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PERSPECTIVE
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Recent progress in the development of bimetallic photocatalysts for hydrogen generation Yvonne Halpin,a Mary T. Pryce,a Sven Rau,b Danilo Dinic and Johannes G. Vos*a
Received 23rd August 2013, Accepted 26th September 2013 DOI: 10.1039/c3dt52319e www.rsc.org/dalton
In this contribution recent developments in the design and application of bimetallic photocatalysts for the generation of hydrogen via intramolecular processes are assessed. The basic concepts of such assemblies are discussed together with an overview of the factors and molecular issues that affect their potential as photocatalysts. Issues that so far have limited progress are discussed and suggestions for future directions are made.
Introduction Man-made climate change is now recognised as one of the biggest challenges facing mankind. The continuing use of fossil fuels is leading to ever increasing levels of CO2, recently passing the 400 ppm level. The realisation of this has led to the establishment of the UN driven Framework Convention on Climate Change and the Kyoto Protocol in 1997. Since then EU and G8 leaders agreed in 2009 that CO2 emissions must be cut by 80% by 2050. This will require, for example, 95% decarbonisation of the road transport sector and fuel cell vehicles based on hydrogen will be required as part of the transport mix. a
SRC for Solar Energy Conversion, School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland. E-mail: [email protected] b Anorganische Chemie I, Universität Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany c Department of Chemistry, University of Rome “La Sapienza”, Rome, Italy
Yvonne Halpin
Yvonne Halpin obtained a Bachelor of Science degree in Chemical and Pharmaceutical Science in 2006 and in 2010 a PhD from Dublin City University on the electrochemical and spectroscopic analysis of a series of iron, ruthenium and osmium complexes with applications in hydrogen production and molecular electronics. She is at present working as a Postdoctoral Fellow on the development of commercially viable carbon neutral methods for hydrogen generation.
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Therefore, the volume of hydrogen required will increase substantially. This approach has led to the concept of the “hydrogen economy” where the use of energy will not result in CO2 emissions as is the case today. At present carbon neutral hydrogen generation is too expensive however, and in a recent study “A portfolio of power-trains for Europe: a fact based analysis”,1 it is estimated that fuel-cell systems costs will decrease by 90% by 2020. Currently most hydrogen is produced from fossil fuels by Steam Methane Reforming and Coal Gasification.2 The process produces large amounts of CO2 and to make this method environmentally friendly carbon capture is required, which will in turn increase the production costs of hydrogen. At present a small amount (4%) is produced by electrolysis of water but its production costs are several times that of the steam reforming method as expensive metals such as platinum are used. Nevertheless the use of hydrogen in transport applications has been championed by the EC driven “Fuel Cells and Hydrogen Joint Undertaking Task Force”,
Mary Pryce obtained a BSc in Analytical Science from DCU in 1991. A PhD (1991–1994) was subsequently obtained in Organometallic Photochemistry. Post doctoral studies were carried out during 1995 and 1996 at the University of Milan in the area of asymmetric synthesis. Joined the School of Chemical Sciences at DCU in 1997, and is currently employed as a Senior Lecturer. Current research projects focus Mary T. Pryce on; Hydrogen generation, CO2 reduction, CO-releasing molecules, porphyrin chemistry, and organometallic photochemistry.
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Perspective mostly driven by industry, and in California, Germany, Italy, Japan and Australia hydrogen pilot projects have been carried out successfully.3 However, in order to develop a hydrogen based economy, novel environmentally friendly carbon neutral methods for hydrogen generation are urgently required. Many different approaches are advocated as options for energy sources for the future, ranging from nuclear, to wind and wave power, all with concomitant advantages and disadvantages. However, in terms of the potential amount of energy that can be delivered solar energy stands apart, as in one hour enough solar energy hits the earth’s surface to supply the amount consumed through human activity for one year. Although only a small fraction of this energy can be captured, that small fraction represents an important part of our future energy requirements.4 Existing technologies such as photovoltaic cells are well established for energy production, providing solutions at a local level. The prevalent crystalline silicon based solar cells are being augmented by the Grätzel type cell, developed two decades ago. The design of which is based upon a molecular approach where a light absorbing dye is immobilised on a nanocrystalline TiO2 electrode surface to generate a light driven electrical current.5 However, the conversion of light into electrical current has an inherent disadvantage, as is the case for wind and wave technologies in that the energy produced has to be used instantly. Thus the next step in the development of an economy that can rely on sustainable energy production is the development of a technology that can store solar energy. A potential method to achieve this is to mimic nature6 by converting light energy directly into chemical energy, which can be stored. This realisation has led to the development of novel technologies, capable of producing “solar fuels” such as hydrogen from water or CO and formic acid, by solar driven CO2 reduction.
Sven Rau studied chemistry at the Friedrich-Schiller-University of Jena and Dublin City University. He obtained his Diploma in chemistry in 1997 and his PhD in 2000. Following his habilitation in 2007, he moved to the Friedrich-Alexander-University Erlangen-Nuremberg in 2008. Since 2011, he leads the institute for materials and catalysis at the university Ulm. His research interests are focused on the synSven Rau thesis and characterization of photoredoxactive metal complexes their applications as photochemical molecular devices in light driven catalytic conversions, interaction with surfaces and living matter.
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Dalton Transactions Semiconductors, such as TiO2 and related non-molecular nanomaterials are known photocatalysts for the reduction of CO2 7 and the generation of hydrogen8 from water. However, their large band gap does not allow for efficient light absorption in the visible part of the solar spectrum. Since the late 1970s there has been increasing interest in the use of molecular components as solar dyes absorbing in the visible part of the spectrum. In these studies the intermolecular approach was taken where mixtures of a light absorbing moiety and a catalytic centre are utilised to produce solar fuels. In these systems the photosensitiser is irradiated and electron transfer from the excited state of the sensitiser to the catalytic centre occurs. Sacrificial electron donors such as triethylamine (TEA) are added to regenerate the dye (see Fig. 1).9 Extensive studies have shown that the catalytic activity of these systems is often limited by the stability of the molecular components.10 Another drawback in these intermolecular systems is the efficiency, which is inevitably limited by diffusion processes required to bring the sensitiser and the catalytic centre close enough to ensure the occurrence of the electron transfer step. More recently the intramolecular option has been considered and this has led to the development of a wide range of bimetallic complexes that act as intramolecular photocatalytic centres as shown in Fig. 1 and 2. Since 2006 investigations
Fig. 1 Intermolecular versus intramolecular photocatalytic hydrogen production from water. PS = photosensitiser, S = sacrificial agent.
Danilo Dini obtained his PhD in Materials Science at the University of Rome “La Sapienza” (Italy). He held postdoctoral positions in the Institut fur Physikaliche Chemie of Max-PlanckInstitut in Berlin (Germany) with Prof. G. Ertl, and in the University of Tuebingen (Germany) with Prof. M. Hanack. After a period as a researcher at Dublin City University (Ireland) in the group of Prof. J. G. Vos, he joined Danilo Dini the University of Rome “La Sapienza” (Italy) holding a position as Assistant Professor at the Department of Chemistry. His main research interests are photoelectrochemistry, photophysics of conjugated molecular materials and solar energy conversion devices. He has authored 90 peerreviewed articles in international journals.
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Perspective compounds discussed in this overview are shown in Fig. 3. Another important factor to consider is whether the hydrogen is generated by the intramolecular assembly or in an intermolecular manner via decomposition of the photocatalysts into separate components.
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Understanding and optimising photocatalytic behaviour
Fig. 2 Basic mechanism of photocatalytic hydrogen production with 1 as photocatalyst.
focussed on both inter- and intramolecular assemblies have attracted much attention and the results of these studies have been documented in a number of recent reviews.11 The aim of this perspective is to assess progress in intramolecular photocatalysis since our Dalton Transactions review in 2007,12 and focus on issues which should be considered when developing intramolecular photocatalytic assemblies for solar fuel production. Examples of important issues are the design and synthesis of such intramolecular assemblies, since the electronic properties controlling vectorial electron transfer from the sensitiser to the catalyst are central components. This means that the molecular components in the assembly should be chosen with care. The importance of the relationship between the bridging and peripheral ligands and the excitation wavelength will be discussed. The composition of the photocatalytic solution and the involvement of the sacrificial agents are also included. A number of results obtained in our laboratories are discussed and compared with those reported in the literature. Where appropriate, comparisons with intermolecular systems will be made. The structures of the
Han Vos is Emeritus Professor of Inorganic Chemistry at Dublin City University. His research efforts are in the design of supramolecular systems containing transition metal complexes. Of particular interest are the synthesis, photophysical and electrochemical properties of dinuclear and polymeric ruthenium and osmium polypyridyl complexes both in solution and when immobilised on solid subJohannes G. Vos strates. The applications of such compounds as optical and electrochemical sensors and in molecular electronics are investigated. At present his research is directed towards the development of novel environmentally friendly sustainable energy sources using photocatalytic and electrocatalytic pathways.
