to excited electronic states that involve more than one molecular
entity. For the monomeric species, either LC or MLCT excited
states (i.e., pp or dp electronic configurations, respectively)
determine the observed photophysical and photochemical pro-
perties. However, in the stacked planar complexes, metal–-
metal-to-ligand charge-transfer (MMLCT) states (i.e., dsp
electronic configurations) are populated and can be described as
electron density displaced from the interacting metal dz^2 orbitals
to the stacking ligands, which is usually associated to a red-
shifted absorption and emission. Further, dsps electronic con-
figurations can also become relevant when investigating
aggregated species. Modulating the intermolecular distance
allows, for instance, to tune the absorption and emission
wavelengths of the assemblies, as the d–d electronic interaction
is a function of distance. Further, the planar nature of Pt(II)
complexes also favors excimer formation, which can also display
red-shifted emission as compared with the emission of mono-
meric species. Pd(II) shares structural and chemical features
with Pt(II) within the group 10 of the periodic table, even though
its photophysical properties are less appealing. Nonetheless, Pd
complexes are very well known for their catalytic properties,
which might be exploited in photoactive supramolecular
architectures.
In the next section, we will divide the d^6 and d^8 metal
complexes and describe the possible modulation of their
properties upon aggregation.
III. Molecular Systems Based on Aggregates of d^6 Metal ComplexesA. PHOTORESPONSIVEASSEMBLIESBASED ONNONCOVALENT
INTERACTIONS
Synthetic chemists have applied the concepts of multivalency
and cooperativity to supramolecular chemistry to create, for
instance, biomimetic receptors able to recognize small molecules
(29,30), polysaccharides ( 31 ), and DNA ( 32 ). Most of these
systems are based on calixa[n]arenes ( 33 – 35 ), cucurbituril (CB)
(36,37), and cyclodextrine (CD) ( 38 – 40 ) (see Fig. 1), and they
represent ideal guests for assembling photoactive species.
It has also been shown that organic chromophores can be
used to reversibly assemble and disassemble large structures.
For example, Zhang and coworkers ( 41 ) reported the light
controlled assembly of a system based on a-CD and
diazobenzene-containing surfactant. As depicted in Fig. 2, the
54 CRISTIAN A. STRASSERTet al.