinum(II) terpyridyl acetylide chromophore

The cationic complex [Pt(tolylterpyridine)(phenylacetylide)]+ has been used as a photosensitizer for the reduction of aqueous protons in the presence of a sacrificial electron donor to make H2. In this system, triethanolamine (TEOA) acts as the sacrificial reducing agent, methyl viologen (MV2+) serves as an electron transfer agent, and colloidal Pt stabilized by polyacrylate functions as the catalyst for H2 generation. The Pt(II) chromophore undergoes both oxidative and reductive quenching, but H2 is only seen when both TEOA and MV2+ are present. Irradiation of the reaction solution for 10 h with lambda > 410 nm leads to 85 turnovers and an overall yield of 34% based on TEOA. While H2 evolution is maximized for the system at pH 7, it is also seen at pH 5 and 9, in contrast with earlier reports using Ru(bpy)32+ as the photosensitizer. This is the first time that a Pt diimine or terpyridyl complex has been used as the photosensitizer for H2 generation from aqueous protons.     

Light-induced charge separation and photocatalytic hydrogen evolution from water using Ru(II)Pt(II)-based molecular devices: effects of introducing additional donor and/or acceptor sites

In our hopes to improve the photocatalytic efficiency of photo-hydrogen-evolving molecular devices, several new dyads and triads possessing a photosensitizing Ru(bpy)(phen)(2)(2+) (or Ru(phen)(3)(2+)) chromophore (abbreviated as Ru(II)) attached to both/either a phenothiazine moiety (abbreviated as Phz) and/or H(2)-evolving PtCl(2)(bpy) units (abbreviated as Pt), such as Phz-Ru(II)-Pt2 (triad), Ru(II)-Pt2 (dyad), and Ru(II)-Pt3 (dyad), were synthesized and their basic properties together with the photo-hydrogen-evolving characteristics were investigated in detail. The (3)MLCT phosphorescence from the Ru(II) moiety in these systems is substantially quenched due to the highly efficient photoinduced electron transfer (PET). Based on the electrochemical studies, the driving forces for the PET were estimated as -0.07 eV for Phz-Ru(II)-Pt2, -0.24 eV for Ru(II)-Pt2, and -0.22 eV for Ru(II)-Pt3, revealing the exergonic character of the PET in these systems. Luminescence lifetime studies revealed the existence of more than two decay components, indicative of a contribution of multiple PET processes arising from the presence of at least two different conformers in solution. The major luminescence decay components of the hybrid systems [τ(1) = 6.5 ns (Ru(II)-Pt2) and τ(1) = 1.04 ns (Phz-Ru(II)-Pt2) in acetonitrile] are much shorter than those of Phz-free/Pt-free Ru(bpy)(phen)(2)(2+) derivatives. An important finding is that the triad Phz-Ru(II)-Pt2 affords a quite long-lived charge separated (CS) state (τ(CS) = 43 ns), denoted as Phz(+)˙-Ru(Red)-Pt2, as a result of reductive quenching of the triplet excited state of Ru(bpy)(phen)(2)(2+) by the tethering Phz moiety, where Ru(Red) denotes Ru(bpy)(phen)(2)(+). Moreover, the lifetime of Phz(+)˙-Ru(Red)-Pt2 was observed to be much longer than that of Phz(+)˙-Ru(Red). The photocatalytic H(2) evolution from water driven by these systems was examined in an aqueous acetate buffer solution (pH 5.0) containing 4-19% dimethylsulfoxide (solubilising reagent) in the presence of EDTA as a sacrificial electron donor. Dyads Ru(II)-Pt2 and Ru(II)-Pt3 were found to exhibit improved photo-hydrogen-evolving activity compared to the heterodinuclear Ru-Pt dyads developed so far in our group. On the other hand, almost no catalytic activity was observed for Phz-Ru(II)-Pt2 in spite of the formation of a strongly reducing Ru(Red) site (Phz(+)˙-Ru(Red)-Pt2), indicating that the electron transfer from the photogenerated Ru(Red) unit to the PtCl(2)(bpy) unit is not favoured presumably due to the slow electron transfer rate in the Marcus inverted region.     

