Photochemistry and Photophysics of Coordination Compounds

72 N. Armaroli et al.
can be classified in three main categories, i.e. anionic complexes (e.g. alocomplexes),
neutral clusters and cationic complexes. The photochemistry of
Cu(I) complexes, also related to environmental aspects, has already been
reviewed [3, 4], here we will essentially focus on photophysics. Anionic complexes
do not exhibit attractive photophysical properties (e.g. luminescence),
unlike cluster and cationic complexes which show a very rich photophysical
behavior. Among the latter, the most extensively investigated are NN-type
(where NN indicates a chelating imine ligand, typically 1,10-phenanthroline)
or PP-type (where PP denotes a bisphosphine ligand). Both homoleptic
[Cu(NN)2] + and heteroleptic [Cu(NN)(PP)] + motifs have been investigated.
The coordination behavior of Cu(I) is strictly related to its electronic
configuration. The complete filling of d orbitals (d 10 configuration) leads
to a symmetric localization of the electronic charge. This situation favors
a tetrahedral disposition of the ligands around the metal center in order to
locate the coordinative sites far from one another and minimize electrostatic
repulsions (Fig. 1). Clearly, the complete filling of d orbitals prevents d-d
metal-centered electronic transitions in Cu(I) compounds. On the contrary,
such transitions are exhibited by d 9 Cu(II) complexes and cause relatively intense
absorption bands in the visible (VIS) spectral window. The lowest ones
extend into the near infrared (NIR) region (above 800 nm for the Cu(II) aqua
ion) [5] and deactivate via ultrafast non-radiative processes. The fact that
the lowest electronic states of Cu(II) complexes are ultra-short lived make
them far less interesting than Cu(I) complexes from the photophysical point
of view.
Fig. 1 Tetrahedral coordination environment typical of Cu(I) complexes