Mechanism of oxidative shuttling for [2]rotaxane in a Stoddart-Heath molecular switch: density functional theory study with continuum-solvation model

The central component of the programmable molecular switch demonstrated recently by Stoddart and Heath is [2]rotaxane, which consists of a cyclobis-(paraquat-p-phenylene) ring-shaped shuttle [(CBPQT(4+))(PF(6)(-))(4)] encircling a finger and moving between two stations on the finger: tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). We report here a quantum mechanics (QM) study of the mechanism by which movement of the ring (and in turn the on-off switching) is controlled by the oxidation-reduction process. We use B3LYP density functional theory to describe how oxidation of the [2]rotaxane components (in using Poisson-Boltzmann continuum-solvation theory for acetonitrile solution) induces the motions associated with switching (translation of the ring). These calculations support the proposal that oxidation occurs on TTF, leading to repulsion between two positive charge centers (TTF(2+) and CBPQT(4+)) that drives the CBPQT(4+) ring from the TTF(2+) station toward the neutral DNP station. The theory also supports the experimental observation that the first and second oxidation potentials are nearly the same (separated by 0.09 eV in the QM). This excellent agreement between the QM and experiment suggests that QM can be useful in designing new systems.     

A redox-driven multicomponent molecular shuttle

A multicomponent [2]rotaxane designed to operate as a molecular shuttle driven by light energy has been constructed, and its properties have been investigated. The system is composed of (1) a light-fueled power station, capable of using the photon energy to create a charge-separated state, and (2) a mechanical switch, capable of utilizing such a photochemically generated driving force to bring about controllable molecular shuttling motions. The light-fueled power station is, in turn, a dyad comprising (i) a pi-electron-accepting fullerene (C60) component and (ii) a light-harvesting porphyrin (P) unit which acts as an electron donor in the excited state. The mechanical switch is a redox-active bistable [2]rotaxane moiety that consists of (i) a tetrathiafulvalene (TTF) unit as an efficient pi-electron-donor station, (ii) a dioxynaphthalene (DNP) unit as a second pi-electron-rich station, and (iii) a tetracationic cyclobis(paraquat-p-phenylene) (CBPQT4+) pi-electron-acceptor cyclophane, which encapsulates the better pi-electron-donating TTF station. Diethylene glycol spacers were conveniently introduced between the electroactive components in the dumbbell-shaped thread to facilitate the template-directed synthesis of the [2]rotaxane. A modular synthetic approach was undertaken for the overall synthesis of this multicomponent bistable [2]rotaxane, beginning with the syntheses of the P-C60 dyad unit and the two-station TTF-DNP-based [2]rotaxane separately, using conventional synthetic methodologies. These two components were finally stitched together by an esterification to afford the target rotaxane. Its structure was characterized by 1H NMR spectroscopy and mass spectrometry as well as by UV-vis-NIR absorption spectroscopy and voltammetry. The observations reflect remarkable electronic interactions between the various units, pointing to the existence of folded conformations in solution. The redox-driven shuttling process of the CBPQT4+ ring between the two competitive electron-rich recognition units, namely, TTF and DNP, was investigated by electrochemistry and spectroelectrochemistry as a means to verify its operational behavior prior to the photophysical studies related to light-driven operation. The oxidation process of the TTF unit is dramatically hampered in the rotaxane, thereby reducing the efficiency of the shuttling motion. These results confirm that, as the structural complexity increases, the overall function of the system no longer depends simply on its "primary" structure but also on higher-level effects which are reminiscent of the secondary and tertiary structures of biomolecules.     

Degenerate [2]rotaxanes with electrostatic barriers

A synthetic approach to the preparation of [2]rotaxanes (1-5·6PF(6)) incorporating bispyridinium derivatives and two 1,5-dioxynaphthalene (DNP) units situated in the rod portions of their dumbbell components that are encircled by a single cyclobis(paraquat-p-phenylene) tetracationic (CBPQT(4+)) ring has been developed. Since the π-electron-deficient bispyridinium units are introduced into the dumbbell components of the [2]rotaxanes 1-5·6PF(6), there are Coulombic charge-charge repulsions between these dicationic units and the CBPQT(4+) ring in the [2]rotaxanes. Thus, the CBPQT(4+) rings in the degenerate [2]rotaxanes exhibit slow shuttling between two DNP recognition sites on the (1)H NMR time-scale on account of the electrostatic barrier posed by the bispyridinium units, as demonstrated by variable-temperature (1)H NMR spectroscopy. Electrochemical experiments carried out on the [2]rotaxanes 1·6PF(6) and 2·6PF(6) indicate that the one-electron reduced bipyridinium radical cation in the dumbbell components of the [2]rotaxanes serves as an additional recognition site for the two-electron reduced CBPQT(2(˙+)) diradical cationic ring. Under appropriate conditions, the ring components in the degenerate rotaxanes 1·6PF(6) and 2·6PF(6) can shuttle along the recognition sites--two DNP units and one-electron reduced bipyridinium radical cation--under redox control.     

