Chemoresponsive alternating supramolecular copolymers created from heterocomplementary calix[4]pyrroles Jung Su Parka, Ki Youl Yoonb, Dong Sub Kima, Vincent M. Lyncha, Christopher W. Bielawskia,1, Keith P. Johnstonb,1, and Jonathan L. Sesslera,c,1 aDepartment of Chemistry and Biochemistry, 1 University Station-A5300, University of Texas, Austin, TX 78712-0165; bDepartment of Chemical Engineering, 1 University Station C0400, University of Texas, Austin, TX 78712-0231; and cDepartment of Chemistry, Yonsei University, Seoul 120-749, Korea Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved October 27, 2011 (received for review September 19, 2011) The importance of noncovalent interactions in the realm of biolo- varying widely in terms of the specifics, as a general rule, the func- gical materials continues to inspire efforts to create artificial supra- tional response to activating molecular stimuli seen in biological molecular polymeric architectures. These types of self-assembled systems can be traced to substrate-induced structural changes materials hold great promise as environmentally stimuli-respon- involving large, multicomponent systems built up from smaller sive materials because they are capable of adjusting their various fragments through self-assembly. structural parameters, such as chain length, architecture, conforma- These same weak noncovalent interactions also provide the tion, and dynamics, to new surrounding environments upon expo- basis for the fast-evolving field of self-assembled and responsive sure to appropriate external stimuli. Nevertheless, in spite of materials (3–20). Here, important advances have been made of considerable advances in the area of responsive materials, it has late, including the development of self-healing materials (18–20) proved challenging to create synthetic self-assembled materials and stimuli-responsive supramolecular polymers (3–10). Although that respond to highly disparate analytes and whose environmen- a variety of strategies have been advanced for the construction tally induced changes in structure can be followed directly through of supramolecular polymeric systems, to date it has not proved both various spectroscopic and X-ray diffraction analyses. Herein, possible to create artificial, stimulus-responsive, synthetic materi- we report a new set of artificial self-assembled materials obtained als that reflect the full complexity of natural functional systems. by simply mixing two appropriately chosen, heterocomplementary Particularly challenging has been the creation of multicomponent macrocyclic receptors, namely a tetrathiafulvalene-functionalized self-assembled systems that respond in an orthogonal fashion to calix[4]pyrrole and a bis(dinitrophenyl)-meso-substituted calix[4] two or more guests and which give rise to easy-to-discern guest- pyrrole. The resulting polymeric materials, stabilized by combina- dependent multiple readout signals. Even more elusive are respon- tion of donor–acceptor and hydrogen bonding interactions, under- sive polymeric materials that have been fully characterized at the CHEMISTRY go dynamic, reversible dual guest-dependent structural transfor- level of atomic resolution via single crystal X-ray diffraction analysis mations upon exposure to two very different types of external We now report a class of structurally characterized heterocom- chemical inputs, namely chloride anion and trinitrobenzene. The plementary, self-assembled materials obtained from the pairing structure and dynamics of the copolymers and their analyte-depen- of electron-rich tetrathiafulvalene-calix[4]pyrroles (TTF-C4Ps 1, dent responsive behavior was established via single crystal X-ray 2, and 3) (24, 25) and electron poor cis/trans-bis(dinitrophenyl)- crystallography, SEM, heterocomplementary isodesmic analysis, calix[4]-pyrroles (DNP-C4Ps 4 and 5)(SI Appendix). As detailed 1- and 2D NMR, and dynamic light scattering spectroscopies. Our below, these macromolecular materials (e.g., 6) are stabilized via results demonstrate the benefit of using designed heterocomple- a combination of oriented hydrogen bonding and pi–pi donor– mentary interactions of two functional macrocyclic receptors to acceptor interactions. They act as “intelligent” chemoresponsive create synthetic, self-assembled materials for the development “ ” materials, displaying distinctive changes in structure, color, electro- of smart sensory materials that mimic the key biological attri- chemical and spectroscopic features when exposed to appropriate butes of multianalyte recognition and substrate-dependent multi- guests, specifically chloride anion and the test nitroaromatic explo- signaling. sive, trinitrobenzene (TNB). Both the initial supramolecular poly- merization process