Observing Condensed Phase Movement

Once the interactions that control the behavior of a molecular switch in the solid state have been considered, demonstrating actuation in a condensed phase device must still be carried out in convincing fashion. Designing a system in which the different intermolecular, intramolecular, and surface interactions work in unison to achieve molecular actuation in an oriented, stimulus-controlled fashion remains the primary goal in the field of molecular machines. In the aforementioned LB studies,92 the molecules were switched while (1) on a water surface in (2) a Langmuir trough. In the computational studies, the molecules were modeled in each conformation of interest, and then the film properties were calculated and compared. Neither of these investigations, however, constitutes a direct experimental verification of molecular actuation in a condensed phase. Recently, the movement of the CBPQT4+ ring between two stations in a bistable [2]rotaxane in thin films was confirmed by XPS,92 surface-sensitive XR,93 impedance spectroscopy94 measurements, and in a molecular switch tunnel junction74 (MSTJ).

XPS experiments directly demonstrated92 the mechanical movement of the CBPQT4+ ring between DNP and TTF recognition sites in tightly packed LB films of two constitutionally isomeric, amphiphilic, bistable [2]rotaxanes deposited on Si wafers. The amphiphilic, bistable [2]rotaxanes investigated contain DNP and TTF recognition sites as well as a hydrophobic, aromatic stopper at one end and a hydrophilic tris-DEG substituted stopper at the other. The arrangement of the recognition sites relative to the different stoppers distinguishes the two constitutional isomers. Evidence for the redox-controlled mechanical switching was based on the fact that the photoemission intensity of electrons ejected from an atom is exponentially related to the depth of the atom within a film and, therefore, the depths of different atoms within a film can be differentiated. Quantitative analysis demonstrated that the up and down movement of the CBPQT4+ ring from one recognition site to another resulted from the introduction of the oxidizing agent Fe(ClO4)3 into the aqueous subphase of the Langmuir trough. The switching process was observed statically by measuring relative differences in the intensity of N1s nuclei which are only present in the CBPQT4+ ring and by comparing the photoemission of LB films prepared with Fe(ClO4)3 to films without it in the subphase. The changes in intensity correspond to a difference in height of 14 A between the position of the ring in the two different systems, directly supporting the hypothesis that the ring switches between upper and lower recognition sites on the dumbbell in a closely packed monolayer as a result of the introduction of a chemical oxidant.

A similar study93 demonstrating the switching of LB monolayers was carried out with XR, a technique that measures the electron density profile within a monolayer and provides direct structural information about the isomeric structures associated with the location of the ring in the constitutionally isometric bistable [2] rotaxanes. Using a synchrotron x-ray source, XR functions by simultaneously varying incident and grazing angles while recording the intensity pattern that results from the interference of the x-rays reflected from different depths of the sample. This data yields the out-of-plane electron density across the water/monolayer/air interface. As in the XPS experiment, the monolayer was assembled and the XR spectrum was measured both before and after the addition of the Fe(ClO4)3 oxidant to the aqueous subphase. Both constitutional isomers of the bistable [2]rotaxane, having either the TTF recognition unit closer to the hydrophilic stopper and DNP recognition site closer to the hydrophobic stopper or with the DNP recognition site closer to the hydrophilic stopper and the TTF recognition site closer to the hydrophobic stopper, were subjected to the XR experiment. Upon oxidation, a change in electron density corresponding to a 12 A shift of the ring in the direction of the DNP recognition site was observed, a value which is in good agreement with the value of 14 A determined by XPS. This value is well short of the calculated value of 34 A calculated for the fully extended [2] rotaxanes and further confirms that the molecules adopt folded conformations as a result of hydrophilic interactions at the air-water interface. However, neither the XPS nor XR experiments track, in real time, the movement of the ring because the monolayers must first be switched by introduction of an oxidant, and then measured; more recent work94 has attempted to measure the movement of the CBPQT4+ ring in situ.

The rate of electron transfer to the CBPQT4+ ring has been measured and its subsequent movement has been demonstrated in a close-packed, viscous SAM on an Au surface by impedance spectroscopy.94 A [2]rotaxane monolayer consisting of the CBPQT4+ ring and an n-electron-rich diaminobenzene recognition site was self-assembled on a gold surface (Figure 11.8). Ring movement was demonstrated by first reducing the CBPQT4+ ring to its biradical dication followed by subsequent reoxidation to its tetracation, and measuring differences in the rate of electron transfer for each process. The rate of electron-transfer reduction to the biradical was measured to be 80 sec-1, whereas back electron-transfer to form the tetracation was observed to be significantly greater, namely 1100 sec-1. This data can be explained by the fact that reduction of the tetracationic cyclophane to its biradical dication decreases its strength as an electron-acceptor, which, in turn, results in the destruction of the favorable noncovalent interactions that exist between the cyclophane and the n-electron-rich diaminobenzene. Given that the electrode is negatively charged and the biradical dication is positively charged, the cyclophane presumably moves towards the electrode surface. The resulting proximity of the cyclophane to the electrode surface results in the increased electron transfer rate during oxidation. In this manner, impedance spectroscopy was successfully used to measure the movement of the cyclophane in a closely packed monolayer.

