Dennis Cao

Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.

Stimulated Release of Size-Selected Cargos in Succession from Mesoporous Silica Nanoparticles†

Combination drug therapy, a regimen in which multiple drugs with different therapeutic outcomes are used in parallel or in sequence, has become one of the dominant strategies in the clinical treatment of HIV/AIDS, diabetes, and cancer. In cancer therapy, for example, the U.S. Food and Drug Administration (FDA) approved the use in 2006 of Avastin in combination with Carboplatin and Paclitaxel for the initial systemic treatment of patients with lung cancer. Unlike monotherapy, combination therapy maximizes therapeutic efficacy against individual targets and is more likely to overcome drug resistance, while increasing the odds of a positive prognosis and reducing harmful side effects. Drug delivery systems, which administer medically active compounds to diseased cells in a targeted and controlled manner, have gained much attention in the past couple of decades. While polymers, dendrimers, micelles, vesicles, and nanoparticles have all been investigated for their use as possible drug delivery systems, most systems provide either delivery of a single drug or the simultaneous delivery of multiple drugs. Using these systems, however, it is difficult to control the administration order, timing, and dosage of each individual drug in a comprehensive manner. While it is possible to deliver a cocktail of drugs using several different co-administered drug delivery systems, this protocol has disadvantages. For example, it is not easy to expose several co-administered drug delivery systems to the same target at the right time, while also controlling the dosage rates and ratios of each individual drug. In order to administer chemotherapeutic combinations and produce synergistic actions, well-organized multidrug release systems, which can provide combination therapies by controlling the release behavior of each drug individually, need to be invented. Mesoporous silica nanoparticles (MSNs) have attracted widespread interest in the past decade for use in integrated functional systems. They have large surface exteriors and porous interiors that can be harnessed as reservoirs for smallmolecule-drug storage. These MSNs are nontoxic to cells and can undergo cellular uptake into acidic lysosomes by endocytosis when they are 100–200 nm in diameter, thus making them a popular candidate for drug delivery systems. In particular, MSNs can be functionalized with molecular, as well as supramolecular, switches in order to control the release of drug molecules in response to external stimuli. Oncommand release systems, which respond to a range of stimuli, including pH changes, light initiation, competitive binding, redox activation, biological triggers, and temperature changes, have been reported by us and others. To the best of our knowledge, however, all the on-command release systems reported to date cannot release multiple drugs in a step-by-step fashion. Cyclodextrins (CDs), because of their abilities to form inclusion complexes with guest molecules, have been the focus of much research. b-Cyclodextrin (b-CD), which comprises seven a-1,4-linked d-glucopyranosyl units with top and bottom cavities of 6.0 and 6.5 , respectively, has been employed as a gatekeeper in drug delivery systems. [*] Dr. C. Wang, D. Cao, Dr. Y.-L. Zhao, J. W. Gaines, Dr. O. A. Bozdemir, M. W. Ambrogio, Dr. M. Frasconi, Dr. Y. Y. Botros, Prof. J. F. Stoddart Center for the Chemistry of Integrated Systems Department of Chemistry and Department of Material Sciences Northwestern University 2145 Sheridan Road, Evanston, IL 60208 (USA) E-mail: stoddart@northwestern.edu Z. Li, Prof. J. I. Zink Department of Chemistry and Biochemistry and California NanoSystems Institute, University of California Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095 (USA) E-mail: zink@chem.ucla.edu

A Radically Configurable Six-State Compound

Radically Organic Metals such as manganese are relatively stable over a wide range of oxidation states. In contrast, purely organic compounds are rarely susceptible to incremental addition or removal of electrons without accompanying fragmentation or coupling reactions. Barnes et al. (p. 429; see the Perspective by Benniston) report a catenane (a compound comprising interlocked rings) in which the topological structure stabilizes six different states that successively differ by the presence or absence of one or two electrons in the framework. The hepta-oxidized state proved remarkably resilient to oxygen exposure. An interlocked-rings topology stabilizes a wide range of collective oxidation states in a metal-free organic compound. [Also see Perspective by Benniston] Most organic radicals possess short lifetimes and quickly undergo dimerization or oxidation. Here, we report on the synthesis by radical templation of a class of air- and water-stable organic radicals, trapped within a homo[2]catenane composed of two rigid and fixed cyclobis(paraquat-p-phenylene) rings. The highly energetic octacationic homo[2]catenane, which is capable of accepting up to eight electrons, can be configured reversibly, both chemically and electrochemically, between each one of six experimentally accessible redox states (0, 2+, 4+, 6+, 7+, and 8+) from within the total of nine states evaluated by quantum mechanical methods. All six of the observable redox states have been identified by electrochemical techniques, three (4+, 6+, and 7+) have been characterized by x-ray crystallography, four (4+, 6+, 7+, and 8+) by electron paramagnetic resonance spectroscopy, one (7+) by superconducting quantum interference device magnetometry, and one (8+) by nuclear magnetic resonance spectroscopy.

