Comprehensive coordination chemistry II vol 9

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Introduction to Volume This volume aims to give as complete a coverage of the real and possible applications of coordination complexes as is possible in a single volume It is far more wide-ranging in its coverage than the related volume on ‘applications’ in the first edition of CCC (1987) The chapters cover the following areas: (i) use of coordination complexes in all types of catalysis (Chapters 1–11); (ii) applications related to the optical properties of coordination complexes, which covers fields as diverse as solar cells, nonlinear optics, display devices, pigments and dyes, and optical data storage (Chapters 12–16); (iii) hydrometallurgical extraction (Chapter 17); (iv) medicinal and biomedical applications of coordination complexes, including both imaging and therapy (Chapters 18–22); and (v) use of coordination complexes as precursors to semiconductor films and nanoparticles (Chapter 23) As such, the material in this volume ranges from solid-state physics to biochemistry There are a few points to make about the extent and depth of the coverage of material in this volume First, the sheer quantity of material involved necessarily limits the depth of the coverage To take a single example, the use of metal complexes as catalysts for carbonylation reactions is a subject worth a large book in its own right, and covering it in a few tens of pages means that the focus is on recent examples which illustrate the scope of the subject rather than covering encyclopedically all of the many thousands of references on the subject which have appeared since CCC (1987) was published Accordingly the general emphasis of this volume is on breadth rather than depth, with all major areas in which coordination complexes have practical applications being touched on, and extensive citations to more detailed and larger reviews, monographs, and books where appropriate Secondly, many of the chapters contain material which – if a strict definition is applied – is not coordination chemistry, but whose inclusion is necessary to allow a proper picture of the field to be given A great deal of license has been taken with the division between ‘‘coordination’’ and ‘‘organometallic’’ complexes; the formal distinction for the purposes of this series is that if more than 50% of the bonds are metal–carbon bonds then the compound is organometallic However, during a catalytic cycle the numbers of metal–carbon and metal–(other ligand) bonds changes from step to step, and it often happens that a catalyst precursor is a ‘‘coordination complex’’ (e.g., palladium(II) phosphine halides, to take a simple example) even when the important steps in the catalytic cycle involve formation and cleavage of M–C bonds Likewise, many of the volatile molecules described in Chapter 23 as volatile precursors for MOCVD are organometallic metal alkyls; but they can be purified via formation of adducts with ligands such as bipyridine or diphosphines, and it would be artificial to exclude them and cover only ‘‘proper’’ coordination complexes such as diketonates and dithiocarbamates In other fields, Chapter 15, which describes the use of phosphors in display devices, includes a substantial amount of solid-state chemistry (of doped mixed-metal oxides, sulfides, and the like) as well as coordination chemistry; Chapter 13 describes how a CD-R optical disk functions as a prelude to describing the metal complexes used as dyes for recording the information So, some of the material in the volume is peripheral to coordination chemistry; but all of it is material that will be of interest to coordination chemists Thirdly, some obvious applications of coordination chemistry are omitted from this volume if they are better treated elsewhere This is the case when a specific application is heavily associated with one particular element or group of elements, to the extent that the application is more appropriately discussed in the section on that element Essentially all of the coordination chemistry of technetium, for example, relates to its use in radioimmunoimaging; inclusion of this in Chapter 20 of this volume would have left the chapter on technetium in Volume almost empty For the same reason, the applications of actinide coordination complexes to purification, recovery, xv xvi Introduction to Volume and extraction processes involving nuclear fuel are covered in Volume 2, as this constitutes a major part of the coordination chemistry of the actinides In conclusion, it is hoped that this volume will be a stimulating and valuable resource for readers who are interested to see just how wide is the range of applications to which coordination chemistry can be put If nothing else it will help to provide an answer to the eternally irritating question which academics get asked at parties when they reveal what they for a living: ‘‘But what’s it for?’’ M D Ward Bristol, UK February 2003 COMPREHENSIVE COORDINATION CHEMISTRY II From Biology to Nanotechnology Second Edition Edited by J.A McCleverty, University of Bristol, UK T.J Meyer, Los Alamos National Laboratory, Los Alamos, USA Description This is the sequel of what has become a classic in the field, Comprehensive Coordination Chemistry The first edition, CCC-I, appeared in 1987 under the editorship of Sir Geoffrey Wilkinson (Editor-in-Chief), Robert D Gillard and Jon A McCleverty (Executive Editors) It was intended to give a contemporary overview of the field, providing both a convenient first source of information and a vehicle to stimulate further advances in the field The second edition, CCC-II, builds on the first and will survey developments since 1980 authoritatively and critically with a greater emphasis on current trends in biology, materials science and other areas of contemporary scientific interest Since the 1980s, an astonishing growth and specialisation of knowledge within coordination chemistry, including the rapid development of interdisciplinary fields has made it impossible to provide a totally comprehensive review CCC-II provides its readers with reliable and informative background information in particular areas based on key primary and secondary references It gives a clear overview of the state-of-the-art research findings in those areas that the International Advisory Board, the Volume Editors, and the Editors-in-Chief believed to be especially important to the field CCC-II will provide researchers at all levels of sophistication, from academia, industry and national labs, with an unparalleled depth of coverage Bibliographic Information 10-Volume Set - Comprehensive Coordination Chemistry II Hardbound, ISBN: 0-08-043748-6, 9500 pages Imprint: ELSEVIER Price: USD 5,975 EUR 6,274 Books and electronic products are priced in US dollars (USD) and euro (EUR) USD prices apply world-wide except in Europe and Japan.EUR prices apply in Europe and Japan See also information about conditions of sale & ordering procedures -http://www.elsevier.com/wps/find/bookconditionsofsale cws_home/622954/conditionsofsale, and links to our regional sales officeshttp://www.elsevier.com/wps/find/ contact.cws_home/regional GBP 4,182.50 030/301 Last update: 10 Sep 2005 Volumes Volume 1: Fundamentals: Ligands, Complexes, Synthesis, Purification, and Structure Volume 2: Fundamentals: Physical Methods, Theoretical Analysis, and Case Studies Volume 3: Coordination Chemistry of the s, p, and f Metals Volume 4: Transition Metal Groups - Volume 5: Transition Metal Groups and Volume 6: Transition Metal Groups - 12 Volume 7: From the Molecular to the Nanoscale: Synthesis, Structure, and Properties Volume 8: Bio-coordination Chemistry Volume 9: Applications of Coordination Chemistry Volume 10: Cumulative Subject Index 10-Volume Set: Comprehensive Coordination Chemistry II COMPREHENSIVE COORDINATION CHEMISTRY II Volume 9: Applications of Coordination Chemistry Edited by M.D Ward Contents Metal complexes as catalysts for polymerization reactions (V Gibson, E.L Marshall) Metal complexes as hydrogenation catalysts (C Pettinari, D Martini, F Marchetti) Metal complexes as catalysts for addition of carbon monoxide (P.W.N M van Leeuwen, C Claver) Metal complexes as catalysts for oxygen, nitrogen and carbon-atom transfer reactions (Tsutomu Katsuki) Metal complexes as catalysts for H-X (X = B,CN, Si, N, P) addition to CC multiple bonds (M Whittlesey) Metal complexes as catalysts for C-C cross-coupling reactions (I Beletskaya, A.V Cheprakov) Metal complexes as catalysts for carbon-heteroatom cross-coupling reactions (J.F Hartwig) Metal complexes as Lewis acid catalysts in organic synthesis (S