Photochemistry and Photophysics of Coordination Compounds

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-