No abstract

Electronics that are soft, conformal, and stretchable are highly desirable for wearable electronics, prosthetics, and robotics. Among the various available electronic materials, single walled carbon nanotubes (SWNTs) and their network have exhibited high mechanical flexibility and stretchability, along with comparable electrical performance to traditional rigid materials, e.g. polysilicon and metal oxides. Unfortunately, SWNTs produced en masse contain a mixture of semiconducting (s-) and metallic (m-) SWNTs, rendering them unsuitable for electronic applications. Moreover, the poor solubility of SWNTs requires the introduction of insulating surfactants to properly disperse them into individual tubes for device fabrication. Compared to other SWNT dispersion and separation methods, e.g., DNA wrapping, density gradient ultracentrifugation, and gel chromatography, polymer wrapping can selectively disperse s-SWNTs with high selectivity (>99.7%), high concentration (>0.1 mg/mL), and high yield (>20%). In addition, this method only requires simple sonication and centrifuge equipment with short processing time down to 1 h. Despite these advantages, the polymer wrapping method still faces two major issues: (i) The purified s-SWNTs usually retain a substantial amount of polymers on their surface even after thorough rinsing. The low conductivity of the residual polymers impedes the charge transport in SWNT networks. (ii) Conjugated polymers used for SWNT wrapping are expensive. Their prices ($100-1000/g) are comparable or even higher than those of SWNTs ($10-300/g). These utilized conjugated polymers represent a large portion of the overall separation cost. In this Account, we summarize recent progresses in polymer design for selective dispersion and separation of SWNTs. We focus particularly on removable and/or recyclable polymers that enable low-cost and scalable separation methods. First, different separation methods are compared to show the advantages of the polymer wrapping methods. In specific, we compare different characterization methods used for purity evaluation. For s-SWNTs with high purity, i.e., >99%, short-channel (smaller than SWNT length) electrical measurement is more reliable than optical methods. Second, possible sorting mechanism and molecular design strategies are discussed. Polymer parameters such as backbone design and side chain engineering affect the polymer-SWNT interactions, leading to different dispersion concentration and selectivity. To address the above-mentioned limiting factors in both polymer contamination and cost issues, we describe two important polymer removal and cycling approaches: (i) changing polymer wrapping conformation to release SWNTs; (ii) depolymerization of conjugated polymer into small molecular units that have less affinity toward SWNTs. These methods allow the removal and recycling of the wrapping polymers, thus providing low-cost and clean s-SWNTs. Third, we discuss various applications of polymer-sorted s-SWNTs, including flexible/stretchable thin-film transistors, therm...

CONSPECTUS: Within the framework of miniaturization of electromechanical devices, the development of a redox-switchable molecular gripper as a tool for nanorobotics is appealing both from an academic and from a practical perspective. Such a tool should be able to controllably grab a molecular cargo, translocate it over considerable distances and time scales, and release it. Resorcin[4]arene cavitands seem to be an ideal platform for the development of molecular grippers due to their ability to adopt two spatially well-defined conformations: an expanded kite and a contracted vase. Furthermore, they possess "legs" for functionalization and attachment to metal surfaces. While changes in temperature, pH, and metal-ion concentration were known to induce conformational switching, redox-switchable cavitands remained a challenge. In this Account, we describe our efforts toward the development of a new class of resorcin[4]arene cavitands that are redox-switchable. First, we introduced the naphthoquinone moiety as a redox-active wall component and showed that cavitands containing four quinone walls strongly prefer the open kite conformation in both the quinone and hydroquinone redox states, while cavitands that contain two quinone and two quinoxaline walls can adopt both the vase and the kite conformations depending on solvent but not on redox state. Next, in order to introduce a driving force for the conformational switching process in diquinone cavitands, we designed cavitands with hydrogen bond acceptor groups on the quinoxaline walls. These acceptors were sought to establish hydrogen bonds with the hydroquinone groups in the reduced redox state, thereby stabilizing the vase form. Oxidation to the quinone state would remove these interactions, switching the cavitand back to the kite form. Cavitands equipped with different hydrogen bond acceptor groups were synthesized and evaluated. We found that carboxamide moieties are best suited to assist redox-induced switching of conformational and binding properties. With the goal of further increasing association constants and reducing guest-exchange rates via steric congestion, we exchanged the naphthoquinone with the triptycene-quinone moiety. The congesting influence of the triptycene-quinone moiety on the binding properties was quantified both in the presence and in the absence of additional hydrogen bond interactions that stabilize the vase form. X-ray crystallographic studies provided insights into the solid-state structures of the cavitands in different solvents and redox states. A significant enhancement of association constants and reduction in guest release rates was observed in the reduced redox state compared with the top-open system, yielding redox-switchable cavitand baskets. These studies represent a step towards the development of redox-switchable molecular grippers on metal surfaces. Future challenges will consist in the development of cavitands that will no longer rely on an external proton source for the switching process, allowing redox-switching to be perfor...

The development of semiquinone-based resorcin[4]arene cavitands expands the toolbox of switchable molecular grippers by introducing the first paramagnetic representatives. The semiquinone (SQ) states were generated electrochemically, chemically, and photochemically. We analyzed their electronic, conformational, and binding properties by cyclic voltammetry, ultraviolet/visible (UV/vis) spectroelectrochemistry, electron paramagnetic resonance (EPR) and transient absorption spectroscopy, in conjunction with density functional theory (DFT) calculations. The utility of UV/vis spectroelectrochemistry and EPR spectroscopy in evaluating the conformational features of resorcin[4]arene cavitands is demonstrated. Guest binding properties were found to be enhanced in the SQ state as compared to the quinone (Q) or the hydroquinone (HQ) states of the cavitands. Thus, these paramagnetic SQ intermediates open the way to six-state redox switches provided by two conformations (open and closed) in three redox states (Q, SQ, and HQ) possessing distinct binding ability. The switchable magnetic properties of these molecular grippers and their responsiveness to electrical stimuli has the potential for development of efficient molecular devices.

Semiconducting, single-walled carbon nanotubes (SWNTs) are promising candidates for applications in thin-film transistors, solar cells, and biological imaging. To harness their full potential, however, it is necessary to separate the semiconducting from the metallic SWNTs present in the as-synthesized SWNT mixture. While various polymers are able to selectively disperse semiconducting SWNTs, the subsequent removal of the polymer is challenging. However, many applications require semiconducting SWNTs in their pure form. Toward this goal, we have designed a 2-ureido-6[1H]-pyrimidinone (UPy)-based H-bonded supramolecular polymer that can selectively disperse semiconducting SWNTs. The dispersion purity is inversely related to the dispersion yield. In contrast to conventional polymers, the polymer described herein was shown to disassemble into monomeric units upon addition of an H-bond-disrupting agent, enabling isolation of dispersant-free, semiconducting SWNTs.