Stimuli-Directed Colorimetric Interconversion of Helical Polymers Accompanied by a Tunable Self-Assembly Process

In layman’s language, stimuli responsive polymers are materials that possess the capability to alter their chemical and/physical properties in response to some external stimuli. These materials have been intensively studied over the years for a diverse range of applications, e.g., for on-demand drug delivery, tissue generation/repair, biosensing, smart coatings, and artificial muscles. In recent times dynamic helical polymers, such as poly(phenylacetylene)s (PPAs), have caught the attention of many researchers as they represent a very interesting class of macromolecules, mainly unique due to their stimuli-responsive properties.

Interestingly, their helical sense and elongation can be manipulated by the action of external stimuli such as chiral additives, polarity changes or metal ions. It has already been established that changes on the elongation of the polymer can be accompanied by colorimetric changes, although the parameters or mechanism that govern these stretching or compression fluctuations in the polymer still remain unclear.

In a recent publication, researchers, namely: Dr. Rafael Rodríguez, Prof. Emilio Quiñoá, Prof. Ricardo Riguera, and Prof. Félix Freire, from University of Santiago de Compostela in Spain, presented a study where they opted to make a step forward in the study of stimuli-responsive helical polymers by acting selectively on the flexible fragment at the pendant group made by an achiral extensor and a chiral center. Their work was inspired by the fact that the kind of helical sense control they were proposing had never been explored, particularly, being usually the chiral moiety of a poly(phenylacetylene)s (PPA) the one affected by the presence of different external stimuli. Their work is currently published in the research journal, Small.

In brief, in order to proceed with the proposed design, the team took into consideration the fact that the elongation degree of a helical PPA (easily detected by CD and UV) was related to the steric hindrance of the pendant group attached to the phenyl ring. Bearing this in mind, they devised a pendant group that adopted the equilibrium between two interconvertible linear and bent structures.

The authors showed that through the manipulation of the conformational composition of an achiral spacer, it became possible, not only to activate/deactivate the chiral teleinduction commanded from a remote chiral fragment, but also to modulate drastically the 3D structure adopted by the polymer, i.e., from helical to planar or vice versa which in their case accompanied by a color change of the solution. In addition, the extended/bent equilibrium of the pendant group was demonstrated in the monomer unit (RMN), and was clearly evidenced in poly-1, by the two completely different scaffolds adopted in polar and low polar solvents characterized by using various techniques.

In summary, introduction of an interconvertible bent/extended structural motif in the pendants of a poly(phenylacetylene) allowed the polymer to switch from a predominant one-handed helix induced through chiral teleinduction to a quasiplanar helical structure and vice versa by the presence of an external stimuli. Altogether, the strategy presented here opened new way to the development of new sensors and stimuli responsive materials based on the reported type of behavior.