Redox-controllable amphiphilic [2]rotaxanes

With the fabrication of molecular electronic devices (MEDs) and the construction of nanoelectromechanical systems (NEMSs) as incentives, two constitutionally isomeric, redox-controllable [2]rotaxanes have been synthesized and characterized in solution. Therein, they both behave as near-perfect molecular switches, that is, to all intents and purposes, these two rotaxanes can be switched precisely by applying appropriate redox stimuli between two distinct chemomechanical states. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by i) two pi-electron rich recognition sites-a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) moiety-with ii) a rigid terphenylene spacer placed between the two recognition sites, and then terminated by iii) a hydrophobic tetraarylmethane stopper at one end and a hydrophilic dendritic stopper at the other end of the dumbbells, thus conferring amphiphilicity upon these molecules. A template-directed protocol produces a means to introduce the tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), which contains two pi-electron accepting bipyridinium units, mechanically interlocked around the dumbbell-shaped components. Both the TTF unit and the DNP moiety are potential stations for CBPQT(4+), since they can establish charge-transfer and hydrogen bonding interactions with the bipyridinium units of the cyclophane, thereby introducing bistability into the [2]rotaxanes. In both constitutional isomers, (1)H NMR and absorption spectroscopies, together with electrochemical investigations, reveal that the CBPQT(4+) ring is predominantly located on the TTF unit, leading to the existence of a single translational isomer (co-conformation) in both cases. In addition, a model [2]rotaxane, incorporating hydrophobic tetraarylmethane stoppers at both ends of its dumbbell-shaped component, has also been synthesized as a point of reference. Molecular synthetic approaches were used to construct convergently the dumbbell-shaped compounds by assembling progressively smaller building blocks in the shape of the rigid spacer, the TTF unit and the DNP moiety, and the hydrophobic and hydrophilic stoppers. The two amphiphilic bistable [2]rotaxanes are constitutional isomers in the sense that, in one constitution, the TTF unit is adjacent to the hydrophobic stopper, whereas in the other, it is next to the hydrophilic stopper. All three bistable [2]rotaxanes have been isolated as green solids. Electrospray and fast atom bombardment mass spectra support the gross structural assignments given to all three of these mechanically interlocked compounds. Their photophysical and electrochemical properties have been investigated in acetonitrile. The results obtained from these investigations confirm that, in all three [2]rotaxanes, i) the CBPQT(4+) cyclophane encircles the TTF unit, ii) the CBPQT(4+) cyclophane shuttles between the TTF and DNP stations upon electrochemical or chemical oxidation/reduction of the TTF unit, and iii) folded conformations are present in which the CBPQT(4+) cyclophane, while encircling the TTF unit, interacts through its pi-accepting bipyridinium exteriors with other pi-donating components of the dumbbells, especially those located within the stoppers.     

Ground-state equilibrium thermodynamics and switching kinetics of bistable [2]rotaxanes switched in solution, polymer gels, and molecular electronic devices