The switching behavior of amphiphilic, bistable [2]rotaxanes has also been confirmed by studying74 the redox-controlled switching behavior in solution, polymer gels, SAMs, and MSTJs. The switching cycles can be detected by a number of experimental observations in the cyclic voltammogram (CV) studied, providing information which can be used to understand switching behavior across different environments. The first oxidation potential of the GSCC corresponds to the one-electron oxidation of TTF to TTF+^ and occurs at +490 mV, while the same oxidation of TTF to TTF+^ in the MSCC is +310 mV (potentials referenced to an Ag/AgCl electrode).72,73 Knowing that relaxation of the MSCC back to the GSCC is temperature-dependant, thermal activation parameters may be quantified by time- and temperature-dependent CV measurements. The ratio of MSCC to GSCC is dependent on the structural


Au S


Au S

Ring movement

Ring movement

FIGURE 11.8 Demonstration of the mechanical movements of the tetracationic cyclophane CBPQT4+ along a diaminobenzene-containing thread that is tethered to a gold surface. A two-electron reduction of the CBPQT4+ ring erases the favorable binding interactions that exist between the ring host and diaminobenzene guest with an electron transfer rate of 80 Hz. The resulting dicationic CBPQT2+" is attracted to and moves toward the gold surface. A two-electron oxidation of the CBPQT2+" returns the cyclophane host back to its tetracationic state and restores the favorable noncovalent interactions between the CBPQT4+ host and diaminobenzene guest. The rate of oxidative electron transfer is 1100 Hz, which is indicative of the close proximity of the reduced ring to the gold surface and is demonstrative of the reversible redox-controllable mechanical motions of the interlocked molecule on a gold surface.

features of the particular bistable molecules, whereas changes in kinetic parameters (lifetime and decay) are more sensitive to physical environment. Thus CV was used to quantify the ratios of different coconformations by measuring the integration of oxidation peaks corresponding to MSCC and GSCC, in solution, polymer gels, and SAMs. While at equilibrium, the NMSCC/NGSCC ratios for the different amphiphilic, bistable molecules were very similar in all three environments, relaxation rates from the MSCC to the GSCC slowed down and lifetimes increased, as the viscosity increased from solution to polymer gel to SAMs.

Using the knowledge that the switching mechanism appears to be universal across the three environments, this analysis was extended to a functioning nanodevice incorporating the amphiphilic bistable [2]rotaxane molecular switches in MSTJs.74 The detailed procedures regarding MSTJ manufacture, design and operation have been reviewed extensively70,82,95 and will not be discussed in great detail here. A brief description, however, of those aspects relevant to this discussion will be made. The MSTJs investigated contained a Langmuir deposited layer of amphiphilic bistable [2]rotaxanes — which contain TTF and DNP recognition sites along their dumbbells along with a hydrophobic stopper at one end and a hydrophilic stopper at the other — sandwiched between an n-type poly(silicon) bottom electrode and a metallic top electrode. These MSTJs have been used in molecular logic devices — such as a 64 kbit RAM — and are able to function because they can be switched reversibly between low conductance states and hysteretic, high-conductance states. Several critical pieces of evidence indicate that the MSCC is the high-conductance state: (1) high conductance occurs subsequent to an oxidizing bias being passed between the electrodes which, in other environments, would lead to the MSCC; (2) the lower first-oxidizing potential of the MSCC in CV experiments suggests that it is of higher conductance than the GSCC; (3) modeling78 has shown that switching to the MSCC causes a narrowing of the HOMO-LUMO gap with respect to the GSCC, which, in turn, would lead to a higher conductance state. However, the switching could not be tested directly by CV in MSTJs, so other methods were used to determine that the switching mechanism in MSTJs was consistent with the switching mechanism in other environments. The equilibrium thermodynamics could be inferred within the MSTJs by assuming that the high and low conductance states correspond to the MSCC and GSCC, respectively. Thus, the temperature-dependent switching characteristics, normalized against the transport characteristics of an MSTJ, provide insight into the thermodynamics of the molecules within the junctions. Conductance changes in MSTJs are measured as remnant molecular signatures, hysteretic switching loops that represent the normalized current response to a voltage pulse as the gate voltage is swept. Because the current stays on after the bias has been removed, the high conductance is most likely a result of the bistable [2] rotaxane remaining in the MSCC before thermal relaxation to the GSCC, which reduces the conductance to the normalized baseline. The remnant molecular signatures, along with the decay rate to the baseline current, can be used to determine both NMSCC/NGSCC and provide thermodynamic data, the decay rate, and ultimately provide kinetic data. Using this hypothesis, it has been shown that, even in a full device setting, amphiphilic bistable [2]rotaxanes demonstrate switching thermodynamics that are controlled by structure while switching kinetics in these systems are still predominantly controlled by environmental factors.

