Photodynamic Control of the Chain Length in Supramolecular Polymers: Switching an Intercalator into a Chain Capper.
2020
Supramolecular systems are intrinsically
dynamic and sensitive
to changes in molecular structure and external conditions. Because
of these unique properties, strategies to control polymer length,
composition, comonomer sequence, and morphology have to be developed
for sufficient control over supramolecular copolymerizations. We designed
photoresponsive, mono acyl hydrazone functionalized benzene-1,3,5-tricarboxamide
(m-BTA) monomers that play a dual role in the coassembly
with achiral alkyl BTAs (a-BTA). In the E isomer form, the chiral m-BTA monomers intercalate
into stacks of a-BTA and dictate the chirality of the
helices. Photoisomerization to the Z isomer transforms
the intercalator into a chain capper, allowing dynamic shortening
of chain length in the supramolecular aggregates. We combine optical
spectroscopy and light-scattering experiments with theoretical modeling
to show the reversible decrease in length when switching from the E to Z isomer of m-BTA in
the copolymer with inert a-BTA. With a mass-balance thermodynamic
model, we gain additional insights into the composition of copolymers
and length distributions of the species over a broad range of concentrations
and mixing ratios of a-BTA/m-BTA. Moreover,
the model was used to predict the impact of an additive (chain capper
and intercalator) on the chain length over a range of concentrations,
showing a remarkable amplification of efficiency at high concentrations.
By employing a stimuli-responsive comonomer in a mostly inert polymer,
we can cooperatively amplify the effect of the switching and obtain
photocontrol of polymer length. Moreover, this dynamic decrease in
chain length causes a macroscopic gel-to-sol phase transformation
of the copolymer gel, although 99.4% of the organogel is inert to
the light stimulus.
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