Photochemistry and Photophysics of Coordination Compounds

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188 S. Campagna et al. 3D-networked structures. In these systems, the Ru(II) polypyridine subunits carry their own photoluminescence properties in the networked assemblies. Finally, it has been demonstrated that the photoexcitation of suitable Ru(II) complexes can inhibit biological functions; for example, in Ru(II)labeled oligonucleotides DNA polymerase is inhibited by a photocrosslinking process [387]. On the basis of these and similar results, which indicate strong and even complete inhibition of gene transcription by photoexcited Ru(II) complexes [398], it has been proposed that properly designed compounds can be ideal candidates for a phototherapy with implemented fiber-optic light source [398–400]. It should be noted that the photoactivity of Ru(II) complexes for phototherapy does not depend on the presence of oxygen: this could represent a real advantage as compared to other dyes used in photodynamic therapy [398]. 7 Dye-Sensitized Photoelectrochemical Solar Cells One of the most important developments involving Ru(II) polypyridine complexes in the last two decades is related to the design of dye-sensitized photoelectrochemical solar cells, which have outstanding properties for application in the field of solar energy conversion, in particular photovoltaics. Since their appearance in the early 1990s [401, 402], dye-sensitized photoelectrochemical solar cells based on the principle of sensitization of wide-bandgap mesoporous semiconductors have indeed attracted the interest of the scientific community, due to their performances which started the vision of a promising alternative to conventional junction-based photovoltaic devices. For the first time a solar energy device operating on a molecular level showed the stability and the efficiency required for potential practical applications. In the last few years, several excellent review articles have been published in this field [403–405]. These articles indicated that research on dye-sensitized solar cells is strongly multidisciplinary, involving areas such as nanotechnology, materials science, interfacial electron transfer, and supramolecular photochemistry and electrochemistry [405]. Here we mention the main basic aspects and describe a few of recent Ru(II) photosensitizers which exhibit quite interesting performances. 7.1 General Concepts The principle of dye sensitization of semiconductors can be traced back to the end of the 1960s [406]. However, the practical use of these systems was limited for a long time because the efficiencies obtained with single-crystal substrates were too low due to the poor light absorption of the adsorbed
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 189 monolayer of dye molecules. The breakthrough in the field was brought about by the introduction of mesoscopic films made of sintered nanoparticles of a semiconductor metal oxide with a large surface area, which allowed the adsorption, at monolayer coverage, of a much larger number of sensitizer molecules leading to absorbance values, of thin films of a few microns, well above unity [407]. Wide-bandgap semiconductor materials such as TiO2 show a separation between the energy levels of the valence and conduction bands of the order of 3 eV, which means that the electron–hole pair needs to be produced by irradiation with light having a wavelength shorter than 400 nm, i.e., UV light. In order to use sunlight, mainly visible and near-IR, two general approaches have been developed: doping and molecular sensitization. The doping approach is the preferred choice for conventional photovoltaic devices [405]. Dye-sensitized photelectrochemical cells rely on photosensitization. In this process, a photoexcited species, the sensitizer (S), is capable of injecting an electron into the conduction band (CB) or a hole into the valence band (VB) of the semiconductor (Fig. 18). Fig. 18 Schemes for sensitized charge injection in the photoelectrochemical solar cells: a electron injection, b hole injection In fact, when the excited-state energy level of the sensitizer is higher with respect to the bottom of the conduction band, an electron can be injected with no thermal activation barrier in the semiconductor, leaving the sensitizer in its one-electron oxidized form (Fig. 18a). When the excited state is lower in energy with respect to the top of the valence band, an electron transfer (formally a hole transfer) between the semiconductor and the sensitizer can take place, leaving the molecule in its one-electron reduced form (Fig. 18b) [408]. The operation of a dye-sensitized solar cell is schematized in Fig. 19 [403, 405]. The system is comprised of two facing electrodes, a photoanode and a counter electrode, with an electrolyte in between. The transparent conductive photoanode is covered with a thin film (7–10 µm) of a mesoporous
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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X Preface nation Compounds. The ven
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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