inorganic chemistry

• dendrimers with one or more metal complexes as branching
  centers (Fig. 2a and b);


• dendrimers capable of coordinating one or more metal ions in
  their cavities (Fig. 2c and d);


• dendrimers assembled around metal ions without or with
  other ligands (Fig. 2e and f).
  We will focus on the remarkable luminescent properties of
  selected examples of compounds belonging to the above-mentioned
  categories.
  For the sake of simplicity, electronic transitions in metal
  complexes are usually classified on the basis of the predominant
  localization, on the metal or on the ligand(s), of the molecular
  orbitals involved in the transition ( 4 ). This assumption leads
  to the well-known classification of the electronic excited states
  of metal complexes into three types, namely, metal-centered
  (MC), ligand-centered (LC), and charge-transfer (CT). The CT
  excited states can be further classified as ligand-to-metal
  charge-transfer (LMCT) and metal-to-ligand charge-transfer
  (MLCT).

II. Intrinsic Photochemical and Photophysical Properties of

Organic Dendrimers

Organic dendrimers are constituted by molecular components
that can absorb light and, often, undergo luminescence. It is
therefore worthwhile recalling some fundamental aspects of the
processes that follow light excitation of an organic molecule and
then the complications that may arise because of the interactions
among nearby molecular components.
Figure 3a shows a schematic energy level diagram for a mole-
cule that could be a chromophoric and luminescent unit of an
organic dendrimer. In most cases, the ground state of a molecule
is a singlet state (S 0 ), and the excited states are either singlets
(S 1 , S 2 , etc.) or triplets (T 1 ,T 2 , etc.). In principle, transitions
between states having the same spin value are allowed, whereas
those between states of different spin are forbidden. Therefore,
the electronic absorption bands observed in the UV–visible spec-
trum of molecules usually correspond to S 0 !Sn transitions.
When a molecule is excited to upper singlet excited states, it usu-
ally undergoes a fast and 100% efficient radiationless deactiva-
tion (internal conversion, ic) to the lowest excited singlet,S 1.
Such an excited state undergoes deactivation via three compet-
ing processes: nonradiative decay to the ground state (internal

108 VINCENZO BALZANIet al.