Photochemistry and Photophysics of Coordination Compounds

Photochemistry and Photophysics of Coordination Compounds: Copper 85
Fig. 15 Schematic orbital splitting diagrams for [Cu(NN)2] + . Left, D2d symmetry; right,
D2 symmetry. Grey arrows represent the transitions leading to bands I (— —), band II
(···), and band III (– – –)
A simple picture able to rationalize the MLCT absorption patterns of
these complexes in solution is difficult, nevertheless some general trends have
been found by analyzing the MLCT absorption maxima and molar extinction
coefficients of series of [Cu(NN)2] + compounds and have been reported
elsewhere [15].
The spectrum of [Cu(4)2] + in Fig. 13 exhibits a well-established fingerprint
for complexes with 2,9-aryl substitution of the phenanthroline ring, i.e.
a pronounced low-energy shoulder extending down to 650 nm (band I, see
above), which is practically absent in the case of [Cu(1)2] + .Thispatternisrelated
to the above mentioned intramolecular π-stacking interactions, which
make the transition corresponding to band I more permitted. The absorption
spectrum of [Cu(5)2] + is somewhat different if compared to both [Cu(4)2] +
and [Cu(1)2] + . The low energy band is wider and more intense and the peak
around 440, which is typically the most intense in [Cu(NN)2] + complexes, appears
as just a weak shoulder [74]. This trend is due to the presence of the
cumbersome tert-butyl groups on the phenyl residues, that limit π-π stacking
interactions and make the structure of the complex particularly rigid.
The MLCT absorption profile of [Cu(5)2] + is found to be similar to that of
aCu(I)-catenate complex made of two interlocked 27-membered rings containing
a phenanthroline moiety and exhibiting a very rigid coordination
environment [75]. The structural peculiarity of [Cu(5)2] + , as revealed by the
absorption spectrum, is in accord with its extraordinary kinetic inertness towards
demetallation [74].
Interestingly, it has been found that the complex [Cu(6)2] + (Fig. 14) exhibits
a very weak band above 500 nm despite the presence of phenyl rings in
the 2 and 9 positions [71]. Indeed this confirms the above described model:
the methyl groups in the 3 and 8 position contrast the flattening distortion,