The observation that molecular structure plays a dominant role in the thermodynamic properties of bistable [2] rotaxanes and environmental factors govern their kinetics is vital to the development of surface-bound organic molecular devices based upon these particular molecules. Systematic investigations of the interplay between structure, function, and environment, such as those outlined in this section, enable researchers to fine-tune the dynamic properties of organic molecular machines as they are moved from the solution environment to more condensed phases and onto surfaces. Surface-substrate interactions, molecular packing, and molecular conformation at the organic-solid interface all influence the dynamic properties of functional molecules and affect the ultimate performance of surface-bound molecular machines. With these important considerations in mind, a number of research groups have recently been able to develop molecular machines that operate in condensed phases or on solid supports. The following sections will highlight advancements that have been made in the design, synthesis, fabrication, and modeling of four types of synthetic organic molecular machines — (1) molecular muscle systems, (2) nanovalves, (3) rotors, and (4) surfaces with controllable wettability.

11.3 Molecular Muscle Systems 11.3.1 The Potential of Artificial Muscles

Despite the significant progress that has been made recently, emulating the high strength, high displacement, and low weight of natural muscle tissue (Figure 11.2[b]) with synthetic molecular systems, the challenge is one that remains at the forefront of the field of molecular machinery. Such artificial systems promise to impact a wide range of technologies, from nanoelectromechanical systems to exotic applications such as futuristic body armor. Investigations into actuating systems which switch in response to

FIGURE 11.9 Cu(I)-templated synthesis of an interlocked dimer.

chemical, photochemical, or electrochemical stimuli have run a wide gauntlet of approaches, including hydrogel swelling,96 osmotic expansion of conjugated polymers,97 motions within supramolecular polymer systems, ion intercalation in nanoparticle films,98 and the bending of surfaces in response to DNA hybridization.99 Most of these approaches to actuation rely on bulk material responses, rather than controlled linear motion at the single-molecule level.

No synthetic system reported to date approaches the beauty and efficiency of the sarcomere, the cellular unit responsible for skeletal muscle contraction.100 Muscle contraction occurs as a result of an ATP hydrolysis-driven power stroke that causes myosin and actin filaments to slide over one another. This process is suggestive of a biomimetic approach in which two or more interlocked molecular components may be induced to slide through one another in response to some external stimulus, resulting in the contraction and expansion of the single-molecule system. The challenge is to turn this singular event into a coherent one involving many molecules in a materials setting.

11.3.2 Transition Metal Controlled Actuators

Pioneering work by Sauvage and coworkers resulted in the development of a transition metal templated synthesis of a mechanically interlocked dimer (Figure 11.9).101 The dimer's components consist of a macrocycle containing a bidentate ligand (1,10-phenanthroline) and a complementary ligand on the thread portion. Addition of a stoichiometric amount of Cu(I) ions resulted in the formation of the metal-templated dimer in quantitative yield.102 It is important to note that the formation of dimer structures from self-complementary rotaxane components is a general phenomenon observed with other binding motifs — the dimer structure produces favorable binding interactions while, at the same time, maximizing the entropy of the system relative to alternative oligomeric and polymeric structures.103-105 The generality of this approach bodes well for future actuating systems based on dimers with interlocked topologies.

Elaboration of the rotaxane dimer to a linear actuator was accomplished by incorporating a second binding site onto the thread portion of the dimer. The system now exhibits two potential isomers, an extended coconformation in which metal coordination occurs with the macrocycle and the inner ligand, and a contracted coconformation in which the rings coordinate metal ions at the outer ligand set. Stimuli capable of switching the coordination preference from one site to the other should expand or contract the molecule in a manner reminiscent of myosin and actin motion in natural muscles.

These transition-metal controlled molecular actuators can be switched (Figure 11.10) by means of a sequential metalation/demetalation procedure. The interlocked dicopper complex exists exclusively in the extended coconformation as a result of the preference of Cu(I) for a tetracoordinate-binding geometry. The molecule loses its strong coconformational preference following demetalation using an excess of potassium cyanide; the bulky stoppers, however, at both ends of the threads prevent dissociation of the dimer. The addition of Zn(II) ions to the demetalated dimer produces the contracted isomer in quantitative o-

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