Kobayashi et al.) Supported metal complexes as catalysts (A Choplin, F Quignard) Electrochemical reactions catalyzed by transition metal complexes (A Deronzier, J-C Moutet) Combinatorial methods in catalysis by metal complexes (M.T Reetz) Metal complexes as speciality dyes and pigments (P Gregory) Metal complexes as dyes for optical data storage and electrochromic materials (R.J Mortimer, N.M Rowley) Non-linear optical properties of metal complexes (B Coe) Metal compounds as phosphors (J Silver) Conversion and storage of solar energy using dye-sensitized nanocrystalline TiO2 cells (M Gratzel, Md.K Nazeeruddin) Metal complexes for hydrometallurgy and extraction (P.A Tasker et al.) Metal complexes as drugs and chemotherapeutic agents (N Farrell) Metal complexes as MRI contrast enhancement agents (A.E Merbach et al.) Radioactive metals in imaging and therapy (S Jurisson et al.) Fluorescent complexes for biomedical applications (S Faulkner, J Matthews) Metal complexes for photodynamic therapy (R Bonnett) Coordination complexes as precursors for semiconductor films and nanoparticles (P.O'Brien, N.Pickett) 9.1 Metal Complexes as Catalysts for Polymerization Reactions V C GIBSON and E L MARSHALL Imperial College, London, UK 9.1.1 INTRODUCTION 9.1.2 OLEFIN POLYMERIZATION 9.1.2.1 Introduction 9.1.2.2 Catalyst Survey 9.1.2.2.1 Group metallocene catalysts 9.1.2.2.2 Group non-metallocenes 9.1.2.2.3 Group and rare earth metal catalysts 9.1.2.2.4 Group metal catalysts 9.1.2.2.5 Group metal catalysts 9.1.2.2.6 Group metal catalysts 9.1.2.2.7 Group metal catalysts 9.1.2.2.8 Group 10 metal catalysts 9.1.2.2.9 Main group metal catalysts 9.1.3 POLYMERIZATION OF STYRENES 9.1.3.1 Introduction 9.1.3.2 Coordinative Polymerization of Styrenes 9.1.3.3 Atom Transfer Radical Polymerization of Styrenes 9.1.4 POLYMERIZATION OF ACRYLATES 9.1.4.1 Introduction 9.1.4.2 Anionic Initiators of the group 1, 2, and Metals 9.1.4.3 Well-defined Magnesium and Aluminum Initiators 9.1.4.4 Lanthanide Initiators 9.1.4.5 Early Transition Metal Initiators 9.1.4.6 Atom Transfer Radical Polymerization 9.1.5 RING-OPENING METATHESIS POLYMERIZATION OF CYCLIC ALKENES 9.1.5.1 Introduction 9.1.5.2 Titanacyclobutanes 9.1.5.3 Group Metal Initiators 9.1.5.4 Ruthenium Initiators 9.1.5.5 Acyclic Diene Metathesis 9.1.6 RING-OPENING POLYMERIZATION OF CYCLIC ESTERS 9.1.6.1 Introduction 9.1.6.2 General Features of Lactone Polymerization 9.1.6.3 Aluminum-based Initiators 9.1.6.4 Zinc–Aluminum Oxo-alkoxide Initiators 9.1.6.5 Magnesium and Zinc Initiators 9.1.6.6 Calcium Initiators 9.1.6.7 Tin Initiators 9.1.6.8 Iron Initiators 9.1.6.9 Yttrium and Rare-earth Initiators 9.1.6.10 Titanium and Zirconium Initiators 9.1.7 RING-OPENING POLYMERIZATION OF EPOXIDES 9.1.7.1 Introduction 9.1.7.2 Tetraphenylporphyrin Aluminum and Zinc Initiators 9.1.7.3 Non-porphyrinato Aluminum Initiators 2 3 11 12 13 14 15 15 17 18 18 18 20 23 23 23 24 26 27 29 29 29 29 30 33 36 36 36 37 37 42 42 43 44 45 46 51 52 52 52 54 Metal Complexes as Catalysts for Polymerization Reactions 9.1.7.4 Copolymerization of Epoxides and Carbon Dioxide 9.1.7.5 Copolymerization of Epoxides and Aziridines with Carbon Monoxide 9.1.8 OTHER LIVING COORDINATION POLYMERIZATIONS 9.1.8.1 ROP of N-carboxyanhydrides and -lactams 9.1.8.2 Polymerization of Isocyanates and Guanidines 9.1.9 REFERENCES 9.1.1 54 57 58 58 58 59 INTRODUCTION The period since the mid 1980s has seen a tremendous growth in the use of coordination complexes to catalyze chain growth polymerization processes One of the main advances has been a move away from ill-defined catalysts, where relatively little is understood about the influence of the metal coordination environment on monomer insertion, to precisely defined single-site catalysts where macromolecular parameters such as molecular weight and molecular weight distribution, and microstructural features such as tacticity and monomer placement, can be controlled through the nature of the ligand donor atoms and their attendant substituents For many metal-mediated polymerization reactions it has proved possible to control the kinetics of chain propagation vs chain transfer or chain termination to an extent that ‘‘living’’ polymerizations can be achieved This has made accessible a plethora of new materials with novel topologies and micro- and macro-structural architectures