An Orthogonal Conductance Pathway in Spiropyrans for Well-Defined Electrosteric Switching Single-Molecule Junctions.

While a multitude of studies have appeared touting the use of molecules as electronic components, the design of molecular switches is crucial for the next steps in molecular electronics. In this work, single-molecule devices incorporating spiropyrans, made using break junction techniques, are described. Linear spiropyrans with electrode-contacting groups linked by alkynyl spacers to both the indoline and chromenone moieties have previously provided very low conductance values, and removing the alkynyl spacer has resulted in a total loss of conductance. An orthogonal T-shaped approach to single-molecule junctions incorporating spiropyran moieties in which the conducting pathway lies orthogonal to the molecule backbone is described and characterized. This approach has provided singlemolecule conductance features with good correlation to molecular length. Additional higher conducting states are accessible using switching induced by UV light or protonation. Theoretical modeling demonstrates that upon (photo)chemical isomerization to the merocyanine, two cooperating phenomena increase conductance: release of steric hindrance allows the conductance pathway to become more planar (raising the mid-bandgap transmission) and a bound state introduces sharp interference near the Fermi level of the electrodes similarly responding to the change in state. This design step paves the way for future use of spiropyrans in single-molecule devices and electrosteric switches.

Multistate Switching of Some Ruthenium Alkynyl and Vinyl Spiropyran Complexes.

To study the switching properties of photochromes, we undertook the synthesis and characterization of several ruthenium organometallic complexes of the type [Ru(Cp*)(dppe)(C≡C-SP)] or [Ru(CO)(dppe)(PPh3)Cl(CH═CH-SP)], where SP = spiropyran. The spectroscopic and electrochemical properties of the complexes were determined by careful cyclic voltammetric and spectroelectrochemical experiments. Whereas the mononuclear alkynyl ruthenium complexes undergo one-electron oxidations localized over the metal alkynyl moiety, the oxidation of the mononuclear vinyl ruthenium complexes is centered on the indoline moiety of the spiropyran. Through these studies, we demonstrate access to several stable redox states, in addition to switching states attained via acidochromism and/or photoisomerization.

Multifunctional switching properties of "wire-like" dinuclear ruthenium bis-alkynyl spiropyran complexes.

Multifunctional switches are crucial to the development of smart molecular materials and molecular-electronic applications. Here, we describe the synthesis, structure, and characterization of several spiropyrans functionalized with alkynyl-[Ru(dppe)2] moieties. Through electrochemical and spectroelectrochemical studies, we demonstrate access to several stable redox states, in addition to states accessed via acidochromism and photoisomerisation. Initial protonation was found to occur at the alkynyl functionality followed by acid-induced ring-opening of the spiropyran ligand to form the protonated merocyanine. The protonated merocyanine can be switched from the Z- to E-isomer by using ultraviolet light. The spiropyran was also shown to be an effective insulator for electronic communication across the molecular backbone.

Synthesis and 83Kr NMR spectroscopy of Kr@C60.

Synthesis of Kr@C60 is achieved by quantitative high-pressure encapsulation of the noble gas into an open-fullerene, and subsequent cage closure. Krypton is the largest noble gas entrapped in C60 using 'molecular surgery' and Kr@C60 is prepared with >99.4% incorporation of the endohedral atom, in ca. 4% yield from C60. Encapsulation in C60 causes a shift of the 83Kr resonance by -39.5 ppm with respect to free 83Kr in solution. The 83Kr spin-lattice relaxation time T1 is approximately 36 times longer for Kr encapsulated in C60 than for free Kr in solution. This is the first characterisation of a stable Kr compound by 83Kr NMR.

Isotope-Coded Maleimide Affinity Tags for Proteomics Applications.