We report on the kinetics and ground-state thermodynamics associated with electrochemically driven molecular mechanical switching of three bistable [2]rotaxanes in acetonitrile solution, polymer electrolyte gels, and molecular-switch tunnel junctions (MSTJs). For all rotaxanes a pi-electron-deficient cyclobis(paraquat-p-phenylene) (CBPQT4+) ring component encircles one of two recognition sites within a dumbbell component. Two rotaxanes (RATTF4+ and RTTF4+) contain tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units, but different hydrophilic stoppers. For these rotaxanes, the CBPQT4+ ring encircles predominantly (>90 %) the TTF unit at equilibrium, and this equilibrium is relatively temperature independent. In the third rotaxane (RBPTTF4+), the TTF unit is replaced by a pi-extended analogue (a bispyrrolotetrathiafulvalene (BPTTF) unit), and the CBPQT4+ ring encircles almost equally both recognition sites at equilibrium. This equilibrium exhibits strong temperature dependence. These thermodynamic differences were rationalized by reference to binding constants obtained by isothermal titration calorimetry for the complexation of model guests by the CBPQT4+ host in acetonitrile. For all bistable rotaxanes, oxidation of the TTF (BPTTF) unit is accompanied by movement of the CBPQT4+ ring to the DNP site. Reduction back to TTF0 (BPTTF0) is followed by relaxation to the equilibrium distribution of translational isomers. The relaxation kinetics are strongly environmentally dependent, yet consistent with a single electromechanical-switching mechanism in acetonitrile, polymer electrolyte gels, and MSTJs. The ground-state equilibrium properties of all three bistable [2]rotaxanes were reflective of molecular structure in all environments. These results provide direct evidence for the control by molecular structure of the electronic properties exhibited by the MSTJs.     

Tetrathiafulvalene radical cation dimerization in a bistable tripodal [4]rotaxane

The template-directed synthesis of a bistable tripodal [4]rotaxane, which has cyclobis(paraquat-p-phenylene) (CBPQT4+) as the pi-electron-deficient rings, and tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene units as the pairs of pi-electron-rich recognition sites located on all three legs of the tripodal dumbbell, is described. The chemical and electrochemical oxidation of the [4]rotaxane and its tripodal dumbbell have allowed us to unravel an unprecedented TTF.+ radical cation dimerization. In fact, two types of TTF dimers, namely, the radical cation dimer [TTF.+]2 and the mixed-valence one [(TTF)2].+, have been observed at room temperature for the tripodal dumbbell, whereas, in the case of the [4]rotaxane, only the radical cation dimer [TTF.+]2 is formed. This anomaly can be explained if it is accepted that most of the neutral TTF units in the [4]rotaxane are encircled by CBPQT4+ rings, which renders the formation of the mixed-valence dimer [(TTF)2].+ highly unfavorable.     

Functionally rigid bistable [2]rotaxanes

Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized and characterized in solution. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by (i) two pi-electron-rich stations-two NP moieties or a MPTTF unit and a NP moiety-with (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminated by (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification was investigated as a means to simplify both molecular structure and switching function previously observed in related bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes the tetrathiafulvalene (TTF) parent core structure. Furthermore, only one translational isomer is observed (> 95 < 5), surprisingly across a wide temperature range (198-323 K), meaning that the CBPQT4+ ring component resides, to all intents and purposes, predominantly on the MPTTF unit in the ground state. As a consequence of these two effects, the assignment of NMR and UV-vis data is more simplified as compared to previous donor-acceptor bistable [2]rotaxanes. This development has not only allowed for much better control over the position of the ring component in the ground state but also for control over the location of the CBPQT4+ ring during solution-state switching experiments, triggered either chemically (1H NMR) or electrochemically (cyclic voltammetry). In this instance, the use of the rigid spacer defines an unambiguous distance of 1.5 nm over which the ring moves between the MPTTF and NP units. The degenerate NP/NP [2]rotaxane was used to investigate the shuttling barrier by dynamic 1H NMR spectroscopy for the movement of the CBPQT4+ ring across the new rigid spacer. It is evident from these measurements that the rigid spacer poses a much lower barrier to the 1.0 nm movement of the CBPQT4+ ring from one station to another as compared with previous systems-a finding that is thought to be a result of the combination of fewer favorable interactions between the spacer and the CBPQT4+ ring and a relatively unimpeded path between the two NP stations. This example augers well for exploiting rigidity during the development of well-defined bistable [2]rotaxanes, which are unencumbered by the excesses of structural conformations that have characterized the first generations of molecular switches based on the donor-acceptor recognition motif.     