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Experimental conditions. In recent years many photocatalytic studies have been carried out,11–14 with the aim of identifying the role played by each component in the intramolecular assembly and the parameters effecting optimisation of the photocatalytic performance. The light source used in these experiments is normally chosen to correspond to the lowest energy band in the UV/Vis spectrum of the sensitiser. The amount of hydrogen produced is usually reported in turnover numbers (TON), or as a turnover frequency (TOF). For the examples discussed from our group photocatalytic experiments were carried out in acetonitrile containing a photocatalyst concentration of 4 × 10−5 M in the presence of 2.15 M TEA as the sacrificial agent and between 0–15% water. The amount of hydrogen obtained was determined by gas chromatography and is the average of three independent measurements. Design features. As outlined in our previous review12 the reasoning behind the investigation of dinuclear photocatalysts for the photogeneration of hydrogen, is the potential of such assemblies to engage in intra- rather than inter-molecular electron transfer processes. The compound [(tbbipy)2Ru(tpphz)PdCl2](PF6)2 (1), where tbbipy = 4,4′-di-tert-butyl-2,2′-bipyridine and tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2′″,3′″-j]phenazine (see Fig. 2) has been studied in great detail and represents the first example of a photocatalyst of this type13 and will be used as a reference. This compound typically produces hydrogen in acetonitrile containing 10% water with a TON of 210 over an 18 hour period using a 470 nm LED radiation source. To ensure that such an assembly is capable of producing hydrogen, efficient electron transfer has to occur from the light absorbing ruthenium centre to the palladium moiety that acts as the catalytic centre. As outlined above, a sacrificial agent, e.g. TEA, is required to re-reduce the light absorbing Ru centre and allows the next excitation to take place. The overall process can be described as shown in Scheme 1. It is important to realise that sacrificial agents such as TEA can create problems and may participate in the photocatalytic system since they decompose after the initial electron transfer and produce both protons and electrons in dark reactions as shown in Scheme 2 below. During the decomposition process two protons and one further electron are produced. The formation of these reactive species may be at least partially responsible for the decomposition of the photocatalytic assemblies. The intramolecular approach discussed here assumes that the ability of the processes outlined in Scheme 1 and Fig. 2 to yield significant amounts of hydrogen will depend on the rate of electron transfer from the light absorbing centre to the catalytic Pd moiety. The approach taken to achieve efficient vectorial electron transfer is that the bridging ligand, tpphz (B),
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Perspective
Dalton Transactions
Fig. 3 Structures of photocatalysts 1–15 discussed in the text. Precursors are the photosensitisers without the catalytic centres attached and when cited these are indicated with “Xa”, where X is 1–15. TON values are after 18 hours unless otherwise stated. * See text.
Scheme 1 Proposed pathway for photocatalytic hydrogen generation. B = bridging ligand, S = solvent.
Scheme 2 sensitiser.
Decomposition pathway of TEA after electron donation to the
has a lower π* energy level than the peripheral ligand (tbbipy in this case) which will generate a downhill energy gradient from the excited state located on the ruthenium centre to the catalytic Pd centre. For this reason a phenazine based
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bridging ligand was introduced. However, when taking this approach one needs to take into account a fast back reaction. To obtain further information concerning the electronic properties of compound 1 detailed investigations have been carried out on the photophysical properties of this compound, including time resolved resonance Raman and picosecond ( ps) transient absorption studies.14 These measurements were carried out in CH3CN and in the absence of a sacrificial agent in order to evaluate the photophysical properties of the assembly under non-catalytic conditions. An overview of the dynamics of electron-excitation transfer processes as evaluated by our study is given in Scheme 3. This scheme shows a comparison of the photophysical behaviour of the mononuclear precursor complex 1a and the dinuclear photocatalyst 1. In both compounds the initially populated singlet metal-to-ligand charge-transfer excited states (1MLCT) are based on the tbbipy/phenanthroline moieties. This first excited state successively undergoes relaxation to a 3 MLCT state, which is located on the phenanthroline part of tpphz bridging ligand. For both compounds the formation of the phenanthroline-centred 3MLCT state from the initially populated mixture of singlet states takes place within 1.2 ps for the precursor 1a and 0.8 ps for compound 1. However, there is a considerable difference in the decay of this triplet excited state. The decay time for 1a is 240 ps, while for 1 a decay time of 5 ps is observed. This decay is assigned to a transition from the equilibrated phenanthroline-centred 3MLCT to
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Dalton Transactions
Scheme 3 Electron transfer dynamics of 1 and its precursor complex 1a. Localisation of the photoexcited electron is illustrated by shaded areas. Reproduced with permission from ref. 14.
an intra-ligand-charge-transfer (ILCT) state localised on the phenazine unit. These data show that the Pd(II) centre significantly influences the electronic structure of the bridging ligand by creating a significantly higher driving force for the formation of the phenazine-centred ILCT state. For 1 this state further decays over 310 ps giving rise to the population of a long-lived ligand to metal charge transfer (LMCT) state on the Pd(II) centre, the decay of which was not observed on the time range accessible. The presence of the excited electron on the phenazine part of the bridging ligand is confirmed by electron paramagnetic resonance spectroscopy. The final reduction of the Pd(II) centre was expected to coincide with the dissociation of a Cl− anion. This was supported by the observation that the catalytic activity of 1 is completely inhibited by the addition of Cl− to the catalytic mixture during catalysis and by DFT calculations.13 Further confirmation for the interaction of the Pd centre with the excited state of the bridging ligand is the fact that while for the precursor complex emission is observed at 650 nm with a lifetime of 240 ns, for 1 a relatively weaker emission at the same wavelength with a lifetime of 10 vs. 100 ns) could represent another contributing factor.26 Similar studies involving a change in bridging ligands have been carried out by Rau and co-workers.27 Structural
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Perspective
Fig. 5 Time-dependence of the TON for H2 evolution in the presence of 10% water for compound 9 (blue) and its precursor 9a with [Pd(CH3CN)2Cl2] in a (1 : 1) ratio (red).
modifications of the bridging ligand of 1 were considered and the photocatalytic properties of compounds 2 and 3 were investigated. For 2, the phenanthroline moiety was brominated and as a result a TON value of 69 was obtained after 18 h of irradiation. For compound 3 the pyrazine ring of tpphz was replaced by an acridine moiety and a value of 139 was found. This compares with a value of 210 for compound 1 under the same conditions (acetonitrile with 10% water and 2 M TEA). This variation in TONs shows that even relatively small structural modifications of the bridging ligand can result in a considerable difference in photocatalytic behaviour. The nature of the catalytic centre. So far the discussion has centred on catalytic centres based on Pd or Pt. In this section some alternative options are considered. Brewer and coworkers have investigated compounds 11 and 12 (see Fig. 3) using an assembly able to act as a 2-electron collector. The trinuclear species consists of two ruthenium centres as the light absorbers and a rhodium electron collector/catalytic centre.28 In their studies they investigated the effect of changing the nature of the halide on the Rh catalytic centre. The photophysical properties of compounds 11 and 12 indicate that the highest occupied molecular orbital (HOMO) is Ru based and dπ in nature, while the lowest unoccupied molecular orbital having dσ* character is located on the Rh centred bridge of the assembly. This orientation of the molecular orbitals results in a low lying Ru → Rh metal-to-metal charge transfer (3MMCT) excited state and the Rh centre takes on the role of electron acceptor in the molecule. Following electron transfer from Ru to the Rh metal centre, Rh+ is formed and it is this species which is responsible for hydrogen generation, halide loss being central to the catalytic process. The results obtained show that replacement of Cl− (11) with Br− (12) did not result in significant photocatalytic activity as the TON value only increased from 28 for the chloride to 38 for the bromide containing compound after 4 hours of irradiation. However, in later studies they observed greatly improved TONs for the
