Photochemistry and Photophysics of Coordination Compounds

86 N. Armaroli et al.
leading to a coordination geometry (and a spectrum) very similar to that of
the much simpler [Cu(1)2] + complex.
Some Cu(I)-bisphenanthroline compounds have also been utilized as receptors
for dicarboxylic acids [76] and spherical inorganic anions [77].
The recognition motif is established by suitably functionalizing one phenyl
residue in the 2 or 9 position of the phenanthroline chelating units, which
are made able to pinch the pertinent substrate. The formation of the supramolecular
adducts is monitored through the substantial changes of the MLCT
absorption bands that occur as a consequence of the modification of the coordination
geometry and related symmetry [76].
2.3
Excited State Distortion: Pulsed X-ray and Transient Absorption Spectroscopy
Upon light excitation of [Cu(NN)2] + complexes the lowest MLCT excited state
is populated, thus the metal center changes its formal oxidation state from
Cu(I) to Cu(II) [15]. The Cu(I) MLCT excited complex undergoes further flattening
compared to its ground state and assumes a geometry similar to that
of ground state Cu(II)-bisphenanthroline complexes [59]. In this excited state
flattened tetrahedral structure a fifth coordination site is made available for
the Cu(II) d 9 ion, that can be filled by nucleophilic species such as solvent
molecules and counterions. The intermediate species thus obtained is termed
“pentacoordinated exciplex”. The process, which is schematically depicted in
Fig. 16, had been proposed by McMillin and coworkers nearly 20 years ago
on the basis of classical photochemical experiments on series of [Cu(NN)2] +
complexes with increasingly nucleophilic counteranions [78].
Fig. 16 Flattening distortion and subsequent nucleophilic attack by solvent, counterion,
or other molecules following light excitation in Cu(I)-phenanthrolines. The size (and position)
of the R substituents is of paramount importance in determining both the extent of
the distortion and the protection of the newly formed Cu(II) ion from nucleophiles
In recent years this hypothesis has been nicely confirmed thanks to the
development of light-initiated time-resolved X-ray absorption spectroscopy