Control over the photochemical outcome of photochromic molecules in solution represents a major challenge, as photoexcitation often leads to multiple competing photochemical and/or supramolecular pathways resulting in complex product mixtures....
Besides their widespread use in coordination chemistry,2 ,2'-bipyridinesa re known for their ability to undergo cis-trans conformational changes in response to metal ions and acids, which has been primarily investigated at the molecularl evel. However,t he exploitation of such conformational switching in self-assemblyh as remained unexplored. In this work, the use of 2,2'-bipyridines as acid-responsive conformationals witches to tune supramolecular polymerizationp rocesses has been demonstrated. To achieve this goal, we have designed ab ipyridine-based linear bolaamphiphile, 1,t hat forms ordered supramolecular polymers in aqueous media throughc ooperative aromatic and hydrophobic interactions. Interestingly,a ddition of acid (TFA) inducest he monoprotonation of the 2,2'-bipyridine moiety, leadingt oas witch in the molecular conformation from a linear (trans)t oaV-shaped (cis)s tate. This increase in molecular distortion alongwith electrostatic repulsions of the positively charged bipyridine-H + units attenuate the aggregation tendency and induce at ransformation from long fibers to shorter thinner fibers. Our findings mayc ontribute to opening up new directions in molecular switches and stimuli-responsives upramolecular materials.
The coordination geometry of d 8 transition metal complexes has been successfully exploited as a tool to tune photophysical properties and self-assembly pathways of supramolecular polymerization processes, with a focus being primarily placed on organic media. Expanding such controlled supramolecular and photophysical properties to assemblies in aqueous media by molecular design is, however, still challenging due to the difficulty in programming noncovalent interactions in water. Herein, we tackle this challenge by analyzing the aqueous selfassembly of amphiphilic Pt(II) complexes of different molecular geometry in order to control self-assembly and metal−metal interactions in aqueous media. To this end, we have designed two Pt(II) complexes, 1 and 2, containing an identical oligophenyleneethynylene (OPE)-based aromatic scaffold that differ in the molecular geometry (linear vs V-shaped) imposed by ligand substitution and studied their comparative self-assembly behavior in aqueous media. Even though both molecules follow the isodesmic mechanism of self-assembly, their structural difference strongly influences the molecular packing in aqueous media, which in turn impacts the photophysical properties (i.e. absence or presence of MMLCT) and the self-assembly outcome. While the molecular geometry for 2 enforces short Pt•••Pt contacts driven by an efficient face-to-face stacking of the OPE backbone, the antiparallel packing of 1 with slight translational offset does not allow the formation of short Pt•••Pt contacts. Such a distinct interplay of interactions for 1 and 2 in aqueous media leads to significant differences in photoluminescence.
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