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Perspective bromide containing compound after changing the catalytic conditions. By changing the solvent from acetonitrile to DMF, increasing the headspace, purging the system of hydrogen and by increasing the concentration of the sacrificial agents the TONs for hydrogen formation increased to 280 after 19.5 h and a value of 820 was found after 50 h.28 Artero and co-workers have carried out detailed investigations on the potential of Co centres as catalysts for hydrogen generation.21 Such an approach is important since one of the drivers in the design of economically viable photocatalytic systems for hydrogen generation from solar power is the cost and availability of the molecular components. The authors critically examined the effect of structural variation on the catalytic efficiency for hydrogen generation by systematic manipulation of the bridging ligand and the nature of the photosensitiser. In these studies they varied the molecular compositions of the bridge as well at the peripheral ligands and investigated compounds 13–15 (see Fig. 3). The Ru–Co complexes exhibit catalytic activity for the photochemical production of hydrogen. In such complexes, the pyridine ring of the bridging ligand, either (4-pyridine)oxazolo-[4,5-f ]phenanthroline (14) or (4-pyridine)imidazolo-[4,5-f ]phenanthroline) (13) is axially coordinated to a cobaloxime centre. A TON of 56 after 4 h (and 103 after 15 h when a 350 nm UV cut-off filter was used) was reported for 14 in acetone using Et3NHBF4 as the proton source. Altering the structure of the bridge in 14, by the replacement of the oxazolo moiety with an imidazolo unit to form compound 13, led to a TON of 104 after 4 h under the same conditions. Thus the nature of the bridge influences directly the capacity of the photocatalyst for generating hydrogen. This was further confirmed by examining the photocatalytic activity of individual Ru and cobaloxime units, which demonstrated no hydrogen was detected in the absence of the bridging ligand. Therefore, the bridging ligand facilitates electron transfer from the Ru photoactive centre to the catalytic cobaloxime centre either through bonds or by an outer-sphere mechanism. An investigation of the iridium analogue (15) of the Ru compound (13), under the same catalytic conditions yielded an enhanced hydrogen production, with a TON of 210 after 15 h. This study elegantly demonstrates how first row transition metals, when coupled with a suitable light harvesting unit, can produce photocatalytic devices that are economically viable for use in the photocatalytic generation of molecular hydrogen. The Ir–Co assembly 4 was considered by Sakai et al.20 Using visible light irradiation for 8 hours a TON value of 20 was reported for this compound. Based on NMR and electronic absorption spectroscopy, the authors suggested that compound 4 forms by self-assembly (the Ir/Co ratio used was 3 : 1). The TON values are given with respect to the Co concentration. This value dropped to 10 for the corresponding multicomponent system using the model compounds [Ir( ppy)2(bipy)]+, and [Co(bipy)3]3+. It is noteworthy that the bridge utilised is not conjugated and thus is not expected to promote intramolecular electron transfer. Is the nature of the photocatalytic process intra- or intermolecular? There has been concern in the literature whether
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Dalton Transactions the intramolecular systems, in particular those containing Pd moieties as catalytic centres, are acting as supramolecular assemblies or that hydrogen is generated by colloids formed after the decomposition of the intramolecular assembly.29,30 In the latter case the process would be inter- rather than intramolecular in nature. To investigate this issue we have carried out a number of comparative experiments with the ruthenium species 8a vs. 8 and 9a vs. 9 (see Fig. 3).26 The difference between the amounts of hydrogen produced by the intra- and intermolecular experiments carried out for the ruthenium based compound 9 and a mixture of its precursor 9a are 125 and 70, respectively, following 8 hours of irradiation (see Fig. 5). No induction period was observed and hydrogen production accelerates reaching the highest productivity after ca. 3.5 h for compound 9, and ca. 5.5 h for the mixture of its precursor in the presence of [Pd(acetonitrile)2Cl2]. The time dependent behaviour is very different with the intramolecular compound having a higher efficiency with respect to the bimolecular mixture, thus indicating that the intramolecular mechanism is the most efficient route for hydrogen generation, and that decomposition leading to colloid formation, does not represent an important contributing factor for producing hydrogen. Artero and co-workers, using cobalt catalysts, also observed higher TONs with the intramolecular mechanism with respect to the corresponding intermolecular route.21 For compound 8 a different behaviour was observed. For the intramolecular compound, a TON of 400 is observed after 18 h of irradiation, whereas the bimolecular mixture gave a similar value of 450 under the same conditions.24 These results suggest that in both cases intermolecular photocatalysis is observed or that the dinuclear compound 8 is formed in situ. Further studies are required to better understand the behaviour observed with this bridging ligand. Intermolecular experiments were also carried out using the iridium compound 5a.16 At 350 nm the 1 : 1 mixtures of 5a with [Pt(CH3CN)2Cl2], and 5a with [Pd(CH3CN)2Cl2] yielded at 350 nm TON values of 117 and 124 respectively, while at 470 nm values of 166 and 245 were obtained. The values obtained at 350 nm are higher than those obtained with the photocatalyst 5 and 6. This could be due to the absence of the Pd or Pt based MLCT states present in the bimetallic compounds that limit the photocatalytic processes at that wavelength of analysis. This observation is a strong indication that at 350 nm the “interand intramolecular” reactions follow a different mechanism, where for the intramolecular case using the bimetallic compounds 5 and 6 the formation of hydrogen is reduced by the presence of Pd or Pt based MLCT states. These states are absent in the intermolecular case. The values obtained for the intermolecular approach at 470 nm are similar to those obtained for compound 5 and 6 that display values of 308 and 254, respectively. From these values no clear pattern can be deduced about the mechanism that controls hydrogen formation at 470 nm.16 Kinetic considerations. The search for effective intramolecular photocatalytic devices has mainly been driven by the argument that electron transfer between the photosensitiser and
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Dalton Transactions the coordinatively bound catalytic centre will be faster with respect to what is observed in the intermolecular case.12,13 As a result of that observation higher efficiencies for hydrogen production are expected via the intramolecular route. In the intermolecular case physical diffusion is required and electron transfer between the photoexcited photosensitiser PS* and the catalyst requires high frequency of reactive collisions.31 The results discussed above show that in most cases the intramolecular photocatalytic process leads to TON values higher than those determined in the analogous intermolecular investigations. However, the improvement is generally less than an order of magnitude. Kinetic measurements have the potential to offer critical insights into the overall process in photocatalytic hydrogen generation for both inter- and intramolecular approaches. To date only a limited number of kinetic studies have been reported for the intramolecular case. To assess the importance of the electron transfer rates between the photosensitiser and the catalyst in the overall photocatalytic process, some typical examples will be considered. The discussion will mainly be concerned with electron transfer from the photosensitiser to the catalytic centre. A good example of a kinetic study on intermolecular systems was carried out by Hamm and co-workers who demonstrated how time resolved IR spectroscopy can be used to determine electron transfer and the quenching rate for a number of rhenium carbonyl photosensitisers with Co(III) glyoxime as catalyst.32,33 The reaction pathways and associated rate data obtained in DMF are outlined in Scheme 4,34 where [Re(NCS)(CO)3bipy] acts as the PS, [Co(dmgH)2] (dmgH2 = dimethylglyoxime) as the catalytic centre, while TEOA acts as the sacrificial agent and [HTEOA][BF4] as the hydrogen donor. Following excitation of the rhenium complex, a 3MLCT based excited state is generated, which undergoes reductive quenching in the presence of TEOA with kq = 9.7 × 107 M−1 s−1. This step is followed by electron transfer (k1 = 1.3 × 108 M−1 s−1) to the cobalt centre (CoII → CoI). The lifetime of the reduced species, PS− varies from 8.9 to 2.7 µs when the concentration of the CoII catalyst is increased from 1 to 3 mM. Following this electron transfer step the cobalt centre is protonated (CoI →
Scheme 4
Perspective CoIII_H). The formation of this cobalt hydride intermediate leads, via a bimolecular reaction, to the generation of H2. This latter reaction is of second order in cobalt (k3 = 25 M−1 s−1) and represents the rate limiting step. Interestingly, this rate is similar to that reported by Espenson et al. who investigated the generation of hydrogen using a hydridocobaloxime.35 They observed two different catalytic processes with variable concentration of the reactants. At high H+ and low cobaloxime concentrations a pseudo first order rate constant of 6 × 10−3 s−1 was observed while at higher concentrations for the hydridocobaloxime a second order process with a rate of 2 × 102 M−1 s−1 was observed. For the rhenium-based system time-resolved infrared spectroscopy confirmed that photo-oxidised TEOA generates a second reducing equivalent (see Scheme 4) which can be transferred to [Re(NCS)(CO)3bipy] (k2e− = 3.3 × 108 M−1 s−1) if no Co(dmgH)2 is present. TONs of 6000 for H2 production with respect to rhenium were observed, while TON values with respect to cobalt never exceeded 1000. This indicates decomposition of the Co(dmgH)2 catalyst. Similar kinetic data have been reported for other systems.36 The fact that the electron transfer process is second order indicates that this process is dependent also on the concentration of the catalyst. Assuming a concentration of 10−4 M for the catalytic centre, a first order rate constant for electron transfer between the two components of 104–105 s−1 is obtained. To the best of our knowledge, no kinetic studies on the photocatalytic generation of hydrogen by intramolecular assemblies have been reported and this discussion will concentrate on the data shown in Scheme 3. This scheme indicates the photophysical properties of 1 and 1a.14 The initial absorption of photons by the photoactive moiety of the multifunctional system leads to the formation of the charge-separated state necessary for the generation of hydrogen. Intramolecular electron transfer does not require thermal collisions between the PS* and the catalytic centre and is thus mainly controlled by the electronic properties of the bridging ligand, the photosensitiser and the catalytic centre. The kinetic data demonstrate sequential electron transfer from the excited photosensitiser (RuII) over the tpphz bridge to the
Reaction scheme for intermolecular photocatalytic hydrogen generation. Reproduced with permission from ref. 34.