The following sections outline the important advances in polymerization catalyst technology for a number of polymerization mechanisms and polymer types Where the emphasis is placed on stereoselective polymerizations, the r and m notation is employed Two adjacent stereocenters of the same configuration are said to form a meso (or m) dyad, whereas a racemic, r dyad contains two centers of opposing stereochemistries If a polymer contains all m junctions, i.e., -RRRRRR- or -SSSSSS-, then it is termed isotactic, whereas a perfectly syndiotactic polymer possesses all r dyads, i.e., -RSRSRS- Different tacticities are often distinguishable by NMR spectroscopy, with the level of detail dependent upon the polymer type 9.1.2 9.1.2.1 OLEFIN POLYMERIZATION Introduction The transition metal catalyzed polymerization of ethylene was first reported by Ziegler in 1955 using a mixture of TiCl4 and Et2AlCl1,2 and was quickly followed by Natta’s discovery of the stereoselective polymerization of propylene.3,4 Polyolefins have since become the most widely produced family of synthetic polymers, the vast majority being produced using heterogeneous Ziegler systems, e.g., TiCl4/MgCl2/Et3Al However, during the 1980s interest grew in the use of well-defined homogeneous catalysts, largely stimulated by the discovery that group metallocenes, in combination with methylaluminoxane (MAO) cocatalyst, afford exceptionally high activities and long-lived polymerization systems More recently, attention has turned towards non-metallocene polymerization catalysts, partly to avoid the growing patent minefield in group cyclopentadienyl systems, but also to harness the potential of other metals to polymerize ethylene on its own and with other monomers A number of reviews have outlined the key developments in molecular olefin polymerization catalyst systems.5–15 Due to the importance of group metallocenes to the development of the field, we include here a brief outline of some of their key features The majority of this section, however, is devoted to advances in non-metallocene catalyst systems Where necessary, catalyst activities have been converted into the units g mmolÀ1 hÀ1 barÀ1 for gaseous monomers such as ethylene and propylene, and g mmolÀ1 hÀ1 for reactions carried out in liquid -olefins such as 1-hexene Activities are classified as very high (>1,000), high (1,000–100), moderate (100–10), low (10–1) and very low ( NCSÀ > H2O > NCN2À > ClÀ The red shift of the absorption maxima in complexes (19) and (20), containing 4,40 -dicarboxy-2,20 -biquinoline (dcbiq) instead of 4,40 -dicarboxy-2,20 -bipyridine (dcbpy) as acceptor ligand, is due to the low energy of the * orbitals of 4,40 -dicarboxy-2,20 -biquinoline The resonance Raman spectra of these complexes for excitation at 568 nm show predominantly bands associated with the dcbpy and dcbiq ligands indicating that the lowest excited state is a ruthenium to dcbpy or dcbiq MLCT state.44–46 Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells 731 Energy Ligand π * π * tuning orbitals ∆E ∆E ∆E Metal t2g orbitals t2g tuning [Ru(dcbpy)3]2+ [Ru(dcbiq)3]2+ [Ru(dcbpy)2(phpy)]+ Figure Tuning of HOMO (t2g) and LUMO (*) orbital energy in various ruthenium polypyridyl complexes (Hphpy ¼ 2-phenylpyridine) The spectral properties of ruthenium polypyridyl complexes can also be tuned by introducing nonchromophoric donor ligands such as NCSÀ, which destabilizes the metal t2g orbitals A comparison of the visible absorption spectra of complexes ((21)–(24)), where the number of nonchromophoric ligands is varied from one to three, shows that the most intense MLCT transition maxima are at 500, 535, 570, and 620 nm, respectively The 70 nm red shift in complex (23) compared to complex (21) is due to an increase in the energy of the metal t2g orbitals caused by introducing the nonchromophoric ligands Complex (24) shows an absorption maximum at 620 nm, and the 50 nm red shift of complex (24) compared to complex (23) reflects the extent of decrease in the LUMO energy, due to substitution of three carboxylic acid groups at the 4,40 ,400 -positions of 2,20 :60 ,200 -terpyridine compared to unsubstituted terpyridine.29,47,48 In an effort to tune further the spectral responses of the ruthenium complexes containing polypyridyl ligands, several groups have explored the influence