Tailoring two-dimensional surfaces with pillararenes based host-guest chemistry

The two-dimensional surfaces have been fueled by the infinite possibility they offe red fo r basic re search,and for novel technologies in nanoelectronics.To realize many of these promises,the effective strategies were to design and control their surface chemistry,which plays a vital role in determining the chemical and physical properties.Macrocyclic host-guest chemistry with the reversible noncovalent interactions between macrocyclic hosts and suitable guests can be readily used for constructing multifunctional surfaces.Macrocyclic pillararenes,possessed the unique structure,have attracted the attentions of researchers in recent years.This feature article covers the recent development of pillararene-based twodimensional inte rfaces,including the fabrication and function of the hybrid composite.The combination of pillararenes and materials platform exhibited the novel property because of the characte ristic of cavity of macrocyclic host and confined spaces of surfaces.We anticipate that this review will be helpful to the researchers working in the fields of supramolecular chemistry and materials science.

A meso–molecular muscle based on copillar[5]arenes

A meso–molecular muscle was prepared by capping the[c2]daisy chain based on a mono-functionalized copillar[5]arene with an imidazolium group in its axle.From its crystal structure,we observed that it was a cyclic dimer composed of two mirror image subcomponents,a pR-and a pS-copillar[5]arene.Their conformations were fixed by the doubly interlocked mechanical bond.By comparison of the^1H NMR and COSY spectra,we found that the length of this meso–molecular muscle could be controlled not only by the solvents,but also by the counter anions.

Self-assembly of partially oxidized pillar[5]arenes induced by intermolecular charge transfer interactions

Supramolecular interactions such as t-stacking interaction and charge transfer interaction have drawn much attention in the design and construction of various supramolecular assemblies.Herein,partially oxidized pillar[5]arene(P5A),pillar[4]arene[1]quinone(P4A1Q),pillar[3]arene[2]quinone(P3A2Q),and pillar[2]arene[3]quinone(P2A3Q)were synthesized by one-step reaction.As indicated by experimental characterization data and density function theory modeling results,charge transfer interaction among partially oxidized P5A plays a significant role in host-host self-assembly behavior and corresponding packing morphology.This work provides a unique strategy for the construction of functional macrocyclic assemblies through host-host self-assembly.

All-in-one functional supramolecular nanoparticles based on pillar[5]arene for controlled generation,storage and release of singlet oxygen

The storage and controlled release of singlet oxygen(^(1)O_(2))have attracted increasing attention due to the wide application and microsecond lifetime of^(1)O_(2)in water.Herein we provide an integrated nanoplatform consisting of a diphenylanthracene derivative,a water-soluble pillar[5]arene and a photosensitizer tetrakis(4-hydroxyphenyl)porphyrin(TPP),that may provide the controlled generation,storage and release of singlet oxygen.We design a new diphenylanthracene derivative with two trimethylammonium bromide groups on both ends that can be well recognized by the pillar[5]arene.The formed nanocarriers can be used to load TPP through their supramolecular self-assembly.The resulting nanoparticles show good water-solubility and uniform spherical morphology.After laser irradiation(660 nm),the nanoparticles exhibit excellent ability for the generation and storage of^(1)O_(2).When the irradiated nanoparticles are heated above 80°C,^(1)O_(2)can be released from the system.Therefore,in this paper we pioneer the use of noncovalent interaction to integrate the diphenylanthracene derivatives and photosensitizers into one functional system,which provides a new strategy for the controlled generation,storage and release of singlet oxygen.We believe this groundbreaking strategy will have a great potential in providing necessary amounts of^(1)O_(2)for the photodynamic therapy of tumors in dark.

Artificial Water Channels—Progress Innovations and Prospects

The design of artificialwater channels(AWCs)aims to mimic natural proteins that effectively mediate the selective transport of water across the bilayer and artificial polymeric membranes.Currently,impressive progress on synthetic backbones,including selfassembled channels,helical unimolecular,and other functional organic molecules,has been made to provide artificial alternatives and applications in real systems for promising product development.In this review,several synthetic channels exploited during the last fewyears are summarized according to their backbone architecture for water transport and ion transport respectively,showing their potential as natural transporter analogs and the importance of AWCs for industrial,environmental,and medical applications.

