•Supramolecular [c3]DCs form on account of radical and anion templations•Molecular [c3]DCs with 18 positive charges are synthesized in near quantitative yields•The air stability of radical [c3]DCs is enhanced because of aggregation•The [c3]DCs undergo “co-conformational” switching upon a redox stimulus Mechanically interlocked molecules (MIMs), which undergo controllable motions between their component parts in more than one dimension, are of interest in controlling nanoconfinement in a three-dimensional space. Cyclic [n]daisy chains ([cn]DCs) are a class of garland-like rotaxanes composed of self-complementary monomers with cross-threaded architectures. They represent a promising new range of mechanically interlocked, multi-dimensional artificial molecular switches. Although [c2]DCs have been widely investigated as prototypes of artificial molecular muscles, selectively and efficiently producing high-order [cn]DCs with n > 2 remains an alluring goal. In this paper, we present an efficient strategy for the synthesis of [c3]DCs in >90% yields by employing geometry restraints along with radical and anion templations. These [c3]DCs display some attractive properties, including enhanced air stability in their radical states as well as redox-switchable 2D coming-and-going expansions and contractions. This research paves the way for the development of switchable cross-linked polymers and mechanically interlocked frameworks as well as for future applications in the shape of adjustable molecular nets and breathable molecular membranes. Mechanically interlocked molecules (MIMs) that undergo controllable internal motions of their component parts in more than one dimension are rare entities in the molecular world. Cyclic [2]daisy chains ([c2]DCs) are a class of MIMs that have been identified as prototypes for molecular muscles. It remains, however, a challenge to synthesize [cn]DCs with n > 2 selectively and efficiently. Herein, we report the design and synthesis of [c3]DCs employing radical and anionic templates. Two mechanically interlocked [c3]DCs with 18 positive charges were obtained in >90% yields. One [c3]DC displayed good air stability in its radical cationic form, while the other underwent reversible “co-conformational” switching between open macrocyclic and closed trisarm-shaped forms under electrochemical control. These findings provide not only two-dimensional MIMs with attractive electronic and switchable properties, but also a starting point for the design of extended molecular arrays, which could become the forerunners of adjustable molecular nets and breathable molecular membranes. Mechanically interlocked molecules (MIMs) that undergo controllable internal motions of their component parts in more than one dimension are rare entities in the molecular world. Cyclic [2]daisy chains ([c2]DCs) are a class of MIMs that have been identified as prototypes for molecular muscles. It remains, however, a challenge to synthesize [cn]DCs with n > 2 selectively and efficiently. Herein, we report the design and synthesis of [c3]DCs employing radical and anionic templates. Two mechanically interlocked [c3]DCs with 18 positive charges were obtained in >90% yields. One [c3]DC displayed good air stability in its radical cationic form, while the other underwent reversible “co-conformational” switching between open macrocyclic and closed trisarm-shaped forms under electrochemical control. These findings provide not only two-dimensional MIMs with attractive electronic and switchable properties, but also a starting point for the design of extended molecular arrays, which could become the forerunners of adjustable molecular nets and breathable molecular membranes. The development of stimuli-responsive mechanically interlocked molecules1Bruns C.J. Stoddart J.F. The Nature of the Mechanical Bond: From Molecules to Machines. 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Engl. 2008; 47: 7470-7474Crossref PubMed Scopus (171) Google Scholar a redox-switchable molecular [c3]DC can be obtained as a side product using a donor-acceptor templating strategy but only in a 2% yield, with the major products turning out to be [c1]DCs (58%) and [c2]DCs (9%). To date, there is only one example54Chang J.C. Tseng S.H. Lai C.C. Liu Y.H. Peng S.M. Chiu S.H. Mechanically interlocked daisy-chain-like structures as multidimensional molecular muscles.Nat. Chem. 2017; 9: 128-134Crossref Google Scholar of switchable molecular [cn]DCs (n = 3 or 4) that has been reported with a good yield (~60%). The [c3]- and [c4]DCs were prepared54Chang J.C. Tseng