Electrochemical Switching of Chromogenic Monolayers Self-Assembled on Transparent Platinum Electrodes.

Transparent, ultrathin Pt electrodes permit the simultaneous electrochemical and spectroscopic investigation of self-assembled monolayers of electrochromic compounds. Voltage stimulations applied to the Pt substrate reversibly alter the redox state of the chemisorbed molecules and, hence, modulate the intensity of the light transmitted through the Pt/monolayer assembly.

Signal communication between molecular switches.

[structure: see text]. Efficient strategies for the communication of signals between molecules must be identified for the development of molecule-based nanoprocessors. We have demonstrated that photoinduced proton transfer can be exploited to implement intermolecular digital transmission. Light inputs operate a three-state molecular switch inducing the transfer of a proton to or from a two-state molecular switch. The signal transduction protocol of the two communicating molecular switches is equivalent to that of a sequential logic circuit incorporating three logic gates.

The magnitude of [C-H...O] hydrogen bonding in molecular and supramolecular assemblies.

Ab initio calculations at the MP2/6-311++G** level on model systems (N-methylpyridinium complexes of dimethyl ether and dimethyl phosphate anion) provide quantitative measures of the large stabilization energies that arise from [C-H...O] contacts in charged systems. These attractive interactions control (i) the self-assembly of bipyridinium-based catenanes and rotaxanes in solution, (ii) the self-organization of left-handed Z-DNA with alternating [dC-dG] sequences in the solid state, and (iii) the binding of pyridinium derivatives with single- and double-stranded DNA. Slightly attractive interactions occur between the donor ether and phosphate moieties and a neutral pyridine molecule in the gas phase. Electrostatic potential and solvation calculations demonstrate that [C-H...O] interactions which involve a cationic [C-H] donor are dominated by electrostatic terms.

Dual-mode "co-conformational" switching in catenanes incorporating bipyridinium and dialkylammonium recognition sites.

Three [2]catenanes and three [3]catenanes incorporating one or two pi-electron-rich macrocyclic polyethers and one pi-electron-deficient polycationic cyclophane have been synthesized in yields ranging from 4 to 38%. The pi-electron-rich macrocyclic components possess either two 1,4-dioxybenzene or two 1.5-dioxynaphthalene recognition sites. The pi-electron-deficient cyclophane components incorporate two bipyridinium and either one or two dialkylammonium recognition sites. The template-directed syntheses of these catenanes rely on i) pi...pi stacking interactions between the dioxyarene and bipyridinium recognition sites, ii) C-H...O hydrogen bonds between some of the bipyridinium hydrogen atoms and some of the polyether oxygen atoms, and iii) C-H...pi interactions between some of the dioxyarene hydrogen atoms and the aromatic spacers separating the bipyridinium units. The six catenanes were characterized by mass spectrometry and by both 1H and 13C NMR spectroscopy. The absorption spectra and the electrochemical properties of the catenanes have been investigated and compared with those exhibited by the component macrocycles and by related known catenanes. Broad and weak absorption bands in the visible region, originating from charge-transfer (CT) interactions between electron-donor and electron-acceptor units, have been observed. Such charge-transfer interactions are responsible for the quenching of the potentially fluorescent excited states of the aromatic units of the macrocyclic polyether components. The redox behavior of these novel compounds has been investigated and correlations among the observed redox potentials are illustrated and discussed. The catenanes undergo co-conformational switching upon one-electron reduction of the two bipyridinium units. One of them--in its reduced form--can be also switched by acid/base inputs and exhibits AND logic behavior. The co-conformational rearrangements induced by the redox and acid/base stimulations lend themselves to exploitation in the development of molecular-level machines and logic gates.

Digital communication through intermolecular fluorescence modulation.

[see reaction]. Ultraminiaturized processors incorporating molecular components can be developed only after devising efficient strategies to communicate signals at the molecular level. We have demonstrated that a three-state molecular switch responds to ultraviolet light, visible light, and H+, attenuating the emission intensity of a fluorescent probe. Intermolecular communication is responsible for the transduction of three input signals into a single optical output. The behavior of the communicating ensemble of molecules corresponds to that of a logic circuit incorporating seven gates.

Chromatography of mechanically interlocked molecular compounds.

Catenanes and rotaxanes are molecules composed of mechanically interlocked components which are not linked to each other by covalent bonds. These molecular assemblies behave as discrete molecules with defined properties significantly different from those of the parent "free" components. High-performance liquid chromatography has been employed successfully to characterize some tetracationic catenanes and rotaxanes incorporating either cyclobis(paraquat-p-phenylene) or cyclobis(paraquat-4,4'-biphenylene) as the charged components and either hydroquinone-containing macrocycles or dumbbell-shaped entities as the neutral components. In each case, significant differences in the retention times of the mechanically interlocked molecular compounds, in comparison with those of their components as their "free" forms, were observed.