Rim-differentiation vs. mixture of constitutional isomers: A binding study between pillar[5]arene-based glycoclusters and lectins from pathogenic bacteria

Macrocycle-based glycoclusters, on account of their promising anti-adhesive properties against bacteria,are potential therapeutic alternatives to classic antibiotics through the much less explored anti-adhesive strategy. In this study, a series of constitutionally-pure pentavalent glycoclusters was prepared by conjugating assorted azido-carbohydrates onto a penta-propargyl rim-differentiated pillar[5]arene(RD-P[5])scaffold through Cu(I)-catalyzed azide–alkyne cycloaddition “click” reactions. Their binding towards therapeutically relevant bacterial lectins, such as Lec A and Lec B from Pseudomonas aeruginosa and concanavalin A(Con A), were evaluated subsequently by isothermal titration calorimetric studies. Most of these isomerfree RD-P[5] pentavalent glycoclusters, except the fucosylated ones, display good affinities to lectins.Nonetheless, the dissociation constants observed are similar to those displayed by an analogous pentavalent glycocluster consisting of four P[5] constitutional isomers, in which the RD-P[5] component merely accounts for 7% in the mixture. Our results revealed that high constitutional purity is not essential for achieving effective multivalent interactions between P[5]-based glycoclusters and lectins, presumably as a result of the conformationally labile nature of the P[5] scaffold. This information provides valuable design principles for low-cost and facile syntheses of glycosylated P[5]s for biomedical applications.

Pillararene-based supramolecular systems for theranostics and bioapplications

As an emerging type of important macrocycles for supramolecular chemistry,pillararenes and their derivatives have been widely studied and applied in numerous fields,which intensively promotes the development of chemistry,materials science and biology.Pillararene-based theranostic systems are of special interest in the biological and medical areas as they have shown very promising results.Owing to easy preparation,reliable guest affinity,good biocompatibility and stability,pillararenes are frequently used to construct functional biomaterials.On one hand,pillararenes can either be used individually or form diversiform self-assemblies such as micelles,nanoparticles and vesicles to increase water solubility and biocompatibility of drugs.On the other hand,it is promising to modify solid materials like framework materials,silica nanoparticles and graphene oxides with pillararene derivatives to enhance their functions and controllability.In this review,we summarize recent endeavors of pillararene-based supramolecular systems with theranostics and other biological applications comprising drug delivery/chemotherapy,photodynamic/photothermal therapy,antimicrobials,bioimaging,etc.By introducing several typical examples,the design principles,preparation strategies,identifications and bio-applications of these pillararene-based supramolecular systems are described.Future challenges and directions of this field are also outlined.

pH-Responsive Host-Guest Complexation between a Water-soluble Pillar[7]Arene and a 2,7-Diazapyrenium Salt and Its Application in Controllable Self-assembly

A novel host-guest recognition motif based on a water-soluble pillar[7]arene （WP7） and a 2,7-diazapyrenium salt （DMDAP） was prepared. According to the integrated results of ^1H NMR, 2D NOESY, UV-vis spectroscopy and fluorescence titration experiments, we demonstrated that the molecular recognition of WP7 to DMDAP in water not only has high association constant but also has pH-responsiveness. Subsequently, we took advantage of this molecular recognition motif to fabricate a supra-amphiphile based on WP7 and an amphiphilic 2,7-diazapyrenium derivative DAPAC. Its controllable self-assembly in water was also investigated by means of TEM and DLS techniques.

Four Pillar[5]arene Constitutional Isomers:Synthesis,Crystal Structures,and Host-Guest Complexation of Their Derivatives with Paraquat in Water

A pillar[5]arene prepared from monomer 1 has four constitutional isomers.Although these constitutional iso-mers have the same molecular formula,their other properties that we have investigated are more or less different.They have different melting points,different NMR spectra and different crystal structures.Furthermore,the corre-sponding four anionic constitutional pillar[5]arene isomers derived from these four constitutional isomers showed different binding abilities with paraquat G in water.The association constants of 3a⊃G,3b⊃G,3c⊃G and 3d⊃G were determined to be(3.33±0.46)×10^(4)L•mol^(−1),(5.00±0.63)×10^(4)L•mol^(−1),(3.33±0.28)×10^(4)L•mol^(−1),and(1.43±0.09)×10^(4)L•mol^(−1),respectively.

Carbon-carbon double bond in pillar[5]arene cavity:Selective binding of cis/trans-olefin isomers

The binding behavior of pillar[5]arenes(P5 As)towards a series of olefin guests((E)-1,4-dichlo robut-2-ene(1 E),(Z)-1,4-dichlo robut-2-ene(1 Z),(E)-but-2-ene-1,4-diol(2 E),and(Z)-but-2-ene-1,4-diol(2 Z),as well as an alkyne derivative 1,4-dichlorobut-2-yne(3))have been studied in organic solution.P5 As exhibit considerable selectivities for the trans-olefin isomers(1 E and 2 E)over their cis-isomers(1 Z and 2 Z).The cis/trans-selective interactions hold the potential of utilizing P5 As to separate olefin isomers.

