Photochemistry and Photophysics of Coordination Compounds

104 N. Armaroli et al.
Fig. 32 Schematic representation describing the relative positions of the potential energy
curves related to the emitting states of Cu4I4py4
same conditions) also indicate that CC excited states are quite distorted relative
to both the GS and XLCT levels. Such distortion was proposed as the
cause for the lack of communication between CC and XLCT states, for the
rigidochromism of the low CC energy band, and for the large reorganization
energies of electron transfer reactions involving CC excited states [143].
In an interesting recent work on clusters of general formula CuXL (L = N–
heteroaromatic ligands), it was shown that the energy of the emitting level
can be finely tuned [149]. The emission is attributed to a MLCT charge transfer
state, because no clear correlation between Cu–Cu distances and emission
maxima was observed and also because the effects of bridging halides were
smaller than those of N-heteroaromatic ligands, therefore the position of the
luminescence band can be varied by increasing the electron-accepting character
of the ligand L, Fig. 33. In these compounds Cu–Cu distances are in
the range 2.9–3.3 ˚A, accordingly the weak metal interaction prevent clustercentered
luminescence. Very recently, density functional theory calculations
have confirmed the involvement of the triplet cluster-centered (CC) and
triplet XLCT excited states as the origin of the dual emission [151].
It must be pointed out, however, that short Cu–Cu separation does not automatically
imply the establishment of metal-metal bonds and the effect of
the bridging ligands has to be taken into account. For example Cotton et al.