Characterization of photochemical processes for H2 production by CdS nanorod-[FeFe] hydrogenase complexes

We have developed complexes of CdS nanorods capped with 3-mercaptopropionic acid (MPA) and Clostridium acetobutylicum [FeFe]-hydrogenase I (CaI) that photocatalyze reduction of H(+) to H(2) at a CaI turnover frequency of 380-900 s(-1) and photon conversion efficiencies of up to 20% under illumination at 405 nm. In this paper, we focus on the compositional and mechanistic aspects of CdS:CaI complexes that control the photochemical conversion of solar energy into H(2). Self-assembly of CdS with CaI was driven by electrostatics, demonstrated as the inhibition of ferredoxin-mediated H(2) evolution by CaI. Production of H(2) by CdS:CaI was observed only under illumination and only in the presence of a sacrificial donor. We explored the effects of the CdS:CaI molar ratio, sacrificial donor concentration, and light intensity on photocatalytic H(2) production, which were interpreted on the basis of contributions to electron transfer, hole transfer, or rate of photon absorption, respectively. Each parameter was found to have pronounced effects on the CdS:CaI photocatalytic activity. Specifically, we found that under 405 nm light at an intensity equivalent to total AM 1.5 solar flux, H(2) production was limited by the rate of photon absorption (~1 ms(-1)) and not by the turnover of CaI. Complexes were capable of H(2) production for up to 4 h with a total turnover number of 10(6) before photocatalytic activity was lost. This loss correlated with inactivation of CaI, resulting from the photo-oxidation of the CdS capping ligand MPA.     

A cobalt-dithiolene complex for the photocatalytic and electrocatalytic reduction of protons

The complex [Co(bdt)(2)](-) (where bdt = 1,2-benzenedithiolate) is an active catalyst for the visible light driven reduction of protons from water when employed with Ru(bpy)(3)(2+) as the photosensitizer and ascorbic acid as the sacrificial electron donor. At pH 4.0, the system exhibits very high activity, achieving >2700 turnovers with respect to catalyst and an initial turnover rate of 880 mol H(2)/mol catalyst/h. The same complex is also an active electrocatalyst for proton reduction in 1:1 CH(3)CN/H(2)O in the presence of weak acids, with the onset of a catalytic wave at the reversible redox couple of -1.01 V vs Fc(+)/Fc. The cobalt-dithiolene complex [Co(bdt)(2)](-) thus represents a highly active catalyst for both the electrocatalytic and photocatalytic reduction of protons in aqueous solutions.     

Reduction of CO2 on a tricarbonyl rhenium(I) complex: modeling a catalytic cycle

Tricarbonyl rhenium(I) complexes, such as Re(bpy)(CO)(3)Cl where bpy = 2,2'-bipyridyl, have demonstrated superior activity in catalyzing CO(2) reduction in the presence of sacrificial electron donors. We have utilized density functional theory (DFT) to investigate a potential pathway for formate production via a rhenium-hydride insertion mechanism in the presence of triethylamine (TEA). On the basis of prior studies, we re-examined the role of TEA and studied a catalytic cycle for CO(2) reduction in which TEA functions as both the hydrogen atom and the electron donor for reducing CO(2) into formate. The catalytic cycle is found to be exothermic with inclusion of solvation and may be viewed as a two-electron reduction of CO(2) because the net result is a transfer of hydride from TEA to CO(2). In addition, we have identified structures of key intermediates in the CO(2)-reduction process and found that the insertion step has a very modest barrier in acetonitrile. These findings provide a molecular-level understanding to formate production via CO(2) reduction mediated by transition-metal complexes. A theoretical investigation is underway to elucidate the formation of carbon monoxide, another common product in Re-catalyzed CO(2) reduction.     

A copper(ii)-ethylenediamine modified polyoxoniobate with photocatalytic H(2) evolution activity under visible light irradiation

A new dimer polyoxoniobate [Cu(en)(2)](11)K(4)Na(2)[KNb(24)O(72)H(9)](2)·120H(2)O (1) has been synthesized and systematically characterized. Visible light photocatalytic H(2) evolution activity was researched with 1 as the visible-light photosensitizer and catalyst, cobaloximes [Co(III)(dmgH)(2)pyCl] as the co-catalysts, and triethylamine (TEA) as the sacrificial electron donor.     

