The Construction Of Redox-Responsive Ferrocene/Ferrocenium-Pillar[6]arene-Based Self-Assembled Supramolecular Architectures

Redox-responsive host-guest interactions exhibited great applications in the field of supramolecular chemistry.Especially,the redox-responsive host-guest interactions based on macrocyclic hosts have attracted increasing research interests,for their advantages in constructing responsive supramolecular self-assembled architectures.Ferrocene could form redox-responsive host-guest inclusion complexes with macrocyclic hosts such as cyclodextrin,calixarene,and cucurbituril.And these inclusion complexes have been widely used in constructing redox-responsive supramolecular self-assembled architectures.However,most of these current researches were based on the reduced state ferrocene in the redox couple ferrocene/ferrocenium,and few researches have focused on the oxidized state ferrocenium.This paper performed comprehensive studies on the redox-responsive host-guest interactions between ferrocene/ferrocenium and pillar[6]arene as well as its self-assembled architectures in organic solvent.Moreover,the supramolecular cross-linked polymeric network and its corresponding organogel based on the host-guest inclusion complex of ferrocenium has been achieved for the first time.Part 1.A novel redox-responsive inclusion complex between per-butylated pillar[6]arene and ferrocene/ferrocenium has been studied.Various experiments demonstrated that ferrocenium,as well as its analogue cobaltocenium,are strongly included in the cavity of pillar[6]arenes,indicating these motifs are suitable guests for the pillar[6]arenes.Meanwhile,the reduced species ferrocene only exhibits extremely weak binding affinity with pillar[6]arene.As a result,a redox-responsive host-guest inclusion complex can be formed between ferrocenium and pillar[6]arene.More importantly,these strong interactions can prohibit the decomposition of ferrocenium in the presence of oxygen.Part 2.A supramolecular cross-linked polymeric network and its corresponding organogel have been successfully constructed based on the pillar[6]arene-ferrocenium redox-controllable interactions.Pillar[6]arene copolymers with different molecular weight and PDI are synthesized by "traditional" free radical polymerization or reversible addition/fragmentation chain-transfer(RAFT)polymerization with acrylate functionalized pillar[6]arene and methyl acrylate monomer.When mixed with another linear polymer with pendant ferrocene groups,the polymer blends can form a redox-responsive polymeric network:no host-guest interaction between the polymer chains exist in their reduced state;while supramolecular network is formed when the ferrocene groups were oxidized into ferrocenium and further included into the cavity of pillar[6]arene.Rheology studies demonstrate that low molecular weight samples exhibited frequency-dependent transient-network behaviors while high molecular weight samples exhibit typical gel behaviors.Furthermore,the viscoelastic properties of such supramolecular network can be easily controlled by different external stimuli including redox stimulus and competing host/guest reagents.Part 3.A cholesterol functionalized pillar[6]arene 4-3 has been successfully synthesized as a low molecular mass organo-gelator.It is found that 4-3 can gel a few non-polar organic solvent such as cyclohexane and n-hexane,and form a pillararene-based low-molecular mass organogel.Moreover,a cationic guest(ferrocenylmethyl)trimethylammonium 4-4 with tetrakis[3,5-bis-(trifluoromethyl)phenyl]borate counter ion is synthesized,which can complex with 4-3 in the mixed solvent(cyclohexane/1-hexanol = 10/1,v/v)and further affect its gelation properties.Interestingly,comparing with 4-3,the mixture exhibits a higher Tgel-sol in the presence of low equiv.of 4-4(<0.5 equiv.),but demonstrates an inhibited gelation property with much lower Tgel-sol in the presence of higher equiv.of 4-4.In conclusion,we have established comprehensive studies on the redox-responsive host guest interactions between ferrocene/ferrocenium and pillar[6]arenes,and we further used them to construct stimuli-responsive polymeric network and organogels.These researches will pave a way for further functional studies in this field.