Bilge Baytekin

The Mosaic of Surface Charge in Contact Electrification

Electrification caused by rubbing two objects creates patches of positive and negative charge on both surfaces. When dielectric materials are brought into contact and then separated, they develop static electricity. For centuries, it has been assumed that such contact charging derives from the spatially homogeneous material properties (along the material’s surface) and that within a given pair of materials, one charges uniformly positively and the other negatively. We demonstrate that this picture of contact charging is incorrect. Whereas each contact-electrified piece develops a net charge of either positive or negative polarity, each surface supports a random “mosaic” of oppositely charged regions of nanoscopic dimensions. These mosaics of surface charge have the same topological characteristics for different types of electrified dielectrics and accommodate significantly more charge per unit area than previously thought.

Mass spectrometric studies of non-covalent compounds: why supramolecular chemistry in the gas phase?

Supramolecular chemistry has progressed quite a long way in recent decades. The examination of non-covalent bonds became the focus of research once the paradigm that the observed properties of a molecule are due to the molecule itself was revised, and researchers became aware of the often quite significant influence of the environment. Mass spectrometry and gas-phase chemistry are ideally suited to study the intrinsic properties of a molecule or a complex without interfering effects from the environment, such as solvation and the effects of counterions present in solution. A comparison of data from the gas phase, i.e. the intrinsic properties, with results from condensed phase, i.e. the properties influenced by the surroundings of the molecule, can consequently contribute significantly to the understanding of non-covalent bonds. This review provides insight into the often-underestimated power of mass spectrometry for the investigation of supramolecules. Through example studies, several aspects are discussed, including determination of structure in solution and the gas phase, ion mobility studies to reveal the formation of zwitterionic structures, stereochemical issues, analysis of reactivity of supramolecular compounds in the condensed and in the gas phase, and the determination of thermochemical data.

Control of Surface Charges by Radicals as a Principle of Antistatic Polymers Protecting Electronic Circuitry

Dissipating Static The accumulation of a static charge on polymers and other insulators often causes little more than a slight annoyance but it can lead to the destruction of sensitive electrical equipment. Thus, approaches are required that prevent and dissipate static electricity through improved electrical conductivity, or that ensure complete discharge before a contact with a key piece of equipment. Baytekin et al. (p. 1368) show that surface charges will colocalize with radicals on the surface of a polymer, and that the addition of free radical scavengers causes a discharge of the surface as the charges are removed. The approach was used successfully to produce coatings that protected electronic circuits from damage caused by electrostatic discharge. Removal of radicals destabilizes surface charges, providing a means for rapid dissipation of static electricity. Even minute quantities of electric charge accumulating on polymer surfaces can cause shocks, explosions, and multibillion-dollar losses to electronic circuitry. This paper demonstrates that to remove static electricity, it is not at all necessary to “target” the charges themselves. Instead, the way to discharge a polymer is to remove radicals from its surface. These radicals colocalize with and stabilize the charges; when they are scavenged, the surfaces discharge rapidly. This radical-charge interplay allows for controlling static electricity by doping common polymers with small amounts of radical-scavenging molecules, including the familiar vitamin E. The effectiveness of this approach is demonstrated by rendering common polymers dust-mitigating and also by using them as coatings that prevent the failure of electronic circuitry.

What really drives chemical reactions on contact charged surfaces

Although it is known that contact-electrified polymers can drive chemical reactions, the origin of this phenomenon remains poorly understood. To date, it has been accepted that this effect is due to excess electrons developed on negatively charged surfaces and to the subsequent transfer of these electrons to the reactants in solution. The present study demonstrates that this view is incorrect and, in reality, the reactions are driven by mechanoradicals created during polymer-polymer contact.