Structures and properties of self-assembled monolayers of bistable [2]rotaxanes on Au (111) surfaces from molecular dynamics simulations validated with experiment

Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)) (denoted as the ring), between a tetrathiafulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system located along a dumbbell component. When the ring is encircling the TTF unit, this co-conformation of the rotaxane is the most stable and thus designated the ground-state co-conformer (GSCC), whereas the other co-conformation with the ring surrounding the DNP ring system is less favored and so designated the metastable-state co-conformer (MSCC). We report here the structure and properties of self-assembled monolayers (SAMs) of a bistable [2]rotaxane on Au (111) surfaces as a function of surface coverage based on atomistic molecular dynamics (MD) studies with a force field optimized from DFT calculations and we report several experiments that validate the predictions. On the basis of both the total energy per rotaxane and the calculated stress that is parallel to the surface, we find that the optimal packing density of the SAM corresponds to a surface coverage of 115 A(2)/molecule (one molecule per 4 x 4 grid of surface Au atoms) for both the GSCC and MSCC, and that the former is more stable than the latter by 14 kcal/mol at the optimum packing density. We find that the SAM retains hexagonal packing, except for the case at twice the optimum packing density (65 A(2)/molecule, the 3 x 3 grid). For the GSCC and MSCC, investigated at the optimum coverage, the tilt of the ring with respect to the normal is theta = 39 degrees and 61 degrees, respectively, while the tilt angle of the entire rotaxane is psi = 41 degrees and 46 degrees , respectively. Although the tilt angle of the ring decreases with decreasing surface coverage, the tilt angle of the rotaxane has a maximum at 144 A(2)/molecule (the 4 x 5 grid/molecule) of 50 degrees and 51 degrees for the GSCC and MSCC, respectively. The hexafluorophosphate counterions (PF(6)(-)) stay localized around the ring during the 2 ns MD simulation. On the basis of the calculated density profile, we find that the thickness of the SAM is 40.5 A at the optimum coverage for the GSCC and 40.0 A for MSCC, and that the thicknesses become less with decreasing surface coverage. The calculated surface tension at the optimal packing density is 45 and 65 dyn/cm for the GSCC and MSCC, respectively. This difference suggests that the water contact angle for the GSCC is larger than for the MSCC, a prediction that is verified by experiments on Langmuir-Blodgett monolayers of amphiphilic [2]rotaxanes.     

The role of physical environment on molecular electromechanical switching

The influences of different physical environments on the thermodynamics associated with one key step in the switching mechanism for a pair of bistable catenanes and a pair of bistable rotaxanes have been investigated systematically. The two bistable catenanes are comprised of a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring, or its diazapyrenium-containing analogue, that are interlocked with a macrocyclic polyether component that incorporates the strong tetrathiafulvalene (TTF) donor unit and the weaker 1,5-dioxynaphthalene (DNP) donor unit. The two bistable rotaxanes are comprised of a CBPQT4+ ring, interlocked with a dumbbell component in which one incorporates TTF and DNP units, whereas the other incorporates a monopyrrolotetrathiafulvalene (MPTTF) donor and a DNP unit. Two consecutive cycles of a variable scan rate cyclic voltammogram (10-1500 mV s(-1)) performed on all of the bistable switches (approximately 1 mM) in MeCN electrolyte solutions (0.1 M tetrabutylammonium hexafluorophosphate) across a range of temperatures (258-303 K) were recorded in a temperature-controlled electrochemical cell. The second cycle showed different intensities of the two features that were observed in the first cycle when the cyclic voltammetry was recorded at fast scan rates and low temperatures. The first oxidation peak increases in intensity, concomitant with a decrease in the intensity of the second oxidation peak. This variation changed systematically with scan rate and temperature and has been assigned to the molecular mechanical movements within the catenanes and rotaxanes of the CBPQT4+ ring from the DNP to the TTF unit. The intensities of each peak were assigned to the populations of each co-conformation, and the scan-rate variation of each population was analyzed to obtain kinetic and thermodynamic data for the movement of the CBPQT4+ ring. The Gibbs free energy of activation at 298 K for the thermally activated movement was calculated to be 16.2 kcal mol(-1) for the rotaxane, and 16.7 and 19.2 kcal mol(-1) for the bipyridinium- and diazapyrenium-based bistable catenanes, respectively. These values differ from those obtained for the shuttling and circumrotational motions of degenerate rotaxanes and catenanes, respectively, indicating that the detailed chemical structure influences the rates of movement. In all cases, when the same bistable compounds were characterized in an electrolyte gel, the molecular mechanical motion slowed down significantly, concomitant with an increase in the activation barriers by more than 2 kcal mol(-1). Irrespective of the environment--solution, self-assembled monolayer or solid-state polymer gel--and of the molecular structure--rotaxane or catenane--a single and generic switching mechanism is observed for all bistable molecules.     

