Photochemistry and Photophysics of Coordination Compounds

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166 S. Campagna et al. charge-separated state, did not take place in the triad with D = PTZ (PTZ = phenothiazine subunit), the formation of such a charge-separated species was inferred for the species with D = DPPA (DPPA = diphenylamino moiety) at 150 K, although it could not be evidenced spectroscopically because it did not accumulate as a consequence of a fast recombination rate [282]. In spite of less control of distance and orientation between donor and acceptor, better results have been reported for a series of systems exemplified by 42 [283–285]. The components of this series of triads are a tris-bipyridine Ru(II) chromophore covalently linked to one or two phenothiazine electron donors and to quaternized bipyridinium electron acceptors. The saturated alkyl chains bridging the molecular components are electrically insulating and flexible. This latter point is apparently a drawback since it does not allow for control of geometry. Moreover, even the octahedral arrangement of the bpy subunits adds some difficulties in defining the real structure: for example, geometrical isomers can also exist, since each bpy of 42 is nonsymmetric. The compound 42 exhibited formation of the D + –P–A – chargeseparated state in dichloromethane at room temperature with an initially reported efficiency of about 26%. Such a value was later corrected to about 86% by using a slightly different solvent (1,2-dichloroethane) [283, 286, 287]. Once formed, the charge-separated state decayed with a relatively fast rate (kCR = 6.3 × 10 6 s –1 ), corresponding to a lifetime of about 160 ns. On the basis of redox data, the charge-separated state stored about 1.3 eV. Similar complexes were prepared that differed from one another by the length of the alkyl chains connecting the subunits and/or by changing
Photochemistry and Photophysics of Coordination Compounds: Ruthenium 167 the methylene chains connecting the quaternary nitrogens (and, as a consequence, the reduction potential of the electron acceptor) [283–287]. In a homogeneous series of experiments performed on such species, however, the efficiency of charge separation does not change appreciably, remaining larger than 0.80, although the driving forces and the rate constants of the various electron transfer steps, as obtained by independent studies performed on isolated dyads of the type D–P or P–A, were different. In D–P ∗ –A systems, the fully developed charge-separated state can be obtained, in principle, by two different routes (excluding direct electron transfer from D to A): (1) a route initiated by oxidative quenching, that is, the series of events described by the sequence D–P ∗ –A, D–P + –A – ,D + –P–A – ;and (2) a route initiated by reductive quenching, described by the sequence D– P ∗ –A, D + –P – –A, D + –P–A – . Both routes can also take place simultaneously. The comparison between the photophysical properties of the various triads and the corresponding isolated dyads of this family of compounds indicated that the emission decay rates of any D–P–A triad never differed by more than a factor of two from those of the P–A dyads, although the absolute decay rate values changed by over a factor of 10 3 (over the whole collection of compounds). This prompted the authors to attribute the initial quenching event in all the D–P–A triads of this family to oxidative electron transfer, with formation of the D–P + –A – intermediate, with the route initiated by reductive quenching playing a negligible role [288]. However, in all the P–A dyad systems, it was always impossible to detect the A – radical anion [284], indicating that back electron transfer in the P–A dyads was faster than the forward, oxidative electron transfer. This posed some problems in justifying the efficiency of formation of the fully developed charge-separated state, where apparently reduction of P + from D in D–P + –A – species efficiently competes with back electron transfer in the intermediate. In fact, this looks somewhat puzzling because the reductive electron transfer in D–P ∗ dyads is reported to be of the order of 10 6 s –1 [289], while oxidative electron transfer in P ∗ –A dyads ranges from 10 10 to 10 7 s –1 [284, 285, 290–293] and, based on the circumstances mentioned above, back electron transfer in P + –A – (and for extension in D–P + –A – ), opposing the formation of the fully developed charge-separated state, could be even faster. To justify the experimental data, electron transfer from D to P + in D–P + –A – should be about 1 × 10 10 s –1 or faster. Therefore, the exceptional properties of these compounds as far as the efficiency of charge separation is concerned remained largely unexplained. A recent paper has shed light on the photophysical behavior of these triads [288]. A series of new experiments, including transient absorption measurements, emission decay, and a careful examination of the ground-state absorption spectra of the triads and of various separated dyad components, suggested that in the D–P–A triads of this family an association between the tethered phenothiazine electron donor subunit and the Ru(II) chromophore takes place, in a folded conformation. The association is already present in the
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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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