Solute–solvent interactions are key for the assembly and proper functioning of biomacromolecules and play important roles in many fields of organic and polymer chemistry. Despite numerous reports describing the effects of (chiral) solvents on helical conformations of (supramolecular) polymers, the combination of chiral solvents and chiral monomers is unexplored. Here we report diastereomeric differences in the supramolecular polymerization of enantiomers of chiral triphenylene-2,6,10-tricarboxamides in chiral chlorinated solvents. Competition between the preferences induced by the stereocentres of the assembled monomers and those present in the solvent molecules results in unforeseen temperature-dependent solvation effects. By combining experiments and mathematical modelling, we show that the observed differences between enantiomers originate from the combined additive entropic effects of stereocentres present in the monomer and in the solvent. Remarkably, copolymerizations show that the chiral solvent can bias the copolymer helicity and thereby overrule the helical preference of the monomers. Our results highlight the importance of cumulative solvation effects in supramolecular polymerizations.
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All data supporting the findings are included in the manuscript and Supplementary Information. Individual data files are also available upon request from the corresponding authors.
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We thank L. de Windt for providing BTA and SBTA monomers, C. Kulkarni for fruitful discussions about solvation, and B. Markvoort and H. ten Eikelder for discussion on the calculations. This work was partially supported by the European Union’s Horizon 2020 research and innovation programme (Marie Skłodowska‐Curie grant agreement number 642083), the National Science Center, Poland (MINIATURA 3, grant number 2019/03/X/ST3/02148) and NWO (TOP-PUNT grant 10018944). Part of this work was carried out on the Dutch national e-infrastructure with the support of the SURF Cooperative.
The authors declare no competing interests.
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Extended Data Fig. 1 Spectroscopic characterization of TTAs 1 in bulk and concentrated CHCl3 solutions.
a) FT-IR spectra of TTAs 1 in bulk; b) FT-IR spectra of (S)-1 in CHCl3 at different concentrations; c) CD-spectra of (S)-1 at 512 μM at 20 °C and −5 °C; d) UV-Vis spectra of (S)-1 at 20 °C and −5 °C; AFM image of 100 μM of e) (S)-1 and f) (R)-1 spin-coated from 1-chlorooctane. FT-IR spectra in bulk (a) show absorption bands that are characteristic for helical supramolecular polymers based on three-fold hydrogen bond in previously studied, analogous benzene-1,3,5-tricarboxamides49. This absorption band pattern is maintained in concentrated solutions of chloroform (b). CD spectra of the same solutions at 512 μM (c) show a positive CD effect which saturates at −5 °C, thereby confirming full aggregation, which is also supported by UV-Vis spectra (d), which show hypsochromic shift upon cooling from molecularly dissolved state. Therefore, based on these multiple spectroscopic methods, we conclude that all experiments discussed below, which show the same CD pattern, are indicative of the formation of helically arranged monomers, stabilized by threefold hydrogen bonds within the supramolecular polymers. In addition, the AFM images of solutions of (S)-1 and (R)-1 spin-coated from 1-chlorooctane show long, fibrous structures obtained from 1-chlorooctane.
Extended Data Fig. 2 Cooling rate and concentration dependency on supramolecular polymerization of (R)-1.
a) VT-CD of (R)-1 in 1-chlorooctane at 300 μM at different cooling rates; b) VT-CD of (R)-1 in 1-chlorooctane at different concentrations at a cooling rate of 0.5 K min−1; c) VT-CD of (R)-1 in (S)-ClMeBu at different concentrations at a cooling rate of 0.5 K min−1; d) VT-CD of (R)-1 in (S)-CldMeOct at different concentrations at a cooling rate of 0.1 K min−1. Dashed lines in (b), (c) and (d) signify helix inversion temperature. Superimposable VT-CD of (R)-1 at 300 μM at variable cooling rate (a) show that the two competing helical states P and M are in thermodynamic equilibrium. When the concentration of the monomer is lowered (b), the stable aggregates are formed below the transition temperature, thus only M-helical aggregates for (R)-1 are observed. When the same experiments is carried in other solvents (c and d), only the transition temperature is shifted, while the competition between the states PR and MR remains monomer-concentration independent, supporting our thesis that it is the differential solvation that drives the thermodynamic equilibrium towards one of the helical states.
