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Furthermore, in dendritic structure containing several fluorophores multiple
excitation can lead to singlet-singlet annihilation and thus to fluorescence depolar-
ization. This channel of depolarization is not discussed in the present paper; for an
example, see Ref. [18].
11.3.1 Dendrimers Containing Multiple Dansyl Units at the Periphery
Poly(propylene amine) dendrimers G2
D
,G3
D
,G4
D
, andG5
D
(Scheme 11.5) contain
8, 16, 32, and 64 peripheral dansyl units
, respectively. The photophysical properties
are very close to that of the dansyl model compound [19].
Concerning the mechanisms of fluorescence depolarization, energy migration is
not highly efficient in these compounds because dansyl shows a very large Stokes
shift between the absorption (
D
l
max
¼
340 nm) and emission (
l
max
¼
515 nm) bands.
Indeed, according to F
orster equation [20], the overlap integral between absorption
andemissionspectraJ is 3
€
10
18
cm
3
/M and the distance value where the rate of
energy transfer and of intrinsic deactivation are equal (R
0
) is about 1.1 nm,
compared to an average estimated distance of about 1.3 nm for G4
(a radius of
about 2 nmhas been reported) [21]. Therefore, energymigration is not very efficient
compared to intrinsic deactivation and is not supposed to significantly contribute to
fluorescence depolarization.
D
11.3.1.1 Steady-State Properties
The fluorescence anisotropy value in dichlor-
omethane (Figure 11.2, dark gray bars) is practically zero for
and increases with
increasing dendrimer generation [22]. In a dichloromethane/propylene glycol 1:30
(v/v) mixture, r
ss
(Figure 11.2, light gray bars) is higher than in pure dichlor-
omethane because of the increased viscosity.Indeed,alsointhecaseofmodel
compound
D
dendrimer, nonzero values of r
ss
have been measured, different from the results in dichloromethane solution. The
steady-state anisotropy measured for the dendrimers Gn
D
and the smallest investigated G2
D
in dichloromethane/
propylene glycol 1:30 (v/v) is (i) much lower (about 0.2) than the maximum
anisotropy value r
0
¼
D
0.31 for dansyl fluorophore [23], suggesting that motion is still
present under these experimental conditions, (ii) increasing with dendrimer mo-
lecular mass, (iii) but much less sensitive to this parameter, compared to dichlor-
omethane solution (the same r
ss
values have been obtained for G3
,Fig-
ure 11.2). These results suggest that fluorescence depolarization takes place by two
processes, a slower one whose rate decreases with increasing dendrimer generation,
and a faster one that is not largely dependent on dendrimer generation and that is the
dominant one in more viscous solvents. We assign the slow process to the global
rotation of the dendrimer, and the fast one to local motions of the peripheral dansyl
units appended to the dendritic scaffold. The close similarity of steady-state
anisotropy observed for G3
D
-G5
D
in the more viscous mixture of
solvents is because local motions are not strongly influenced by the dendrimer
generation and they are mainly responsible for fluorescence depolarization.
D
,G4
D
,andG5
D
