(donor) ground state molecule; for example, between excited
states of aromatic molecules (electron acceptors) and amines
(electron donors). In dendrimers containing a variety of compo-
nents, both exciplex and excimer formation can take place (see
e.g., Refs.(8,9)). In such a case, as many as three different types
of luminescence can be observed, namely,“monomer”emission,
exciplex emission, and excimer emission (Fig. 4b).
If there is a nonnegligible electronic interaction between adja-
cent chromophoric units already in the ground state, the absorp-
tion spectrum of the species may substantially differ from the
sum of the absorption spectra of the component units. When
the units have the same chemical nature, the interaction leads
to formation of dimers. When the two units are different, the
interaction is usually CT in nature with formation of CT
complexes. Excitation of such dimers leads to an excited state
that is substantially the same as the corresponding excimers,
and excitation of the CT ground state complexes leads to an
excited state that is substantially the same as that of the
corresponding exciplexes.
III. Dendrimers with One or More Metal Complexes as
Branching CentersIn this section, we will describe selected examples in which
metal complexes constitute the core (Fig. 2a) or the branching
points (Fig. 2b) of the dendritic architecture. Therefore, the den-
drimer does not exist by itself as a ligand.
A. A METALCOMPLEX ASCORE
Dendrimer 12 þ(Fig. 5) is a classical example of a dendrimer
built around a metal complex core. In this compound, the 2,2^0 -
bipyridine ligands, that constitute the first coordination sphere
of the Ru^2 þion, carry branches containing 1,3-dimethoxybenzene
and 2-naphthyl chromophoric units separated by aliphatic con-
nectors ( 10 ). Since the interchromophoric interactions are weak,
the absorption spectrum of 12 þis substantially equal to the sum-
mation of the spectra of [Ru(bpy) 3 ]^2 þ, which is characterized by a
broad spin-allowed Ru!bpy metal-to-ligand (MLCT) band
around 450 nm ( 11 ), and of the chromophoric groups contained
in the branches, which show very intense bands in the near UV
region.
PHOTOCHEMISTRY & PHOTOPHYSICS OF METAL COMPLEXES 113