Temperature-controlled helical inversion of asymmetric triphenylamine-based supramolecular polymers; difference of handedness at the micro- and macroscopic levels

Abstract

This study describes the temperature-controlled helicity inversion of supramolecular polymers based on N-triphenylamines (TPAs) bearing different numbers of d-alanine and/or glycine moieties. Despite chiral branching differences, the three investigated TPAs form helical supramolecular polymers via a typical nucleation-elongation model, characteristic of a cooperative assembly. The supramolecular polymer TPA-3, composed of symmetric tri-alanine segments, was more thermodynamically stable compared with TPA-1 and TPA-2, which had one or two alanine segments. More interestingly, at the microscopic level, temperature-dependent circular dichroism (CD) measurements revealed that the left-handed (M-type) helicity of supramolecular polymers TPA-1 and TPA-2 was inverted to the right-handed (P-type) helicity during heating. The (+/-) pattern of the vibrational circular dichroism (VCD) signal in the region of amide groups of self-assembled TPA-1 was also converted to the (-/+) pattern of the VCD signal. This indicates that the conformation of achiral gycine groups with the left-handed orientation was changed to the conformation of the glycine groups with the right-handed orientation. However, at the macroscopic level, the left- and the right-handed helical fibers were observed to coexist in self-assembled TPA-1 and TPA-2 at lower and higher temperatures. In contrast, no helicity conversion with the increase in temperature was observed for self-assembled TPA-3, which exhibited an ambiguous left-handed helical structure with a long helical pitch. Furthermore, the origin of the helical inversion at the microscopic level was supported via computational simulations. The helical inversion was possibly due to a molecular conformational change of flexible glycine moieties during the formation of supramolecular polymers. Based on AFM observations and DFT calculations, we conclude that even though either the positive or the negative signal of supramolecular polymers TPAs was obtained in CD spectra, the right- and the left-handed supramolecular polymer TPAs co-existed at the microscopic and macroscopic levels. Thus, the handedness of supramolecular polymers TPAs was determined by relative distribution of the right- and left-handed helical fibers.