FULL PAPER DOI: 10.1002/chem.201101904 Cucurbituril-Modulated Supramolecular Assemblies: From Cyclic Oligomers to Linear Polymers Hai Qian, Dong-Sheng Guo, and Yu Liu*[a] Abstract: Employing bis(p-sulfonatoca- (ITC), and gel permeation chromatog- HBV4+, and reinforcing the host–guest lix[4]arenes) (bisSC4A) and N’,N’’- raphy (GPC) experiments. The ternary inclusion of bisSC4A with HBV4+, hexamethylenebis(1-methyl-4,4ACHTUNGRE ’-bipyri- supramolecular polymer participated which prompts the formation of dinium) (HBV4+) as monomer building by CB[8] is constructed on the basis of a linear polymer. Moreover, the CB[8]- blocks, the assembly morphologies can host–guest interactions by bisSC4A participated ternary assembly could be modulated by cucurbit[n]uril and the [2]pseudorotaxane HBV4+ disassemble into the molecular loop (CB[n]) (n=7, 8), achieving the inter- @CB[8], which is characterized by HBV2+@CB[8] and free bisSC4A after esting topological conversion from means of AFM, DLS, NMR spectros- reduction of HBV4+ to HBV2+, cyclic oligomers to linear polymers. copy, thermogravimetric analysis whereas the CB[7]-based assembly re- The binary supramolecular assembly (TGA), UV/Vis spectroscopy, and ele- mained unchanged after the reduction. fabricated by HBV4+ and bisSC4A mental analysis. CB[n] plays vital roles CB[8] not only controlled the topologi- units, forms an oligomeric structure, in rigidifying the conformation of cal conversion of the supramolecular which was characterized by NMR spec- assemblies, but also improved the troscopy, atomic force microscopy redox-responsive assembly/disassembly Keywords: calixarenes · cucurbit- (AFM), transmission electron micros- property practically. urilsACHTUNGRE · redox response · supramolec- copy (TEM), dynamic light scattering ular chemistry · polymers (DLS), isothermal titration calorimetry Introduction (CDs) have been extensively studied to build various supra- molecular polymers.[6] In early 1999, Reinhoudt and co- Supramolecular polymers, defined as polymeric arrays of workers reported molecular threads composed of intercon- monomer units brought together by noncovalent interac- nective, linear host–guest complexation of b-CD-calix[4]ar- tions, coalesce the advantages of both polymer science and ene couples.[7] Recently, Harada and co-workers described supramolecular chemistry.[1] Comparing with the covalent supramolecular fibrils based on host–guest interactions of polymerization and/or cross-linking, the noncovalent route modified CDs, which could disassemble by adding competi- has been demonstrated as a promising approach towards tive guests or hosts.[8] The same group prepared a stilbene- novel smart design principle for responsive materials capa- bridged bis(ACHTUNGREb-CD) dimer and firstly achieved the conforma- ble of self-repairing and self-healing.[2] In this context, it tional change from pinching-type dimer to supramolecular emerges to be a significantly interesting topic to construct polymer triggered by photoirradiation.[9] Thus, it can be seen supramolecular polymers with different topologies, and that host–guest-directed supramolecular polymers can be endow them with responsive capability to external stimuli.[3] not only smoothly switched between assembly and disassem- Host–guest interactions are an alternative, robust candi- bly, but also tuned to topological conversion by adjusting date for supramolecular polymerization among the family of their binding affinities and/or geometries. noncovalent forces, ascribing to their diverse binding selec- Calixarenes,[10] composed of phenolic units linked by tivity.[4] It is also well known that macrocyclic hosts always methylene groups, represent the third generation of supra- show distinguishable inclusion affinities to different sub- molecular host molecules. It remains an immature project to strates, even to homologues and isomers.[5] Among these construct supramolecular polymers based on calixarenes, macrocyclic compounds, crown ethers and cyclodextrins compared to construct cyclodextrin-based supramolecular polymers. Only a few examples have been occasionally re- [11] [a] H. Qian, Dr. D.-S. Guo, Prof. Dr. Y. Liu ported, where the cavities of calixarenes were not suffi- Department of Chemistry ciently exploited.[12] Haino and co-workers reported supra- State Key Laboratory of Elemento-Organic Chemistry molecular polymeric nano networks based on strong binding Nankai University, Tianjin, 300071 (P.R. China) between ditopic calix[5]arene and dumbbell C .[13] Parisi Fax : (+86)22-2350-3625 60 E-mail: [email protected] and co-workers outlined a general strategy for constructing Supporting information for this article is available on the WWW noncovalent assemblies from modular calix[5]arenes with under http://dx.doi.org/10.1002/chem.201101904. iterative inclusion of alkylammonium components.