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Perspective catalytic PdII centre within a few nanoseconds.14 In successive studies Rau and co-workers reported the photocatalytic properties for compound 3. Transient studies indicated that photoinduced intraligand charge-transfer dynamics takes place on the sub-ps timescale. This is in stark contrast to the mononuclear building block 3a where photoinduced chargetransfer from the photoactive Ru-unit to the catalytically active Pd-centre occurred on the sub-ns timescale.27b This brief and by no means exhaustive discussion indicates that for intramolecular photocatalytic systems the first order rate constant for electron transfer from the photosensitiser to the catalytic centre is in the order of 1010–1011 s−1 and is considerably faster than the first order rate constant observed for intermolecular systems where rates of 103–105 s−1 are commonly found. However, the turnover numbers observed for the intramolecular assemblies do not reflect these differences in electron transfer rates. It seems clear that other factors strongly affect the photocatalytic processes. These may include photophysical parameters such as the lifetime and quantum yield of the photoexcited PS* molecule (see below) and its collisional deactivation, the nature of the catalytic cycle (it may result in first or second order dependence and is generally slow for the intermolecular case), and quenching rates by sacrificial agents.36 A phenomenon specific of the intramolecular process which should also be considered in more detail is the possibility of back electron transfer from the photoinduced charge-separated state with the restoration of the ground state. Here, the nature of the bridging unit plays a crucial role since, ideally, it should promote electron transfer in a unidirectional fashion and exclusively to the catalytic moiety. For this reason non-conjugated bridging ligands have been adopted. It is however unclear whether this leads to increased efficiency.11i A clear example of the importance of the lifetime of the excited state is reported by McCormick et al.37 for the intermolecular hydrogen generation using modified rhodamine and a cobalt based glyoximate. These researchers obtained increased TON values when a modified rhodamine molecule was used as a photosensitiser and Co(III)-dimethylglyoximate as catalyst for H2 production. When in rhodamine the O atom was substituted with S or Se, turnover frequencies (TOF) were 0, 1700 and 5500 h−1, respectively. In this system the 3PS excited state is >1000 times longer than the singlet state (0.12–2.5 ns), and is sufficiently long lived to facilitate bimolecular electron transfer which is required for the completion of H2 formation. The ΦH2 yields are 0%, 12.2% and 32.8% respectively for these dyes and correlate directly with the quantum yields for singlet oxygen generation, ΦO2. The ΦO2 yields are 0.05, 0.17 to 0.67 for rhodamines with O, S and Se, respectively, as heteroatoms. The increase of TOF values with the increase of the atomic weight of the heteroatom of rhodamine is very impressive and determines the efficiency of the photosensitiser. These increases are explained by assuming that in case of the S and Se compounds the PS* is located on a triplet excited state, while for the O compound population of a singlet excited state is expected. It is worth noting that the lifetimes obtained vary
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Dalton Transactions between 1–2 ns. These values are still very short when compared to inorganic photosensitisers such as Ru and Ir complexes and it is somewhat surprising that such short lifetimes yield such high efficiencies. Previously the group reported increased TONs for hydrogen formation in a xanthene dye when H was replaced with a halide. This was attributed to enhanced intersystem crossing (ISC) promoted by the heavy atom effect.38
Concluding remarks In this overview the central assumption made in the design of intramolecular catalysts is the need for bridging ligands that allow interaction between the photosensitiser and the catalytic centre. This was also the starting point of our Frontier Paper in 2007. The use of non-conjugated bridging ligands has also been suggested on the basis that this would prevent a fast back reaction.11i This may indeed be the case for systems discussed here, but there is no direct evidence to support this. The results obtained show that interaction between the photosensitiser and the catalytic centre occurs and that this is a necessary step for the production of H2. For the bimetallic photocatalysts a reduced emission lifetime with respect to that observed for the precursors suggests that communication between the photosensitiser and the catalytic centre is taking place. Further evidence is obtained from changes in spectroscopic features such as absorption or emission spectra. It is also important to note that TONs show a clear dependence on the nature of the bridge as well as the peripheral ligands. These observations show that it is not only important to tune the energies of bridging ligands with those of the peripheral ligands but also optimise their electronic coupling. This creates a range of additional parameters that should be considered in the optimisation of the electronic interaction in the bimetallic compounds. An important observation is that for the intermolecular hydrogen generation by the rhenium compound in Scheme 4, the rate determining step is not associated with electron transfer between the sensitiser and the catalyst, but with hydrogen formation at the catalytic centre, which has a second order rate constant of 25 M−1 s−1. Therefore, for intramolecular systems like those considered here, the electron transfer processes involving the photosensitisers are not rate limiting. To allow further advances in this area there is an immediate need for detailed kinetic and photophysical studies on a wide range of assemblies to identify the rate determining step of the intramolecular mechanism. One of the crucial points is represented by the possibility that when certain dinuclear assemblies are used, hydrogen is not generated by the bimetallic compound but by uncoordinated Pd moieties in the colloids formed upon decomposition of the assembly. In a similar manner, the weak coordination strength of first row transition metals such as cobalt raises the question of how crucial is the de-coordination of such metal centres. In some of the compounds discussed above Pd colloids do form,
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Dalton Transactions but the results obtained thus far suggest that their formation is based upon decomposition of the compounds, at a stage where hydrogen formation decreases. Interestingly the behaviour of compound 8a indicates that with such a precursor the photocatalyst 8 is obtained by self-assembly of its mononuclear precursor and the Pd salt in solution.24 This observation together with loss of chloride for compound 1 are a reminder that often the compounds used in catalytic processes are often pre-catalysts rather than catalysts themselves. Further evidence for intramolecular behaviour is obtained from the analysis of the photocatalytic behaviour of compound 9. This compound shows that the time dependence and TON values for hydrogen formation are different in passing from the intra (9) to the intermolecular case (9a), with hydrogen obtained in the intramolecular case, indicating that hydrogen generation is intra- rather than intermolecular in nature.26 Further research on this issue is taking place at present but the results discussed above show that intramolecular photocatalysis is the main contributor to hydrogen generation. Another important issue is the stability of the compounds. In the majority of the photocatalytic systems investigated the stability of reactants is limited, irrespective of whether the inter- or intramolecular approach is taken, the exception being the Ir type compounds investigated by Bernhard and coworkers.17,18 It is not always clear what the driving force for the decomposition of the photocatalysts is, i.e. whether this is photochemical in nature or related to formation of radical cations by the sacrificial agent as shown in Scheme 2. The recent results obtained in our laboratories indicate that the sacrificial agents play an important role in the decomposition of the photocatalyst. It was found that while photocatalysts such as 1 are photostable in acetonitrile for 8 hours, while in the presence of sacrificial agents such as TEA the photostability is much reduced.39 Similar behaviour is observed for other systems the decomposition of which is accompanied by the formation of colloids. Apart from the loss of the catalysts from the bridging ligand one may also expect photochemically induced changes in the photosensitiser. These might include ligand loss or chemical rearrangements in the peripheral or the bridging ligand. Such processes are found in the photosynthesis modules in plants but in nature repair mechanisms exist. The introduction of preventative steps or an effective repair mechanism need to be developed to further stabilise the supramolecular assemblies. In recent reviews extensive11 TON values for a wide range of inter- and intramolecular systems have been reported and conclusions have been drawn concerning the efficiency of the approaches taken. However, the assessment of the photocatalytic potential of the various photocatalytically active compounds and mixtures is quite problematic since the experimental parameters used in the literature vary widely. Important variables include the nature of the proton carrier (for example, water or Et3NHBF4), the solvent used, pH, the nature and concentration of the sacrificial agent, its optical absorption properties, the optical absorption properties of the catalytic centre in the ground and the reduced state.40
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Perspective In addition, variations in the reaction procedures such as the type of light source used, the wavelength range and the irradiation duration, the geometry and volume of the photoreactor, and the volume of the head space need to be taken into account. A typical example of the importance of parameters such as the size of the headspace and the solvent are the results of Brewer and co-workers discussed above. Finally, it is important to note the potential effect of sacrificial agents on the photocatalytic processes via dark reactions; for example, it was found that a mixture of DMF, water and TEA produces formic acid without any irradiation.41 In addition, the need for sacrificial agents is a serious problem that limits studies in this area. The utilisation of sacrificial agents prevents the development of a sustainable, carbon neutral energy producing photocatalytic technology based on the principles used so far. Ultimately, the most suitable solution is the immobilisation of photocatalysts on active surfaces for creating photocatalytic or photoelectrocatalytic devices which do not rely on sacrificial materials. In conclusion, the effectiveness of the photocatalytic system under investigation depends on a wide range of factors, and this at present prohibits a meaningful comparison between inter- and intramolecular photocatalytic mechanisms of H2 formation. Therefore, more studies are needed where detailed kinetic analyses are carried out for well-defined photocatalytic cycles using systematic variations of the experimental conditions.
Acknowledgements This research is supported by the SFI under grants no. 07/SRC/ B1160 and 08/RFP/ CHE1349 and the German Research Association (DFG SFB 583, SR and GRK 1626; MPG). DD acknowledges the financial support from the University of Rome “LA SAPIENZA” through the program Ateneo 2012 (protocol no. C26A124AXX).
References 1 http://www.zeroemissionvehicles.eu A portfolio of powertrains for Europe: a fact based analysis. 2 NREL Technical report NREL/TP-581-40605. 3 Building the global hydrogen economy: Technologies and opportunities, BCC Research, May 2010. 4 (a) N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 15729; (b) N. Armaroli and V. Balzani, Angew. Chem., Int. Ed., 2007, 46, 52; (c) M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446. 5 (a) B. O’Regan and M. Grätzel, Nature, 1991, 353, 737; (b) A. Hagfeldt and M. Grätzel, Acc. Chem. Res, 2000, 33, 269. 6 L. Sun, L. Hammarström, B. Åkermark and S. Styring, Chem. Soc. Rev., 2001, 30, 36.