Rewiring Chemistry: Algorithmic Discovery and Experimental Validation of One‐Pot Reactions in the Network of Organic Chemistry

In 2005 and 2006, we published the first reports on the representation of all synthetic knowledge as a giant network in which molecule “nodes” are connected by reaction “arrows” (Figure 1). In these early works, we focused on the topological structure and evolution of this network and demonstrated the scale-free network topology, existence of hub molecules central to organic synthesis, exponential growth of the network in time, correlations between molecular masses, trends in reactivity based on network connectivity, and more. While our analyses had little applicability to the everyday synthetic practice, we envisioned that such a junction between network theory and synthesis would one day be achieved. Now, we are reporting, in three consecutive communications, the extension of chemicalnetwork concepts into methods directly relevant to experimental chemistry: 1) discovery of one-pot reactions; 2) optimization of multiple reaction pathways, and 3) the detection and blocking of synthetic pathways leading to dangerous chemicals. The first communication in this series addresses one of the most important challenges in organic chemistry: namely, how to “wire” individual reactions into sequences that could be performed in one pot. One-pot reactions save resources and time by avoiding isolation, purification, characterization, and production of chemical waste after each synthetic step. Sometimes, such reactions are identified by chance or, more often, by careful inspection of individual steps that are to be wired together; even this latter process, however, is invariably subjective and depends on the knowledge and intuition of any individual chemist (or group of chemists) involved. Herein, we show that the discovery of one-pot reactions can be facilitated by computational methods. We first describe algorithms that identify possible onepot reactions within the network of all known synthetic knowledge and then demonstrate that the computationally predicted sequences can indeed be carried out experimentally in good overall yields. The experimental examples are chosen to “rewire” small networks of reactions around practically important chemicals: quinoline scaffolds, quinoline-based enzyme inhibitors, and thiophene derivatives. In this way, we replace individual synthetic connections with two-, three-, or even four-step one-pot sequences. The network of organic chemistry (NOC; Figure 1) is constructed from reactions reported in the chemical literature since 1779 and nowadays stored in chemical databases. Pruning the raw data to remove catalysts, solvents, substances that do not participate in reactions, and duplicate or incomplete reactions, leaves about 7 million reactions and about 7 million substances on which further analyses are based. This dataset is translated into a network by representing chemical substances as network nodes, and the reactions as arrows directed from the reaction s substrates to products. At first glance, this giant network of chemistry might look akin to the metabolic networks of biochemical reactions. In reality, however, metabolic networks are true chemical systems comprising reactions that can, in most cases, occur concurrently within the same reaction medium (that is, in Figure 1. a) A small (ca. 5500 nodes, ca. 0.1% of the total) fragment of the network of organic chemistry (NOC), where individual nodes represent the molecules and arrows represent reactions. The representation in b) has the reaction arrows colored by the times these reactions were first reported. This representation emphasizes the fact that NOC, by itself, is not a “coherent” giant chemical system but only a repository of reactions discovered separately, without regard for their mutual compatibility. At best, it can be said that there was a “coherent” interest in certain areas of chemistry (for example, synthetic activity around the Penicillin V node in the 1960s, following the first total synthesis).

Mechanoradicals created in "polymeric sponges" drive reactions in aqueous media.

The study of mechanochemical phenomena in polymers dates back to the work of Staudinger, who attributed the reduction in the molecular weight of polymers under mastication to the mechanical rupture of the constituent macromolecules. Subsequent studies have established that these and other related effects are a result of the homolytic cleavage of covalent bonds and creation of radicals within stressed polymers. Technologically, mechanochemical treatment has been used to adjust the rheological properties of rubbers, to degrade biopolymers, such as starch, to desired molecular weights, to dehalogenate hazardous polymer contaminants, to polymerize or copolymerize through vigorous milling and/or grinding, and it has also been used as the basis of mechanochromism. Yet, these applications remain scarce and specialized, and in virtually all everyday systems where polymers are subject to mechanical stresses, for example, in the tires of road vehicles, the soles of walking shoes, and so on, the chemical energy of homolytically broken bonds is not being harnessed in any purposeful way. Herein, we show how a significant fraction of this ubiquitous, and in some sense “free” (for otherwise not used), energy can be retrieved if the polymers that are being deformed are in contact with water. Under these conditions, the mechanoradicals that are created within the polymer migrate to the polymer/water interface, at which they produce H2O2, which can then drive several types of chemical processes, such as nanoparticle synthesis, dye bleaching, or fluorescence. The amount of H2O2 that is produced scales with the interfacial area and is in the order of tens of mg m 2 for 1 J of mechanical energy input, and the overall efficiency of the mechanical-tochemical energy conversion is, depending on the polymer used, from approximately 7 % up to a remarkable 30% for soft, “spongy” polymers. Deformable polymer “sponges” that drive aqueous-phase radical reactions can be construed as solid-state chemical reagents that convert mechanical energy into chemical energy in a “clean”, environmentally friendly fashion. On the other hand, the fact that polymers under stress produce potentially harmful free radicals might raise concerns about the safety of polymer-based medical implants. We used the flexible polymers poly(dimethylsiloxane) (PDMS, Dow Corning, Sylgard 184), Tygon (Saint-Gobain Corp. #R-3603), and poly(vinyl chloride) (PVC, VWR, #60985-534), all of which gave qualitatively similar results. Typically, hollow polymer tubes (inner diameter 0.6–0.8 cm, outer diameter 1.5 cm, height ca. 7.5 cm) were filled with water or an aqueous solution of a desired reagent, and were compressed mechanically with strains of up to 40% (Figure 1a). In all cases, the deformations were nondestructive and reversible at the macroscopic level, as shown by the forcedisplacement curves of the squeezed polymers not changing over many compression/release cycles (see Figure S10 in the Supporting Information). Measurements with a Faraday cup connected to a high-precision electrometer (Keithley, 6517B) confirmed that the compressed pieces did not generate any measurable static electricity (surface charge densities were

