Photochemistry and Photophysics of Coordination Compounds

98 N. Armaroli et al.
prisingly, PPh3. Although POP provides the best emission performance when
combined with phenanthroline ligands, a better result is found here for PPh3.
This shows that subtle and combined steric and electronic effects of both P–P
and N–N ligands are crucial for an enhanced light output, highlighting that
general rules able to predict the photophysical behavior of [Cu(NN)(PP)] +
complexes are not easy to draw. From an electronic point of view, both POP
and PPh3 units promote the usual blue shift of the MLCT state compared to
[Cu(NN)2] + compounds, as predictable by the substantially higher oxidation
potential of the Cu(I) center of heteroleptic [Cu(NN)(PP)] + complexes.
Dinuclear Cu(I) complexes have also been synthesized and investigated,
two of them (A and B) are depicted in Fig. 27. Despite the presence of a P–
P-type ligand, complex A shows a luminescence band peaked at 700 nm with
a lifetime of 320 ns in the solid state [132]. The X-ray crystal structure indicates
a distorted tetrahedral geometry which, combined to the scarcely
protective 2,5-bppz N–N ligand (2,5-bis(2-pyridil)pyrazine) leads to a weakly
red-emitting compound. By changing the N-N ligand (Fig. 27, B), a stronger
Fig. 27 Chemical structures of heteroleptic Cu(I) complexes A and B