Electron transfer and hydrogen generation from a molecular dyad: platinum(II) alkynyl complex anchored to [FeFe] hydrogenase subsite mimic

A PS-Fe(2)S(2) molecular dyad 1a directly anchoring a platinum(II) alkynyl complex to a Fe(2)S(2) active site of a [FeFe] H(2)ase mimic, and an intermolecular system of its reference complexes 1b and 2, have been successfully constructed. Time-dependence of H(2) evolution shows that PS-Fe(2)S(2)1a as well as complex 2 with 1b can produce H(2) in the presence of a proton source and sacrificial donor under visible light irradiation. Spectroscopic and electrochemical studies on the electron transfer event reveal that the reduced Fe(I)Fe(0) species generated by the first electron transfer from the excited platinum(II) complex to the Fe(2)S(2) active site in PS-Fe(2)S(2)1a and complex 2 with 1b is essential for photochemical H(2) evolution, while the second electron transfer from the excited platinum(II) complex to the protonated Fe(I)Fe(0) species is thermodynamically unfeasible, which might be an obstacle for the relatively small amount of H(2) obtained by PS-Fe(2)S(2) molecular dyads reported so far.     

Proton transfer in hydrogen-bonded pyridine/acid systems: the role of higher aggregation

In this work the role of higher molecular aggregation in the proton transfer processes within hydrogen bond (H-bond) is investigated. The H-bonded complex consisting of 4-cyanopyridine (CyPy) with trichloroacetic acid (TCA) has been studied in the solutions of acetonitrile, carbon tetrachloride, chloroform and dichloroethane as solvent by FTIR spectroscopy and quantum chemical DFT calculations. In order to illustrate the effect of increasing H-bond strength FTIR investigations have also been performed on solutions of CyPy with H(2)O, acetic-, trifluoroacetic- and methanesulfonic acids. Proton states in the H-bond have been monitored using vibrational CyPy ring modes in FTIR spectra. The stabilization of the CyPy/TCA complex in its protonated form upon increasing polarity of the solvent has been evidenced. It was shown that formation of the CyPy/(TCA)(2) aggregates in the solutions favors the proton transfer process. An X-ray diffraction study has been performed on a single 1 : 2 co-crystal of pyridine/3,5-dinitrobenzoic acid. The H-bond motif found in this system exhibits the same connectivity by strong hydrogen bonds N-H(+)[dot dot dot]O(-) and O-H[dot dot dot]O as that in the CyPy/(TCA)(2) complex predicted by DFT calculation. Certain discrepancies are observed in C-H[dot dot dot]O connectivity only. The networks of H-bonds in both assemblies differ from those usually pictured for 1 : 2 base/carboxylic acid complexes in the literature.     

Effect of sacrificial electron donors on hydrogen generation over visible light–irradiated nonmetal-doped TiO2 photocatalysts

Hydrogen generation over carbon-, nitrogen- and sulfur-doped TiO₂ semiconductor photocatalysts (represented as C–TiO₂, N–TiO₂ and S–TiO₂, respectively) under visible light irradiation has been achieved using various sacrificial electron donors, namely triethanolamine, diethanolamine, monoethanolamine, triethylamine, MeOH, EtOH, EDTA, l-ascorbic acid and phenol. The highest initial rate of H₂ production was found to be in the range 1,000–2,200 μmol/g/h at ambient conditions when triethanolamine was used as sacrificial electron donor. The efficacy of hydrogen production over these photocatalysts depends strongly on the nature of the sacrificial electron donor and decreases in the following order: C–TiO₂ > S–TiO₂ > N–TiO₂. The results of the present studies suggest that the rate of H₂ production is not simply governed by the reduction potential of the sacrificial electron donor but also by the kinetic barrier of the electron transfer process.     