of the position of the carboxylic acid substituents.49–51 Comparison of complexes ((25)–(28)) shows the extent HOMO and LUMO tuning The lowest MLCT absorption maximum of complex (25) is at 510 nm in ethanol By substituting two carboxylic acid groups at the 4,40 -positions of the 2,2-bipyridine ligands in (22), the MLCT maximum was red-shifted to 535 nm However, on substitution at the 5,50 positions of 2,20 -bipyridine in (26), the absorption maximum shifted further into the red (580 nm) In contrast, the MLCT maximum was blue shifted (500 nm) by substituting at the 6,60 positions of 2, 20 -bipyridine, in (27) The enhanced red response of complexes containing the 5,50 -dicarboxylic acid-2,20 -bipyridine ligand is due to a decrease in the energy of the * orbitals, which makes them attractive as sensitizers for nanocrystalline TiO2 films Bignozzi and co-workers found that the IPCE of complexes having the 5,50 -dicarboxylic acid-2,20 -bipyridine ligands were lower than the analogous complexes that contain 4,40 -dicarboxylic acid-2,20 -bipyridine.49 They rationalized the low efficiency of these sensitizers containing 5,50 -dicarboxylic acid-2,20 -bipyridine ligands in terms of low excited state redox potentials In search of new sensitizers that absorb strongly in the visible region of the spectrum, Arakawa and co-workers have developed several sensitizers based on 1,10-phenanthroline ligands Among these new compounds, (29) and (30) are noteworthy; they show an intense and broad MLCT absorption band at 525 nm in ethanol The energy levels of the LUMO and HOMO for (29) were estimated to be À1.02 and 0.89 eV vs SCE, respectively, which are slightly more positive than those of the sensitizer (22) These sensitizers, when anchored onto a TiO2 surface, yield more than 85% photon to electron injection efficiencies.8,52,53 732 Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells HO2C [+] NCN HO2C N N Y N N Ru N N N N HO2C N HO2C N N (19) Y = NCS– (20) Y = Cl– (18) CO2H [+] HO2C N N N N N Ru NCS NCS N N HO2C NCS HO2C (22) (21) [–] NCS N [–] HO2C NCS N NCS NCS Ru Ru NCS N CO2H N Ru N N N N Ru N NCS N N HO2C CO2H (24) (23) HO2C HO2C N N N N Ru Ru NCS N NCS (25) N N NCS N NCS CO2H (26) CO2H Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells 9.16.3.4 733 Osmium Complexes Although there are a large number of osmium polypyridyl complexes, very few have been used as sensitizers in dye-sensitized solar cells Osmium complexes have several advantages compared to their ruthenium analogues: in particular, osmium has a stronger ligand field splitting compared to ruthenium, and the spin–orbit coupling leads to excellent response in the red region by enhancing the intensity of the ‘‘forbidden’’ singlet-triplet MLCT transitions.54 Heimer et al found that the complex (31) is extremely stable under irradiation in a homogeneous aqueous solution compared to the analogous ruthenium complex.55 The greater photostability for osmium is consistent with a stronger crystal field splitting of the metal d-orbitals, which inhibits efficient population of d-d excited states Lewis and co-workers have developed osmium-based sensitizers (31) and (32), and found nearly 80% incident monochromatic photon-to-current conversion efficiencies.56 9.16.3.5 MLCT Transitions in Geometrical Isomers Isomerization is another approach for tuning the spectral properties of metal complexes.57–59 The UV–vis absorption spectrum of the trans-dichloro complex (35) in DMF solution shows at least three MLCT absorption bands in the visible region at 690, 592, and 440 nm Alternatively, the cis-dichloro complex (33) in DMF solution shows only two distinct broadbands in the visible region at 590 and 434 nm, assigned as MLCT transitions The lowest energy MLCT band in the trans-complexes ((35)–(37)) is significantly red shifted compared to that in the corresponding cis- complexes ((22), (33), (34)) (Figure 9) This red shift is due to stabilization of the LUMO of the dcbpy ligand in the trans species relative to the cis species The red shift (108 nm) of the lowest energy MLCT absorption of the trans-dichloro complex (35) compared that of the trans-dithiocyanato complex (37) is due to the strong -donor property of the ClÀ ligand compared to NCSÀ The chloride ligands cause destabilization of the metal t2g orbitals, raising them in energy