Photodynamic Control of the Chain Length in Supramolecular Polymers: Switching an Intercalator into a Chain Capper

Abstract

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 gelto-sol phase transformation of the copolymer gel, although 99.4% of the organogel is inert to the light stimulus.

Dissecting the Role of the Sergeants in Supramolecular Helical Catalysts: From Chain Capping to Intercalation

“…Finally, the third category of sergeants, constituted of BTA Ala , BTA Cha cyclo and BTA Phe , yields short assemblies embedding few sergeants. Here, we must stress out that a precise distinction of the role of these sergeants between chain cappers [15b, 23] and competitors, [15a] in which the sergeant decreases the length of l‐BTA stacks by capping or by sequestrating l‐BTA monomers, respectively, is challenging. However, for BTA Phe , the DP n value of 24 extracted from SANS data at low q translates into a fraction of capper in stacks of 1/24=4 % if all chain‐ends are BTA Phe molecules.…”

“…Homochiral helices are present for f s 0 ≥39 % and f s 0 ≥50 % with BTA Leu and BTA Cha mixtures, respectively. Homogeneous distribution of the ligand and the sergeants within these long co‐assemblies generates an optimal chiral environment for the catalytic sites which in turn provide the optimal selectivity in the catalytic reaction of reference. Class b ( BTA Nle and BTA Abu ): These sergeants also co‐assemble efficiently with l‐BTA but partly act as chain cappers [15b, 23] leading to moderately long helices (DP n ≈40 for f s 0 =39 %). The fact that these helices are not perfectly homochiral, even though the fraction of sergeants in stacks is important, might be explained either by the fact that the number of sergeants per stacks is insufficient to fully bias the assemblies (i.e.…”

How to Determine the Role of an Additive on the Length of Supramolecular Polymers?

“…Our group developed an additive able to switch reversibly from an intercalator to a chain capper upon light irradiation. 71 This additive has a similar core to the BTA monomer, but contains a photoswitchable acyl hydrazone group on one side arm. The chain shortening induced by light was explained by the steric hindrance generated at the chain ends upon photoisomerization of the acyl hydrazone from the E to the Z isomer.…”

Controlling the Length of Cooperative Supramolecular Polymers with Chain Cappers

“…The design of a chain capper is straightforward because it is a monotopic derivative of the ditopic monomer. However, in cooperative supramolecular polymerization, the noncovalent analogue of covalent chain‐growth polymerization, the design of a chain capper is far from trivial and only two examples have been formally reported in organic solvent . The challenge in designing an effective chain capper arises from the same molecular features that make the polymerization cooperative: a correlated reactivity of the two binding sites of the monomer.…”