Photochemistry of Ruthenium Complexes - Essay Example

4 November 2008 Photochemistry of Ruthenium Complexes Background and significance of the topic Photochemistry is the utilization of light to produce energy. This is most illustrated in the process of photosynthesis which is unique to plants. Plants, through specialized structures called chloroplasts (or chromophores), trap the energy coming from the visible spectrum of sunlight. Chromophores are able to trap energy due their large conjugated double bond systems. Once a photon, or stream of light particles, is absorbed, its energy is transferred to the electrons in the double bonds. This raises the energy level of electrons from the ground state to the excited state. In packed systems, this will lead to two events: the transfer of excitation energy or the transfer of the electron itself to a neighboring complex that has a lower energy level. Eventually, the series of reactions will result in the production of NADPH and ATP. The net photochemical reaction involves the splitting of water, which is the ultimate electron donor, to ½ O2, 2H+ and 2 electrons (summarized in Mathews and Van Holde). The photochemical reaction in photosynthesis was the basis for developing inorganic systems that can make use of the high potential of solar energy. This is especially important now that supply of fossil fuels like gasoline and diesel is affected by political, climate and environmental events. The recent erratic changes in price and supply coupled with the high demand for fossil fuels increased the search for renewable and cheap sources of energy. Sunlight is free and harnessing solar energy is one of the main objectives for developing photochemical technologies. In this context, ruthenium complexes have very important roles to play. Ruthenium complexes have long been studied for their many uses in the energy, chemical and lately, in the medical industry. Properties of Ruthenium Ruthenium (Ru) is a rare transition metal which falls under the platinum group in the periodic table of elements. Its atomic number is 44, and has oxidation states ranging from -2 to +8 but the most common are +2, +3 and +4. In its elemental state, ruthenium is easily oxidized by air to form ruthenium oxide, RuO4. It does not react with acids but easily reacts with bases and halogens. Small amounts of Ru are added to platinum, palladium, gold and titanium to produce hard and tarnish-resistant alloys. Ruthenium, as part of an alloy or when complexed with other compounds, is also utilized in other applications like catalytic reactions, electrolytic protection, optic sensors, microelectronics, organic and polymer synthesis (Dragutan and Dragutan), anti-cancer agents (Dougan, Habtemariam and McHale) and protein hydrodynamics (Terpetschnig, Szmacinski and Malak). Photochemistry of Ruthenium Complexes In biological systems, minute amounts of heavy metals play many roles in DNA synthesis, enzyme catalysis and overall metabolism. Prominent are the metalloproteins where proteins have metal centers that are indispensable in the performance of cellular functions (Mathews and Van Holde). Thus, it is not a novel concept to make use of metals in mimicking biological systems especially the photochemical energy harvesting system in photosynthesis. The capacity of metals to transfer light energy (Balzani, Credo and Scandola) is of premium interest in the area of solar energy production. Ruthenium complexes are prevalent in the field of photochemistry because of their capacity to strongly absorb visible light and are photochemically and thermally stable. Ruthenium, due to its many oxidation states, has been used for many years in producing complexes with different ligands for use in the solar energy production industry. Among the most investigated are the photochemical reactions of the Ru(II) polypyridyl complex, ruthenium (II)(2,2’-bipyridine) or Ru(bpy)3+2 with the basic molecular structure presented in Figure 1. This complex is also named as tris(2,2’-bipyridine) ruthenium(II). In this paper, the complex will be referred to as Ru(bpy)3+2. To this basic structure, modifications have been made like addition of secondary ligands and substitution of the primary bipyridine ligands. The complex is formed from the reaction of ruthenium trichloride with bipyridine in the presence of hypophosphoric acid. Pyridines are heterocyclic organic or carbon compounds that have a single nitrogen moiety. Note that pyridines have 3 double bonds within a single pyridine ring. The pyridine Nitrogen individually binds covalently to the ruthenium metal center and in the process shares their electrons with the metal. This lends stability to the complex and increases efficiency of photon trapping and energy transfer. During the complexation reaction Ru(III) is reduced to Ru(II). Ru(bpy)3+2 is chiral and six-coordinated, with three bipyridines surrounding the Ru(II) center. Ru(bpy)3+2 is a versatile chromophore, has a long half-life of 600 nanoseconds during its excited state which allows it to transfer this energy and strong luminescence (low photodissociation) at room temperature. The partial mechanism for the conversion of light energy to chemical energy by Ru(bpy)3+2 was first reported in 1976 (Sprintschnik, Sprintschnik and Kirsch). The researchers reacted Ru(bpy)3+2 with dihydrocholesteryl esters (long-chained compounds) which yielded long-tailed surfactant complexes that were insoluble in water. The compounds were spread sheets of wet glass under continuous light irradiation. It was observed that the complexes gave rise to a steady stream of molecular hydrogen and oxygen. Apparently, the dihydrocholesteryl esters lowered the barrier to the electron transfer process. Currently, the mechanism for the energy transfer has been elucidated (de Paula). When light photons are absorbed by Ru(bpy)3+2, two events are possible: transfer of energy or transfer of electrons to other molecules or compounds (acceptors or quenchers) that are present in the solution. Energy transfer involves the transfer of energy from a higher excitation state to a lower excitation state. Ru(bpy)3+2 has been studied as a photosensitizer for both the oxidation and reduction of water. Upon absorbing a photon, Ru(bpy)3+2converts to the triplet state, Ru(bpy)3+2* which transfers an electron from a ligand, to an oxidant in solution. Ru(bpy)3+2* which has high reducing power can reduce many compounds including methylviologen, a renewable electron carrier (Ohtani, Kobayashi and Ohno). This reaction has been utilized in photocatalysis. The transfer of electrons oxidizes Ru(bpy)3+2 to Ru(bpy)3+3 while producing an anion in the solution. Free energy is produced by the electron transfer which is used to generate H2 and O2 from water. The accompanying reaction also reverts the Ru(bpy)3+3 to the original Ru(bpy)3+2. (1) 2 Ru(bpy)3+3 + H2O → 2 Ru(bpy)3+2 + ½ O2 + 2H+ (2) Q- + H+ → Q + ½ H2; where Q is the acceptor compound in the reaction solution. Ru(bpy)3+3 is a very strong oxidant and is therefore able to oxidize water to its component elements. Subsequently, the H2 and O2 may be utilized as fuel in fuel cells where H2 is “split” to give water and free energy where the energy released can be used in producing work. The reaction is catalytic, therefore there are no by-products that can pollute the environment like what happens when fossil fuels are utilized. In its lowest excited state, Ru(bpy)3+2 makes use of metal to ligand charge transfer (MTLC). This property permits the control of the ligands and easy incorporation and replacement of anion binding sites (Choi and Hamilton). This property has resulted in the many derivatives made from the basic Ru(bpy)3+2 structure. Studies have measured the Stark spectra of metal tris(2,2′-bipyridine) complexes of Ru in the ligand-centered region in the near UV. The results show that excitation into these ligand-centered absorption bands of the metal complexes resulted in substantially larger than expected from purely ligand-centered transitions. Thus, it is possible that ligand-centered excited states occur with MLCT states (Hug and Boxer). The family of derivatives of Ru(bpy)3+2 is enormous. These complexes are widely used in biodiagnostics, photovoltaics and organic light-emitting diode. Some Important Uses Of Ru(bpy)3+2and Its Derivatives The molecular structures of ruthenium complexes have the same structure as tris (2,2’-bipyridine)ruthenium(II). Changing the ligand either singly or totally results in the same general purpose but different specificity of functions. 1. Dye-sensitized solar cells (DSC) were invented by Michael Gratzel in his laboratory Ecole Polytechnique Fédérale de Lausanne. DSC provide a cheaper alternative to the use of solar cells that have been traditionally made up of silicon. The solar energy is harvested by the sensitizer, the yellow dye, which is a ruthenium complex with derivatives of the bipyridine ligand specifically 2,2-bipyridyl-4,4-dicarboxylate. Electrons are transferred from the excited Ru complex into the conduction band of the semiconductor oxide. Subsequent redox reactions regenerates the dye through an organic solvent (usually iodide/ triiodide) which itself is regenerated through reduction of triiodide at the counter electrode. The major shortcoming of earlier DSC is the low thermal stability; however, the need for a means to use clean energy sources have revved up the research on improving thermal stability. A modification of the ruthenium ligands to produce the amphiphilic ruthenium sensitizer cis-RuLL(SCN)2 where L = 4,4-dicarboxylic acid-2,2-bipyridine, and L = 4,4-dinonyl-2,2-bipyridine in together with a quasi-solid-state polymer gel electrolyte resulted in higher thermostabiity (Wang, Zakeeruddin and Moser). The new Ru complex which had heterogeneous ligands appeared to have significant contribution to thermal stability. 2. Photocatalysis. Ruthenium complexes have been used to facilitate the fixation of some compounds (which could be in excess) into forms that are potentially useful. The use of Ru(bpy)3+2 to catalyze, in the presence of light, the reduction of carbon dioxide to formic acid (Ramaraj and Premkumar). This opens the way for carbon dioxide fixation in solution instead of metal or solid matrices. 