Contribution of disulfide S2(2-) anions to the crystal and electronic structures in ternary sulfides, Ba12In4S19, Ba4M2S8 (M=Ga, In)

Three semiconducting ternary sulfides have been synthesized from the mixture of elements with about 20% excess of sulfur (to establish oxidant rich conditions) by solid-state reactions at high temperature. Ba(12)In(4)S(19) ≡ (Ba(2+))(12)(In(3+))(4)(S(2-))(17)(S(2))(2-), 1, crystallizes in the trigonal space group R ?3 with a = 9.6182(5) ?, b = 9.6182(5) ?, c = 75.393(7) ?, and Z = 6, with a unique long period-stacking structure of a combination of monometallic InS(4) tetrahedra, linear dimeric In(2)S(7) tetrahedra, disulfide S(2)(2-) anions, and isolated sulfide S(2-) anions that is further enveloped by Ba(2+) cations. Ba(4)In(2)S(8) ≡ (Ba(2+))(4)(In(3+))(2)(S(2-))(6)(S(2))(2-), 2, crystallizes in the triclinic space group P ?1? with a = 6.236(2) ?, b = 10.014(4) ?, c = 13.033(5) ?, α = 104.236(6)°, β = 90.412(4)°, γ = 91.052(6)°, and Z = 2. Ba(4)Ga(2)S(8) ≡ (Ba(2+))(4)(Ga(3+))(2)(S(2-))(6)(S(2))(2-), 3, crystallizes in the monoclinic P2(1)/c with a = 12.739(5) ?, b = 6.201(2) ?, c = 19.830(8) ?, β = 104.254(6)° and Z = 4. Compounds 2 and 3 represent the first one-dimensional (1D) chain structure in ternary Ba/M/S (M = In, Ga) systems. The optical band gaps of 1 and 3 are measured to be around 2.55 eV, which agrees with their yellow color and the calculation results. The CASTEP calculations also reveal that the disulfide S(2)(2-) anions in 1-3 contribute mainly to the bottom of the conduction bands and the top of valence bands, and thus determine the band gaps.     

Linear artificial molecular muscles

Two switchable, palindromically constituted bistable [3]rotaxanes have been designed and synthesized with a pair of mechanically mobile rings encircling a single dumbbell. These designs are reminiscent of a "molecular muscle" for the purposes of amplifying and harnessing molecular mechanical motions. The location of the two cyclobis(paraquat-p-phenylene) (CBPQT(4+)) rings can be controlled to be on either tetrathiafulvalene (TTF) or naphthalene (NP) stations, either chemically ((1)H NMR spectroscopy) or electrochemically (cyclic voltammetry), such that switching of inter-ring distances from 4.2 to 1.4 nm mimics the contraction and extension of skeletal muscle, albeit on a shorter length scale. Fast scan-rate cyclic voltammetry at low temperatures reveals stepwise oxidations and movements of one-half of the [3]rotaxane and then of the other, a process that appears to be concerted at room temperature. The active form of the bistable [3]rotaxane bears disulfide tethers attached covalently to both of the CBPQT(4+) ring components for the purpose of its self-assembly onto a gold surface. An array of flexible microcantilever beams, each coated on one side with a monolayer of 6 billion of the active bistable [3]rotaxane molecules, undergoes controllable and reversible bending up and down when it is exposed to the synchronous addition of aqueous chemical oxidants and reductants. The beam bending is correlated with flexing of the surface-bound molecular muscles, whereas a monolayer of the dumbbell alone is inactive under the same conditions. This observation supports the hypothesis that the cumulative nanoscale movements within surface-bound "molecular muscles" can be harnessed to perform larger-scale mechanical work.     