and the ensuing analyte-induced changes were cooperativity ∣ sensing ∣ supramolecular chemistry ∣ supramolecular followed in solution by UV-visible (UV-vis) and 1H-, NOESY-, and polymers ∣ dynamic materials diffusion ordered nuclear magnetic resonance (DOSY-NMR) spectroscopies, as well as dynamic light scattering (DLS). The n recent years, considerable effort has been devoted to the associated structures were also determined in the solid state via Idevelopment of artificial supramolecular polymeric materials single crystal X-ray diffraction analyses (Figs. 1 and 2). (1–20). Much of the interest in these kinds of systems stems from an appreciation that they act as rudimentary models for some of the most fundamental, yet complex systems, found in nature. Author contributions: J.S.P., C.W.B., and J.L.S. designed research; J.S.P., K.Y.Y., D.S.K., and Living systems typically exploit multiple noncovalent interactions V.M.L. performed research; J.S.P. contributed new reagents/analytic tools; J.S.P., K.Y.Y., D.S.K., V.M.L., C.W.B., K.P.J., and J.L.S. analyzed data; and J.S.P., C.W.B., K.P.J., and J.L.S. to drive the formation and stabilization of a wide variety of com- wrote the paper. plex polymeric architectures for the implementation of function, The authors declare no conflict of interest. codifying diversity, and imparting complexity. Because of their stabilization, in part by multiple weak forces, including hydrogen This article is a PNAS Direct Submission. bonds, metal-ligand interactions, and donor–acceptor interac- Data deposition: The crystallographic data have been deposited in the Cambridge Crystallographic Data Centre, Cambridge CB2 1EZ, United Kingdom, http://www.ccdc. tions, many of these architectures undergo considerable modifi- cam.ac.uk (CSD reference nos. CCDC-831195 to CCDC-831205). cation in their structure and function upon application of some 1To whom correspondence may be addressed. E-mail: [email protected], kpj@ external stress or exposure to a specific chemical trigger, as illu- che.utexas.edu, or [email protected]. strated by various denaturation and renaturation processes invol- This article contains supporting information online at www.pnas.org/lookup/suppl/ ving DNA (21), proteins (22), and lipid bilayers (23). Although doi:10.1073/pnas.1115356108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1115356108 PNAS ∣ December 27, 2011 ∣ vol. 108 ∣ no. 52 ∣ 20913–20917 Downloaded by guest on September 29, 2021 Fig. 1. Schematic representation of the self-association between the TTF-C4P monomers 1, 2, and 3 and the heterocomplementary DNP-C4P monomers 4 and 5. Also shown is the chemoresponsive behavior seen upon the addition of either TEACl or TNB. TTF-C4Ps, like other calix[4]pyrroles, can adopt several con- Solid-State Characterization of Alternating Supramolecular Copoly- formations, including the limiting cone and 1,3-alternate forms. mers 6. In order to characterize the solid-state products obtained In previous work, we found that these macrocycles would bind as the result of these mixing experiments, pentane was slowly planar, electron-deficient nitroaromatics, including TNB, with a diffused into the initial chloroform solutions. In all cases, this 1∶2 stoichiometry in their 1,3-alternate forms in the absence of crystallization procedure afforded long, brownish needle-shaped anions (24, 25). In contrast, the corresponding anion-stabilized materials except for the combination of 3 and 5 (SI Appendix, cone forms would interact with larger electron-deficient species, Sections 3-1). The nature of these solids differed significantly as evidenced by the anion-triggered binding of C60 (26) or the in terms of both morphology and color from the starting materials stabilization of a long-lived electron transfer state as the result 1–5 when subject to similar pentane-chloroform crystallization 6a–e of supramolecular capsule formation (27). The fact that we could procedures. Detailed SEM studies of the materials obtained 1–3 4 5 stabilize such discrete supramolecular structures led us to consid- from the mixtures of and either or revealed the formation er that doubly functionalized electron-deficient guest species, of well-defined, long needle-like fibers from the five combina- such as 4 and 5, could be utilized as ditopic macrocyclic building tions. Collectively, these data provide support for the inference blocks to create self-assembled polymeric arrays. We also postu- that the solid material was obtained as the result of a highly A C lated that the resulting self-assembled systems would prove directional 1D crystal growing process (see Fig. 2 and , and SI Appendix, Figs. S10–S12). responsive to multiple chemical stimuli. Specifically, we predicted 6a 6d 1 þ 4 2 þ 5 that the