S.H. Lai C.C. Liu Y.H. Peng S.M. Chiu S.H. Mechanically interlocked daisy-chain-like structures as multidimensional molecular muscles.Nat. Chem. 2017; 9: 128-134Crossref Google Scholar using a metal-templated protocol and isolated from the mixture of competing cyclic DCs. Here, we report the design (Figure 1B) and synthesis of two [c3]DCs, namely, [c3]DC2·18PF6 and [c3]DC12·18PF6, based on a combination of radical55Trabolsi A. Khashab N. Fahrenbach A.C. Friedman D.C. Colvin M.T. Cotí K.K. Benítez D. Tkatchouk E. Olsen J.C. Belowich M.E. et al.Radically enhanced molecular recognition.Nat. Chem. 2010; 2: 42-49Crossref PubMed Scopus (222) Google Scholar and anion56Vickers M.S. Beer P.D. Anion templated assembly of mechanically interlocked structures.Chem. Soc. Rev. 2007; 36: 211-225Crossref PubMed Google Scholar, 57Cai J. Sessler J.L. Neutral CH and cationic CH donor groups as anion receptors.Chem. Soc. Rev. 2014; 43: 6198-6213Crossref PubMed Google Scholar, 58Lee S. Chen C.H. Flood A.H. A pentagonal cyanostar macrocycle with cyanostilbene CH donors binds anions and forms dialkylphosphate [3]rotaxanes.Nat. Chem. 2013; 5: 704-710Crossref PubMed Scopus (227) Google Scholar, 59Bunchuay T. Docker A. Martinez-Martinez A.J. Beer P.D. 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Moreover, the geometry of the macrocycle, based on [c3]DC1218+ in solution, can be switched (Figure 1B) conveniently and reversibly between its oxidized cyclic [c3]DC1218+ and reduced trisarm-shaped [c3]DC129(⋅+) states by employing an electrochemical stimulus. These findings demonstrate the potential of the [c3]DCn18+ to act as 2D switchable or movable linking units for future uses in polymeric materials or molecular nanotechnology. At the outset, we employed (Figure 2A ) the radical-based recognition61Cai K. Shi Y. Cao C. Vemuri S. Cui B. Shen D. Wu H. Zhang L. Qiu Y. Chen H. et al.Tuning radical interactions in trisradical tricationic complexes by varying host-cavity sizes.Chem. Sci. 2020; 11: 107-112Crossref PubMed Google Scholar,62Cai K. Mao H. Liu W.G. Qiu Y. Shi Y. Zhang L. Shen D. Chen H. Jiao Y. Wu H. et al.Highly stable organic bisradicals protected by mechanical bonds.J. Am. Chem. Soc. 2020; 142: 7190-7197Crossref PubMed Scopus (10) Google Scholar between mCBPQT2(⋅+) and BIPY⋅+ in templating the synthesis of [c3]DC29(⋅+) and [c3]DC129(⋅+). Although mCBPQT2(⋅+), in its binding of BIPY⋅+, had a smaller association constant (Ka = 8.9 × 103 M‒1 in MeCN) than that (Ka = 3.9 × 104 M‒1 in MeCN) obtained for the para analog, CBPQT2(⋅+), it has several advantages when approaching the design of [c3]DCs. First of all, the m-xylylene linker can be functionalized at its 5-position easily while retaining the local C2v symmetry.63Low symmetric monomers—i.e., a CBPQT4+ derivative substituted on one of the p-xylylene units—will not only result in highly complicated NMR spectra for the [cn]DCs, but will also make it difficult to grow crystals and obtain ordered crystal superstructures and structures. Second, the synthesis of mCBPQT2(⋅+) derivatives is straightforward and the yields are more often than not satisfactory.50Coutrot F. Romuald C. Busseron E. A new pH-switchable dimannosyl [c2]daisy chain molecular machine.Org. Lett. 2008; 10: 3741-3744Crossref PubMed Scopus (174) Google Scholar Third, the BIPY⋅+⊂mCBPQT2(⋅+) pairs can be conveniently and reversibly switched between radical pairs and highly positively charged polycations by redox stimuli, making them good candidates for the construction of electrochemically switchable [c3]DCs. Finally, both the host (mCBPQT2(⋅+)) and the guest (BIPY⋅+) are rigid species—an important property when it comes to preorganizing the conformations of the self-complementary monomers—and as a consequence, not only high efficiency but also good selectivity is achieved during synthesis of [c3]DCs.Figure 3ESI MS and TWIM MS Spectra of [c3]DC2·18PF6 and [c3]DC12·18PF6Show full caption(A) ESI MS (top) and TWIM MS (bottom) spectra of [c3]DC2·18PF6. The insets show the comparison between the theoretical and experimental isotope patterns of the 5+ peaks of [c3]DC2·18PF6.