Photoinduced water oxidation by a tetraruthenium polyoxometalate catalyst: ion-pairing and primary processes with Ru(bpy)3(2+) photosensitizer

The tetraruthenium polyoxometalate [Ru(4)(μ-O)(4)(μ-OH)(2)(H(2)O)(4)(γ-SiW(10)O(36))(2)](10-) (1) behaves as a very efficient water oxidation catalyst in photocatalytic cycles using Ru(bpy)(3)(2+) as sensitizer and persulfate as sacrificial oxidant. Two interrelated issues relevant to this behavior have been examined in detail: (i) the effects of ion pairing between the polyanionic catalyst and the cationic Ru(bpy)(3)(2+) sensitizer, and (ii) the kinetics of hole transfer from the oxidized sensitizer to the catalyst. Complementary charge interactions in aqueous solution leads to an efficient static quenching of the Ru(bpy)(3)(2+) excited state. The quenching takes place in ion-paired species with an average 1:Ru(bpy)(3)(2+) stoichiometry of 1:4. It occurs by very fast (ca. 2 ps) electron transfer from the excited photosensitizer to the catalyst followed by fast (15-150 ps) charge recombination (reversible oxidative quenching mechanism). This process competes appreciably with the primary photoreaction of the excited sensitizer with the sacrificial oxidant, even in high ionic strength media. The Ru(bpy)(3)(3+) generated by photoreaction of the excited sensitizer with the sacrificial oxidant undergoes primary bimolecular hole scavenging by 1 at a remarkably high rate (3.6 ± 0.1 × 10(9) M(-1) s(-1)), emphasizing the kinetic advantages of this molecular species over, e.g., colloidal oxide particles as water oxidation catalysts. The kinetics of the subsequent steps and final oxygen evolution process involved in the full photocatalytic cycle are not known in detail. An indirect indication that all these processes are relatively fast, however, is provided by the flash photolysis experiments, where a single molecule of 1 is shown to undergo, in 40 ms, ca. 45 turnovers in Ru(bpy)(3)(3+) reduction. With the assumption that one molecule of oxygen released after four hole-scavenging events, this translates into a very high average turnover frequency (280 s(-1)) for oxygen production.     

Photocatalytic hydrogen production

The efficient storage of solar energy in chemical fuels, such as hydrogen, is essential for the large-scale utilisation of solar energy systems. Recent advances in the photocatalytic production of H(2) are highlighted. Two general approaches for the photocatalytic hydrogen generation by homogeneous catalysts are considered: HX (X = Cl, Br) splitting involving both proton reduction and halide oxidation via an inner-sphere mechanism with a single-component catalyst; and sensitized H(2) production, employing sacrificial electron donors to regenerate the active catalyst. Future directions and challenges in photocatalytic H(2) generation are enumerated.     

Mechanism of four-electron reduction of dioxygen to water by ferrocene derivatives in the presence of perchloric acid in benzonitrile, catalyzed by cofacial dicobalt porphyrins

The selective two-electron reduction of dioxygen occurs in the case of a monocobalt porphyrin [Co(OEP)], whereas the selective four-electron reduction of dioxygen occurs in the case of a cofacial dicobalt porphyrin [Co(2)(DPX)]. The other cofacial dicobalt porphyrins [Co(2)(DPA), Co(2)(DPB), and Co(2)(DPD)] also catalyze the two-electron reduction of dioxygen, but the four-electron reduction is not as efficient as in the case of Co(2)(DPX). The micro-superoxo species of cofacial dicobalt porphyrins were produced by the reactions of cofacial dicobalt(II) porphyrins with dioxygen in the presence of a bulky base and the subsequent one-electron oxidation of the resulting micro-peroxo species by iodine. The superhyperfine structure due to two equivalent cobalt nuclei was observed at room temperature in the ESR spectra of the micro-superoxo species. The superhyperfine coupling constant of the micro-superoxo species of Co(2)(DPX) is the largest among those of cofacial dicobalt porphyrins. This indicates that the efficient catalysis by Co(2)(DPX) for the four-electron reduction of dioxygen by Fe(C(5)H(4)Me)(2) results from the strong binding of the reduced oxygen with Co(2)(DPX) which has a subtle distance between two cobalt nuclei for the oxygen binding. Mechanisms of the catalytic two-electron and four-electron reduction of dioxygen by ferrocene derivatives will be discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The turnover-determining step in the Co(OEP)-catalyzed two-electron reduction of dioxygen is an electron transfer from ferrocene derivatives to Co(OEP)(+), whereas the turnover-determining step in the Co(2)(DPX)-catalyzed four-electron reduction of dioxygen changes from the electron transfer to the O-O bond cleavage of the peroxo species of Co(2)(DPX), depending on the electron donor ability of ferrocene derivatives.     