Ground-State Thermodynamics of Bistable Redox-Active Donor–Acceptor Mechanically Interlocked Molecules

Fashioned through billions of years of evolution, biological molecular machines, such as ATP synthase, myosin, and kinesin, use the intricate relative motions of their components to drive some of lifes most essential processes. Having control over the motions in molecules is imperative for life to function, and many chemists have designed, synthesized, and investigated artificial molecular systems that also express controllable motions within molecules. Using bistable mechanically interlocked molecules (MIMs), based on donor-acceptor recognition motifs, we have sought to imitate the sophisticated nanoscale machines present in living systems. In this Account, we analyze the thermodynamic characteristics of a series of redox-switchable [2]rotaxanes and [2]catenanes. Control and understanding of the relative intramolecular movements of components in MIMs have been vital in the development of a variety of applications of these compounds ranging from molecular electronic devices to drug delivery systems. These bistable donor-acceptor MIMs undergo redox-activated switching between two isomeric states. Under ambient conditions, the dominant translational isomer, the ground-state coconformation (GSCC), is in equilibrium with the less favored translational isomer, the metastable-state coconformation (MSCC). By manipulating the redox state of the recognition site associated with the GSCC, we can stimulate the relative movements of the components in these bistable MIMs. The thermodynamic parameters of model host-guest complexes provide a good starting point to rationalize the ratio of GSCC to MSCC at equilibrium. The bistable [2]rotaxanes show a strong correlation between the relative free energies of model complexes and the ground-state distribution constants (K(GS)). This relationship does not always hold for bistable [2]catenanes, most likely because of the additional steric and electronic constraints present when the two rings are mechanically interlocked with each other. Measuring the ground-state distribution constants of bistable MIMs presents its own set of challenges. While it is possible, in principle, to determine these constants using NMR and UV-vis spectroscopies, these methods lack the sensitivity to permit the determination of ratios of translational isomers greater than 10:1 with sufficient accuracy and precision. A simple application of the Nernst equation, in combination with variable scan-rate cyclic voltammetry, however, allows the direct measurement of ground-state distribution constants across a wide range (K(GS) = 10-10(4)) of values.

Selective isolation of gold facilitated by second-sphere coordination with α-cyclodextrin

Gold recovery using environmentally benign chemistry is imperative from an environmental perspective. Here we report the spontaneous assembly of a one-dimensional supramolecular complex with an extended {[K(OH2)6][AuBr4](α-cyclodextrin)2}n chain superstructure formed during the rapid co-precipitation of α-cyclodextrin and KAuBr4 in water. This phase change is selective for this gold salt, even in the presence of other square-planar palladium and platinum complexes. From single-crystal X-ray analyses of six inclusion complexes between α-, β- and γ-cyclodextrins with KAuBr4 and KAuCl4, we hypothesize that a perfect match in molecular recognition between α-cyclodextrin and [AuBr4]− leads to a near-axial orientation of the ion with respect to the α-cyclodextrin channel, which facilitates a highly specific second-sphere coordination involving [AuBr4]− and [K(OH2)6]+ and drives the co-precipitation of the 1:2 adduct. This discovery heralds a green host–guest procedure for gold recovery from gold-bearing raw materials making use of α-cyclodextrin—an inexpensive and environmentally benign carbohydrate.

Assembly of Supramolecular Nanotubes from Molecular Triangles and 1,2-Dihalohydrocarbons