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Dalton Transactions 24 G. Singh Bindra, M. Schulz, A. Paul, S. Soman, R. Groarke, J. Inglis, M. T. Pryce, W. R. Browne, S. Rau, B. J. Maclean and J. G. Vos, Dalton Trans., 2011, 40, 10812. 25 J. Hirschmann, G. Singh Bindra, S. Soman, A. Paul, R. Groarke, M. Schulz, M. T. Pryce, W. R. Browne, S. Rau and J. G. Vos, Dalton Trans., 2011, 40, 10545. 26 G. Singh Bindra, M. Schulz, A. Paul, R. Groarke, S. Soman, J. L. Inglis, W. R. Browne, M. Pfeffer, S. Rau, B. J. MacLean, M. T. Pryce and J. G. Vos, Dalton Trans., 2012, 41, 13050. 27 (a) C. Kuhnt, M. Karnahl, S. Rau, M. Smitt, B. Dietzek and J. Popp, ChemPhysChem, 2011, 12, 2101; (b) M. Karnahl, C. Kuhnt, F. W. Heinemann, M. Schmitt, S. Rau, J. Popp and B. Dietzek, Chem. Phys., 2012, 393, 65. 28 (a) M. Elvington, J. Brown, S. M. Arachchige and K. J. Brewer, J. Am. Chem. Soc., 2007, 129, 10644; (b) M. Elvington and K. J. Brewer, Inorg. Chem., 2006, 45, 5242; (c) S. M. Arachchige, J. Brown and K. J. Brewer, J. Photochem. Photobiol., A, 2008, 197, 13; (d) K. Rangan, S. M. Arachchige, J. R. Brown and K. J. Brewer, Energy Environ. Sci., 2009, 2, 410; (e) S. M. Arachchige, R. Shaw, T. A. White, V. Shenoy, H.-M. Tsui and K. J. Brewer, ChemSusChem, 2011, 4, 514. 29 P. Du, J. Schneider, L. Fan, W. Zhao, U. Patel, F. N. Castellano and R. Eisenberg, J. Am. Chem. Soc., 2008, 130, 5056. 30 P. Lei, M. Hedlund, R. Lomoth, H. Rensmo, O. Johansson and L. Hammarström, J. Am. Chem. Soc., 2008, 130, 26. 31 T. G. Gray, A. S. Veige and D. G. Nocera, J. Am. Chem. Soc., 2004, 126, 9760. 32 M. Guttentag, A. Rodenburg, C. Bachmann, A. Senn, P. Hamm and R. Alberto, Dalton Trans., 2013, 42, 334. 33 M. Guttentag, A. Rodenberg, R. Kopelent, B. Probst, C. Buchwalder, M. Brandstätter, P. Hamm and R. Alberto, Eur. J. Inorg. Chem., 2012, 59. 34 B. Probst, A. Rodenberg, M. Guttentag, P. Hamm and R. Alberto, Inorg. Chem., 2010, 49, 6453. 35 T. Chao and J. H. Espenson, J. Am. Chem. Soc., 1978, 100, 129. 36 (a) C. V. Krishnan, B. S. Brunschwig, C. Creutz and N. J. Sutin, J. Am. Chem. Soc., 1985, 107, 2005; (b) E. D. Cline, S. E. Adamson and S. Bernhard, Inorg. Chem., 2008, 47, 10378; (c) P. Du, J. Schneider, G. Luo, W. W. Brennessel and R. Eisenberg, Inorg. Chem., 2009, 48, 4952. 37 T. M. McCormick, B. D. Calitree, A. Orchard, N. D. Kraut, F. V. Bright, M. R. Detty and R. Eisenberg, J. Am. Chem. Soc., 2010, 132, 15480. 38 T. Lazarides, T. McCormick, P. Du, G. Luo, B. Lindley and R. Eisenberg, J. Am. Chem. Soc., 2009, 131, 9192. 39 H. M. Y. Ahmed, PhD Thesis, Dublin City University, 2011. 40 L. E. Roy, G. Scalmani, R. Kobayashi and E. R. Batista, Dalton Trans., 2009, 6719. 41 A. Paul, D. Connolly, M. Schulz, M. T. Pryce and J. G. Vos, Inorg. Chem., 2012, 51, 1977.
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PERSPECTIVE
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Recent progress in the development of bimetallic photocatalysts for hydrogen generation Yvonne Halpin,a Mary T. Pryce,a Sven Rau,b Danilo Dinic and Johannes G. Vos*a
Received 23rd August 2013, Accepted 26th September 2013 DOI: 10.1039/c3dt52319e www.rsc.org/dalton
In this contribution recent developments in the design and application of bimetallic photocatalysts for the generation of hydrogen via intramolecular processes are assessed. The basic concepts of such assemblies are discussed together with an overview of the factors and molecular issues that affect their potential as photocatalysts. Issues that so far have limited progress are discussed and suggestions for future directions are made.
Introduction Man-made climate change is now recognised as one of the biggest challenges facing mankind. The continuing use of fossil fuels is leading to ever increasing levels of CO2, recently passing the 400 ppm level. The realisation of this has led to the establishment of the UN driven Framework Convention on Climate Change and the Kyoto Protocol in 1997. Since then EU and G8 leaders agreed in 2009 that CO2 emissions must be cut by 80% by 2050. This will require, for example, 95% decarbonisation of the road transport sector and fuel cell vehicles based on hydrogen will be required as part of the transport mix. a
SRC for Solar Energy Conversion, School of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland. E-mail: [email protected] b Anorganische Chemie I, Universität Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany c Department of Chemistry, University of Rome “La Sapienza”, Rome, Italy
Yvonne Halpin
Yvonne Halpin obtained a Bachelor of Science degree in Chemical and Pharmaceutical Science in 2006 and in 2010 a PhD from Dublin City University on the electrochemical and spectroscopic analysis of a series of iron, ruthenium and osmium complexes with applications in hydrogen production and molecular electronics. She is at present working as a Postdoctoral Fellow on the development of commercially viable carbon neutral methods for hydrogen generation.
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Therefore, the volume of hydrogen required will increase substantially. This approach has led to the concept of the “hydrogen economy” where the use of energy will not result in CO2 emissions as is the case today. At present carbon neutral hydrogen generation is too expensive however, and in a recent study “A portfolio of power-trains for Europe: a fact based analysis”,1 it is estimated that fuel-cell systems costs will decrease by 90% by 2020. Currently most hydrogen is produced from fossil fuels by Steam Methane Reforming and Coal Gasification.2 The process produces large amounts of CO2 and to make this method environmentally friendly carbon capture is required, which will in turn increase the production costs of hydrogen. At present a small amount (4%) is produced by electrolysis of water but its production costs are several times that of the steam reforming method as expensive metals such as platinum are used. Nevertheless the use of hydrogen in transport applications has been championed by the EC driven “Fuel Cells and Hydrogen Joint Undertaking Task Force”,
Mary Pryce obtained a BSc in Analytical Science from DCU in 1991. A PhD (1991–1994) was subsequently obtained in Organometallic Photochemistry. Post doctoral studies were carried out during 1995 and 1996 at the University of Milan in the area of asymmetric synthesis. Joined the School of Chemical Sciences at DCU in 1997, and is currently employed as a Senior Lecturer. Current research projects focus Mary T. Pryce on; Hydrogen generation, CO2 reduction, CO-releasing molecules, porphyrin chemistry, and organometallic photochemistry.
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Perspective mostly driven by industry, and in California, Germany, Italy, Japan and Australia hydrogen pilot projects have been carried out successfully.3 However, in order to develop a hydrogen based economy, novel environmentally friendly carbon neutral methods for hydrogen generation are urgently required. Many different approaches are advocated as options for energy sources for the future, ranging from nuclear, to wind and wave power, all with concomitant advantages and disadvantages. However, in terms of the potential amount of energy that can be delivered solar energy stands apart, as in one hour enough solar energy hits the earth’s surface to supply the amount consumed through human activity for one year. Although only a small fraction of this energy can be captured, that small fraction represents an important part of our future energy requirements.4 Existing technologies such as photovoltaic cells are well established for energy production, providing solutions at a local level. The prevalent crystalline silicon based solar cells are being augmented by the Grätzel type cell, developed two decades ago. The design of which is based upon a molecular approach where a light absorbing dye is immobilised on a nanocrystalline TiO2 electrode surface to generate a light driven electrical current.5 However, the conversion of light into electrical current has an inherent disadvantage, as is the case for wind and wave technologies in that the energy produced has to be used instantly. Thus the next step in the development of an economy that can rely on sustainable energy production is the development of a technology that can store solar energy. A potential method to achieve this is to mimic nature6 by converting light energy directly into chemical energy, which can be stored. This realisation has led to the development of novel technologies, capable of producing “solar fuels” such as hydrogen from water or CO and formic acid, by solar driven CO2 reduction.
Sven Rau studied chemistry at the Friedrich-Schiller-University of Jena and Dublin City University. He obtained his Diploma in chemistry in 1997 and his PhD in 2000. Following his habilitation in 2007, he moved to the Friedrich-Alexander-University Erlangen-Nuremberg in 2008. Since 2011, he leads the institute for materials and catalysis at the university Ulm. His research interests are focused on the synSven Rau thesis and characterization of photoredoxactive metal complexes their applications as photochemical molecular devices in light driven catalytic conversions, interaction with surfaces and living matter.
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Dalton Transactions Semiconductors, such as TiO2 and related non-molecular nanomaterials are known photocatalysts for the reduction of CO2 7 and the generation of hydrogen8 from water. However, their large band gap does not allow for efficient light absorption in the visible part of the solar spectrum. Since the late 1970s there has been increasing interest in the use of molecular components as solar dyes absorbing in the visible part of the spectrum. In these studies the intermolecular approach was taken where mixtures of a light absorbing moiety and a catalytic centre are utilised to produce solar fuels. In these systems the photosensitiser is irradiated and electron transfer from the excited state of the sensitiser to the catalytic centre occurs. Sacrificial electron donors such as triethylamine (TEA) are added to regenerate the dye (see Fig. 1).9 Extensive studies have shown that the catalytic activity of these systems is often limited by the stability of the molecular components.10 Another drawback in these intermolecular systems is the efficiency, which is inevitably limited by diffusion processes required to bring the sensitiser and the catalytic centre close enough to ensure the occurrence of the electron transfer step. More recently the intramolecular option has been considered and this has led to the development of a wide range of bimetallic complexes that act as intramolecular photocatalytic centres as shown in Fig. 1 and 2. Since 2006 investigations
Fig. 1 Intermolecular versus intramolecular photocatalytic hydrogen production from water. PS = photosensitiser, S = sacrificial agent.
Danilo Dini obtained his PhD in Materials Science at the University of Rome “La Sapienza” (Italy). He held postdoctoral positions in the Institut fur Physikaliche Chemie of Max-PlanckInstitut in Berlin (Germany) with Prof. G. Ertl, and in the University of Tuebingen (Germany) with Prof. M. Hanack. After a period as a researcher at Dublin City University (Ireland) in the group of Prof. J. G. Vos, he joined Danilo Dini the University of Rome “La Sapienza” (Italy) holding a position as Assistant Professor at the Department of Chemistry. His main research interests are photoelectrochemistry, photophysics of conjugated molecular materials and solar energy conversion devices. He has authored 90 peerreviewed articles in international journals.