(B) ESI MS (top) and TWIM MS (bottom) spectra of [c3]DC12·18PF6. The insets show the comparison between the theoretical and experimental isotope patterns of the 5+ peaks of [c3]DC12·18PF6.View Large Image Figure ViewerDownload Hi-res image Download (PPT) Low symmetric monomers—i.e., a CBPQT4+ derivative substituted on one of the p-xylylene units—will not only result in highly complicated NMR spectra for the [cn]DCs, but will also make it difficult to grow crystals and obtain ordered crystal superstructures and structures. (A) ESI MS (top) and TWIM MS (bottom) spectra of [c3]DC2·18PF6. The insets show the comparison between the theoretical and experimental isotope patterns of the 5+ peaks of [c3]DC2·18PF6. (B) ESI MS (top) and TWIM MS (bottom) spectra of [c3]DC12·18PF6. The insets show the comparison between the theoretical and experimental isotope patterns of the 5+ peaks of [c3]DC12·18PF6. Another crucial choice to be made in favoring the production of [c3]DC is the bridge unit that covalently links the mCBPQT2(⋅+) ring to the BIPY⋅+ unit. In the research described in this article, a single methylene group was chosen (Figure 2A) as the linker, not only because it is convenient synthetically but also because it introduces some necessary rigidity. These structural features provide the corresponding hermaphroditic monomer with a preorganized conformation that makes the formation of the unwanted [c1]DCs or [c2]DCs unfavorable. Instead, preferential formation of [c3]DC is favored and occurs as expected. The synthesis of the self-complementary monomer with an alkyne tail, namely, M2-CC·6PF6, is straightforward. See Supplemental Sections B and C. Starting from (3,5-bis(bromomethyl)phenyl)methanol, M2-CC·6PF6 was prepared in five steps involving a sequence of SN2 substitutions with an overall yield of ~30%. Moreover, the synthesis of M2-CC·6PF6 can be scaled up to a gram scale quite easily, while the functionalized tail of the monomer can also be altered at will—for example, M2-N3·6PF6 and M12-N3·6PF6 in Scheme 1—so as to enable the modification of the [c3]DCn18+ structure. The self-assembly of the self-complementary monomer M2-CC·6PF6 was investigated using visible/NIR (Vis/NIR) absorption spectroscopy under reducing conditions. The reduction of the colorless MeCN solution of M2-CC·6PF6 with an excess of Zn dust was followed by filtration, giving a deep-purple-colored solution. The Vis/NIR spectrum (Figure 2B) of this solution exhibited an intense NIR absorption band centered at 1,060 nm, an observation that is a characteristic61Cai K. Shi Y. Cao C. Vemuri S. Cui B. Shen D. Wu H. Zhang L. Qiu Y. Chen H. et al.Tuning radical interactions in trisradical tricationic complexes by varying host-cavity sizes.Chem. Sci. 2020; 11: 107-112Crossref PubMed Google Scholar of BIPY⋅+⊂mCBPQT2(⋅+) complex formation. This observation tells us that some daisy-chain-like assemblies—cyclic and/or acyclic—must have been present in the reduced solution. We also recorded Vis/NIR spectra of the 0.10 mM MeCN solution of M2-CC·6PF6 in its reduced state at different temperatures. The temperature-dependent Vis/NIR spectra (Figure 2B) show that the intensity of the NIR absorption band decreased gradually upon heating to 351 K. Meanwhile, the absorbance of the peak near 600 nm increased upon heating, and two clear isosbestic points were observed at 766 and 536 nm. These observations indicate that the supramolecular assembly dissociated into monomers at high temperature, and this transition involved two optically different states—the daisy-chain assemblies and their monomers. In order to obtain deeper insight into this self-assembly process, we performed thermodynamic analyses on the temperature-dependent Vis/NIR spectroscopic data of M2-CC3(⋅+) in MeCN. See Supplemental Section F. Based on the van 't Hoff analysis (Figure S27), the apparent64For a 1:1 mixture of a host and a guest in solution, the association constant Ka can be calculated by applying a van ’t Hoff analysis to the temperature-dependent absorption spectroscopic data. For a self-complementary monomer in which the host and the guest units are covalently linked, using this same analysis of the temperature-dependent absorption spectroscopic data, we can obtain the apparent association constant Kapp. enthalpy (ΔHa = −13.8 ± 0.3 kcal·mol−1) and entropy (ΔS = −24.4 ± 0.9 cal·mol−1·K−1) of binding were determined for the BIPY⋅+⊂mCBPQT2(⋅+) pair in M2-CC3(⋅+) in MeCN. Hence, the apparent binding constant associated with the BIPY⋅+⊂mCBPQT2(⋅+) complex in M2-CC3(⋅+) at 298 K is approximately 4.9 × 104 M−1, a Ka value that is significantly larger than that (8.9 × 103 M−1) of the host–guest complex, MV⋅+⊂mCBPQT2(⋅+). Accordingly, the self-assembly of M2-CC3(⋅+) leading to the formation of supramolecular daisy chains is a strongly cooperative65Hunter C.A. Anderson H.L. What is cooperativity?.Angew. Chem. Int. Ed. Engl. 2009; 48: 7488-7499Crossref PubMed Scopus (576) Google Scholar process, implying that these chains are very like