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Figure 4.19
(a, b) STM images of
on highly oriented pyrolytic graphite
(HOPG). (c) Molecular model of two adjacent B-type ribbons
(each made from 8 molecules), fitting the unit cell parameters
of the STM images.
6
Besides templating-ion addition and removal, the supramolecular
organization of derivative
can be changed in another way. In
chloroform the guanosine derivative, templated by alkali metal
ions, assembles via H-bonding in G-quartet-based
6
-symmetric
octamers; the polar guanine bases are located into the inner part
of the assembly and act as a scaffold for the terthienyl pendants.
But, upon changing the medium polarity, e.g., in the more polar (and
H-bond competing) acetonitrile, different aggregates are observed
in which the terthiophene moieties are
D 4
π
π
stacked in a helicoidal
(left-handed) arrangement in the central core, and the guanine
bases (free from hydrogen bonding) are located at the periphery and
exposed to the solvent [37]. The system can be switched back and
forth by subsequent addition of chloroform and acetonitrile (Fig.
4.20).
-
Figure 4.20
The solvent-controlled interconversion between two
supramolecular architectures (K + -templated octamer and
π
π
stacked terthiophene chains) of lipophilic guanosine
derivative
-
(the guanine is represented in red, while the
terthiophene moiety in yellow).
6
The solvent-induced switching can be easily followed by
CD spectroscopy (Fig. 4.21): the exciton couplet in the guanine
chromophore absorption region observed in chloroform disappears
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