inorganic chemistry

The kinetic constants of the deactivation processes usually can-
not be measured directly. What can be easily measured is the
lifetime (t) of an excited state:

tðS 1 Þ¼

1

ðkicþkflþkiscÞ

ð 1 Þ

tðT 1 Þ¼

1

ðk^0 iscþkphÞ

ð 2 Þ

The orders of magnitude oft(S 1 ) andt(T 1 ) are approximately
10 ^9 – 10 ^7 and 10^3 – 100 s, respectively.
Other quantities that can be measured are the quantum yields
of fluorescence and phosphorescence:

Ffl¼

kfl

ðkicþkflþkiscÞ

ð 3 Þ

Fph¼

kph

ðk^0 iscþkphÞ





kisc

ðkicþkflþkiscÞ



ð 4 Þ

Deactivation of an excited state in fluid solution can occur not
only by the above-mentioned intrinsic (first order) decay
channels but also by interaction with other species (called
“quenchers”) following second order kinetics. The two most
important types of interactions are those leading to energy
(Eq. 6) or electron (Eqs. 7 and 8) transfer (A and B stand for
excited molecules):

Aþhn!A ð 5 Þ

AþB!AþB ð 6 Þ

AþB!Aþþ B ð 7 Þ

AþB!Aþ Bþ ð 8 Þ

In both cases, the luminescence of the species A is quenched,
and in the case of energy transfer, it can be replaced by the lumi-
nescence of species B (sensitization process). In dendrimers,
energy- and electron-transfer processes can also occur among
nearby molecular components incorporated in the dendrimer
structure. For example, in a dendrimer containing A and B com-
ponent units, excitation of A may be followed by energy (Eq.10)
or electron (Eqs. 11 and 12) transfer to B ( 5 ) (Fig. 3b):

110 VINCENZO BALZANIet al.