The Self-Assembly of Lipophilic Guanosine Derivatives - Molecular Self-Assembly: Advances and Applications - page 113

addition of potassium ions (Fig. 4.17, right). Addition of the cryptand
reverts the surface structure to the original ribbon-like pattern. The
designs can be switched reversibly for hours [32b].
This supramolecular dynamer, besides being of importance as
a model system to mimic the formation-annihilation of G-quartet-
based architectures, which might be of biological significance in the
frame of nucleic acid telomerase, expands the possibilities offered
by the scaffolding concept.
Molecular packing of open-shell molecules is crucial in originating
magnetic properties, such as ferromagnetism, in supramolecular
assemblies. Switching of the spin-spin interactions in organic
radicals is of particular interest for the development of molecular-
scale magnetic devices. When the spin exchange occurs only
through space, complete control of this interaction can be obtained
by modifying the spatial arrangement of radical centers. Strong
through-space interactions between organic radicals are difficult to
achieve because unpaired spins have to be located at a short distance
and only few successful reports have been published [33]. We have
shown that the scaffolding of the persistent radical unit 4-carbonyl-
2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) can be achieved by
taking advantage of the K

+

-templated self-assembly of guanosine
derivatives into highly directional hydrogen-bonded networks (Fig.
4.18). TEMPO-functionalized derivative of type 1b in the presence of

potassium ions forms a -symmetric octameric assembly in which

the nitroxyl moieties show a weak electron spin-spin exchange
interaction. Reversible interconversion fuelled by cation release and
complexation allows the switching between discrete quartet-based
assemblies and molecularly dissolved TEMPO-

D 4

(or its ribbon-like
supramolecular oligomers), thus controlling the intermolecular
weak spin-spin interactions [34]. More drastic magnetic changes,
as a consequence of reversible assembly/disassembly, are observed
for TEMPO derivative of type

1b

, where the presence of two open-
shell residues in the molecule leads, in the presence of templating
metal ions, to an octameric

1a

-symmetric assembly with 16 open-
shell moieties confined in a sphere with a diameter of ca. 30 Å [35].
Oligothiophenes are the most promising semiconducting
materials for organic electronics. In this context, the control of
the self-assembly through molecular engineering in thiophene-
based architectures is an important issue in order to direct and

C 4