Chemistry Reference
In-Depth Information
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(a)
(b)
Heating
Cooling
Figure 7.12
Molecular structure of the pyrene-based gelator and photographs
acquired under UV light of: (a) the gel state, and (b) the sol phase.
72
Below the micrographs, schematic illustrations showing the self-assembly
of the pyrene-based gelator in the two states are shown.
Thereafter, Kamikawa and Kato designed pyrene-based gelators containing
dendritic oligopeptides that were capable of ecient gelation of a wide variety
of organic solvents
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(Figure 7.12). The hydrogen bonds of peptide residues
together with the p-p interactions between the pyrene moieties facilitated the
self-assembly, and prompted the gelation process. Self-assembled structures
consisting of helical columnar assemblies were characterised by XRD, UV-
visible absorption and circular dichroism (CD) spectroscopies. Surprisingly, the
pyrene-based gelator shown in Figure 7.12 exhibited a drastic emission colour
change between the gel and sol phases under UV irradiation. The gel state
displayed a blue monomer emission, with bands located at 396 and 412 nm,
whereas the sol phase displayed a bright-green emission located around 480
nm, a band characteristic of pyrene excimers (Figures 7.12a and b). The re-
markable luminescent properties of pyrene derivatives incorporating alkynyl
units when they form gels in which gelled DMF, toluene, or cyclohexane has
also been shown.
73
The use of laser scanning confocal microscopy to charac-
terise the photoluminescence showed that it was independent of superstructure
size and that fibre entanglement prevents the ecient formation of excimers.
Apart from their optical properties, it was shown that the xerogels could be-
have as active layers in organic field-effect transistors.