Evidence for a Bulky Unit of a Pillar[5]arene Flipping in the Solid State

It is well known that pillar[5]arenes have two most stable conformations(pS and pR)in their crystal structures.Because of the intramolecular H-bonding interactions,substituents,temperature,solvent and so on,the rotational behaviors of the phenolic units on pillararenes are also common.This paper showed some other kinds of conforma-tions in the functionalized pillar[5]arenes and gave evidence for a bulky unit(1,4-methoxycarbonylmethoxybenzene unit)flipping in the solid state.The presence of hydrogen bonding facilitated the intermolecular self-assembly in terms of energy-minimized packing in the crystals.Thus,the main driving force for the flipping of this bulky unit might be both the intramolecular hydrogen bonding between the phenolic units on pillararenes and quadrupolar hy-drogen bonding between the host and water.This paper helps us to have a better understanding on the conforma-tions of pillar[5]arenes.

Synthesis of a Pillar[5]arene with Both Hydroxyl and Methoxycarbonyl-Methoxy Groups and Its Host-Guest Complexation with a Bis(imidazolium)Salt

By the introduction of methoxycarbonyl-methoxy groups and hydroxyl groups into a pillar structure,a pillararene entirely with two types of functional groups was successfully prepared,which can form a stable 1∶1 complex with a bis(imidazolium)salt in CHCl3/acetone solution(V∶V=1∶1).

Hybrid vesicles of pillar[5]arene/silica:Host-guest complexation and application in pH-triggered release

Integrating silica with organic nanoparticles can generate unique properties.Here pillar[5]arene/silica hybrid vesicles were constructed based on the amphiphilic and rigid properties of pillararenes,as well as the catalytic hydrolysis of tetraethoxysilane.Such vesicles exhibited the high strength of silica and unique molecular recognition of pillararenes,both of which could tune the pH-trigge red release behavior.Furthermore,a rhodamine B derivative with hexyl group(RhB-C6) was synthesized,which can form a complex with the pillar[5]arene.Based on the host-guest interaction and high strength of silica,the hybrid vesicles could load more RhB-C6 and the rhodamine B was released more slowly compared with the organic vesicles.

Naphthalene Diimide Templated Synthesis of Pillar[6]arenes

A facile method for the selective synthesis of pillar[6]arenes has been developed.Due to a strong electron do-nor-acceptor interaction between the electron-deficient naphthalene diimide and a precursor for the pillararene,1,4-diisobutoxy-2,5-bis(methoxymethyl)benzene,naphthalene diimide has been demonstrated to act as a template for selective synthesis of pillar[6]arene in a modest yield(30%).

Voltammetric Behavior of 1,4-Dimethoxypillar[m]arene[n]quinones

The electrochemical properties of a series of 1,4-dimethoxypillar[m]arene[n]quinones （DMP[m]A[n]Qs） and the interactions between individual quinone units have been investigated on glassy carbon electrode in acetonitrile. All the quinone units showed relative electron uptake behavior except 1,4-dimethoxypillar[5]quinones （DMP[5]Q）. The results have shown that the electrochemical behavior of the DMP[m]A[n]Qs is comparatively different from that of their related linear quinone analogues. The resultant properties were attributed to the close proximity of redox-active sites as well as the delocalization of electrons on the aromatic rings. Another aspect to be considered responsible for their electronic properties was suggested to be the electrostatic repulsions between adjacent quinone units in these complex structures. Current studies provide a better understanding on the voltammetric behavior of pillararene derivatives with different numbers of quinone units as well as their future scope in certain future electrochemical applications.

Supramolecular Prodrug Micelles Constructed by Drug-Drug Conjugate with Water Soluble Pillar[6]arene for Controllable and Rapid Drug Release

The first attempt of constructing supramolecular prodrug micelles based on the host-guest interaction between water-soluble pillar[6]arene(WP6)and a novel doxorubicin(DOX)-based drug-drug conjugate(G)is reported,in which the drug-drug conjugate is synthesized by conjugating anticancer drug DOX with another drug isoniazide via the acid-cleavable hydrazone bond.The obtained WP6⊃G micelles are stable under physiological conditions,whereas the cumulative release of DOX is approximate to 100%within 30 min at pH 5.5 by simulating the endo-lysosomal environment at 37℃.Therefore,this WP6-based prodrug micelles can achieve efficient DOX ac-cumulation in the acidic tumor cells within a short period of time,which is very important and valuable;meanwhile it can also reduce the unexpected premature burst release under physiological condition.Furthermore,cellular in-ternalization and localization experiments demonstrated that these prodrug micelles enter cancer cells mainly via endocytosis and can lead to significant drug accumulation in SKOV3 cancer cells,implying its promising future for cancer therapy.

