Synthesis,Properties And Applications Of Photo-Responsive Macrocyclic Anion Receptors

Anions are ubiquitous in nature and are not only closely related to energy,materials and environmental issues,but also crucial in the fields of life and pharmaceutical science.As such,research on anion chemistry is of great scientific significance and practical value in solving problems such as major diseases,resource shortages and environmental pollution.The past decades have witnessed tremendous advances in anion recognition chemistry.Nevertheless,in most,if not all,current anion recognition systems,the anion binding and release processes are not controllable.Light is considered as a remote and efficient stimulus without introduction of any pollution,featuring easy operability and controllability,and it has been widely utilized in host-guest systems,e.g.anion recognition systems.However,most of the known light-responsive anion receptors have been reported to be acyclic ones,which prove far from ideal for binding anions due to their open recognition characteristics.This dissertation thus focuses on the design and synthesis of photo-responsive macrocyclic anion receptors or ion-pair receptors,the systematic studies on photo-responsive behaviors of the anion receptors for anion binding and release,as well as their applications in the field of anion（including ion-pair）extraction and separation.Anion extraction and separation controlled by light-responsive macrocyclic anion receptors could save time and energy for,inter alia,the back-extraction process,which will be beneficial for pushing the supramolecular extraction forward along the direction of high efficiency and low energy consumption.The main contents of this dissertation include four chapters as below:1.Photo-responsive macrocyclic HP₂O₇^3-（Pyrophosphate,PPi）receptor Ⅱ-1 was synthesized for the first time by combining an azobenzene building block with two urea motifs.Receptor Ⅱ-1-trans has a weak binding affinity to PPi and can be quantitatively isomerized to Ⅱ-1-cis configuration under 365-370 nm irradiation.The latter can strongly bind PPi through cooperative multiple NH hydrogen bonding to form Ⅱ-1-cis@PPi complexes.As inferred from the single crystal structure,¹H NMR titration and density functional theory（DFT）calculations,Ⅱ-1-cis has a high selectivity and binding affinity for PPi and the resulting Ⅱ-1-cis@PPi complex can dissociate over the course of the cis→trans isomerization under 420-425 nm irradiation,thus releasing PPi guests.The above binding-release process can be repeatedly for many cycles by simply changing the irradiation wavelength（365-370nm or 420-425 nm）.2.The photo-responsive macrocyclic sulfate receptors Ⅲ-1～4 were synthesized by integrating azobenzene unit with polyamides moieties into a macrocycle.The trans receptor Ⅲ-1（Ⅲ-2,Ⅲ-3,or Ⅲ-4）displays a bow-shaped conformation and the four amide NHs are not able to cooperatively bind anionic guests,allowing for a relatively weak binding affinity(～10²M^-1)toward sulfate.However,after irradiation with 365-370 nm light,the trans isomers can quantitatively photo-isomerize to the corresponding cis isomers.As a result,the four amide NHs work cooperatively to hold the four oxygen atoms of sulfate,resulting high selectivity and strong binding affinity(～10⁴M^-1)toward sulfate.Subsequently,upon irradiation with 420-425 nm light once again,the cis macrocyclic receptors could be photo-isomerized back to the trans conformation,releasing sulfate anions.The binding and release process can be efficiently repeatedly regulated for a couple of cycles simply by changing the irradiation wavelength（365-370 nm/420-425 nm）.Furthermore,the macrocyclic receptor Ⅲ-4-cis with enhanced solubility in chloroform was found able to recognize and extract SO₄^2-（as its tetramethylammonium salt）,wherein the sulfate release can be triggered by light,thus avoiding a tedious and energy-consuming back-extraction step and facilitating the use of light-responsive anion receptors in real-world scenarios.3.A light-responsive macrocyclic ion-pair receptor Ⅳ-1 was prepared by incorporating a functional acylhydrazone unit into a macrocycle with an alkoxy tether.As suggested by single-crystal structure,¹H NMR spectroscopic titration,fluorescence spectroscopic titration and density functional theory（DFT）calculations,both the E and Z conformers of Ⅳ-1 are relatively stable,and the multiple intramolecular hydrogen bonds in the Z conformer enable it to be more stable than the corresponding E conformer,which may account for the observation that Ⅳ-1-（E）was found able to be quantitatively photo-isomerized to Ⅳ-1-（Z）under light irradiation（<425 nm）but Ⅳ-1-（Z）was not able to be transformed back into Ⅳ-1-（E）only by light.Instead,it was found capable of being completely isomerized back to Ⅳ-1-（E）under acid stimulation.More importantly,Ⅳ-1-（E）can act as an ion-pair receptor for capturing NaCl,NaBr or NaI,and it was found able to selectively pluck LiCl out from mixed solid salts（LiCl,NaCl,KCl,and CsCl）to the chloroform layer.Meanwhile,it can also selectively extract CsF from mixed solid salts（CsF,CsCl,CsBr,CsI,CsNO₃）to organic layer.Once the organic layer with the LiCl@Ⅳ-1-（E）or CsF@Ⅳ-1-（E）was separated and exposed to irradiation（<425 nm）,either LiCl or CsF was observed to be released from the complex in the organic layer,allowing for the practical recycling of the ion-pair receptor as Ⅳ-1-（Z）.4.We have synthesized a light-responsive ion-pair receptor Ⅴ-1 by linking the azobenzene and alkoxy tethers with two thiourea groups.The fact that was able to serve as an ion pair receptor for capturing CsF was evidenced by single-crystal structure and density functional theory（DFT）calculations.Per the ¹H NMR spectroscopic analysis and solid-liquid extraction studies carried out in CDCl₃,receptor Ⅴ-1-cis exhibits high selectivity toward CsF over other ion pairs,i.e.NaF,NaCl,NaBr,KF,KCl,KBr,CsCl and CsBr.Importantly,controllable extraction and photo triggered release of CsF from the receptor can be achieved due to excellent photoisomerization properties of macrocyclic anion receptor Ⅴ-1 under irradiation（365-370 nm/420-425 nm）.