Structural fine-tuning of (-donor-spacer-acceptor-spacer-)_n type foldamers. effect of spacer segment length, temperature, and metal-ion complexation on the folding process

Abstract

The synthesis of a series of polymers (PDA-nOE) containing an alternating arrangement of electron rich (donor) and electron deficient (acceptor) aromatic units, which are linked by flexible oligo(oxyethylene) (nOE) spacers, is reported. The length of the oligo(oxyethylene) spacer was varied from tetra(oxyethylene) (n = 4) to hexa(oxyethylene) (n = 6), to examine the relative propensities of these polymers to form folded structures by virtue of intrachain charge-transfer interactions between the adjacent donor and acceptor units. Comparison of the proton NMR and UV-visible spectra of the three different polymers clearly reveals that PDA-4OE, which has the shortest tetra(oxyethylene) spacer, exhibits the greatest propensity to fold in the nascent form. A significant upfield shift of the aromatic protons upon lowering of temperature confirmed this greater tendency of the PDA-4OE to fold; the spectra of polymers with longer spacers, on the other hand, exhibited a very weak temperature dependence indicating their sluggishness to fold in nascent form. However, in the presence of suitable alkali-metal ions (or a polar solvent), the spectra of the polymers with longer oligo(oxyethylene) spacers also exhibit a much stronger temperature dependence (similar to the PDA-4OE case), implying the occurrence of a metal-ion (and solvophobic effect) assisted formation of folded structures. These NMR studies corroborate well with similar variable temperature UV-visible spectral studies, wherein the intensities of the D-A charge-transfer band is monitored as a signature of the folded structure; there were, however, some distinct differences in the nature of the variation in the case of nascent polymer samples that appear to reflect the differences in sensitivities of the two spectral techniques to the folding process. In an effort to better understand the folding process, model compounds of the type donor-spacer-acceptor and analogous DAD and ADA were synthesized. Comparison of the spectral changes seen in model compounds, both as a function of temperature and during alkali-metal ion titrations, with those seen in the analogous polymers helped confirm that the folded structures in the polymer clearly must involve intrachain charge-transfer interactions that exceeds simple pairing of adjacent donor and acceptor units; i.e., the acceptors in the folded polymer are sandwiched between donors and vice versa, thereby forming extended stacks.