Diverse structures and adsorption properties of quasi-Werner-type copper(II) complexes with flexible and polar axial bonds

The quasi-Werner-type copper(II) complex, [Cu(PF(6))(2)(4-mepy)(4)] (1), in which 4-mepy is the 4-methylpyridine ligand, has flexible and polar axial bonds of Cu-PF(6). Flexibility of the Cu-PF(6) bonds induces diverse and unprecedented guest-inclusion structures, such as {[Cu(PF(6))(2)(4-mepy)(4)][Cu(PF(6))(4-mepy)(4)(acetone)]·PF(6)·4acetone} (γ-1?2.5acetone), {[Cu(PF(6))(2)(4-mepy)(4)][Cu(PF(6))(4-mepy)(4)(2-butanone)]·PF(6)·3.5(2-butanone)} (γ-1?2.25(2-butanone)), {[Cu(PF(6))(2)(4-mepy)(4)][Cu(PF(6))(4-mepy)(4)(H(2)O)]·PF(6)·4benzene} (γ-1?0.5H(2)O·2benzene), and {[Cu(PF(6))(2)(4-mepy)(4)]·2benzene} (γ-1?2benzene). Exposure of the dense form, α-1, to benzene vapor affords the benzene-inclusion complex {[Cu(PF(6))(2)(4-mepy)(4)]·2benzene} (γ-1?2benzene), all benzene guests of which are easily removed by vacuum drying, reforming guest-free, dense α-1' with smaller sized crystals than α-1. In contrast to α-1, which shows almost no CO(2) adsorption, α-1' adsorbs CO(2) gas with structural transformations, this being the first example that exhibits adsorption of gas in a dense Werner-type complex and a drastic change in adsorption properties depending on the size of the crystals.     

Molecular dynamics simulation of amphiphilic bistable [2]rotaxane langmuir monolayers at the air/water interface

Bistable [2]rotaxanes display controllable switching properties in solution, on surfaces, and in devices. These phenomena are based on the electrochemically and electrically driven mechanical shuttling motion of the ring-shaped component, cyclobis(paraquat-p-phenylene) (CBPQT(4+)), between a monopyrrolotetrathiafulvalene (mpTTF) unit and a 1,5-dioxynaphthalene (DNP) unit located along a dumbbell component. The most stable state of the rotaxane (CBPQT(4+)@mpTTF) is that in which the CBPQT(4+) ring encircles the mpTTF unit, but a second less favored metastable co-conformation with the CBPQT(4+) ring surrounding the DNP (CBPQT(4+)@DNP) can be formed experimentally. For both co-conformations of an amphiphilic bistable [2]rotaxane, we report here the structure and surface pressure-area isotherm of a Langmuir monolayer (LM) on a water subphase as a function of the area per molecule. These results from atomistic molecular dynamics (MD) studies are validated by comparing with experiments based on similar amphiphilic rotaxanes. For both co-conformations, we found that as the area per molecule increases the thickness of the LM decreases while the molecular tilt increases. Both co-conformations led to similar LM thicknesses at the same packing area. From the simulated LM systems, we calculated the electron density profiles of the monolayer as a function of area per molecule, which show good agreement with experimental analyses from synchrotron X-ray reflectivity measurements of related systems. Decomposing the overall electron density profiles into component contributions, we found distinct differences in molecular packing in the film depending upon the co-conformation. Thus we find that the necessity of allowing the tetracationic ring to become solvated by water leads to differences in the structures for the two co-conformations in the LM. At the same packing area, the value of the overall tilt angle does not seem to be sensitive to whether the CBPQT(4+) ring is encircling the mpTTF or the DNP unit. However, the conformation of the dumbbell does depend on the location of the CBPQT(4+) ring, which is reflected in the segmental tilt angles of the mpTTF and DNP units. Using the Kirkwood-Buff formula in conjunction with MD calculations, we find the surface pressure-area isotherms for each co-conformation in which the CBPQT(4+)@mpTTF form has smaller surface tension and therefore larger surface pressure than the CBPQT(4+)@DNP at the same packing area, differences that decreases with increasing area per molecule, which is verified experimentally.     

Raman microscopy of synthetic goudeyite (YCu6(AsO4)2(OH)6 x 3H2O)

Raman microscopy has been used to study the molecular structure of a synthetic goudeyite (YCu(6)(AsO(4))(3)(OH)(6) x 3H(2)O). These types of minerals have a porous framework similar to that of zeolites with a structure based upon (A(3+))(1-x)(A(2+))(x)Cu(6)(OH)(6)(AsO(4))(3-x)(AsO(3)OH)(x). Two sets of AsO stretching vibrations were found and assigned to the vibrational modes of AsO(4) and HAsO(4) units. Two Raman bands are observed in the region 885-915 and 867-870 cm(-1) region and are assigned to the AsO stretching vibrations of (HAsO(4))(2-) and (H(2)AsO(4))(-) units. The position of the bands indicates a C(2v) symmetry of the (H(2)AsO(4))(-) anion. Two bands are found at around 800 and 835 cm(-1) and are assigned to the stretching vibrations of uncomplexed (AsO(4))(3-) units. Bands are observed at around 435, 403 and 395 cm(-1) and are assigned to the nu(2) bending modes of the HAsO(4) (434 and 400 cm(-1)) and the AsO(4) groups (324 cm(-1)).     

