A Remarkable Multitasking Double Spiropyran: Bidirectional Visible-Light Switching of Polymer-Coated Surfaces with Dual Redox and Proton Gating.

Smart or functional surfaces that exhibit complex multimodal responsivity, e.g., to light, heat, pH, etc., although highly desirable, require a combination of distinct functional units to achieve each type of response and present a challenge in achieving combinations that can avoid cross-talk between the units, such as excited-state quenching. Compounds that exhibit multiple switching modalities help overcome this challenge and drastically reduce the synthetic cost and complexity. Here we show that a bis-spiropyran photochrome, which is formed through coupling at the indoline 5-position using redox chemistry, exhibits pH-gated photochromism, with opening of the spiro moiety by irradiation with UV light and the expected reversion by either heating or irradiation with visible light gated by protonation/deprotonation. Remarkably, when the photochrome is oxidized to its dicationic form, bis-spiropyran(2+), visible light can be used instead of UV light to switch between the spiro and merocyanine forms, with locking and unlocking of each state achieved by protonation/deprotonation. The formation of the bis-spiropyran unit by electrochemical coupling is exploited to generate "smart surfaces", i.e., polymer-modified electrodes, avoiding the need to introduce an ancillary functional group for polymerization and the concomitant potential for cross-talk. The approach taken means not only that the multiresponsive properties of the bis-spiropyran are retained upon immobilization but also that the effective switching rate can be enhanced dramatically.

Light Emission as a Probe of Energy Losses in Molecular Junctions.

Visible light emission was observed for molecular junctions containing 5-19 nm thick layers of aromatic molecules between carbon contacts and correlated with their current-voltage behaviors. Their emission was compared to that from Al/AlOx/Au tunnel junctions, which has been previously attributed to transport of carriers across the AlOx layer to yield "hot carriers" which emit light as they relax within the Au contact. The maximum emitted photon energy is equal to the applied bias for the case of coherent tunneling, and such behavior was observed for light emission from AlOx and thin (<5 nm) molecular junctions. For thicker films, the highest energy observed for emitted photons is less than eVapp and exhibits an energy loss that is strongly dependent on molecular layer structure and thickness. For the case of nitroazobenzene junctions, the energy loss is linear with the molecular layer thickness, with a slope of 0.31 eV/nm. Energy loss rules out coherent tunneling as a transport mechanism in the thicker films and provides a direct measure of the electron energy after it traverses the molecular layer. The transition from elastic transport in thin films to "lossy" transport in thick films confirms that electron hopping is involved in transport and may provide a means to distinguish between various hopping mechanisms, such as activated electron transport, variable range hopping, and Poole Frankel transport.

Preparation and post-functionalization of hyperbranched polyurea coatings.

Postfunctionalizable hyperbranched polyurea coatings were prepared by the bulk polycondensation of AB2 monomers on preactivated silicon substrates. As previously shown, AB2 monomers were prepared, comprising a secondary amino group (A) and two blocked isocyanates (B) connected by hexyl spacers, in a single step and in quantitative yields. Covalent anchoring of the coatings on substrates was accomplished by reacting the secondary amino group in the focal point of the polymers with the blocked isocyanates (BIs) of the covalently attached coupling agent. The BIs in the top layer of the coatings were storage-stable under ambient conditions but well-modifiable with amino- or hydroxyl-functional compounds on heating. Attachment of polyethylene glycol or perfluoro-1-decanol afforded hydrophilic or hydrophobic surfaces. Immobilization and quaternization of polyethylenimines yielded highly charged surfaces. The coatings were extensively characterized by a number of techniques, such as Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, ellipsometry, and contact -angle measurements.

Comparing graphene growth on Cu(111) versus oxidized Cu(111).

