Gokhan Barin

High hopes: can molecular electronics realise its potential?

Manipulating and controlling the self-organisation of small collections of molecules, as an alternative to investigating individual molecules, has motivated researchers bent on processing and storing information in molecular electronic devices (MEDs). Although numerous ingenious examples of single-molecule devices have provided fundamental insights into their molecular electronic properties, MEDs incorporating hundreds to thousands of molecules trapped between wires in two-dimensional arrays within crossbar architectures offer a glimmer of hope for molecular memory applications. In this critical review, we focus attention on the collective behaviour of switchable mechanically interlocked molecules (MIMs)--specifically, bistable rotaxanes and catenanes--which exhibit reset lifetimes between their ON and OFF states ranging from seconds in solution to hours in crossbar devices. When these switchable MIMs are introduced into high viscosity polymer matrices, or self-assembled as monolayers onto metal surfaces, both in the form of nanoparticles and flat electrodes, or organised as tightly packed islands of hundreds and thousands of molecules sandwiched between two electrodes, the thermodynamics which characterise their switching remain approximately constant while the kinetics associated with their reset follow an intuitively predictable trend--that is, fast when they are free in solution and sluggish when they are constrained within closely packed monolayers. The importance of seamless interactions and constant feedback between the makers, the measurers and the modellers in establishing the structure-property relationships in these integrated functioning systems cannot be stressed enough as rationalising the many different factors that impact device performance becomes more and more demanding. The choice of electrodes, as well as the self-organised superstructures of the monolayers of switchable MIMs employed in the molecular switch tunnel junctions (MSTJs) associated with the crossbars of these MEDs, have a profound influence on device operation and performance. It is now clear, after much investigation, that a distinction should be drawn between two types of switching that can be elicited from MSTJs. One affords small ON/OFF ratios and is a direct consequence of the switching in bistable MIMs that leads to a relatively small remnant molecular signature--an activated chemical process. The other leads to a very much larger signature and ON/OFF ratios resulting from physical or chemical changes in the electrodes themselves. Control experiments with various compounds, including degenerate catenanes and free dumbbells, which cannot and do not switch, are crucial in establishing the authenticity of the small ON/OFF ratios and remnant molecular signatures produced by bistable MIMs. Moreover, experiments conducted on monolayers in MSTJs of molecules designed to switch and molecules designed not to switch have been probed directly by spectroscopic and other means in support of MEDs that store information through switching collections of bistable MIMs contained in arrays of MSTJs. In the quest for the next generation of MEDs, it is likely that monolayers of bistable MIMs will be replaced by robust crystalline extended structures wherein the switchable components, derived from bistable MIMs, are organised precisely in a periodic manner.

Photoinduced memory effect in a redox controllable bistable mechanical molecular switch

Mechanically interlocked molecules (MIMs) in the form of multiand bistable rotaxanes in which the ring component can be switched between different co-conformations in response to external stimuli, constitute an artificial molecular switch. They are of importance when it comes to the development of integrated systems and devices, such as responsive surfaces, molecule-based muscles and actuators, 5] nanovalves for controlled drug delivery, and molecular electronic devices (MEDs). Although the operation of bistable molecular switches is based on classical switching processes between thermodynamically stable states, it has become clear that the fulfillment of useful functions will only become possible if the rates of the mechanical movement between such states can also be controlled. This approach was used recently to implement ratchet-type mechanisms which are essential ingredients for the construction of molecular motors, and is of considerable relevance for the development of sequential logic devices such as flip-flops and memories. For all these purposes, the ability to be able to adjust the shuttling kinetics by modulating the corresponding energy barriers through external stimuli in a convenient, efficient, and reversible manner is a goal which still poses a considerable challenge to chemists. Herein, we discuss the performance of a molecular switch in the form of a bistable [2]rotaxane (Scheme 1), which 1) undergoes relative mechanical movements of its ring and

A catenated strut in a catenated metal-organic framework

Mechanically interlocked molecules (MIMs), in the form of donor–acceptor [2]catenane-containing struts of exceptional length, have been incorporated into a three-dimensional catenated metal–organic framework (MOF) at precise locations and with uniform relative orientations. Catenation is expressed simultaneously within the struts and the framework.

Metal-Organic Frameworks Incorporating Copper-Complexed Rotaxanes

MOFs on the move: A copper-coordinated [2]pseudorotaxanate which reacts with zinc nitrate to form threefold interpenetrated networks retains most of its solution-state chemistry, including its ability to undergo electronic switching of some of the copper(I) ions under redox control.