The nature of the [TTF]˙+···[TTF]˙+ interactions in the [TTF]2(2+) dimers embedded in charged [3]catenanes: room-temperature multicenter long bonds

The properties of tetrathiafulvalene dimers ([TTF](2)(2+)) and the functionalized ring-shaped bispropargyl (BPP)-functionalized TTF dimers, [BPP-TTF](2)(2+), found at room temperature in charged [3]catenanes, were evaluated by M06L calculations. The results showed that their isolated [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are energetically unstable towards dissociation. When enclosed in the 4(+)-charged central cyclophane ring of charged [3]catenanes (CBPQT(4+)), [TTF](2)(2+) and [BPP-TTF](2)(2+) dimers are also energetically unstable with respect to leaving the CBPQT(4+) ring; since the barrier for the exiting process is only about 3 kcal mol(-1), that is, within the reach of thermal energies at room temperature (neutral [TTF](2)(0) dimers are stable within the CBPQT(4+) ring). However, the [BPP-TTF](2)(2+) dimers in charged [3]catenanes cannot exit, because this would imply breaking the covalent bonds of the BPP-TTF(+) macrocycle. Finally, it was shown that the [TTF](2)(2+), [BPP-TTF](2)(2+) dimers, and charged [3]catenanes are energetically stable in solution and in crystals of their salts, in the first case due to the interactions with the solvent, and in the second case mostly due to cation-anion interactions. In these environmental conditions at room temperature the TTF units of the [BPP-TTF](2)(2+) dimers make short contacts, thus allowing their SOMO orbitals to overlap: a room-temperature multicenter long bond is formed, similar to those previously found in other [TTF](2)(2+) salts and their solutions.     

Synthesis and characterization of a metallocycle-based molecular shuttle

Kinetically stable metallocycle-based molecular shuttles of [2]rotaxanes 4a and 4b, along with [3]rotaxanes 5a and 5b, have been prepared using the rhenium(I)-bridged metallocycle 2 and the dumbbell components containing two stations, 3a and 3b. The rotaxanes were self-assembled by hydrogen bonding interactions upon heating a Cl(2)CHCHCl(2) solution containing their components at 70 degrees C. Each rotaxane was isolated in pure form by silica gel chromatography under ordinary laboratory conditions and fully characterized by elemental analysis and various spectroscopic methods. The (1)H NMR signals for the amide NH and the methylene -(CH(2))(4)- of the station were considerably changed when occupied by the metallocycle. In [2]rotaxane 4b, which has a larger naphthyl spacer, the occupied and unoccupied stations gave widely separated signals in the (1)H NMR spectroscopy at room temperature, but averaged signals of two stations were observed in [2]rotaxane 4a, which has a smaller phenyl spacer. This is attributed to the shuttling of the metallocycle between two stations. The coalescence temperature experiment gave a shuttling rate of approximately 670 s(-)(1) at 19 degrees C in CDCl(3), corresponding to an activation free energy (DeltaG()) of 13.3 kcal/mol. With respect to the relative position of the chloride in the rhenium(I) center, two diastereomers are possible in the [2]rotaxane and three diastereomers are possible in the [3]rotaxane. In fact, the rotaxanes exist as diastereomeric mixtures in nearly equal amounts of all possible diastereomers on the basis of the amide NH signals of the station in the (1)H NMR spectroscopy.     