Precise control of molecular assembly is a challenging goal facing supramolecular chemists. Herein, we report the highly specific assembly of a range of supramolecular nanotubes from the enantiomeric triangular naphthalenediimide-based macrocycles (RRRRRR)- and (SSSSSS)-NDI-Δ and a class of similar solvents, namely, the 1,2-dihalo-ethanes and -ethenes (DXEs). Three kinds of supramolecular nanotubes are formed from the columnar stacking of NDI-Δ units with a 60° mutual rotation angle as a result of cooperative [C-H···O] interactions, directing interactions of the [X···X]-bonded DXE chains inside the nanotubes and lateral [X···π] or [π···π] interactions. They include (i) semiflexible infinite nanotubes formed in the gel state from NDI-Δ and (E)-1,2-dichloroethene, (ii) rigid infinite nonhelical nanotubes produced in the solid state from NDI-Δ and BrCH2CH2Br, ClCH2CH2Br, and ClCH2CH2I, and (iii) a pair of rigid tetrameric, enantiomeric single-handed (P)- and (M)-helical nanotubes formed in the solid state from the corresponding (RRRRRR)- and (SSSSSS)-NDI-Δ with ClCH2CH2Cl. In case (i), only the electron-rich C═C double bond of (E)-1,2-dichloroethene facilitates the gelation of NDI-Δ. In cases (ii) and (iii), the lengths of anti-DXEs determine the translation of the chirality of NDI-Δ into the helicity of nanotubes. Only ClCH2CH2Cl induces single-handed helicity into the nanotubes. The subtle interplay of noncovalent bonding interactions, resulting from the tiny structural variations involving the DXE guests, is responsible for the diverse and highly specific assembly of NDI-Δ. This research highlights the critical role that guests play in constructing assembled superstructures of hosts and offers a novel approach to creating supramolecular nanotubes.

Mechanical bonds and topological effects in radical dimer stabilization

While mechanical bonding stabilizes tetrathiafulvalene (TTF) radical dimers, the question arises: what role does topology play in catenanes containing TTF units? Here, we report how topology, together with mechanical bonding, in isomeric [3]- and doubly interlocked [2]catenanes controls the formation of TTF radical dimers within their structural frameworks, including a ring-in-ring complex (formed between an organoplatinum square and a {2+2} macrocyclic polyether containing two 1,5-dioxynaphthalene (DNP) and two TTF units) that is topologically isomeric with the doubly interlocked [2]catenane. The separate TTF units in the two {1+1} macrocycles (each containing also one DNP unit) of the isomeric [3]catenane exhibit slightly different redox properties compared with those in the {2+2} macrocycle present in the [2]catenane, while comparison with its topological isomer reveals substantially different redox behavior. Although the stabilities of the mixed-valence (TTF2)(•+) dimers are similar in the two catenanes, the radical cationic (TTF(•+))2 dimer in the [2]catenane occurs only fleetingly compared with its prominent existence in the [3]catenane, while both dimers are absent altogether in the ring-in-ring complex. The electrochemical behavior of these three radically configurable isomers demonstrates that a fundamental relationship exists between topology and redox properties.

Interface‐Engineered Bistable [2]Rotaxane‐Graphene Hybrids with Logic Capabilities

The use of high-quality graphene as a local probe in combination with photo excitation helps to establish a deep mechanistic understanding of charge generation/quenching processes under lying the graphene/environment interface. By combining a non-destructive bottom-up assembly technique with sensitive graphene-based transistors, a bistable [2]rotaxane-graphene hybrid device, which exhibits a symmetric mirror-image photoswitching effect with logic capabilities, is produced.

Lock-arm supramolecular ordering: A molecular construction set for cocrystallizing organic charge transfer complexes

Organic charge transfer cocrystals are inexpensive, modular, and solution-processable materials that are able, in some instances, to exhibit properties such as optical nonlinearity, (semi)conductivity, ferroelectricity, and magnetism. Although the properties of these cocrystals have been investigated for decades, the principal challenge that researchers face currently is to devise an efficient approach which allows for the growth of high-quality crystalline materials, in anticipation of a host of different technological applications. The research reported here introduces an innovative design, termed LASO-lock-arm supramolecular ordering-in the form of a modular approach for the development of responsive organic cocrystals. The strategy relies on the use of aromatic electronic donor and acceptor building blocks, carrying complementary rigid and flexible arms, capable of forming hydrogen bonds to amplify the cocrystallization processes. The cooperativity of charge transfer and hydrogen-bonding interactions between the building blocks leads to binary cocrystals that have alternating donors and acceptors extending in one and two dimensions sustained by an intricate network of hydrogen bonds. A variety of air-stable, mechanically robust, centimeter-long, organic charge transfer cocrystals have been grown by liquid-liquid diffusion under ambient conditions inside 72 h. These cocrystals are of considerable interest because of their remarkable size and stability and the promise they hold when it comes to fabricating the next generation of innovative electronic and photonic devices.

Size-selective pH-operated megagates on mesoporous silica materials.

pH-responsive megagates have been fabricated around mesoporous silica material SBA-15 in order to mechanize the mesopores. These megagates remain closed in neutral conditions, but open at pH 5. The capping components of the megagates were designed to be capable of controlling pores up to 6.5 nm in diameter. Selectivity of payloads with different sizes can be achieved through the use of different capping components. The operation of the megagates was demonstrated by time-resolved fluorescence spectroscopy which is capable of monitoring the release of both the payload and the cap. This study opens up new possibilities in the field of controllable release.