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

Scheme 1. Molecular Structure of (a) Mono Acyl Hydrazone Functionalized BTA (m-BTA), (b) Achiral Octyl BTA (a-BTA) Monomers, and (c) Copolymer Formed by Intercalation of Chiral m-BTA into Helical Stacks of a-BTA

Figure 1

(a) Light-induced isomerization from the planar E isomer to the Z isomer of m-BTA. (b) Spectral changes with prolonged times of irradiation with UV light of λ = 310 or 405 nm. The band corresponding to the E isomer at λ = 308 nm decreases, and a new band from the Z isomer arises around λ = 375–405 nm when irradiated with UV₃₁₀ (c = 50 μM in MCH). (c) Decrease of the E isomer band upon irradiation for solutions of c = 10–50 μM in MCH.

Figure 2

Copolymerization of chiral acyl hydrazone functionalized E-m-BTA with a-BTA. (a) CD melting curves of copolymer mixtures (cooling rate = 1 K min^–1). (b) CD maximum determined by averaging the Δε value at 223 nm in (a) between 10 and 15 °C for the different ratios of E-m-BTA/a-BTA (error bars indicate standard deviation). (c) UV melting curves showing decrease in T_e with increasing mol % E-m-BTA of the copolymer mixtures. (d) Normalized CD and absorbance melting curves (c_tot = 25 μM in MCH, signal probed at λ = 223 nm).

Figure 3

Theoretical modeling of the copolymerization of E-m-BTA/a-BTA. (a) Calculated P_tot – M_tot over a range of temperatures (curves of 0 and 100 mol % m-BTA overlap at zero) and (b) with increasing mol % m-BTA at 25 °C. (c) M_tot + P_tot curves for the copolymer mixtures with increasing mol % of E-m-BTA. (d) Specimen plot of a 5 mol % mixture of E-m-BTA in a-BTA (calculations based on c_tot = 25 μM in MCH, ΔH_MAB = −46.5 kJ mol^–1, ΔS_AB = −54.2 J mol^–1 K^–1, and ΔH_P^pen = 2 kJ mol^–1).

Figure 4

Effect of light irradiation on m-BTA/a-BTA copolymers. (a) Decrease in CD intensity of 10 mol % m-BTA in a-BTA after 15 min of UV₃₁₀ irradiation (c_tot = 25 μM in MCH). Inset: gel-to-sol transformation of a copolymer gel of 10 mol % m-BTA in a-BTA (6 wt % in MCH) upon irradiation.

Figure 5

Decrease in copolymer length determined by SLS when switching m-BTA from an intercalating comonomer (E-m-BTA) to a chain capper (Z-m-BTA) with light. (a) Scattering profiles of 10 mol % m-BTA in a-BTA before, after UV₃₁₀, and after UV₄₀₅ irradiation. (b) Decrease in R_θ for 5 mol % m-BTA in a-BTA at an angle of 30° followed during E to Z isomerization and Z to E back-isomerization. (c) Effect of light stimulus on the chain length of the copolymers of m-BTA/a-BTA over a range of copolymer compositions. (d) Deviation of chain length in a-BTA/m-BTA stacks before and after irradiation with increasing mol % m-BTA. Total concentration was c = 2.0 mM in MCH in all cases.

Figure 6

Influence of concentration on decrease of chain length for dilution of a-BTA (black lines), for m-BTA as an intercalator (red lines), and as a chain capper (blue lines). The effects of the additive on the chain length predicted by the thermodynamic model are small at lower concentrations but become much more pronounced at higher concentration.