Photochemistry and Photophysics of Coordination Compounds

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Photochemistry and Photophysics of Coordination Compounds: Copper 87 (LITR-XAS) [79]. This pump-and-probe technique allows one to catch the transient oxidation state of a metal atom as well as its surrounding structures following photoexcitation via an ultrafast laser source; accordingly, it is particularly suited to investigating the process depicted in Fig. 16. Practically, the information obtained via LITR-XAS is a sort of snapshot of electronic excited states in disordered media (e.g. solution), which are generated via a UV-VIS femtosecond pump laser and subsequently probed with a 30–100 ps intense X-ray pulse produced by 3rd generation large synchrotron facilities [80]. This technique has been applied in solution studies to [Cu(1)2] + and unambiguously confirmed that, in the thermally equilibrated MLCT excited state, the copper ion is pentacoordinated both in poorly donor (toluene) [81] or highly donor (CH3CN) [82] solvents; in addition, the copper ion has thesameoxidationstateasthecorrespondinggroundstateCu(II) complex in both cases. Analogous investigations have been carried out also on Cu(I) complexes as solid crystals (“photocrystallography”) [55, 83]. LITR- XAS studies of [Cu(1)2] + in solution have been complemented by optical time-resolved spectroscopy, which evidenced spectroscopic features in the ps timescale, associated to excited state structural rearrangements, possibly flattening distortion [82]. We have also carried out femtosecond transient absorption studies on [Cu(7)2] + and [Cu(8)2] + in CH2Cl2 (Fig. 17) [84]. These complexes are characterized by alkyl- and more cumbersome phenyl-residues in the 2 and 9 position of the phenanthroline ligand, which imparts rather different photophysical properties (i.e. shape of UV-VIS absorption, luminescence spectra, excited state lifetime) [85]. Despite this diversity, femtosecond transient absorption spectra have revealed a dynamic process lasting 15 ps in both cases Fig. 18. Fig. 17 Ligands 7 (2,9-bis(4-n-butylphenyl)-1,10-phenanthroline) and 8 (2,9-di-n-hexyl- 1,10-phenanthroline) Specific assignment of the observed spectral variation to (i) flattening distortion or (ii) extra ligand pick-up, two processes that might also occur sim-
88 N. Armaroli et al. Fig. 18 Transient absorption spectral changes observed for [Cu(8)2] + in CH2Cl2 at λexs = 400 nm (150 fs Ti:Sapphire laser pulse). The spectra were recorded at delays of 2, 5, 10, 25, 100, 1000 ps following the excitation pulse. In the inset are depicted spectral decays at two selected wavelengths, λobs = 520 (full circles) and 590 (half-empty circles) ultaneously, is not straightforward. Access to the fifth coordinating position is likely to be less favored for the more congested phenyl–phenanthroline complex [Cu(7)2] + . Hence the identical rate constant observed for the two compounds in CH2Cl2, aswellasinCH3CN for another [Cu(NN)2] + complex [82], is likely to be associated with the flattening distortion which is expected to be less solvent- and ligand-dependent than picking up an external unit for coordination expansion. Transient absorption studies leading to similar results have also been carried out with monophenanthroline complexes [86]. 2.4 Emissive Excited State(s) and Luminescence Spectra The first report on [Cu(NN)2] + luminescence in fluid solution dates back to 1980, when it was shown that, upon excitation into the MLCT band region, [Cu(1)2] + exhibits a luminescence spectrum peaking around 700 nm and an excited state lifetime of 54 ns in air-equilibrated CH2Cl2 [87]. Luminescence from [Cu(NN)2] + complexes is observed in poorly electron donor solvents, typically CH2Cl2. At room temperature, the emission band is wide and exhibit λmax peaking between 680 and 740 nm, with rather low quantum yield (Φem 10 –3 –10 –4 ) [15]. Excited state lifetimes in CH2Cl2 solution are strongly dependent on the degree of excited state distortion and the protection to-
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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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258 Author Index Volumes 251-280 Be
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Subject Index Alkyne bridges 148 An
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Subject Index 273 Rh(III) cyclometa
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Photochemistry and Photophysics of Coordination Compounds

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