Defect Creation by Linker Fragmentation in Metal–Organic Frameworks and Its Effects on Gas Uptake Properties

We successfully demonstrate an approach based on linker fragmentation to create defects and tune the pore volumes and surface areas of two metal-organic frameworks, NU-125 and HKUST-1, both of which feature copper paddlewheel nodes. Depending on the linker fragment composition, the defect can be either a vacant site or a functional group that the original linker does not have. In the first case, we show that both surface area and pore volume increase, while in the second case they decrease. The effect of defects on the high-pressure gas uptake is also studied over a large temperature and pressure range for different gases. We found that despite an increase in pore volume and surface area in structures with vacant sites, the absolute adsorption for methane decreases for HKUST-1 and slightly increases for NU-125. However, the working capacity (deliverable amount between 65 and 5 bar) in both cases remains similar to parent frameworks due to lower uptakes at low pressures. In the case of NU-125, the effect of defects became more pronounced at lower temperatures, reflecting the greater surface areas and pore volumes of the altered forms.

A Multistate Switchable [3]Rotacatenane

Rotacatenanes are exotic molecular compounds that can be visualized as a unique combination of a [2]catenane and a [2]rotaxane, thereby combining both the circumrotation of the ring component (rotary motion) and the shuttling of the dumbbell component (translational motion) in one structure. Herein, we describe a strategy for the synthesis of a new switchable [3]rotacatenane and the investigation of its switching properties, which rely on the formation of tetrathiafulvalene (TTF) radical π-dimer interactions-namely, the mixed-valence state (TTF(2) )(+.) and the radical-cation dimer state (TTF(+.) )(2) -under ambient conditions. A template-directed approach, based on donor-acceptor interactions, has been developed, resulting in an improved yield of the key precursor [2]catenane, prior to rotacatenation. The nature of the binding between the [2]catenane and selected π-electron-rich templates has been elucidated by using X-ray crystallography and UV/Vis spectroscopy as well as isothermal titration microcalorimetry. The multistate switching mechanism of the [3]rotacatenane has been demonstrated by cyclic voltammetry and EPR spectroscopy. Most notably, the radical-cation dimer state (TTF(+.) )(2) has been shown to enter into an equilibrium by forming the co-conformation in which the two 1,5-dioxynaphthalene (DNP) units co-occupy the cavity of tetracationic cyclophane, thus enforcing the separation of TTF radical-cation dimer (TTF(+.) )(2) . The population ratio of this equilibrium state was found to be 1:1. We believe that this research demonstrates the power of constructing complex molecular machines using template-directed protocols, enabling us to make the transition from simple molecular switches to their multistate variants for enhancing information storage in molecular electronic devices.

A redox-active reverse donor–acceptor bistable [2]rotaxane

The synthesis and the dynamic behavior of a bistable [2]rotaxane, based on a reverse donor–acceptor motif containing naphthalene diimide (NpI) and 4,4′-bipyridinium (BIPY2+) as two electron-deficient stations and bis-1,5-dioxynaphthalene[38]crown-10 (BDNP38C10) as the electron-rich ring, is described. A functionalized tetraarylmethane moiety has been incorporated between the two stations in order to control the free energy barrier for the shuttling of the BDNP38C10 on the dumbbell component. The bistable [2]rotaxane was synthesized using the so-called “threading-followed-by-stoppering” approach and characterized by NMR spectroscopy and mass spectrometry. Initially, the BDNP38C10 ring resides on the NpI station on account of the synthetic approach employed in the synthesis of the bistable [2]rotaxane. 1H NMR spectroscopy was used to follow the equilibration process between the two translational isomers of the bistable [2]rotaxane—namely, NpI ⊂ BDNP38C10 and BIPY2+ ⊂ BDNP38C10. After 72 h, equilibrium was reached with a 3 : 2 ratio of the two translational isomers in favor of the NpI ⊂ BDNP38C10 co-conformation in CD3CN. The rate of relaxation of the crown ether from NpI ⊂ BDNP38C10 back to BIPY2+ ⊂ BDNP38C10 was associated with a rate constant of 2.2 ± 0.3 × 10−5 s−1 (t1/2 = 3.4 h), corresponding to a free energy of activation of 23.8 ± 0.1 kcal mol−1. Cyclic voltammetry (CV) reveals that the BDNP38C10 ring can be enticed to pass over the speed bump onto the neutral BIPY0 unit upon the generation of the NpI2− dianion, even although the neutral BIPY0 has presumably little or no affinity for the BDNP38C10 ring.