[14] In our Chem. Eur. J. 2012, 18, 5087 – 5095 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 5087 previous works,[15] we also fabricated nano-supramolecular linear/netlike assemblies from the bis(p-sulfonatocalix[5]ar- enes) and porphyrins, and more recently, electrochemical stimulus-responsive supramolecular polymers from bis(p-sul- fonatocalix[4]arenes) and viologen dimers. Cucurbit[n]urils (CB[n]s), a family of novel pumpkin-shaped macrocyclic hosts, containing a different number of glycoluril units, were extensively exploited in host–guest chemistry.[16] Especially, CB[8] has been occasionally employed for supramolecular polymerization and cross-linking[17] by utilizing multiple host-stabilized charge-transfer interactions.[18] As a natural evolution of our ongoing program concern- ing supramolecular polymers based on calixarene macrocy- cles, we report here the fascinating topological conversion of assemblies, controlled by the complexation of additional CB[8]. The complexation of N’,N’’-hexamethylenebis(1- Figure 1. 1H NMR spectra of 1.0 mm HBV4+ in the a) absence and 4+ methyl-4,4’-bipyridinium) (HBV ) with bis(p-sulfonatoca- b) presence of one equivalent bisSC4A, and c) 1.0 mm free bisSC4A. The ~ lix[4]arenes) (bisSC4A) leads to binary cyclic oligomeric as- solvent is D2O and “ ” represents the proton of 2,2-dimethyl-2-silapen- semblies (Scheme 1). Although interestingly, upon addition tane-5-sulfonate (DSS), which was added as an external reference. lene-bridged bisviologen (HBV4+), a species with a longer spacer. The obvious evidence for host–guest complexation between bisSC4A and HBV4+ was initially presented by 1H NMR experiments (Figure 1). The aromatic and methyl protons (H1–4) of HBV4+ undergo pronounced upfield chem- ical shifts owing to the ring current effect of the aromatic nuclei of calixarene. The Dd values are 2.18 for H1, 2.28 for H2, 1.30 for H3, and 0.36 ppm for H4. However, the aliphatic spacer protons (H5–8) have almost negligible shifts. The Dd values are 0.12 for H5, 0.13 for H6, 0.04 for H7, and 0.04 ppm for H8. It indicates that HBV4+ is captured by bisSC4A with methyl-pyridinium moieties deeply immersed [19] Scheme 1. Structural illustration of bis(p-sulfonatocalix[4]arenes) into the cavity, whereas the hexamethylene spacer is lo- (bisSC4A), N’,N’’-hexamethylenebis(1-methyl-4,4’-bipyridinium) (HBV4+) cated outside. and cucurbit[8]uril (CB[8]) (counterions are omitted for clarity). Isothermal titration calorimetry (ITC) measurements sup- plied the quantitative information (Table 1) for the host– guest complexation, both binding affinity and thermodynam- of CB[n](n=7, 8), a ternary supramolecular linear polymer ic origin. The high association constant of bisSC4A with was formed by the complexation of bisSC4A with [2]pseu- HBV4+ was obtained as (1.15Æ0.01)106 mÀ1, fitted by dorotaxane (HBV4+@CB[n]). Moreover, we can operate using the “one set of binding sites” model (assuming that all practically the assembly/disassembly of the obtained poly- the binding sites for the interacting species are identical, n:n mer by redox stimulus, benefiting from the introduction of complexation is dealt with a 1:1 binding model; DH8 and CB[8]. TDS8 represent the values per host site, in which the experi- Results and Discussion Table 1. Complex stability constant (KS), enthalpy (DH8), and entropy changes (TDS8) for the intermolecular complexation of bisSC4A with HBV4+, SC4A with 2+[a] 4+ Construction of a binary supramolecular cyclic oli- MV , and bisSC4A with HBV @CB[n] in aqueous solution (pH 7.0) at 298.15 K. 4+ gomer based on bisSC4A and HBV : Viologens, Complex KS DH8 TDS8 ACHTUNGREmÀ1 ACHTUNGRE À1 ACHTUNGRE À1 one class of dicationic redox couples, are strongly [ ] [kJmol ] [kJ mol ] 4+ 6 included by p-sulfonatocalix[n]arene (SCnA) (n=4, HBV @bisSC4A (1.15Æ0.01)10 (À29.6Æ0.2) (4.96Æ0.18) MV2+@SC4A (9.26 0.08)105 ( 27.2 0.2) (6.79 0.18) 5) to form a stable complex.[19] Previously, we built Æ À Æ Æ HBV4+@CB[8]/bisSC4A[b] (1.85Æ0.01)106 (À47.9Æ0.4) (À12.1Æ0.4) a supramolecular linear polymer based on the com- HBV4+@CB[7]/bisSC4A[b] (1.86Æ0.21)107 (À42.0Æ0.6) (À0.543Æ0.290) plexation of bisSC4A with ethylene-bridged bisviol- [a] Methyl viologen. [b] ITC titrations of ternary supramolecular polymers based on 4+ [15b] ogen (EBV ). We studied herein the binding bisSC4A with HBV4+@CB[n] revealed relatively large associated errors due to limited and assembly behavior of bisSC4A with hexamethy- solubility of the ternary complexes. 5088 www.chemeurj.org 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2012, 18, 5087 – 5095 Cucurbituril-Modulated Supramolecular Assemblies FULL PAPER mental DH8 value was divided