closer to the ligand * orbitals, resulting in lower energy MLCT transitions 9.16.3.6 Sensitizers Containing Functionalized Hybrid Tetradentate Ligands The main drawback of the trans complexes discussed above is their thermal and photo-induced isomerization back to the cis configuration In an effort to stabilize the trans configuration of an octahedral ruthenium complex, and integrate the concepts of donor and acceptor ligands in a single complex, Renouard et al have developed functionalized hybrid tetradentate ligands and their ruthenium complexes ((38)–(47)).60 In these complexes the donor units of the tetradentate ligand (benzimidazole in (38) and (39) and tert-butylpyridine in (40) and (41)) tune the metal t2g orbital energies, and the acceptor units (methoxycarbonyl) tune the * molecular orbital energies The use of a tetradentate ligand will inhibit the trans- to cis-isomerization process The axial coordination sites are used further to fine-tune the spectral and redox properties, and to stabilize the hole that is being generated on the metal, after injection of an electron into the conduction band 0.8 Absorbance 0.6 0.4 (22) (37) 0.2 0.0 300 400 500 600 λ(nm) 700 800 Figure UV–visible absorption spectra of the complexes (22) and (37) in ethanol solution at room temperature 734 Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells HO2C HO2C HO2C CO2H N N Ru NCS N N N N N CO2H CN N NCS CN CO2H CO2H (27) (28) CO2H HO2C CO2H CO2H N N HO2C N N N Ru NCS NCS NCS (29) (30) CO2H HO2C N Os Os CN N CO2H N N N N [2+] CO2H N N N CN HO2C NCS N CO2H HO2C N Ru N HO2C CO2H Ru N HO2C (31) CO2H CO2H (32) CO2H HO2C HO2C CO2H N Ru Ru HO2C N N X N N N N N X (33) X = Cl– (34) X = H2O CO2H X HO2C X (35) X = Cl– (36) X = H2O (37) X = NCS– CO2H Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells 735 The trans-dichloro and dithiocyanate complexes show MLCT transitions in the entire visible and near IR region The lowest energy MLCT transition band of the trans-dichloro complexes is around 700 nm in DMF solution, and the complexes show weak and broad emission signals above 950 nm The absorption and emission maxima of the trans-dithiocyanate complexes are blue shifted compared to its trans-dichloro analogues due to the strong  acceptor property of the NCSÀ ligands compared to ClÀ, which is consistent with the electrochemical properties of these complexes The RuIII/II redox potentials of the thiocyanate complexes were more positive (by %350 mV) than those of the corresponding dichloro complexes This is in good agreement with the Ligand Electrochemical Parameters scale, according to which the potential of the RuIII/II couple in the thiocyanate complex should be $340 mV more positive than in the analogous dichloride complex.61 The iox/ired peak current is substantially greater than unity due to the oxidation of the thiocyanate ligand subsequent to the oxidation of the ruthenium(II) center The electronic spectrum of (38) was calculated by intermediate neglect of differential overlay (INDO/S) and compared with the experimental data Geometry optimization of (38) produced a structure with C2-point group symmetry The three HOMOs (HOMO, HOMO-1, and HOMO-2) for complex (38) are mostly formed from 4d(Ru) orbitals and their contribution ranges from 52% to 84% The dyz Ru orbital is directed toward the tetradentate ligand and coupled to the -orbitals of the ligand, and is thereby delocalized to a greater extent than the other d(t2g) orbitals The LUMOs are almost entirely localized on the ligand (Figure 10) Extensive -back-donation between metal 4d and ligand * orbitals is observed Complex (45) when anchored on a TiO2 layer shows an IPCE of 75%, yielding a current density of 18 mA cmÀ2 under standard AM 1.5 sunlight.60 9.16.3.7 Hydrophobic Sensitizers An important aspect of dye-sensitized solar cells is water-induced desorption of the sensitizer from the surface Extensive efforts have been made to overcome this problem by introducing hydrophobic properties in the ligands Complexes that contain hydrophobic ligands ((48)–(53)) have several advantages compared to cis-dithiocyanato-bis(2,20 -bipyridyl-4,40 -dicarboxylate)ruthenium(II) (22): (i) The ground state pKa is higher due to acceptor and donor properties of the 4, 40 -dicarboxylic acid and the hydrophobic ligand, respectively (ii) The dye uptake