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Dalton Transactions
Perspective compounds discussed in this overview are shown in Fig. 3. Another important factor to consider is whether the hydrogen is generated by the intramolecular assembly or in an intermolecular manner via decomposition of the photocatalysts into separate components.
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Understanding and optimising photocatalytic behaviour
Fig. 2 Basic mechanism of photocatalytic hydrogen production with 1 as photocatalyst.
focussed on both inter- and intramolecular assemblies have attracted much attention and the results of these studies have been documented in a number of recent reviews.11 The aim of this perspective is to assess progress in intramolecular photocatalysis since our Dalton Transactions review in 2007,12 and focus on issues which should be considered when developing intramolecular photocatalytic assemblies for solar fuel production. Examples of important issues are the design and synthesis of such intramolecular assemblies, since the electronic properties controlling vectorial electron transfer from the sensitiser to the catalyst are central components. This means that the molecular components in the assembly should be chosen with care. The importance of the relationship between the bridging and peripheral ligands and the excitation wavelength will be discussed. The composition of the photocatalytic solution and the involvement of the sacrificial agents are also included. A number of results obtained in our laboratories are discussed and compared with those reported in the literature. Where appropriate, comparisons with intermolecular systems will be made. The structures of the
Han Vos is Emeritus Professor of Inorganic Chemistry at Dublin City University. His research efforts are in the design of supramolecular systems containing transition metal complexes. Of particular interest are the synthesis, photophysical and electrochemical properties of dinuclear and polymeric ruthenium and osmium polypyridyl complexes both in solution and when immobilised on solid subJohannes G. Vos strates. The applications of such compounds as optical and electrochemical sensors and in molecular electronics are investigated. At present his research is directed towards the development of novel environmentally friendly sustainable energy sources using photocatalytic and electrocatalytic pathways.
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Experimental conditions. In recent years many photocatalytic studies have been carried out,11–14 with the aim of identifying the role played by each component in the intramolecular assembly and the parameters effecting optimisation of the photocatalytic performance. The light source used in these experiments is normally chosen to correspond to the lowest energy band in the UV/Vis spectrum of the sensitiser. The amount of hydrogen produced is usually reported in turnover numbers (TON), or as a turnover frequency (TOF). For the examples discussed from our group photocatalytic experiments were carried out in acetonitrile containing a photocatalyst concentration of 4 × 10−5 M in the presence of 2.15 M TEA as the sacrificial agent and between 0–15% water. The amount of hydrogen obtained was determined by gas chromatography and is the average of three independent measurements. Design features. As outlined in our previous review12 the reasoning behind the investigation of dinuclear photocatalysts for the photogeneration of hydrogen, is the potential of such assemblies to engage in intra- rather than inter-molecular electron transfer processes. The compound [(tbbipy)2Ru(tpphz)PdCl2](PF6)2 (1), where tbbipy = 4,4′-di-tert-butyl-2,2′-bipyridine and tpphz = tetrapyrido[3,2-a:2′,3′-c:3″,2″-h:2′″,3′″-j]phenazine (see Fig. 2) has been studied in great detail and represents the first example of a photocatalyst of this type13 and will be used as a reference. This compound typically produces hydrogen in acetonitrile containing 10% water with a TON of 210 over an 18 hour period using a 470 nm LED radiation source. To ensure that such an assembly is capable of producing hydrogen, efficient electron transfer has to occur from the light absorbing ruthenium centre to the palladium moiety that acts as the catalytic centre. As outlined above, a sacrificial agent, e.g. TEA, is required to re-reduce the light absorbing Ru centre and allows the next excitation to take place. The overall process can be described as shown in Scheme 1. It is important to realise that sacrificial agents such as TEA can create problems and may participate in the photocatalytic system since they decompose after the initial electron transfer and produce both protons and electrons in dark reactions as shown in Scheme 2 below. During the decomposition process two protons and one further electron are produced. The formation of these reactive species may be at least partially responsible for the decomposition of the photocatalytic assemblies. The intramolecular approach discussed here assumes that the ability of the processes outlined in Scheme 1 and Fig. 2 to yield significant amounts of hydrogen will depend on the rate of electron transfer from the light absorbing centre to the catalytic Pd moiety. The approach taken to achieve efficient vectorial electron transfer is that the bridging ligand, tpphz (B),
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Perspective
Dalton Transactions
Fig. 3 Structures of photocatalysts 1–15 discussed in the text. Precursors are the photosensitisers without the catalytic centres attached and when cited these are indicated with “Xa”, where X is 1–15. TON values are after 18 hours unless otherwise stated. * See text.
Scheme 1 Proposed pathway for photocatalytic hydrogen generation. B = bridging ligand, S = solvent.
Scheme 2 sensitiser.
Decomposition pathway of TEA after electron donation to the
has a lower π* energy level than the peripheral ligand (tbbipy in this case) which will generate a downhill energy gradient from the excited state located on the ruthenium centre to the catalytic Pd centre. For this reason a phenazine based
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bridging ligand was introduced. However, when taking this approach one needs to take into account a fast back reaction. To obtain further information concerning the electronic properties of compound 1 detailed investigations have been carried out on the photophysical properties of this compound, including time resolved resonance Raman and picosecond ( ps) transient absorption studies.14 These measurements were carried out in CH3CN and in the absence of a sacrificial agent in order to evaluate the photophysical properties of the assembly under non-catalytic conditions. An overview of the dynamics of electron-excitation transfer processes as evaluated by our study is given in Scheme 3. This scheme shows a comparison of the photophysical behaviour of the mononuclear precursor complex 1a and the dinuclear photocatalyst 1. In both compounds the initially populated singlet metal-to-ligand charge-transfer excited states (1MLCT) are based on the tbbipy/phenanthroline moieties. This first excited state successively undergoes relaxation to a 3 MLCT state, which is located on the phenanthroline part of tpphz bridging ligand. For both compounds the formation of the phenanthroline-centred 3MLCT state from the initially populated mixture of singlet states takes place within 1.2 ps for the precursor 1a and 0.8 ps for compound 1. However, there is a considerable difference in the decay of this triplet excited state. The decay time for 1a is 240 ps, while for 1 a decay time of 5 ps is observed. This decay is assigned to a transition from the equilibrated phenanthroline-centred 3MLCT to
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Dalton Transactions
Scheme 3 Electron transfer dynamics of 1 and its precursor complex 1a. Localisation of the photoexcited electron is illustrated by shaded areas. Reproduced with permission from ref. 14.
an intra-ligand-charge-transfer (ILCT) state localised on the phenazine unit. These data show that the Pd(II) centre significantly influences the electronic structure of the bridging ligand by creating a significantly higher driving force for the formation of the phenazine-centred ILCT state. For 1 this state further decays over 310 ps giving rise to the population of a long-lived ligand to metal charge transfer (LMCT) state on the Pd(II) centre, the decay of which was not observed on the time range accessible. The presence of the excited electron on the phenazine part of the bridging ligand is confirmed by electron paramagnetic resonance spectroscopy. The final reduction of the Pd(II) centre was expected to coincide with the dissociation of a Cl− anion. This was supported by the observation that the catalytic activity of 1 is completely inhibited by the addition of Cl− to the catalytic mixture during catalysis and by DFT calculations.13 Further confirmation for the interaction of the Pd centre with the excited state of the bridging ligand is the fact that while for the precursor complex emission is observed at 650 nm with a lifetime of 240 ns, for 1 a relatively weaker emission at the same wavelength with a lifetime of 10 vs. 100 ns) could represent another contributing factor.26 Similar studies involving a change in bridging ligands have been carried out by Rau and co-workers.27 Structural
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Perspective
Fig. 5 Time-dependence of the TON for H2 evolution in the presence of 10% water for compound 9 (blue) and its precursor 9a with [Pd(CH3CN)2Cl2] in a (1 : 1) ratio (red).
