Abstract
Besides their widespread use in coordination chemistry, 2,2’‐bipyridines are known for their ability to undergo cis–trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecular level. However, the exploitation of such conformational switching in self‐assembly has remained unexplored. In this work, the use of 2,2’‐bipyridines as acid‐responsive conformational switches to tune supramolecular polymerization processes has been demonstrated. To achieve this goal, we have designed a bipyridine‐based linear bolaamphiphile, 1, that forms ordered supramolecular polymers in aqueous media through cooperative aromatic and hydrophobic interactions. Interestingly, addition of acid (TFA) induces the monoprotonation of the 2,2’‐bipyridine moiety, leading to a switch in the molecular conformation from a linear (trans) to a V‐shaped (cis) state. This increase in molecular distortion along with electrostatic repulsions of the positively charged bipyridine‐H+ units attenuate the aggregation tendency and induce a transformation from long fibers to shorter thinner fibers. Our findings may contribute to opening up new directions in molecular switches and stimuli‐responsive supramolecular materials.
Highlights
Subtle conformational changes of biomacromolecules, triggered by physiological stimuli, regulate various complex biological events, such as protein folding and membrane transport.[1]
Between December 2010 and September 2015, he headed an independent junior research group in the same institute focusing on the self-assembly and selfsorting of p-systems
We initially investigated the responsiveness of ligand 1 to the addition of trifluoroacetic acid (TFA) in aqueous solution by UV/Vis titration studies
Summary
Subtle conformational changes of biomacromolecules, triggered by physiological stimuli, regulate various complex biological events, such as protein folding and membrane transport.[1]. PH,[2d] or cation/anions.[2e] In recent years, different types of molecular switches derived from diarylethenes,[4a] spiropyrans,[4b] spirooxazines,[4c] fulgides,[4d] and flavylium[4e] have been widely explored, with some promising applications in bioimaging,[5a] drug delivery,[5b] organic light-emitting diodes,[5c] molecular electronics[5d] and catalysis.[5e] In this regard, 2,2’-bipyridine represents an archetypal example of a molecular switch that can change its conformation from the linear trans-state to the V-shaped cis-state by acid-induced protonation as well as by metal complexation.[6] To date, conformational switching of 2,2’-bipyridines has been mainly investigated at the molecular level, for example to obtain molecular hinges[7a] and cavitands.[7b] to the best of our knowledge, bipyridines have not been exploited as conformational switches in supramolecular polymerization We foresee that this concept would broaden the range of applications of bipyridines and complement the existing arsenal of tools in stimuli-responsive supramolecular materials. We have designed a bolaamphiphilic derivative 1, in which a central 2,2’-bipyridine unit is conjugated at the 4,4’-positions with oligophenyleneethynylene (OPE) fragments bearing hydrophilic triethyleneglycol (TEG) chains (Scheme 1, for synthesis and characterization, see the Supporting Information) Interestingly, we found that the acid-induced trans-to-cis conformational change of the 2,2’-bipyridine unit, along with the electrostatic repulsion resulting from protonation, immensely impact the molecular packing and attenuate the aggregation tendency of 1, leading to a transformation from long fibers to shorter thinner fibers
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