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Helical foldamers, oligomers that can adopt a compact and folded conformation, often
display complex dynamic conformational changes in a solution similar to most of the
biopolymers. Integration of stimuli-responsive components into foldamers may provide
means to control such conformational changes through external modulation, and thus give
access to functions such as controlled guest release or applications in material science. One
such component is 1,8-diaza-anthracene which forms a stable ‘anti-parallel’ photodimer upon
light irradiation. Recently, we have demonstrated a ‘parallel’ arrangement of several such
motifs in novel helix-sheet-helix architectures. In these cases, folding of the helix components
overcomes local dipolar repulsions between aromatic layers that arise from the parallel
arrangement of diaza-anthracenes in the sheets. This provided us an opportunity to explore
the photodimerization of diaza-anthracenes within the folded structures. Remarkably, the
predefined arrangement of the diaza-anthracene in the sheets facilitates their ‘parallel’
photodimerization, for the first time. This phenomenon has been utilized to control the
dynamic conformational exchange of several helix-sheet-helix foldamer architectures in
solution. We have also explored a spring-like function of longer sequence with multiple diazaanthracene
stacks upon photoirradiation as a result of global stiffening and length extension
of the molecule. Moreover, the inherent cavity present in these foldamers was used for
controlled guest binding and release by a light stimulus. In this presentation, some of these
exciting aspects of the foldamer chemistry will be described in detail. |