Photochemistry and Photophysics of Coordination Compounds

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-