Calixarene/pillararene-based supramolecular selective binding and molecular assembly

Calixarenes and pillararenes are cyclic oligomers containing repeating units of phenol and methane. The modification of ionized groups bring calixarenes and pillararenes derivates both good water solubility and strong binding affinities towards various organic/inorganic/biological guest molecules in both water and the solid state. Meanwhile, the pre-organized structures of these compounds are indispensable in constructing of stimuli-responsive supramolecular assembly. With these properties, the supramolecular selective binding and molecular assembly based on calixarene/pillararene are widely used in enzyme tandem assay, stimuli-responsive nanostructure, drug delivery, organelle fluorescent imaging, photo-controlled morphological change, organic and inorganic hybrid material, solid tunable photo-luminescence and so on. This review summarized the recent research progresses on the calixarene/pillararene-based supramolecular systems and discussed the expectation of the future development.

The emerging applications of pillararene architectures in supramolecular catalysis

Over the last decade,numerous research efforts have been devoted to pillar[n]arenes since their debut.The popularity of pillararenes is a reflection of current research trend in supramolecular and macrocyclic chemistry in general.Among the vast applications(such as chemosensors,drug delivery,transmembrance channels,and separation)of pillararenes,their utilization in catalysis is a relatively less explored area.However,soaring attention has been paid by researchers in recent years and this field has seen gradual increasing publications.Therefore,in this review we will discuss progress in the emerging applications of pillararene architectures in catalysis based on various reaction genre including reduction,oxidation,coupling,decomposition and others.Furthermore,this review not only focuses on the pillararenes based current progress in catalysis,but also provides the signs for future development in this research field.

Enhanced photocatalytic hydrogen production performance of pillararene-doped mesoporous TiO_(2) with extended visible-light response

Pillararenes are a new type of supramolecular hosts,and they have been widely applied in drug delivery,catalysis,separation process,and sensors.However,they have rarely been used to produce hydrogen.Here,we report that pillararenes were used as functional molecules to explore photocatalysts and efficiently promoted hydrogen production from water.The most common and easily synthesized pdimethoxy pillar[5]arene(PI-OMe) was employed to form an organic-inorganic hybrid material with titanium dioxide(TiO_(2)),denoted as PI-OMe-TiO_(2).using a convenient sol-gel method.When the material was loaded with Pt nanoparticles,the resulting Pt/PI-OMe-TiO_(2)had a good activity and stability in catalyzing water splitting to produce hydrogen under visible light.The optimized catalyst Pt/PI-OMe-TiO_(2)(5.2 wt%)had a photocatalytic hydrogen production rate of 1736 μmol g^(-1)h^(-1) under visible light(λ> 420 nm)irradiation.The catalyst with a Pt loading of 0.5 wt% and a PI-OMe content of 5.2 wt% also showed good long-term durability after 10 cycles of 50 h testing.The total amount of hydrogen produced was65.01 mmol/g,and the corresponding turnover number(TON) value was 2084.Our findings suggest that pillararene derivatives are promising functional molecules to make efficient and stable hybrid photocatalysts with TiO_(2)and open a new door to hydrogen production using visible light.

Hexnut[12]arene and its derivatives: Synthesis, host-guest properties, and application as nonporous adaptive crystals

Hexnut[12]arene(HN[12])and its derivatives,a new class of sixfold macrocyclic arenes,were designed and synthesized in reasonable yield by a one-pot reaction at room temperature using dimethoxymethane as a methylene source.HN[12],which bears a large,symmetric,and rigid cavity,was easily functionalized at both the methylene bridges and the hydroquinone units.A water-soluble fluorescent HN[12]was synthesized and used as a host to encapsulate benzyl viologen dichloride in water with a high binding affinity of(3.4±0.2)×10^(6)M^(-1).The nonporous adaptive crystal(NAC)of HN[12]was found to capture not only inorganic molecules(iodine)but also trace amounts of large organic molecules(basic fuchsine)from water,which greatly expands the scope of NACs for adsorption.