onto the TiO2 surface is enhanced significantly, because of the increased ground state pKa, and the decreased charge on the complex (only two ionizable carboxylic acid units instead of four) (iii) The long aliphatic chains interact laterally and decrease the back electron transfer to the oxidized redox couple (see Section 9.16.4.5) (iv) The hydrophobicity of the ligand increases the stability of solar cell towards waterinduced desorption Figure 10 Pictures of the frontier orbitals of the complex (38) 736 Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells MeO2C CO2Me MeO2C CO2Me X N HN N X NH Ru N N N X N Ru N N Y N (38) X = Cl– X (39) X = NCS– (40) X = Cl–, Y = tBu (41) X = NCS–, Y = tBu (42) X = Cl– Y = H (43) X = NCS–, Y = H MeO HO2C OMe CO2H X N X N N Ru N N Ru N N X N X (44) X = Cl– (46) X = Cl– (45) X = NCS– (47) X = NCS– X HO2C X N N N Ru NCS N HO2C NCS (48) X = Me (49) X = tBu (50) X = C6H13 (51) X = C9H19 (52) X = C13H27 (53) X = C16H33 Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells 737 The photocurrent action spectra of these complexes show broad features covering a large part of visible spectrum, and display a maximum at around 550 nm, where the incident monochromatic IPCE exceeds 85% These hydrophobic complexes show excellent stability towards waterinduced desorption when used as CT photosensitizers in nanocrystalline TiO2-based solar cells.62 The rate of electron transport in dye-sensitized solar cells is a major element of the overall efficiency of the cells The electrons injected into the conduction band from the excited state of the dye can traverse the TiO2 network and can be collected at the transparent conducting glass, or can react either with oxidized dye molecules or with the oxidized redox couple (recombination) The reaction of injected electrons in the conduction band with the oxidized redox mediator (I3À) gives undesirable dark currents, reducing significantly the charge-collection efficiency, and thereby decreasing the total efficiency of the cell (Figure 11) Several groups have tried to reduce the recombination reaction by using sophisticated device architecture such as composite metal oxides with different band gaps as the semiconductor.63,64 Gregg et al have examined surface passivation by deposition of insulating polymers.65 We have used additional spacer units between the dye and the TiO2 surface to reduce the recombination reaction, but with little success (Nazeeruddin et al personal observations) Nevertheless, by using TiO2 films containing hydrophobic sensitizers that contains long aliphatic chains ((50)–(53)) the recombination reaction was suppressed considerably The most likely explanation for the reduced dark current is that the long chains of the sensitizer interact laterally to form an aliphatic network as shown in Scheme 1, thereby preventing triiodide from reaching the TiO2 surface 9.16.3.8 Near IR Sensitizers Phthalocyanines possess intense absorption bands in the near-IR region and are known for their excellent stability, rendering them attractive for photovoltaic applications.66 They have been repeatedly tested in the past as sensitizers of wide band gap oxide semiconductors but gave poor incident photon-to-electric current conversion yields of under 1%, which is insufficient for solar cell applications.67–70 One of the reasons for such low efficiencies is aggregation of the dye on the TiO2 surface This association often leads to undesirable photophysical properties, such as self-quenching and excited state annihilation However, the advantage of this class of complexes is the near-IR response, which is very strong with extinction coefficients of close to 50,000 MÀ1 cmÀ1 at 650 nm, compared to the ruthenium polypyridyl complexes, which have much smaller extinction coefficients at this wavelength TiO2 (2) –1.0 S+/S∗ e– E / V vs SCE –0.6 –0.2 e– (1) (3) (5) I3–/I– 0.2 (6) 0.6 (4) S+/S 1.0 Figure 11 Illustration of the interfacial CT processes in a nanocrystalline dye-sensitized solar cell S / Sỵ/S* represent the sensitizer in the ground, oxidized and excited state, respectively Visible light absorption by the sensitizer (1) leads to an excited state, followed by electron injection (2) onto the conduction band of TiO2 The oxidized sensitizer (3) is reduced by the IÀ/I3À redox couple (4) The injected electrons into