Extended Data Fig. 3 Influence of chemical structure and e.e. of the chiral solvent on the polymerization of n-1.
a) CD spectra of n-1 in different chiral, non-racemic solvents; b) VT-CD of n-1 of the samples from experiments showed on a); c) VT-CD of n-1 in 1-chloro-2-methylbutane at variable enantiomeric excess (e.e.) of the solvent. Optical path length: 0.1 cm. Complete mirror-symmetry breaking is achieved with use of the optically active solvents as confirmed by the values of the calculated molar circular dichroism from CD spectra in chiral solvents (a), Δε, of n-1, which are identical to the ones calculated for (S)-1 and (R)-1. The values were calculated using equation: ∆ε = (CD effect)/(32980 ∙ c ∙ l). Wherein c is the concentration of TTA in mol L-1 and l is the optical path length in cm. The determined values are: Δεn-1 = 182 L mol−1 cm-1 (in (S)-CldMeOct) and Δε(R)-1 = 187 L mol−1 cm−1 (in 1-chlorooctane). Optically active solvents induce only a single helical preference in the regime measured, as no transition between helical states is observed on the VT-CD (b). VT-CD of n-1 in ClMeBu at various e.e. shows non-linear dependency of the helical bias on the e.e. of the solvent (c). The results show an interesting insight into the solvation of n-1. It is clearly visible that at higher temperatures the helical bias is lower for all solutions below 25% of the solvent e.e. The increase of the bias is visible only at lower temperatures, where solvation is higher.
Extended Data Fig. 4 Influence of chemical structure and e.e. of the chiral solvent on the polymerization of (R)-1 and (S)-1.
a) VT-CD of (R)-1 in chiral solvents at rate 0.5 K min−1; b) VT-CD of (S)-1 and (R)-1 in (S)-CldMeOct (cooling rate 0.1 K min−1, 30 µM); c) VT-CD of (S)-1 in chiral solvents; d) VT-CD at 266 nm of (R)-1 (25 µM) and (S)-1 (20 µM) in (R)-CldMeOct (cooling rate 0.1 K min−1); e) VT-CD of (R)-1 in ClMeBu with varying e.e. of the solvent. VT-CD experiments of (R)-1 and (S)-1 in chiral solvents (a, c) show different profiles. Mirror-symmetry of the interactions is depicted by VT-CD of (S)-1 and (R)-1 in (S)-CldMeOct (b) and (R)-CldMeOct (d). Due to mirror-symmetry in interactions between (S)-1 / (S)-CldMeOct and (R)-1/ (R)-CldMeOct and vice versa, the helix inversion temperatures are identical. The sensitivity of these systems towards chiral solvation is further corroborated with the VT-CD of (R)-1in ClMeBu at varied e.e of the solvent (e). As the e.e is varied from racemic to enantiopure (S)-ClMeBu, the relative stabilities of the P and M helical aggregates are shifted and the transition temperature between the two helical states shifts to a lower temperature. Please note that concentrations in Figures b and d are different due to measurement error as well as concentrations of (S)-1 and (R)-1 in Figure d are also not equal. Since we assumed concentration-independence, we did not correct the experiments.
a) Micro-DSC curves of (S)-1 in (−)-MntCl (500 μM, 1 K min−1); b) VT-CD of (S)-1 solutions used for Micro-DSC experiments. Micro-DSC of (S)-1 in (−)-MntCl at 500 µM reveals a very small but reversible enthalpy change (a) which is observed around the temperature where the P-M transition occurs in the VT-CD experiments of the same samples (b). The exothermic transition upon cooling suggests that higher solvation is observed at low temperatures, which is a common phenomenon in crystals. The thermal hysteresis between heating and cooling, which is visible on the VT-CD curves is caused by high concentration which can alter the kinetics of the process due to high viscosity of the solution, but does not affect the thermodynamics.
Extended Data Fig. 6 Thermodynamic parameters obtained from the global fits of the homopolymerization of 1.