Donor–Acceptor Oligorotaxanes Made to Order

Five donor-acceptor oligorotaxanes made up of dumbbells composed of tetraethylene glycol chains, interspersed with three and five 1,5-dioxynaphthalene units, and terminated by 2,6-diisopropylphenoxy stoppers, have been prepared by the threading of discrete numbers of cyclobis(paraquat-p-phenylene) rings, followed by a kinetically controlled stoppering protocol that relies on click chemistry. The well-known copper(I)-catalyzed alkyne-azide cycloaddition between azide functions placed at the ends of the polyether chains and alkyne-bearing stopper precursors was employed during the final kinetically controlled template-directed synthesis of the five oligorotaxanes, which were characterized subsequently by (1)H NMR spectroscopy at low temperature (233 K) in deuterated acetonitrile. The secondary structures, as well as the conformations, of the five oligorotaxanes were unraveled by spectroscopic comparison with the dumbbell and ring components. By focusing attention on the changes in chemical shifts of some key probe protons, obtained from a wide range of low-temperature spectra, a picture emerges of a high degree of folding within the thread protons of the dumbbells of four of the five oligorotaxanes-the fifth oligorotaxane represents a control compound in effect-brought about by a combination of C-H···O and π-π stacking interactions between the π-electron-deficient bipyridinium units in the rings and the π-electron-rich 1,5-dioxynaphthalene units and polyether chains in the dumbbells. The secondary structures of a foldamer-like nature have received further support from a solid-state superstructure of a related [3]pseudorotaxane and density functional calculations performed thereon.

Carbohydrate-Mediated Purification of Petrochemicals

Metal-organic frameworks (MOFs) are known to facilitate energy-efficient separations of important industrial chemical feedstocks. Here, we report how a class of green MOFs-namely CD-MOFs-exhibits high shape selectivity toward aromatic hydrocarbons. CD-MOFs, which consist of an extended porous network of γ-cyclodextrins (γ-CDs) and alkali metal cations, can separate a wide range of benzenoid compounds as a result of their relative orientation and packing within the transverse channels formed from linking (γ-CD)6 body-centered cuboids in three dimensions. Adsorption isotherms and liquid-phase chromatographic measurements indicate a retention order of ortho- > meta- > para-xylene. The persistence of this regioselectivity is also observed during the liquid-phase chromatography of the ethyltoluene and cymene regioisomers. In addition, molecular shape-sorting within CD-MOFs facilitates the separation of the industrially relevant BTEX (benzene, toluene, ethylbenzene, and xylene isomers) mixture. The high resolution and large separation factors exhibited by CD-MOFs for benzene and these alkylaromatics provide an efficient, reliable, and green alternative to current isolation protocols. Furthermore, the isolation of the regioisomers of (i) ethyltoluene and (ii) cymene, together with the purification of (iii) cumene from its major impurities (benzene, n-propylbenzene, and diisopropylbenzene) highlight the specificity of the shape selectivity exhibited by CD-MOFs. Grand canonical Monte Carlo simulations and single component static vapor adsorption isotherms and kinetics reveal the origin of the shape selectivity and provide insight into the capability of CD-MOFs to serve as versatile separation platforms derived from renewable sources.

Mechanostereochemistry and the mechanical bond

The chemistry of mechanically interlocked molecules (MIMs), in which two or more covalently linked components are held together by mechanical bonds, has led to the coining of the term mechanostereochemistry to describe a new field of chemistry that embraces many aspects of MIMs, including their syntheses, properties, topologies where relevant and functions where operative. During the rapid development and emergence of the field, the synthesis of MIMs has witnessed the forsaking of the early and grossly inefficient statistical approaches for template-directed protocols, aided and abetted by molecular recognition processes and the tenets of self-assembly. The resounding success of these synthetic protocols, based on templation, has facilitated the design and construction of artificial molecular switches and machines, resulting more and more in the creation of integrated functional systems. This review highlights (i) the range of template-directed synthetic methods being used currently in the preparation of MIMs; (ii) the syntheses of topologically complex knots and links in the form of stable molecular compounds; and (iii) the incorporation of bistable MIMs into many different device settings associated with surfaces, nanoparticles and solid-state materials in response to the needs of particular applications that are perceived to be fair game for mechanostereochemistry.