the conduction band may react either with the oxidized redox couple (5) or with an oxidized dye molecule (6) 738 Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells TiO2 OOC NCS N NCS Ru N N N OOC e– I3– OOC N N N Ru NCS N OOC NCS Scheme (i) Ruthenium phthalocyanines The ruthenium phthalocyanine complex (54) shows a visible absorption band at 650 nm (" 49,000 MÀ1 cmÀ1) and a phosphorescence band at 895 nm The triplet state lifetime is 474 ns under anaerobic conditions The emission is entirely quenched when complex (54) is adsorbed onto a nanocrystalline TiO2 film The very efficient quenching of the emission of (54) was found to be due to the electron injection from the excited singlet/triplet state of the phthalocyanine into the conduction band of the TiO2.71 The photocurrent action spectrum, where the IPCE value is plotted as a function of wavelength, shows 60% IPCE with a maximum at around 660 nm (Figure 12) These are by far the highest conversion efficiencies obtained with phthalocyanine-type sensitizers It is fascinating to note that this class of dye injects electrons efficiently into the conduction band of TiO2, despite the fact that the orbitals of the axial pyridine ligands (bearing the carboxylate anchors) not participate in the phthalocyanine-based –* excitation which is responsible for the 650 nm absorption band This phenomenon shows that the electronic coupling of the excited state of the dye to the Ti (3d ) conduction band manifold is strong enough through this axial mode of attachment to render charge injection very efficient These results establish a new pathway for grafting dyes to oxide surfaces through axially attached pyridine ligands (ii) Phthalocyanines containing 3d metals Several zinc(II) and aluminum(III) phthalocyanine derivatives substituted with carboxylic acid and sulfonic acid groups were anchored to nanocrystalline TiO2 films and tested for their photovoltaic behavior.72,73 Interestingly, zinc(II)-2,9,16,23-tetracarboxyphthalocyanine (55) exhibited 45% monochromatic current conversion efficiency at 700 nm It is shown that electron injection to TiO2 occurs from the excited singlet state of the phthalocyanine derivatives The Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells CO2H 739 CO2H CO2H N N N N N HO2C N Ru N N N N N N N N CO2H N N Zn N N HO2C HO2C CO2H (55) (54) [2+] CO2H HO2C HO2C CO2TBA N N Ru N N NCS N N Ru HO2C N N C N CN Ru NCS TBAO2C N N N N N N HO2C (TBA = tetrabutylammonium) CN C Ru N N N HO2C (57) (56) O N X X N N Ph N N N Co Ph Ph O P P Ru O O N N N X X N (58) X = Me (59) X = C4H9 (60) X = C13H27 O Ph P Ph Ph (61) O 740 Conversion and Storage of Solar Energy using Dye-sensitized Nanocrystalline TiO2 Cells Figure 12 Photocurrent action spectra of nanocrystalline TiO2 films sensitized by bis(3,4-dicarboxypyridine) RuII (1,4,8,11,15,18,22,25-octamethyl-phthalocyanin) (54) The incident photon to current conversion efficiency is plotted as a function of wavelength inherent problem of aggregation in this class of compounds is reduced considerably by introducing 4-tert-butylpyridine and 3 ,7 -dihydroxy-5 ... Coordination Chemistry Volume 10: Cumulative Subject Index 10-Volume Set: Comprehensive Coordination Chemistry II COMPREHENSIVE COORDINATION CHEMISTRY II Volume 9: Applications of Coordination Chemistry. .. and Volume 6: Transition Metal Groups - 12 Volume 7: From the Molecular to the Nanoscale: Synthesis, Structure, and Properties Volume 8: Bio -coordination Chemistry Volume 9: Applications of Coordination. .. Monoxide 9. 1.8 OTHER LIVING COORDINATION POLYMERIZATIONS 9. 1.8.1 ROP of N-carboxyanhydrides and -lactams 9. 1.8.2 Polymerization of Isocyanates and Guanidines 9. 1 .9 REFERENCES 9. 1.1 54 57 58 58 58 59
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Xem thêm: Comprehensive coordination chemistry II vol 9 , Comprehensive coordination chemistry II vol 9 , Immobilization studies (see also Chapter 9.9), 4 Metal Complexes as Catalysts for Oxygen, Nitrogen, and Carbon-atom Transfer Reactions, 5 Metal Complexes as Catalysts for H-X (X=B, CN, Si, N, P) Addition to CC Multiple Bonds, BBR3464. A trinuclear platinum clinical agent, Site-specific intrastrand and interstrand cross-links of BBR3464. Bending, protein recognition and nucleotide excision repair

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