Macrocycle functionalized supramolecular gels and the host-guest chemistry of dendrimers in the gas phase

The present research contains two parts: The first explores the macrocycle functionalized supramolecular gels and the second part investigates the host- guest chemistry of dendrimers by mass spectrometry. Although these two parts of the research were investigated under different conditions, one in the condensed phase and one in the gas phase, the same crown ether macrocycle was used as the basic building block in both cases. Herein, we reported the usefulness of macrocycle chemistry, how it benefits the construction of novel soft materials with multiple responsiveness, and how the molecular motion can be investigated in the gas phase by tandem mass spectrometric experiments. The detailed achievements can be summarized as follows: Part A: Construction of macrocycle functionalized supramolecular gels 1\. We designed and synthesized a new type of crown ether-functionalized bis(urea) organogelators which were equipped with two benzo[21]crown-7 moiety at both terminals. The host-guest chemistry of the crown ether plus the urea unit endow the gel with multiple chemical responsiveness: its gel-sol transition can be switched by several different molecular recognition events: K+ binding to the crown ether, formation of pseudorotaxane with secondary amine and the interaction of Cl- anion with urea unit. The most intriguing part of this research is that we took advantage by exploring chemical stimuli to disclose the sophisticated signal networks when several of these chemical signals exist in the gel system. Through rational design of the gel system as well as the inputs of stimuli (type, amount, sequence), we realized seven types of chemical logic gates (including OR, AND, XOR, NOT, NOR, XNOR and INHIBIT gates), which controlled the macroscopic gel-sol transition operated in a logical manner. The resulting supramolecular gels system showed great potential by simply choosing and arranging the known responsive properties to endow the gel materials with ‘smart thinking’. 2\. We further extended the crown- functionalized gels system to a large extent. One direction was to propose a strategy to utilize the selective responsive supramolecular gel as an immobilization and recycling media for Pickering emulsion. We designed and synthesized new crown ether-appended chiral gelators which show impressive gelation ability in a variety of organic solvents, particularly in acetonitrile. Due to the crown ether, the resultant supramolecular gel remains sensitive to multiple chemical stimuli and the sol-gel phase transitions can be reversibly triggered by host-guest interactions. We demonstrated that the 3D network in the gel can entrap an enzyme (CalB, lipase B from Candida antarctica) and released it upon addition of potassium ions. The obtained environmental responsive gel system was extended to be utilized as a novel matrix for Pickering emulsion as a proof of concept. The newly developed gel matrix not only contributes to solving the phase problem that frequently occurs when enzymes are used to catalyze organic reactions in unpolar organic media, but also makes the recycling of the entrapped enzyme possible by chemical stimuli. 3\. The crown ether-appended chiral gelator shows much broader gelation abilities, even in ionic liquids. The detailed gelation ability in different ionic liquids was evaluated, as well as the thermal stability. The ionogels showed impressive a high thermal stability. In comparison with conventional organic solvents, ionic liquids significantly increase the mechanical strength of the corresponding gels. The storage modulus G´ is impressively high and is even comparable with cross-linked protein fibers. Moreover, all the ionogels displayed rare rheopexy (shear thickening) phenomenon instead of thixotropy at low shear rates. Intriguingly, the ionogel even possesses a rapid self-recovery ability. Part B: Investigation of host-guest chemistry of dendrimers in the gas phase 1\. Mass spectrometry aided by hydrogen/deuterium exchange (HDX) experiments provides evidence to a molecular motion occurring within a complex composed of a partly protonated amino-terminated polypropylene amin (POPAM) and POPAM dendrimer fully functionalized with benzo[21]-crown-7 on all branches. The multiple molecular recognition events make the complex system suitable for a multivalent study, particularly from the molecular dynamic motion aspect. Since the crown ethers can efficiently protect the ammonium group against HDX, the complete exchange of all labile N-H hydrogen atoms in the dendritic complex unravel the highly dynamic binding motion inside. Each appended crown is mobile enough to move from one ammonium binding site to another, thus driving the two dendrimers swirling around each other without completely losing all contact at any time. Schematically, such an intracomplex motion is quite similar to two “rock ‘n roll” dancers. So far, this is the first study providing evidence for the microdynamics within multivalent complexes. Accordingly, the marriage of macrocycle and supramolecular gels bring numerous opportunities for designing novel supramolecular gels which would be difficult to achieve by other building blocks. Even for matured dendrimer chemistry, new insights are available when the host-guest chemistry of crown macrocycle is introduced. Consequently, the future of macrocycles is bright.