Retrieving and converting energy from polymers: deployable technologies and emerging concepts

Every year, several tens of million tonnes of polymers end up as waste. Contrary to popular belief and optimistic media stories of ever-improving recycling efforts, only a small fraction – indeed, a few percent – of this polymeric litter is actually being recycled and reused. In the U.S., some 3 million tonnes of plastics are recycled annually, yet over 28 million tonnes age unproductively – if the energy of this waste could be harnessed at a moderate 50% efficiency, the greater Chicago area would glow and spark almost all year round! Fortunately, significant progress is being made in technologies that aim at retrieving energy from waste polymers. On the large, often industrial, scales, polymers are being incinerated, pyrolized, and chemically degraded. Some of these technologies can produce viable fuels at costs as low as

Light‐Harvesting in Multichromophoric Rotaxanes

0.75 per gallon, some five times smaller than what Mr Smith nowadays pays at the gas pump. There is also plenty of interesting, exploratory science done at smaller scales, where polymers are used in electrostatic or piezoelectric generators, or as materials converting mechanical to chemical energy. Several proof-of-concept devices have been shown to produce enough energy to power personal electronic devices or drive laboratory-scale chemical reactions. Together, the large- and small-scale technologies constitute a realistic strategy to retrieve a sizeable fraction of energy stored in polymers that would otherwise be only presenting a serious environmental concern.

Programmable multilayers of nanometer-sized macrocycles on solid support and stimuli-controlled on-surface pseudorotaxane formation

Two rotaxanes with benzyl ether axles and tetralactam wheels were synthesized through an anion template effect. They carry naphthalene chromophores attached to the stopper groups and a pyrene chromophore attached to the wheel. The difference between the two rotaxanes is represented by the connecting unit of the naphthyl chromophore to the rotaxane axle: a triazole or an alkynyl group. Both rotaxanes exhibit excellent light-harvesting properties: excitation of the naphthalene chromophores is followed by energy transfer to the pyrene unit with efficiency higher than 90% in both cases. This represents an example of light-harvesting function among chromophores belonging to mechanically interlocked components, that is, the axle and the wheel of the rotaxanes.

Estimating chemical reactivity and cross-influence from collective chemical knowledge

Mechanically interlocked molecules (MIMs) such as rotaxanes and catenanes are capable of mechanical motion on the nanoscale and are therefore promising prototypes for molecular machines in recent nanotechnology. However, most of the existing examples are isotropically distributed in solution, which prohibits concerted movement and with it the generation of macroscopic effects. Thus, arranging them in ordered arrays is of huge interest in recent research. We report the deposition of quite densely packed multilayers of tetralactam macrocycles on gold surfaces by metal-coordinated layer-by-layer self-assembly. Linear dichroism effects in angle-resolved NEXAFS spectra indicate a preferential orientation of the macrocycles. The sequence of the metal ions can be programmed by the use of different transition metal ions at each deposition step. Additionally, reversible on-surface pseudorotaxane formation was successfully realized by repeated uptake and release of axle molecules inside the macrocycles cavities.