Construction And Self-assembly Of Functional Pillarenes

Supramolecular chemistry is an interdisciplinary subject, investigating functionalmolecular aggregates with complicated and well-defined structures formed throughnoncovalent interactions. Continued interests have been paid to the design andpreparation of supramolecular macrocyclic compounds due to their important roles inthe development of supramolecular chemistry. Pillar[n]arenes (or pillarenes), as a newfamily of supramolecular host molecules with tubular-shape and rigid macrocyclicstructures, advanced significantly in recent years. A major topic in supramolecularchemistry is the contruction of functional supramolecular assemblies throughself-assembly. In this dissertation, systematic researches have been carried outcentering on the construction and self-assembly of functional pillarenes. Thisdissertation is mainly about the synthesis and mechanistic investigation of pillarenes,the construction of supramolecular polymers and fluorescent enhancement materialsinduced by host-guest complexation based on functional pillarenes.In the first part, some catalytic systems have been employed for the synthesis ofpillar[n]arenes. However, these reported systems suffer from harsh reaction conditions,low reaction yield, and/or relatively long reaction routes, which definitely have notmet the goal required for the rapid development of this emerging field. Moreover, amechanism for pillar[n]arene synthesis has not yet been elucidated clearly withexperimental details. We found out that trifluoromethanesulfonic acid (TfOH) couldbe an efficient catalyst for the cyclization of monomers in the synthesis ofpillar[n]arenes and their derivatives. Meanwhile, various methods (ESR and HRMSanalysis) have been employed to probe the mechanism of solution-phase pillarenesynthesis, suggesting a free radical intermediate initially and a Friedel-Craftsalkylation process during the coupling and ring-closure stage.In the second part, employing the catalytic system that we developed above, we have designed and prepared a tertiary ammonium salt functionalized pillar[5]arenereceptor1, and successfully constructed a multi-responsive supramolecular polymerfrom1through self-assembly. First, the self-aggregation at low concentration andself-assembly at high concentration of monomer1were investigated by means of acombination of various techniques, i.e.,1H NMR, ROESY, DOSY, Viscometry andSEM. Meanwhile, the self-aggregation constant of monomer1obtained through1HNMR studies at low concentrations provides a new approach for the investigation ofthe thermodynamic origin of molecular aggregation, which allows us to furtherunderstand molecular recognition and self-assembly in supramolecular chemistry, andthus design novel functional supramolecular assemblies. Furthermore, the linearsupramolecular polymer from1exhibited pH-, counter ion-, and solventcomposition-induced efficiently reversible disassembly-reassembly. On the other hand,supramolecular nanofibers were fabricated from1by electrospinning techniquesuccessfully. Moreover, the structures of supramolecular nanofibers werecharacterized by means of SEM, TEM and AFM analysis.Fabrication of stimuli-responsive fluorescent materials possessing switchableemissions under external stimuli is a challenging task. In the third part, combining theinvestigation results above, we present a new class of pillarene-based fluorescenceenhancement materials induced by host-guest complexation. We have synthesized twoanthracene mono-functionalized pillar[5]arenes M1and M2. M1exhibits trongfluorescence in chloroform due to the complexation behavior between tertiaryammonium salt moiety and pillar[5]arene moiety. Subsequently, the fluorescentenhancement mechanism induced by host-guest complexation was investigatedthrough theoretical calculation. It is indicated that host-guest complex would formlarge steric hindrance and assist in restricting the intramolecular motions ofanthracene unit. Furthermore, the mechanism for the emission enhancement wasverified by means of time-resolved fluorescence measurements. Compared to M2, M1possesses larger proportion in radiative decay process due to the host-guestcomplexation. Therefore, M1exhibited strong fluorescent emission and high quantumyield. Significantly, the fluorescent emission of M1could be reversibly manipulated by temperature, pH and counter anion adjustments, which is promising for thefabrication of multi-responsive supramolecular fluorescent sensor and smartsupramolecular materials.