modifications of the bridging ligand of 1 were considered and the photocatalytic properties of compounds 2 and 3 were investigated. For 2, the phenanthroline moiety was brominated and as a result a TON value of 69 was obtained after 18 h of irradiation. For compound 3 the pyrazine ring of tpphz was replaced by an acridine moiety and a value of 139 was found. This compares with a value of 210 for compound 1 under the same conditions (acetonitrile with 10% water and 2 M TEA). This variation in TONs shows that even relatively small structural modifications of the bridging ligand can result in a considerable difference in photocatalytic behaviour. The nature of the catalytic centre. So far the discussion has centred on catalytic centres based on Pd or Pt. In this section some alternative options are considered. Brewer and coworkers have investigated compounds 11 and 12 (see Fig. 3) using an assembly able to act as a 2-electron collector. The trinuclear species consists of two ruthenium centres as the light absorbers and a rhodium electron collector/catalytic centre.28 In their studies they investigated the effect of changing the nature of the halide on the Rh catalytic centre. The photophysical properties of compounds 11 and 12 indicate that the highest occupied molecular orbital (HOMO) is Ru based and dπ in nature, while the lowest unoccupied molecular orbital having dσ* character is located on the Rh centred bridge of the assembly. This orientation of the molecular orbitals results in a low lying Ru → Rh metal-to-metal charge transfer (3MMCT) excited state and the Rh centre takes on the role of electron acceptor in the molecule. Following electron transfer from Ru to the Rh metal centre, Rh+ is formed and it is this species which is responsible for hydrogen generation, halide loss being central to the catalytic process. The results obtained show that replacement of Cl− (11) with Br− (12) did not result in significant photocatalytic activity as the TON value only increased from 28 for the chloride to 38 for the bromide containing compound after 4 hours of irradiation. However, in later studies they observed greatly improved TONs for the
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Perspective bromide containing compound after changing the catalytic conditions. By changing the solvent from acetonitrile to DMF, increasing the headspace, purging the system of hydrogen and by increasing the concentration of the sacrificial agents the TONs for hydrogen formation increased to 280 after 19.5 h and a value of 820 was found after 50 h.28 Artero and co-workers have carried out detailed investigations on the potential of Co centres as catalysts for hydrogen generation.21 Such an approach is important since one of the drivers in the design of economically viable photocatalytic systems for hydrogen generation from solar power is the cost and availability of the molecular components. The authors critically examined the effect of structural variation on the catalytic efficiency for hydrogen generation by systematic manipulation of the bridging ligand and the nature of the photosensitiser. In these studies they varied the molecular compositions of the bridge as well at the peripheral ligands and investigated compounds 13–15 (see Fig. 3). The Ru–Co complexes exhibit catalytic activity for the photochemical production of hydrogen. In such complexes, the pyridine ring of the bridging ligand, either (4-pyridine)oxazolo-[4,5-f ]phenanthroline (14) or (4-pyridine)imidazolo-[4,5-f ]phenanthroline) (13) is axially coordinated to a cobaloxime centre. A TON of 56 after 4 h (and 103 after 15 h when a 350 nm UV cut-off filter was used) was reported for 14 in acetone using Et3NHBF4 as the proton source. Altering the structure of the bridge in 14, by the replacement of the oxazolo moiety with an imidazolo unit to form compound 13, led to a TON of 104 after 4 h under the same conditions. Thus the nature of the bridge influences directly the capacity of the photocatalyst for generating hydrogen. This was further confirmed by examining the photocatalytic activity of individual Ru and cobaloxime units, which demonstrated no hydrogen was detected in the absence of the bridging ligand. Therefore, the bridging ligand facilitates electron transfer from the Ru photoactive centre to the catalytic cobaloxime centre either through bonds or by an outer-sphere mechanism. An investigation of the iridium analogue (15) of the Ru compound (13), under the same catalytic conditions yielded an enhanced hydrogen production, with a TON of 210 after 15 h. This study elegantly demonstrates how first row transition metals, when coupled with a suitable light harvesting unit, can produce photocatalytic devices that are economically viable for use in the photocatalytic generation of molecular hydrogen. The Ir–Co assembly 4 was considered by Sakai et al.20 Using visible light irradiation for 8 hours a TON value of 20 was reported for this compound. Based on NMR and electronic absorption spectroscopy, the authors suggested that compound 4 forms by self-assembly (the Ir/Co ratio used was 3 : 1). The TON values are given with respect to the Co concentration. This value dropped to 10 for the corresponding multicomponent system using the model compounds [Ir( ppy)2(bipy)]+, and [Co(bipy)3]3+. It is noteworthy that the bridge utilised is not conjugated and thus is not expected to promote intramolecular electron transfer. Is the nature of the photocatalytic process intra- or intermolecular? There has been concern in the literature whether
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Dalton Transactions the intramolecular systems, in particular those containing Pd moieties as catalytic centres, are acting as supramolecular assemblies or that hydrogen is generated by colloids formed after the decomposition of the intramolecular assembly.29,30 In the latter case the process would be inter- rather than intramolecular in nature. To investigate this issue we have carried out a number of comparative experiments with the ruthenium species 8a vs. 8 and 9a vs. 9 (see Fig. 3).26 The difference between the amounts of hydrogen produced by the intra- and intermolecular experiments carried out for the ruthenium based compound 9 and a mixture of its precursor 9a are 125 and 70, respectively, following 8 hours of irradiation (see Fig. 5). No induction period was observed and hydrogen production accelerates reaching the highest productivity after ca. 3.5 h for compound 9, and ca. 5.5 h for the mixture of its precursor in the presence of [Pd(acetonitrile)2Cl2]. The time dependent behaviour is very different with the intramolecular compound having a higher efficiency with respect to the bimolecular mixture, thus indicating that the intramolecular mechanism is the most efficient route for hydrogen generation, and that decomposition leading to colloid formation, does not represent an important contributing factor for producing hydrogen. Artero and co-workers, using cobalt catalysts, also observed higher TONs with the intramolecular mechanism with respect to the corresponding intermolecular route.21 For compound 8 a different behaviour was observed. For the intramolecular compound, a TON of 400 is observed after 18 h of irradiation, whereas the bimolecular mixture gave a similar value of 450 under the same conditions.24 These results suggest that in both cases intermolecular photocatalysis is observed or that the dinuclear compound 8 is formed in situ. Further studies are required to better understand the behaviour observed with this bridging ligand. Intermolecular experiments were also carried out using the iridium compound 5a.16 At 350 nm the 1 : 1 mixtures of 5a with [Pt(CH3CN)2Cl2], and 5a with [Pd(CH3CN)2Cl2] yielded at 350 nm TON values of 117 and 124 respectively, while at 470 nm values of 166 and 245 were obtained. The values obtained at 350 nm are higher than those obtained with the photocatalyst 5 and 6. This could be due to the absence of the Pd or Pt based MLCT states present in the bimetallic compounds that limit the photocatalytic processes at that wavelength of analysis. This observation is a strong indication that at 350 nm the “interand intramolecular” reactions follow a different mechanism, where for the intramolecular case using the bimetallic compounds 5 and 6 the formation of hydrogen is reduced by the presence of Pd or Pt based MLCT states. These states are absent in the intermolecular case. The values obtained for the intermolecular approach at 470 nm are similar to those obtained for compound 5 and 6 that display values of 308 and 254, respectively. From these values no clear pattern can be deduced about the mechanism that controls hydrogen formation at 470 nm.16 Kinetic considerations. The search for effective intramolecular photocatalytic devices has mainly been driven by the argument that electron transfer between the photosensitiser and
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Dalton Transactions the coordinatively bound catalytic centre will be faster with respect to what is observed in the intermolecular case.12,13 As a result of that observation higher efficiencies for hydrogen production are expected via the intramolecular route. In the intermolecular case physical diffusion is required and electron transfer between the photoexcited photosensitiser PS* and the catalyst requires high frequency of reactive collisions.31 The results discussed above show that in most cases the intramolecular photocatalytic process leads to TON values higher than those determined in the analogous intermolecular investigations. However, the improvement is generally less than an order of magnitude. Kinetic measurements have the potential to offer critical insights into the overall process in photocatalytic hydrogen generation for both inter- and intramolecular approaches. To date only a limited number of kinetic studies have been reported for the intramolecular case. To assess the importance of the electron transfer rates between the photosensitiser and the catalyst in the overall photocatalytic process, some typical examples will be considered. The discussion will mainly be concerned with electron transfer from the photosensitiser to the catalytic centre. A good example of a kinetic study on intermolecular systems was carried out by Hamm and co-workers who demonstrated how time resolved IR spectroscopy can be used to determine electron transfer and the quenching rate for a number of rhenium carbonyl photosensitisers with Co(III) glyoxime as catalyst.32,33 The reaction pathways and associated rate data obtained in DMF are outlined in Scheme 4,34 where [Re(NCS)(CO)3bipy] acts as the PS, [Co(dmgH)2] (dmgH2 = dimethylglyoxime) as the catalytic centre, while TEOA acts as the sacrificial agent and [HTEOA][BF4] as the hydrogen donor. Following excitation of the rhenium complex, a 3MLCT based excited state is generated, which undergoes reductive quenching in the presence of TEOA with kq = 9.7 × 107 M−1 s−1. This step is followed by electron transfer (k1 = 1.3 × 108 M−1 s−1) to the cobalt centre (CoII → CoI). The lifetime of the reduced species, PS− varies from 8.9 to 2.7 µs when the concentration of the CoII catalyst is increased from 1 to 3 mM. Following this electron transfer step the cobalt centre is protonated (CoI →
Scheme 4
Perspective CoIII_H). The formation of this cobalt hydride intermediate leads, via a bimolecular reaction, to the generation of H2. This latter reaction is of second order in cobalt (k3 = 25 M−1 s−1) and represents the rate limiting step. Interestingly, this rate is similar to that reported by Espenson et al. who investigated the generation of hydrogen using a hydridocobaloxime.35 They observed two different catalytic processes with variable concentration of the reactants. At high H+ and low cobaloxime concentrations a pseudo first order rate constant of 6 × 10−3 s−1 was observed while at higher concentrations for the hydridocobaloxime a second order process with a rate of 2 × 102 M−1 s−1 was observed. For the rhenium-based system time-resolved infrared spectroscopy confirmed that photo-oxidised TEOA generates a second reducing equivalent (see Scheme 4) which can be transferred to [Re(NCS)(CO)3bipy] (k2e− = 3.3 × 108 M−1 s−1) if no Co(dmgH)2 is present. TONs of 6000 for H2 production with respect to rhenium were observed, while TON values with respect to cobalt never exceeded 1000. This indicates decomposition of the Co(dmgH)2 catalyst. Similar kinetic data have been reported for other systems.36 The fact that the electron transfer process is second order indicates that this process is dependent also on the concentration of the catalyst. Assuming a concentration of 10−4 M for the catalytic centre, a first order rate constant for electron transfer between the two components of 104–105 s−1 is obtained. To the best of our knowledge, no kinetic studies on the photocatalytic generation of hydrogen by intramolecular assemblies have been reported and this discussion will concentrate on the data shown in Scheme 3. This scheme indicates the photophysical properties of 1 and 1a.14 The initial absorption of photons by the photoactive moiety of the multifunctional system leads to the formation of the charge-separated state necessary for the generation of hydrogen. Intramolecular electron transfer does not require thermal collisions between the PS* and the catalytic centre and is thus mainly controlled by the electronic properties of the bridging ligand, the photosensitiser and the catalytic centre. The kinetic data demonstrate sequential electron transfer from the excited photosensitiser (RuII) over the tpphz bridge to the
Reaction scheme for intermolecular photocatalytic hydrogen generation. Reproduced with permission from ref. 34.
