Photochemistry and Photophysics of Coordination Compounds

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Photochemistry and Photophysics of Coordination Compounds: Copper 95 which in principle can occur both via energy- and electron transfer, was evidenced by monitoring sensitized singlet oxygen luminescence in the NIR region [29, 36]. Optically pure dicopper trefoil knots with [Cu(NN)2] + -type cores have been reported to quench the emission of the Λ or ∆ forms of Tb(III) and Eu(III) complexes, a very rare example of enantioselective luminescence quenching [118]. [Cu(NN)2] + complexes have also been used as substrates for DNA binding, trying to take advantage of the sensitivity of the luminescence of Cu(I)phenanthrolines to the local environment [63]. The structure of the associates has not been clarified: both electrostatic binding and intercalation of the aromatic ligands between adjacent bases are possible. Cu(I)-porphyrins seem to be more promising substrates for DNA [63]. 3 Heteroleptic Diimine/Diphosphine [Cu(NN)(PP)] + Complexes 3.1 Photophysical Properties Heteroleptic Cu(I) complexes containing both N- and P-coordinating ligands, [Cu(NN)(PP)] + , have been studied since the late 1970s [119]. The replacement of one N-N ligand with a P-P unit is often aimed at improving the emission properties. Accordingly, the relentless quest for highly performing luminescent metal complexes [7] has sparked revived interest in these compounds in recent years [120–122]. The absorption and luminescence spectrum of [Cu(dbp)(POP)] + (dbp = 2,9-butyl-1,10-phenanthroline and POP = bis[2-(diphenylphosphino)phenyl] ether) is reported in Fig. 24, as a representative example for this class of compounds [123]. Substantial blue-shifts of the lower-energy bands are observed compared to typical spectra of [Cu(NN)2] + compounds (see Sect. 2). UV spectral features above 350 nm are due to ligand-centered transitions whereas those in the 350–450 nm window are attributed to MLCT levels. [Cu(NN)(PP)] + complexes are subject to dramatic oxygen quenching, as deduced from the strong difference in excited state lifetimes passing from air-equilibrated to oxygen-free CH2Cl2 solution, 250 ns and 17 600 ns in the case of [Cu(dbp)(POP)] + [123]. The character of the emitting state in [Cu(NN)(PP)] + complexes has been discussed since their first characterization [119] and now its MLCT nature is established experimentally and theoretically [120, 124, 125]. The electron-withdrawing effect of the P–P unit on the metal center tends to disfavor the Cu(I)→N–N electron donation, as also reflected by the higher oxidation potential of the Cu(I) center compared to [Cu(NN)2] + compounds [126], leading to a blue shift of MLCT transitions. This, according to the energy gap law [127], explains the emission enhance-
96 N. Armaroli et al. Fig. 24 Absorption and (inset) emission spectra of [Cu(dbp)(POP)] + in CH2Cl2 ment of [Cu(NN)(PP)] + , that typically falls in the green spectral window, compared to weaker red-emitting [Cu(NN)2] + complexes. The luminescence efficiency of MLCT excited states in [Cu(NN)(PP)] + compounds is strongly solvent- and oxygen-dependent because it can be decreased by exciplex quenching [128, 129], in line with what is observed for the [Cu(NN)2] + analogues (see above). Therefore, the geometry of [Cu(NN)(PP)] + complexes plays a central role in addressing the extent of luminescence efficiency, even though this is hard to predict a priori. A variety of bidentate phosphine ligands has been prepared to coordinate Cu(I) in tandem with phenanthroline-type units: bis[2-(diphenylphosphino) phenyl]ether (POP), triphenylphosphine (PPh3), bis(diphenylphosphino) ethane (dppe), and bis(diphenyl-phosphino)methane (dppm), represent some recent examples [120, 122, 123, 130, 131], Fig. 25. Among them, the family of mononuclear [Cu(phen)(POP)] + complexes proposed by McMillin (see Fig. 25 for the PP-type ligands), where phen indicates a variably substituted 1,10 phenanthroline, shows an impressive emission efficiency compared to [Cu(NN)2] + compounds [120]. Especially on passing from pristine phenanthroline to dimethyl- or diphenyl-substituted analogues, and thanks to the efficient steric and electron-withdrawing effects of the POP ligand, remarkable emission quantum yields (Φem ∼ 0.15 in CH2Cl2 oxygen-free solution) and long lifetimes (∼ 15 µs) have been measured. On the contrary, the replacement of the POP ligand with two PPh3 units, gives less remarkable results due to the lower geometric rigidity which leads to weak and red-shifted emissions comparable to those of
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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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