Photochemistry and Photophysics of Coordination Compounds

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192 S. Campagna et al. ing intrinsic short excited-state lifetimes, provided that their intrinsic lifetime (that is, the reverse of the summation of the radiative and radiationless decay of the “isolated” chromophores) does not compete with the timescale for photoinjection. The electronic coupling between MLCT states and TiO2 conduction band is so efficient that electron injection takes place in the ultrafast regime. In particular, for the complex [Ru(dcbpy)2(NCS)2] (dcbpy=4,4 ′ -carboxylbipyridine), also called N3 or “red dye” (70), biphasic kinetics was reported for photoinduced electron injection [266, 267, 270]: the first ultrafast component was estimated to have a risetime of 28 fs and the slower component was reported to occur within the 1–50 ps time range [270, 271]. This behavior was initially interpreted on the basis of a two-state mechanism, the fast and slow components being attributed to injection from the MLCT singlet and triplet states, respectively. Also, the singlet-state injection was from nonthermalized vibronic states. Whereas injection from the singlet state was later confirmed (although with risetime slightly different, shorter than 20 fs), the origin of the picosecond component has been questioned: it has been recently proposed that such a “slow” component of electron injection arises from sensitizer molecules which are loosely attached to the semiconductor or are present in aggregated forms [271]. Dozens of Ru(II) complexes have been explored as sensitizers in dyesensitized solar cells of this type. We mention here some of the species exhibiting the best performances. The above mentioned N3 complex has very interesting properties: it shows a photoaction spectrum dominating almost the entire visible region, with incident photon-to-current conversion efficiency (IPCE) of the order of 90% between 500–600 nm. Short-circuit photocurrents exceeding 17 mA/cm 2 in simulated A.M. 1.5 sunlight and opencircuit photovoltages of the order of 0.7 V were obtained by using the couple I – 3 /I– as redox electrolyte [415]. For the first time a photoelectrochemical device was found to give an overall conversion efficiency of 10%. These performances, in part expected for the high reducing ability of the 3 MLCT state (ca. – 1 eV vs SCE) and the positive ground-state oxidation potential (+ 0.85 eV
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 193 vs SCE), contrast with the lower IPCE observed for other sensitizers having comparable ground- and excited-state properties [416]. It has been suggested that a peculiar molecular level property of the cis-[Ru(dcbpy)2(NCS)2] complex could affect one of the key processes of the cell mechanism. This view is consistent with the results of photoelectron spectroscopy and INDO/S calculations indicating that the Ru d orbitals interact strongly with the π orbitals of NCS, resulting in MOs of mixed nature [417]. In particular, the calculations show that the sulfur 3p orbitals give a considerable contribution to the HOMO of the complex. Hole delocalization across the NCS ligands can thus be responsible for an increased electronic matrix element for the electron transfer reaction involving TiO2/Ru III NCS and I – . This would lead to an increase of the rate constant of the reductive process, and as a consequence of IPCE. Even better photoelectrochemical performances, compared to those given by the complex [Ru(dcbpy)2(NCS)2], are featured by a species based on the terpyridine ligand [418]: TiO2 electrodes covered with the complex [Ru(L)(NCS)3] (L=4,4 ′ ,4 ′′ -tricarboxy-2,2 ′ :6 ′ ,2 ′′ -terpyridine) display very efficient panchromatic sensitization covering the whole visible spectrum and extend the spectral response at the near-IR region up to 920 nm, with maximum IPCE values comparable to that obtained with the dithiocyanate complex. Another species based on a substituted terpyridine is the mixed ligand complex [Ru(HP-terpy)(dmb)(NCS)], where P-terpy = 4-phosphonato- 2,2 ′ :6,2 ′′ -terpyridine and dmb = 4,4 ′ -dimethyl-2,2 ′ -bipyridine [419]. A quantitative study of dye adsorption on TiO2 has shown that complexes containing the phosphonated terpyridine ligand adsorb more efficiently and strongly, giving an adsorption constant about 80 times larger than that for the dicarboxy bipyridine compounds. Since one of the problems encountered with the carboxy polypyridine class of sensitizers is the desorption from the semiconductor surface in the presence of water, the search for new anchoring groups is advisable. Along this line of research, complexes based on the derivatization of 2,2 ′ -bipyridine with a phenylboronic functionality were prepared. The photoaction spectra of TiO2 electrodes sensitized with the [Ru(4phenylboronic-2,2 ′ -bipyridine)2(CN)2] complex showed IPCE values comparable to those observed for [Ru(dcbH2)2(CN)2], indicating that the new type of linkage does not reduce the electronic coupling between sensitizer and semiconductor [405]. 7.3 Supramolecular Sensitizers Besides mononuclear Ru(II) complexes, multinuclear (supramolecular) compounds, as well as chromophore–acceptor or chromophore–donor dyads made of Ru(II) species and organic quenchers, have been used as sensitizers. There are several aims that inspired the design of such systems:
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280 Topics in Current Chemistry Edi
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Photochemistry and Photophysics of
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Volume Editors Prof. Vincenzo Balza
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Topics in Current Chemistry Also Av
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X Preface nation Compounds. The ven
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Contents Photochemistry and Photoph
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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