Chemistry Reference
In-Depth Information
equipment, since it undergoes full depolarization during the excited state lifetime.
The anisotropy decays for eosin encapsulated in G4
with 0.1:1 or 7:1 ratio are shown
in the inset of Figure 11.4. The rotational relaxation time is lower when more eosin
molecules are encapsulated inside a dendrimer, but estimation of reliable
B
values is
precluded by the biexponential nature of the eosin fluorescence decay, as previously
observed [29]. These qualitative time-resolved results and the steady-state measure-
ments suggest that (i) when, at most, one eosin molecule is encapsulated inside G4
y
B
its motion is slowed down, so that
values are higher than in pure
dichloromethane solution, and (ii) upon encapsulation of more than one eosin
molecule per dendrimer an additional mechanism, that is, energy migration, con-
tributes to depolarize fluorescence.
r
ss
and
y
11.5.2 Dendrimer with a Cyclam Core Associated to a Molecular Clip
by a Zn(II) Ion
Dendrimer
(Scheme 11.10), consisting of a 1,4,8,11-tetraazacyclotetradecane
(cyclam) core with appended 12 dimethoxybenzene and 16 naphthyl units [31], is
able to coordinate Zn
2
þ
metal ions by its cyclam core [32]. This is demonstrated by
dramatic changes in the fluorescence spectra of the dendrimer. On the other hand, a
molecular clip
D
C
(Scheme 11.10), constituted by two anthracene sidewalls and a
benzene bridging units containing two sulfate groups in the para position [33], is able
to bind Zn
2
þ
ions giving rise to the complex
(Zn
2
þ
), as demonstrated by small
changesintheabsorptionandemissionspectra[34].ByadditionofZn
2
þ
ion to
an equimolar solution of
C
in dichloromethane/acetonitrile 1:1 (v/v), a
self-assembled system constituted by a dendrimer, a Zn
2
þ
ion, and a clip is formed:
C
D
and
C
(Zn
2
þ
)
D
, as demonstrated by efficient energy transfer from naphthalene fluor-
ophores of
[35]. The self-assembly process can also
be proved by fluorescence anisotropy measurements on the clip fluorescence. The
measured r
ss
value (Figure 11.5) (a) is very low (r
ss
D
to anthracene groups of
C
C
in solution at room temperature, (b) it does not show any appreciable change upon
addition of 1 equiv. of Zn
2
þ
ions, and (c) it is doubled (r
ss
¼
0.01) in the case of the clip
0.02) upon addition of 1
equiv. of Zn
2
þ
and 1 equiv. of dendrimer
. For the systems reported in Figure 11.5,
fluorescence depolarization can take place by one or more of the following
mechanisms: global rotation, local motion, energy migration between the two
anthracene sidewalls of the clip. The last process is not supposed to change rate
constant since just one molecular clip
D
C
is present in the self-assembled complex
C
(Zn
2
þ
)
. Local motions of the anthracene sidewalls are not expected to change in
the three situations. On the other hand, global rotation is significantly slowed down
because of an increase of the rotor hydrodynamic volume in going from
D
C
to
C
(Zn
2
þ
)to
(Zn
2
þ
)
is
easily revealed by changes in steady-state fluorescence anisotropy in solution. This
example is reminiscent of the biological applications of fluorescence anisotropy,
where association between biological macromolecules is revealed by this
technique [12].
C
D
. Therefore, association between dendrimer
D
and clip
C