Visible-light photoredox catalysis: dehalogenation of vicinal dibromo-, α-halo-, and α,α-dibromocarbonyl compounds

vic-Dibromo-, α-halo-, or α,α-dibromocarbonyl compounds can be efficiently dehalogenated using catalytic tris(2,2'-bipyridyl)ruthenium dichloride (Ru(bpy)(3)Cl(2)) in combination with 1,5-dimethoxynaphthalene (DMN) and ascorbate as sacrificial electron donor. For this process, a visible light promoted photocatalytic cycle is proposed that involves the reduction of carbon halogen bonds via free radical intermediates.     

Remote site photosubstitution in metalloporphyrin-rhenium tricarbonylbipyridine assemblies: photo-reactions of molecules with very short lived excited states

The synthesis is reported of a series of metalloporphyrins (and the corresponding free-base porphyrin), mono-meso-substituted with a bipyridyl group via an amide link at the 4-position of one phenyl group: [Re(CO)(3)(Pic)Bpy-MTPP][OTf], where M = Mg, Zn, Pd or 2H, Pic = 3-picoline, Bpy = 2,2'-bipyridine, TPP = tetraphenylporphyrin. The photochemical reactions of the assemblies with the sacrificial electron donor triethylamine have been investigated by IR spectroscopy and compared to the behaviour of analogues of the type Bpy-MTPP without rhenium. Selective long-wavelength irradiation of the metalloporphyrin unit in the presence of excess picoline leads to reduction at the rhenium bipyridine centre. In the absence of 3-picoline, the latter is not reduced, but substituted by added halide or by the THF solvent. Mechanistic analysis highlights the differences between the zinc and magnesium chelate on the one hand and the palladium porphyrin on the other. The free-base assembly, [Re(CO)(3)(Pic)Bpy-H(2)TPP][OTf] is unreactive. The zinc and magnesium porphyrin assemblies initially coordinate Et(3)N before undergoing photo-induced inner-sphere electron transfer from the triethylamine to form a charge-shifted excited state of the assembly. In contrast, the palladium-based dyad reacts via outer-sphere reductive quenching of a porphyrin-based excited state. The substitution products are postulated to form by a mechanism involving an electron-transfer chain.     

Photocatalytic H2 evolution from neutral water with a molecular cobalt catalyst on a dye-sensitised TiO2 nanoparticle

We report on a self-assembled system comprising of a molecular H(2) production cobalt catalyst attached on a ruthenium dye-sensitised TiO(2) nanoparticle. Visible light irradiation of the dispersed nanoparticles in the presence of the sacrificial electron donor triethanolamine produces H(2) photocatalytically in pH neutral water and at room temperature.     

Ionic liquids as a novel medium for photochemical reactions. Ru(bpy)(3)2+/ viologen in imidazolium ionic liquid as a photocatalytic system mimicking the oxido-reductase enzyme

Ionic liquids are suitable media which stabilize charged intermediates favoring those mechanisms that occur through charge separation. We have used ionic liquids to develop a photocatalytic system to perform the reduction of a carbonyl group to alcohol, thus mimicking the behavior of the reductase enzymes. The photochemical cycle is based on the well-known electron transfer from the Ru(bpy)(3)2+ complex in its excited state, acting as electron donor to MV2+, which acts as electron acceptor. The initial electron transfer process can be promoted upon selective Ru(bpy)(3)2+ excitation by visible light. By means of laser flash photolysis we have provided evidence of the nature and lifetimes of the intermediates involved in the photocatalytic system. Thus, the initial electron transfer between Ru(bpy)(3)2+ triplets and viologen MV2+ forms the MV*+ radical cation, which upon accepting an H* atom from a suitable hydrogen atom donor, forms the corresponding dihydropyridine MVH+ reducing agent.     

