Tuning the Extent of Chiral Amplification by Temperature in a Dynamic Supramolecular Polymer

Abstract

Here, we report on the strong amplification of chirality observed in supramolecular polymers consisting of benzene-1,3,5-tricarboxamide monomers and study the chiral amplification phenomena as a function of temperature. To quantify the two chiral amplification phenomena, i.e., the sergeants-and-soldiers principle and the majority-rules principle, we adapted the previously reported sergeants-and-soldiers model, which allowed us to describe both amplification phenomena in terms of two energy penalties: the helix reversal penalty and the mismatch penalty. The former was ascribed to the formation of intermolecular hydrogen bonds and was the larger of the two. The latter was related to steric interactions in the alkyl side chains due to the stereogenic center. With increasing temperature, the helix reversal penalty was little affected and remained rather constant, showing that the intermolecular hydrogen bonds remain intact and are directing the helicity in the stack. The mismatch penalty, however, was found to decrease when the temperature was increased, which resulted in opposite effects on the degree of chiral amplification when comparing the sergeants-and-soldiers and the majority-rules phenomena. While for the former a reduction in mismatch penalty resulted in a decrease in degree of chiral amplification, for the latter it resulted in a stronger chiral amplification effect. By combining the sergeants-and-soldiers and majority-rules phenomena in a diluted majority-rules experiment, we could further determine the effect of temperature on the degree of chiral amplification. Extending the experiments to different concentrations revealed that the relative temperature, i.e., the temperature relative to the critical temperature of elongation, controls the degree of chiral amplification. On the basis of these results, it was possible to generate a general "master curve" independent of concentration to describe the temperature-dependent majority-rules principle. As a result, unprecedented expressions of amplification of chirality are recorded.