Isotope-coded affinity tags (ICATs) are valuable tools for mass spectrometry-based quantitative proteomics, in particular, for comparison of protein (cysteine-residue) thiol oxidation state in normal, stressed, and diseased tissue. However, the iodoacetamido electrophile used in most commercial ICATs suffers from poor thiol-selectivity and modest rates of adduct formation, which can lead to spurious results. Hence, we designed and synthesized three ICATs containing thiol-selective N-alkylmaleimide electrophiles (isotope-coded maleimide affinity tags = ICMATs) and assessed these as mass spectrometry probes for ratiometric analysis of lysozyme and muscle proteomes. Two ICMAT pairs containing butylene/D8-butylene linkers were effective MS probes, but not ideal for typical proteomics workflows, because peptides bearing these tags frequently did not coelute with HPLC. A switch to a phenylene/13C6-phenylene linker solved this issue without compromising the efficiency of adduct formation.

A Solid-State Intramolecular Wittig Reaction Enables Efficient Synthesis of Endofullerenes Including Ne@C60 , 3 He@C60 , and HD@C60.

An open-cage fullerene incorporating phosphorous ylid and carbonyl group moieties on the rim of the orifice can be filled with gases (H2 , He, Ne) in the solid state, and the cage opening then contracted in situ by raising the temperature to complete an intramolecular Wittig reaction, trapping the atom or molecule inside. Known transformations complete conversion of the product fullerene to C60 containing the endohedral species. As well as providing an improved synthesis of large quantities of 4 He@C60 , H2 @C60 , and D2 @C60 , the method allows the efficient incorporation of expensive gases such as HD and 3 He, to prepare HD@C60 and 3 He@C60 . The method also enables the first synthesis of Ne@C60 by molecular surgery, and its characterization by crystallography and 13 C NMR spectroscopy.

An Internuclear J-Coupling of 3He Induced by Molecular Confinement.

The solution-state 13C NMR spectrum of the endofullerene 3He@C60 displays a doublet structure due to a J-coupling of magnitude 77.5 ± 0.2 mHz at 340 K between the 3He nucleus and a 13C nucleus of the enclosing carbon surface. The J-coupling increases in magnitude with increasing temperature. Quantum chemistry calculations successfully predict the approximate magnitude of the coupling. This observation shows that the mutual proximity of molecular or atomic species is sufficient to induce a finite scalar nuclear spin-spin coupling, providing that translational motion is restricted by confinement. The phenomenon may have applications to the study of surface interactions and to mechanically bound species.

Synthesis of Ar@C60 using molecular surgery.

Synthesis of Ar@C60 is described, using a route in which high-pressure argon filling of an open-fullerene and photochemical desulfinylation are the key steps for >95% encapsulation of the noble gas. Enrichment by recycling HPLC leads to quantitative incorporation of argon in the product endofullerene, with a mass recovery of tens of milligrams, allowing the first characterisation of fine structure in the solution 13C NMR spectrum.

Chemically and Mechanically Controlled Single-Molecule Switches Using Spiropyrans.

Developing molecular circuits that can function as the active components in electrical devices is an ongoing challenge in molecular electronics. It demands mechanical stability of the single-molecule circuit while simultaneously being responsive to external stimuli mimicking the operation of conventional electronic components. Here, we report single-molecule circuits based on spiropyran derivatives that respond electrically to chemical and mechanical stimuli. The merocyanine that results from the protonation/ring-opening of the spiropyran form showed single-molecule diode characteristics, with an average current rectification ratio of 5 at ±1 V, favoring the orientation where the positively charged end of the molecule is attached to the negative terminal of the circuit. Mechanical pulling of a single spiropyran molecule drives a switch to a more conducting merocyanine state. The mechanical switching is enabled by the strong Au-C covalent bonding between the molecule and the electrodes, which allows the tensile force delivered by the STM piezo to break the molecule at its spiropyran C-O bond.

Enhanced bi-stability in a ruthenium alkynyl spiropyran complex.

The inclusion of a ligated ruthenium moiety to ethynyl spiropyran, 5'-ethynyl-1',3',3'-trimethyl-6-nitrospiro[chromene-2,2'-indoline], has been shown to increase the lifetime of the ring-opened merocyanine form twentyfold. Calculations suggest that the higher barrier to thermal reversion of the merocyanine form of the metal alkynyl complex arises from the capacity for greater delocalisation of charge consequent of the presence of the ruthenium moiety. The complex may provide a different switching mechanism to the 5,5'-dithienylperfluorocyclopentene electrode decoupling seen previously.