Chemistry Reference
In-Depth Information
increases the water-solubility and photostablity of the dyes, both important factors
for biomedical applications. Nevertheless, sometimes rotaxane formation is accom-
panied by a reduction of the fluorescence quantum yields. The modification of the
physicochemical properties of cyanine dyes by cyclodextrin complexation is a
highly selective, structure dependent phenomenon, which is dependent upon the
structure of the dye as well as the size of the host cavity.
3 Dye-Cucurbiturils
CBs are macrocyclic molecules that consist of 5-10 glycoluril repeat units joined
by pairs of methylene bridges (Fig.
7
). Because CBs are capable of binding or
hosting positively charged or neutral molecules within their hydrophobic cavities,
they are useful rotaxane macrocycles to protect dye molecules against aggregation
[
31
-
33
] and environmental impact [
20
,
34
]. CBs are also important for many
applications such as drug delivery [
35
], molecular recognition [
36
,
37
], and supra-
molecular catalysis [
38
,
39
]. Numerous articles and reviews on the synthesis,
chemical modifications [
40
-
42
], spectral, photophysical, and recognition properties
[
43
], and applications of CBs and dye-CB complexes were published [
44
-
46
].
Importantly, CBs with an
odd
number of glycoluril units are highly
hydrophilic
.
Cucurbit[5]uril (CB5) has very small inner volume which greatly limits its use in
hosting dyes. The cavity dimensions of cucurbit[6]uril (CB6) allow accommodation
of up to seven heavy atoms, but it is poorly water-soluble, which limits its biomedi-
cal applications. CB8-CB10 have much larger cavities that can encapsulate more
than one guest dye molecule, which does not prevent aggregation. CB8 has a similar
cavity volume as
-cyclodextrin but displays very low water solubility. CB7 seems
to be the most attractive macrocycle for development of advanced biomedical
markers because it combines sufficient water solubility and appropriate cavity size
for hosting organic dyes with at least 12 heavy atoms.
Encapsulation in a cucurbit[7]uril (CB7) macrocycle was suggested to help
improve solubility, prevent undesirable aggregation, and thereby increase the
quantum yields and fluorescence lifetimes of fluorescent dyes [
46
].
g
O
O
O
NN
NN
NN
R
RR
RR
R
NN
NN
NN
O
O
O
n
CB5
: n = 3;
CB6
: n = 4;
CB7
: n = 5
Fig. 7 Spatial structure of cucurbit[7]uril (CB7)
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