Photochemistry and Photophysics of Coordination Compounds

108 N. Armaroli et al.
which are values comparable to those of [Cu(NN)(POP)] + compounds (see
Sect. 3.1) [163].
This relevant result has been rationalized assuming that the specific complex
structure imposes minimal geometrical changes in the deactivation
process of the luminescent excited state back to the ground state, while maintaining
a significant energy gap. The BHQ ligands accomplish exactly this
requirement by providing considerable steric congestion in the vicinity of
chelation, which stabilizes Cu(I), while also providing interligand π-stacking
that distorts the ground-state geometry and favors the (formal) oxidation of
the metal with little structural change [163]. The same authors later proposed
Cu(I) complexes made of bisphenanthroline ligands with structures identical
to A, B and C (Fig. 36), but no emission data were presented [164].
2-Hydroxy-1,10-phenanthroline (Hophen) is a novel kind of substituted
phenanthroline that was recently proposed (Fig. 37). With Cu(I) it gives origin
to several compounds, as evidenced by X-ray crystallography, including
an unusual neutral dinuclear complex [Cu2(ophen)2] which exists in three
supramolecular isomeric forms and exhibits a broad and weak luminescencebandcenteredaround630
nm, which has been tentatively attributed
to deactivation of an MLCT state [165]. The same ligand, which is characterized
by complicated coordination modes involving both the regular nitrogen
sites and oxygen binding another metal ion (Fig. 37), was also used
to make metal complexes of other d 10 metal ions, i.e. Zn(II), Cd(II) and
Hg(II), which show ligand centered (LC) emission bands in the blue-green
region [166].
Fig. 37 The proposed ketone and hydroxy tautomers of Hophen
Vogler et al. have made several Cu(I) complexes exhibiting emission in different
regions of the VIS spectral window, including blue and red, and having
pure MLCT or mixed MLCT/LLCT character [167, 168]. For instance the complex
depicted in Fig. 38 which is easily accessible from commercially available
productsshowsaweakbutdistinctMLCTredluminescencepeakingat600
nm [167].
Several other unconventional ligands have been utilized recently to make
luminescent Cu(I) complexes such as thia-calix[3]pyridine (orange MLCT