Two classes of alongside charge-transfer interactions defined in one [2]catenane

A [2]catenane, which incorporates hydroquinone (HQ) and a sterically bulky tetrathiafulvalene (TTF) into a bismacrocycle, has been designed to probe the alongside charge-transfer (CT) interactions taking place between a TTF unit and one of the bipyridinium moieties in the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT4+). A template-directed strategy employs the HQ unit as the primary template for formation of the tetracationic cyclophane CBPQT4+, affording the desired [2]catenane structure but as an uncharacteristic green solid. The X-ray crystal structure and detailed 13C NMR assignments have identified a stereoselective preference for catenation about the cis isomer. The 1H NMR spectroscopy, electrochemistry, and X-ray crystallography all confirm that the CBPQT4+ cyclophane encircles the HQ unit, thereby defining a structure which would normally determine a red color. The visible-NIR region of the absorption spectrum displays a band at approximately 740 nm that is unambiguously assigned to a TTF --> CBPQT4+ CT transition on the basis of resonance Raman spectroscopy using 785 nm excitation. The profile of the CT band changes depending on the ratio of the cis- to trans-TTF isomers in the [2]catenane for which the molar absorptivity of each isomer is estimated to be significantly different at epsilon max = 380 and 3690 M-1 cm-1, respectively. Molecular modeling studies confirmed that the observed difference in the absorption spectroscopic profile can be accounted for by both a better overlap of the HOMO(TTF) and LUMO+1(CBPQT4+) as well as a more stable face-to-face (pi...pi) conformation in the trans isomer compared to the edge-to-face cis isomer of the [2]catenane. The latter is arranged for pi-orbital overlap through the sulfur atoms of the TTF unit, thereby defining an [Spi...pi] interaction.     

Extending the nitrogen-heterosuperbenzene family: the spectroscopic, redox, and photophysical properties of "half-cyclized" N-1/2HSB and Its Ru(II) complex

Oxidative cyclodehydrogenation is an important process in the formation of the new graphene, N-(1)/(2)HSB 2. This heteropolyaromatic results from the FeCl(3)-catalyzed oxidative cyclodehydrogenation of 1,2-dipyrimidyl-3,4,5,6-tetra-(4-tert-butylphenyl)benzene. Three new C-C bonds are formed that lock the two pyrimidines in a molecular platform comprising eight fused aromatic rings flanked by two remaining "uncyclized" phenyl rings. Mechanistically intriguing is the fact that N-HSB 1, the product of six C-C bond fusions, is co-synthesized with its "half-cyclized" daughter in this reaction. 1 and 2 have the same bidentate N-atom arrangement. This facilitates formation of the heteroleptic Ru(II) complexes, [Ru(bpy)(2)(2)](PF(6))(2) 4 and [Ru(bpy)(2)(1)](PF(6))(2) 3, which differ in the size and planarity of their aromatic ligands. The new ligand 2 and its complex 4 are characterized by (1)H NMR, IR, ESI-MS, and accurate mass methods. 2 exhibits photophysical properties that are consistent with a reduction of the pi/pi framework, a concomitant increase in the energy of the LUMO, and a blue-shift of the solvent-dependent fluorescence (lambda(em) = 474 nm, phi(F) = 0.55, toluene) as compared to its parent. Complex 4 absorbs throughout the visible region and borders on near-IR emitter character, exhibiting a slightly blue-shifted (3)MLCT emission (868 nm, CH(3)CN) as compared to that of [Ru(bpy)(2)(1)](PF(6))(2) 3 (880 nm, CH(3)CN). Electrochemical analyses permit further elucidation of the intermolecular interactions of 3 and 4. These and the concentration and temperature-dependent NMR spectra of 4 confirm it to be nonaggregating, a direct result of the two uncyclized and rotatable phenyl rings in 2.     

Bispyrrolotetrathiafulvalene-containing [2]catenanes

Two [2]catenanes incorporating bispyrrolotetrathiafulvalene (BPTTF) and weaker aryl donors, hydroquinone (HQ) and 1,5-dioxynaphthalene (DNP), respectively, have been prepared and characterized. These [2]catenanes show a predominant amount (>95:5) of the co-conformation in which either the HQ or the DNP unit is encircled by a tetracationic cyclophane, cyclobis(paraquat-p-phenylene) (CBPQT4+), contrary to what is observed in systems based on the parent tetrathiafulvalene (TTF). These new [2]catenanes act effectively as molecular switches which are always configured in the "on" state.     

Using oppositely charged ions to operate a three-station [2]rotaxane in two different switching modes

A [2]rotaxane undergoes switching of its bis-p-xylyl-[26]crown-6 (BPX26C6) component away from its guanidinium station toward its 2,2'-bipyridyl and carbamate stations upon the addition and removal of Zn(2+) and PO(4)(3-) ions, respectively.