3. Flourophores. Ruthenium complexes with asymmetric have been used to measure macromolecular protein dynamics (Terpetschnig, Szmacinski and Malak). Due to its symmetrical nature, the Ru(bpy)3+2 complex has not been used as anisotropy probes (to measure the polarization of emitted light after excitation of the sample with polarized light). To improve the measurments, the researchers synthesized a less symmetrical Ru-complex, which covalently bonded to proteins allowing for the measure of rotational parameters. 4. DNA intercalation sensors. Ruthenium complexes using different ligand combinations have been used in studies to understand processes and structure of the DNA helix. In the early 1980s, it was shown that tris phenanthroline complexes of ruthenium (II) has enantiomeric selectivity in binding to DNA. This spectroscopic property of the Ru complex was used to distinguish right- and left-handed DNA helices (cited in Nagababu, Kumar and Reddy). Among the processes that have been studied with Ru complexes as tools are the electron transfer rates during metabolic reactions (example, phosphorylation), photocleavage of the DNA strand and electrochemical reactions. The use of Ru complexes and its capacity to produce luminescence have also been employed to detect and locate specific cells and proteins. Notably, there are efforts to utilize these complexes in targeting tumor cells. 5. Anti-cancer agents. Ru complexes are also potential sources of drugs; mainly due to their photochemical properties. It was shown by a recent study theat the use of an iodide ligand and a phenylazopyridine ligand produced inert ruthenium arene complexes [(η6-arene) Ru(azpy)I]+ (where arene = p-cymene or biphenyl, and azpy = N,N-dimethylphenyl- or hydroxyphenyl-azopyridine) in aqueous solution. However, these complexes were found to be highly toxic to certain ovarian and lung cancer cells in humans (Dougan, Habtemariam and McHale). Fluorescence-trapping experiments in cancerous cells suggested that increased amounts of reactive oxygen species led to the observed cytotoxities. The presence of micromolar ruthenium complex concentrations catalyzed the oxidation of glutathione (γ-l-Glu-l-Cys-Gly) to glutathione disulfide in the presence of micromolar ruthenium concentrations. Nevertheless, this being an initial study, the role of ruthenium complexes and its actions need further elucidation. References Balzani, Vicenzo, Alberto Credo and Franco Scandola. "Supramolecular Photochemistry and Photophysics, Energy Conversion and Information-Processing Devices Based on Transition Metal Complexes." Transition Metals in Supramolecular Chemistry. Netherlands: Kluwer Academic Pulbishers, 1994. 1-32. Choi, Kihang and Andrew Hamilton. "Flourescence Sensing of Anions." Encyclopedia of Supramolecular Chemistry. Marcel Dekker Inc., 2004. 566-571. de Paula, J.C. "Photochemistry of a Ru(II) Complex." Chem 302. n.d. Dougan, Sarah J, et al. "Catalytic Organometallic Anticancer Complexes." Proceedings of the National Academy of Science, USA 105.33 (2008): 11628-11633. Downy, Therese Malcom and Timothy A. Nieman. "Chemiluminescence Detection Using Regenerable Tris(2,2’-bipyridyl)ruthenium( I I) Immobilized in Nafion." Anal. Chem. 1992, 64, 261-266 64 (1992): 261-266. Dragutan, Valerian and Ileana Dragutan. "Ruthenium Vinylidene Complexes." Platinum Metals Reviews 48.4 (2004): 148-153. Gratzel, Michael. "Solar Energy Conversion by Dye-Sensitized Photovoltaic Cells." Inorganic Chemistry 44.20 (2005): 6841-6851. Hug, Stephan J and Steven G Boxer. "Dipolar Character of Ligand-centered Transitions in Transition Metal Tris-bipyridyl Complexes." Inorganica Chemica Acta 242.1-2 (1996): 323-327. Mathews, Christopher K and K.C Van Holde. Biochemistry. Second. Menlo Park: The Benjamin Cummings Publishing Company, Inc.,, 1996. Nagababu, P, et al. "DNA Binding and Photocleavage Studies of Cobalt(III) Ethylenediamine Pyridine Complexes." Metal Based Drugs (2008). Ohtani, Hiroyuki, et al. "Nanosecond Spectroscopy on the Mechanism of the Reduction of Methylviologen Sensitized by Metallophthalocyanine." Journal of Physical Chemistry 88 (1984): 4431-4435. Ramaraj, Ramasamy and J. Rajan Premkumar. "Photocatalytic reduction of carbon dioxide by immobilized nickel(II) and ruthenium(II) complexes into a Nafion membrane." Current Science 79.6 (200): 884-886. Sprintschnik, Gerard, et al. "Photochemical reactions in organized monolayer assemblies. III. Photochemical cleavage of water: a system for solar energy conversion using monolayer-bound transition metal complexes." Journal of the American Chemical Society 98.8 (1976): 2337-2338. Terpetschnig, Ewald, et al. "Metal-Ligand Complexes as a New Class of Long-Lived Fluorophores for Protein Hydrodynamics." Biophysical Journal 68 (1995): 342-350. Wang, Peng, et al. "Electrolyte, A Stable Quasi-solid-state Dye-sensitized Solar Cell With an Amphiphilic Ruthenium Sensitizer and Polymer Gel." Nature Materials 2 (2003): 402-407. Read More