The thermodynamic parameters of (R)-1 and (S)-1 in (S)-CldMeOct obtained from the global fits of the thermodynamic mass-balance model to the experimental VT-CD data of all concentrations reported in Fig. 3 in the main text. aΔHe, X and ΔSe, X indicate the enthalpy and entropy of elongation of the polymers with X helicity by addition of a monomer. bNPX indicates the enthalpic nucleation penalty, which relates the formation of X-helical nuclei to the elongation of polymers with that helicity. cR is the gas constant, Δν the difference in the solvent interaction term between the polymer present above and below the inversion temperature and [solv] indicates the concentration of the solvent. In the fits from which those values were obtained ΔSlow T, Δν, NPhigh T and ΔNP were used as fit parameters and the other parameters were derived from these values. See pages 11–13 in the Supplementary Information for details.
Extended Data Fig. 7 Influence of the addition of decalin on the polymerization of (R)-1 and (S)-1 in (S)-CldMeOct.
VT-CD experiments in (S)-CldMeOct / decalin solutions of a) (R)-1; b) (S)-1 at 20 μM (cooling rate: 0.1 K min-1, l = 1 cm). Dilution of (S)-CldMeOct with decalin causes stabilization of the higher solvation state and a gradual shift of the P-M transition temperature (a and b). To explain this trend, we must set the opposite reference than it is shown on the Figure. The reference point is at 0% of (S)-CldMeOct (so at 100% decalin). At this composition the (S)-1 and (R)-1 aggregates are enantiomeric, and the VT-CD curves are mirror images. Titration of the decalin solutions with (S)-CldMeOct causes a non-symmetric response due to the arising diastereomeric relationship between (S)-1 and (R)-1 upon addition of the chiral, non-racemic solvent. Upon addition of (S)-CldMeOct, the M helices of (R)-1 as well as of (S)-1 are destabilized with respect to the P helices. Since M helices of (R)-1 are stable at lower temperatures, the P-M transition temperature of (R)-1 is moved faster towards low temperatures than the P-M transition temperature of (S)-1, which favors P helices at low temperatures. The preference of the solvent towards P helices is clearly visible on the copolymerization experiments (see Extended Data Fig. 8).
Extended Data Fig. 8 Additional plots and thermodynamic characterization of the copolymerization of (S)-1 and (R)-1 in (S)-CldMeOct.
a) VT-CD traces of the copolymerization of (S)-1 and (R)-1 in (S)-CldMeOct at different enantiomeric excesses of the monomer (c = 30 μM, cooling rate = 0.1 K min−1, optical path length = 0.1 cm). The vertical dashed line shows the temperature of intersection of all curves; b) Complete set of selected experimental (squares) and fitted (solid line) data of supramolecular copolymerization of (R)-1 and (S)-1 obtained from global fit; c) extracted CD effects at 266 nm of various compositions at temperatures of 20 °C and 81 °C; d) extracted P-M transition temperatures (Tinv) as function of the monomer composition. The copolymerization of (S)-1 with (R)-1 in (S)-CldMeOct reveals the solvent and the temperature effect on the preferred helicity of the copolymers. The chiral solvent generates an energetic difference between the P and M helices, thereby making them diastereomers. Clear stabilization of the P helix is visible since remarkably, at a temperature of 69 °C, no M helix can be present regardless of the monomer composition (a, dashed line). Moreover, plotting the CD effect at 20 and 81 °C always shows an excess of the P helices (positive CD effect) over M-helices (negative CD effect) with respect to the monomer e.e. (c). Interestingly, the P-M transition temperature with respect to the monomer e.e. shows a quasi-hyperbolic dependency (d), which is not symmetric due to the diastereomeric relationship. This means that addition of (S)-1 to (R)-1 at high (R)-1 content has an opposite effect on relative stability of P and M helices than addition of (R)-1 to (S)-1 at high (S)-1 content.
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Ślęczkowski, M.L., Mabesoone, M.F.J., Ślęczkowski, P. et al. Competition between chiral solvents and chiral monomers in the helical bias of supramolecular polymers. Nat. Chem. (2020). https://doi.org/10.1038/s41557-020-00583-0