Multistate Chiroptical Switch Triggered by Stimuli-Responsive Chiral Teleinduction
Teleinduction of chirality, from a distant chiral center in the pendant, to the helical backbone of a poly(phenylacetylene) (PPA) was demonstrated by using an achiral, flexible and well-organized spacer originating a multistate chiroptical switch. Two PPA series, with pendants formed by one or two consecutive Gly residues C-attached to the PPA backbone and N-attached to (R)-or (S)-αmethoxy-α-trifluormethylacetic acid (MTPA), were prepared and the changes in the helical contain upon modification
... n upon modification of the MTPA conformation by external stimuli (i.e., polarity of the media, metal cations) were examined. Chiral teleinduction, imposing opposite helicities, was observed in the two polymer series due to the parallel β-sheet arrangements of the Gly residues orienting the chiral groups in specific directions along the external part of the helices. This teleinduction can be tuned inducing either the P or M helical sense of the polymer by controlled conformational changes on the chiral moiety attached to the achiral β-sheet spacer. This remote control of the helix can be switched On/Off by favoring/disfavoring the β-sheet arrangement of the Gly residues resorting to changes on the temperature and the addition of destabilizing agents (e.g., TFA). The control of the helical sense of dynamic helical polymers via external stimuli 1-14 has caught the attention of the scientific community due to the potential of these materials as sensors, 7-9 chiroptical switches, 10-12 chiral stationary phases 15 or chiral catalysts, 16-21 among other applications. In these polymers, the helical sense of the backbone is determined by the chirality of the pendant. 1-14 In this way, P or M senses can be selectively obtained depending on the R/S absolute configuration of the pendant chiral group. Studies by the groups of Nolte 22-23 and Yashima, 24 have demonstrated that when the pendant bears more than one chiral center, the helical sense of the polymer, is controlled by the group closer to the backbone. As for the distance between the chiral group and the polymeric backbone, Veciana et al.  have shown that the capability for helical induction decreases when the chiral center is shifted away along the pendant chain. Thus for an effective helical induction, the chiral group should be close to the backbone, limiting very much the structure of the pendants and the applicability of this approach for helical control of the polymer. In order to demonstrate how chiral teleinduction works in helical polymers, and trying to determine which parameters govern this communication mechanism between the distant chiral center and the helical backbone, Veciana and coworkers performed studies in helical polymers where the distant chiral group is connected to the backbone through quite rigid spacers (e.g., tetrathiafulvalene, TTF), which can self-assembly through ππ interactions. 27 Another interesting work in the same field was done by Percec and coworkers introducing connectors with selfassembling properties (e.g., dendrons). 28,29 All these studies, in combination with the different information obtained from the helical polymer field, allow us now to go deeply into the principles for chiral teleinduction in dynamic helical polymers. 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