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Perspective catalytic PdII centre within a few nanoseconds.14 In successive studies Rau and co-workers reported the photocatalytic properties for compound 3. Transient studies indicated that photoinduced intraligand charge-transfer dynamics takes place on the sub-ps timescale. This is in stark contrast to the mononuclear building block 3a where photoinduced chargetransfer from the photoactive Ru-unit to the catalytically active Pd-centre occurred on the sub-ns timescale.27b This brief and by no means exhaustive discussion indicates that for intramolecular photocatalytic systems the first order rate constant for electron transfer from the photosensitiser to the catalytic centre is in the order of 1010–1011 s−1 and is considerably faster than the first order rate constant observed for intermolecular systems where rates of 103–105 s−1 are commonly found. However, the turnover numbers observed for the intramolecular assemblies do not reflect these differences in electron transfer rates. It seems clear that other factors strongly affect the photocatalytic processes. These may include photophysical parameters such as the lifetime and quantum yield of the photoexcited PS* molecule (see below) and its collisional deactivation, the nature of the catalytic cycle (it may result in first or second order dependence and is generally slow for the intermolecular case), and quenching rates by sacrificial agents.36 A phenomenon specific of the intramolecular process which should also be considered in more detail is the possibility of back electron transfer from the photoinduced charge-separated state with the restoration of the ground state. Here, the nature of the bridging unit plays a crucial role since, ideally, it should promote electron transfer in a unidirectional fashion and exclusively to the catalytic moiety. For this reason non-conjugated bridging ligands have been adopted. It is however unclear whether this leads to increased efficiency.11i A clear example of the importance of the lifetime of the excited state is reported by McCormick et al.37 for the intermolecular hydrogen generation using modified rhodamine and a cobalt based glyoximate. These researchers obtained increased TON values when a modified rhodamine molecule was used as a photosensitiser and Co(III)-dimethylglyoximate as catalyst for H2 production. When in rhodamine the O atom was substituted with S or Se, turnover frequencies (TOF) were 0, 1700 and 5500 h−1, respectively. In this system the 3PS excited state is >1000 times longer than the singlet state (0.12–2.5 ns), and is sufficiently long lived to facilitate bimolecular electron transfer which is required for the completion of H2 formation. The ΦH2 yields are 0%, 12.2% and 32.8% respectively for these dyes and correlate directly with the quantum yields for singlet oxygen generation, ΦO2. The ΦO2 yields are 0.05, 0.17 to 0.67 for rhodamines with O, S and Se, respectively, as heteroatoms. The increase of TOF values with the increase of the atomic weight of the heteroatom of rhodamine is very impressive and determines the efficiency of the photosensitiser. These increases are explained by assuming that in case of the S and Se compounds the PS* is located on a triplet excited state, while for the O compound population of a singlet excited state is expected. It is worth noting that the lifetimes obtained vary
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Dalton Transactions between 1–2 ns. These values are still very short when compared to inorganic photosensitisers such as Ru and Ir complexes and it is somewhat surprising that such short lifetimes yield such high efficiencies. Previously the group reported increased TONs for hydrogen formation in a xanthene dye when H was replaced with a halide. This was attributed to enhanced intersystem crossing (ISC) promoted by the heavy atom effect.38
Concluding remarks In this overview the central assumption made in the design of intramolecular catalysts is the need for bridging ligands that allow interaction between the photosensitiser and the catalytic centre. This was also the starting point of our Frontier Paper in 2007. The use of non-conjugated bridging ligands has also been suggested on the basis that this would prevent a fast back reaction.11i This may indeed be the case for systems discussed here, but there is no direct evidence to support this. The results obtained show that interaction between the photosensitiser and the catalytic centre occurs and that this is a necessary step for the production of H2. For the bimetallic photocatalysts a reduced emission lifetime with respect to that observed for the precursors suggests that communication between the photosensitiser and the catalytic centre is taking place. Further evidence is obtained from changes in spectroscopic features such as absorption or emission spectra. It is also important to note that TONs show a clear dependence on the nature of the bridge as well as the peripheral ligands. These observations show that it is not only important to tune the energies of bridging ligands with those of the peripheral ligands but also optimise their electronic coupling. This creates a range of additional parameters that should be considered in the optimisation of the electronic interaction in the bimetallic compounds. An important observation is that for the intermolecular hydrogen generation by the rhenium compound in Scheme 4, the rate determining step is not associated with electron transfer between the sensitiser and the catalyst, but with hydrogen formation at the catalytic centre, which has a second order rate constant of 25 M−1 s−1. Therefore, for intramolecular systems like those considered here, the electron transfer processes involving the photosensitisers are not rate limiting. To allow further advances in this area there is an immediate need for detailed kinetic and photophysical studies on a wide range of assemblies to identify the rate determining step of the intramolecular mechanism. One of the crucial points is represented by the possibility that when certain dinuclear assemblies are used, hydrogen is not generated by the bimetallic compound but by uncoordinated Pd moieties in the colloids formed upon decomposition of the assembly. In a similar manner, the weak coordination strength of first row transition metals such as cobalt raises the question of how crucial is the de-coordination of such metal centres. In some of the compounds discussed above Pd colloids do form,
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Dalton Transactions but the results obtained thus far suggest that their formation is based upon decomposition of the compounds, at a stage where hydrogen formation decreases. Interestingly the behaviour of compound 8a indicates that with such a precursor the photocatalyst 8 is obtained by self-assembly of its mononuclear precursor and the Pd salt in solution.24 This observation together with loss of chloride for compound 1 are a reminder that often the compounds used in catalytic processes are often pre-catalysts rather than catalysts themselves. Further evidence for intramolecular behaviour is obtained from the analysis of the photocatalytic behaviour of compound 9. This compound shows that the time dependence and TON values for hydrogen formation are different in passing from the intra (9) to the intermolecular case (9a), with hydrogen obtained in the intramolecular case, indicating that hydrogen generation is intra- rather than intermolecular in nature.26 Further research on this issue is taking place at present but the results discussed above show that intramolecular photocatalysis is the main contributor to hydrogen generation. Another important issue is the stability of the compounds. In the majority of the photocatalytic systems investigated the stability of reactants is limited, irrespective of whether the inter- or intramolecular approach is taken, the exception being the Ir type compounds investigated by Bernhard and coworkers.17,18 It is not always clear what the driving force for the decomposition of the photocatalysts is, i.e. whether this is photochemical in nature or related to formation of radical cations by the sacrificial agent as shown in Scheme 2. The recent results obtained in our laboratories indicate that the sacrificial agents play an important role in the decomposition of the photocatalyst. It was found that while photocatalysts such as 1 are photostable in acetonitrile for 8 hours, while in the presence of sacrificial agents such as TEA the photostability is much reduced.39 Similar behaviour is observed for other systems the decomposition of which is accompanied by the formation of colloids. Apart from the loss of the catalysts from the bridging ligand one may also expect photochemically induced changes in the photosensitiser. These might include ligand loss or chemical rearrangements in the peripheral or the bridging ligand. Such processes are found in the photosynthesis modules in plants but in nature repair mechanisms exist. The introduction of preventative steps or an effective repair mechanism need to be developed to further stabilise the supramolecular assemblies. In recent reviews extensive11 TON values for a wide range of inter- and intramolecular systems have been reported and conclusions have been drawn concerning the efficiency of the approaches taken. However, the assessment of the photocatalytic potential of the various photocatalytically active compounds and mixtures is quite problematic since the experimental parameters used in the literature vary widely. Important variables include the nature of the proton carrier (for example, water or Et3NHBF4), the solvent used, pH, the nature and concentration of the sacrificial agent, its optical absorption properties, the optical absorption properties of the catalytic centre in the ground and the reduced state.40
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Perspective In addition, variations in the reaction procedures such as the type of light source used, the wavelength range and the irradiation duration, the geometry and volume of the photoreactor, and the volume of the head space need to be taken into account. A typical example of the importance of parameters such as the size of the headspace and the solvent are the results of Brewer and co-workers discussed above. Finally, it is important to note the potential effect of sacrificial agents on the photocatalytic processes via dark reactions; for example, it was found that a mixture of DMF, water and TEA produces formic acid without any irradiation.41 In addition, the need for sacrificial agents is a serious problem that limits studies in this area. The utilisation of sacrificial agents prevents the development of a sustainable, carbon neutral energy producing photocatalytic technology based on the principles used so far. Ultimately, the most suitable solution is the immobilisation of photocatalysts on active surfaces for creating photocatalytic or photoelectrocatalytic devices which do not rely on sacrificial materials. In conclusion, the effectiveness of the photocatalytic system under investigation depends on a wide range of factors, and this at present prohibits a meaningful comparison between inter- and intramolecular photocatalytic mechanisms of H2 formation. Therefore, more studies are needed where detailed kinetic analyses are carried out for well-defined photocatalytic cycles using systematic variations of the experimental conditions.
Acknowledgements This research is supported by the SFI under grants no. 07/SRC/ B1160 and 08/RFP/ CHE1349 and the German Research Association (DFG SFB 583, SR and GRK 1626; MPG). DD acknowledges the financial support from the University of Rome “LA SAPIENZA” through the program Ateneo 2012 (protocol no. C26A124AXX).
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