Efficient photoreduction process of [Ru3(mu3-O)(mu-CH3CO2)6L3]+ by electron-mediation via the viologen dication by excitation of a zinc porphyrin

Photoinduced electron-transfer and electron-mediation processes from the excited triplet state of zinc tetraphenylporphyrin (3ZnTPP) to the hexyl viologen dication (HV2+) in the presence of oxo-acetato-bridged triruthenium clusters, [Ru3(mu3-O)(mu-CH3CO2)6L3]+, have been revealed by the transient absorption spectra in the visible and near-IR regions. By the nanosecond laser-flash photolysis of ZnTPP in the presence of HV2+ and [Ru3(mu3-O)(mu-CH3CO2)6L3]+, the transient absorption bands of the radical cation of ZnTPP (ZnTPP*+) and the reduced viologen (HV*+) were initially observed with the concomitant decay of 3ZnTPP, after which an extra electron of HV*+ mediates to [Ru3(mu3-O)(mu-CH3CO2)6L3]+, efficiently generating [Ru3(mu3-O)(mu-CH3CO2)6L3]0 with high potential. Although back-electron transfer took place between ZnTPP*+ and [Ru3(mu3-O)(mu-CH3CO2)6L3]0 in the diffusion-controlled limit, [Ru3(mu3-O)(mu-CH3CO2)6L3]0 accumulates at a steady concentration upon further addition of 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial donor to re-produce ZnTPP from ZnTPP*+. Therefore, we established a novel system to accumulate [Ru3(mu3-O)(mu-CH3CO2)6L3]0 as an electron pool by the excitation of ZnTPP as photosensitizing electron donor in the presence of HV2+ and BNAH as an electron-mediating reagent and sacrificial donor, respectively. With the increase in the electron-withdrawing abilities of the ligands, the final yields of [Ru3(mu3-O)(mu-CH3CO2)6L3]0 increased.     

Photoinduced electron transfer in platinum(II) terpyridyl acetylide chromophores: reductive and oxidative quenching and hydrogen production

A series of luminescent platinum(II) terpyridyl acetylide complexes, ([Pt(tpy)(CCPh)]ClO4 (1) and [Pt(ttpy)(CC-p-C6H4R)]ClO4, where tpy=terpyridine, ttpy=4'-p-tolylterpyridine, R=H, Cl, Me) (2-4) were studied with regard to excited-state quenching by dialkylated bipyridinium cations as electron acceptors and triethanolamine (TEOA) as an electron donor and the photogeneration of hydrogen from systems containing the chromophore, the dialkylated bipyridinium cations, TEOA, and colloidal Pt as a catalyst. The dialkylated bipyridinium cations include methyl viologen (MV2+) and a series of diquats prepared from 2,2'-bipyridine or 4,4'-dimethyl-2,2'-bipyridine. The quenching rates for the diquats for one of the chromophores (2) are close to the diffusion-controlled limit. The most effective electron acceptor and relay for hydrogen evolution has been found to be 4,4'-dimethyl-1,1'-trimethylene-2,2'-bipyridinium (DQ4) which on photoreduction by the chromohore provides the strongest reducing agent of the diquats studied. The rate of hydrogen evolution depends in a complex way on the concentration of the bipyridinium electron relay, increasing with concentration at low concentrations and then decreasing at high concentrations. The rate of H2 photogeneration also increases with TEOA concentration at low values and eventually reaches a plateau. The most effective system examined to date consists of the chromophore 2 (2.2x10(-5) M), DQ4 (3.1x10(-4) M), TEOA (2.7x10(-2) M), and Pt colloid (6.0x10(-5) M), and has produced 800 turnovers of H2 (67% yield based on TEOA as sacrificial electron donor) after 20 h of photolysis with lambda>410 nm.