Enhanced Stability and Properties of Benzene-1,3,5-Tricarboxamide Supramolecular Copolymers through Engineered Coupled Equilibria

Abstract

Improving the stability of multi-component and functional assemblies such as supramolecular copolymers without impeding their dynamicity is key for their implementation as innovative materials. Up to now, this has been achieved by a trial-and-error approach, requiring the time-consuming characterization of a series of supramolecular coassemblies. We report herein that this is possible to significantly enhance the stability of supramolecular copolymers by a minimal change in the chemical nature of one of the interacting monomers. This is achieved by replacing an ester function by an ether function in the structure of a chiral benzene-1,3,5-tricarboxamide (BTA) monomer, used as "sergeant", coassembled with achiral monomers, the "soldiers". Pseudo-phase diagrams, constructed by probing the nature of the coassemblies with multifarious analytical techniques, confirm that the greater stability of the resulting copolymers is mainly due to the minimization of competing species. This leads to better rheological and catalytic properties of the corresponding supramolecular copolymers. Favouring coassembly over undesired assembly pathways must be considered as a blueprint for the development of better-performing supramolecular multi-component systems.

Keywords: benzene-1,3,5-tricarboxamide; competing assemblies; coupled equilibria; helical catalyst; supramolecular copolymerization.

Scheme 1

a) Coupled equilibria and their influence on the thermodynamic stability of supramolecular copolymers or coassemblies. An achiral monomer (“soldier”, green disk) is copolymerized with either “sergeant 1” (orange disk) or “sergeant 2” (red disk) that differs only by the nature of the lateral groups located in their side chains. The supramolecular helical copolymer formed by coassembly of the “soldier” and “sergeant 2” is more stable because competing species are minimized. For the sake of simplicity, only polymer (stacks, the desired SCP) and competing species are represented. b) Chemical structures of the main BTA monomers investigated in this study.

Figure 1

Structure of the homoassemblies. Schematic representation of the homoassemblies formed by BTA  (R)–Est and by BTA  (R)–Est as determined by FT‐IR, CD, and DOSY analyses performed in toluene and MCH (Figure S1–S4, Tables S1–S2).

Figure 2

Probing the structure and stability of the coassemblies between BTA  P and either BTA  Eth or BTA  Est. a) ITC enthalpograms obtained for toluene solutions containing BTA  P alone, and mixtures of BTA  P with either 50 % of BTA  (R)–Eth or 50 % of BTA  (R)–Est (total concentration in BTA=5.8 mM) injected into pure toluene, versus total BTA concentration in the cell at 293 K. ITC enthalpograms for homoassemblies and coassemblies have been compared in Figure S8. b) CD intensity (λ=295 nm) as a function of the temperature for the mixtures containing BTA  P (5.8 mM) and one of the enantiomers of BTA  Eth or BTA  Est (5.8 mM). Data recorded upon heating (0.5 K . min⁻¹). The elongation temperature (Te) was estimated at the onset of the rising of the CD signal. c) FT‐IR analyses of BTA  P alone (5.8 mM) and of the mixture containing BTA  P (5.8 mM) and 39 % of BTA  (R)–Eth (3.7 mM). Zoom on the N−H and C=O regions. FT‐IR spectra for full and no coassembly have been simulated as indicated in the SI. d) Plot of the fraction of “sergeant” in the stacks (fs_s) as a function of the fraction of “sergeant” initially introduced into the mixtures (fs₀). e) Pseudo‐phase diagram (temperature versus concentration) for the coassemblies between BTA  P and BTA  (R)–Eth (1 : 1 mixture). Data were extracted from ITC (293 K and 323 K) and CD (11.6 mM). For CD, points are taken at the onset (i.e., Te) and 80 % of the plateau of the CD signal. f) Pseudo‐phase diagram for the coassemblies between BTA  P and BTA  (R)–Est (1 : 1 mixture). Data were extracted from ITC (293 K) and CD (11.6 mM). Competing species are small mixed species between BTA  P and BTA  Est or BTA  Eth for which hydrogen bonding network does not exclusively involve amide C=O as acceptors but also ester or ether functions, respectively.

Figure 3

Macroscopic properties of the helical coassemblies. a) Relative viscosity for solutions containing BTA  P and either 50 % of BTA  (R)–Eth or 50 % of BTA  (R)–Est in toluene at 293 K. The indicated concentration corresponds to the total concentration in BTA. Schematic representation of the entanglements (light orange spheres) between stacks constituted of BTA  P and BTA  (R)–Et. b) Enantiomeric excess (ee) in NPnol as a function of the fraction of “sergeants” (either BTA  Eth or BTA  Est) in the helical catalysts. Conversion >85 % (for fs₀<10 %) and conversions >95 % (for fs₀≥10 %). c) Left: CD spectra for solutions containing BTA  P coordinated to copper (BTA  P/[Cu]=4) and either 50 % of BTA  (R)–Eth or 50 % of BTA  (R)–Est at various concentrations in toluene at 293 K. The indicated concentration corresponds to that of BTA  P. The molar extinction coefficient is calculated as Δϵ=θ/(32982×[BTA  P]×l), with θ=ellipticity (in mdeg), [BTA  P]=concentration in BTA  P (in mol . L⁻¹), and l=cell pathlength (in cm). Right: Kuhn anisotropy factor (g) as a function of the concentration of BTA  P for mixtures containing BTA  P coordinated to copper (BTA  P/[Cu]=4) and either 50 % of BTA  (R)–Eth or 50 % of BTA  (R)–Est. The Kuhn anisotropy factor is determined as g=θ²⁹⁵/(32982×Abs²⁹⁵) where θ²⁹⁵ and Abs²⁹⁵ are the ellipticity and UV/Vis absorbance measured at λ=295 nm, respectively.

Figure 4

Supramolecular copolymerization with another “soldier”. ITC enthalpograms obtained for toluene solutions containing BTA  C8 alone, and mixtures of BTA  C8 with either 50 % of BTA  (S)–Eth or 50 % of BTA  (S)–Est (total concentration in BTA=5.0 mM) injected into pure toluene, versus total BTA concentration in the cell at 293 K. Critical concentrations for BTA  C8 only (c*=0.62 mM) and the coassemblies with BTA  (S)–Eth (c*=0.40 mM) correspond to twice the concentrations at the mid‐point of the heat‐flow jump. Full coassembly is not reached for coassemblies with BTA  (S)–Est at the highest concentration studied (0.95 mM). It leads to the estimate that the c* value is superior to 2 mM. In that case, the residual enthalpy (ca. 1.5 kcal . mol⁻¹) at the highest measured concentration may correspond to the disassembly of short stacks or competing species between BTA  C8 and BTA  (S)–Est.