The epitaxial growth of graphene on catalytically active metallic surfaces via chemical vapor deposition (CVD) is known to be one of the most reliable routes toward high-quality large-area graphene. This CVD-grown graphene is generally coupled to its metallic support resulting in a modification of its intrinsic properties. Growth on oxides is a promising alternative that might lead to a decoupled graphene layer. Here, we compare graphene on a pure metallic to graphene on an oxidized copper surface in both cases grown by a single step CVD process under similar conditions. Remarkably, the growth on copper oxide, a high-k dielectric material, preserves the intrinsic properties of graphene; it is not doped and a linear dispersion is observed close to the Fermi energy. Density functional theory calculations give additional insight into the reaction processes and help explaining the catalytic activity of the copper oxide surface.

Thermolubricity of gas monolayers on graphene.

Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. It was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates.

Control of surface wettability using tripodal light-activated molecular motors.

Monolayers of fluorinated light-driven molecular motors were synthesized and immobilized on gold films in an altitudinal orientation via tripodal stators. In this design the functionalized molecular motors are not interfering and preserve their rotary function on gold. The wettability of the self-assembled monolayers can be modulated by UV irradiation.

Microscopic characterisation of suspended graphene grown by chemical vapour deposition.

We present a multi-technique characterisation of graphene grown by chemical vapour deposition (CVD) and thereafter transferred to and suspended on a grid for transmission electron microscopy (TEM). The properties of the electronic band structure are investigated by angle-resolved photoelectron spectromicroscopy, while the structural and crystalline properties are studied by TEM and Raman spectroscopy. We demonstrate that the suspended graphene membrane locally shows electronic properties comparable with those of samples prepared by micromechanical cleaving of graphite. Measurements show that the area of high quality suspended graphene is limited by the folding of the graphene during the transfer.

Oxidative electrochemical aryl C-C coupling of spiropyrans.

The isolation and definitive assignment of the species formed upon electrochemical oxidation of nitro-spiropyran (SP) is reported. The oxidative aryl C-C coupling at the indoline moiety of the radical cation to form covalent dimers of the ring-closed form is demonstrated. The coupling is blocked with a methyl substituent para to the indoline nitrogen.

UV/vis and NIR light-responsive spiropyran self-assembled monolayers.

Self-assembled monolayers of a 6-nitro BIPS spiropyran (SP) modified with a disulfide-terminated aliphatic chain were prepared on polycrystalline gold surfaces and characterized by UV/vis absorption, surface-enhanced Raman scattering (SERS), and X-ray photoelectron spectroscopies (XPS). The SAMs obtained are composed of the ring-closed form (i.e., spiropyran) only. Irradiation with UV light results in conversion of the monolayer to the merocyanine form (MC), manifested in the appearance of an N(+) contribution in the N 1s region of the XPS spectrum of the SAMs, the characteristic absorption band of the MC form in the visible region at 555 nm, and the C-O stretching band in the SERS spectrum. Recovery of the initial state of the monolayer was observed both thermally and after irradiation with visible light. Several switching cycles were performed and monitored by SERS spectroscopy, demonstrating the stability of the SAMs during repeated switching between SP and MC states. A key finding in the present study is that ring-opening of the surface-immobilized spiropyrans can be induced by irradiation with continuous wave NIR (785 nm) light as well as by irradiation with UV light. We demonstrate that ring-opening by irradiation at 785 nm proceeds by a two-photon absorption pathway both in the SAMs and in the solid state. Hence, spiropyran SAMs on gold can undergo reversible photochemical switching from the SP to the MC form with both UV and NIR and the reverse reaction induced by irradiation with visible light or heating. Furthermore, the observation of NIR-induced switching with a continuous wave source holds important consequences in the study of photochromic switches on surfaces using SERS and emphasizes the importance of the use of multiple complementary techniques in characterizing photoresponsive SAMs.

One-pot functionalization of graphene with porphyrin through cycloaddition reactions.

Two types of graphene-based hybrid materials, graphene-TPP (TPP=tetraphenylporphyrin) and graphene-PdTPP (PdTPP=palladium tetraphenylporphyrin), were prepared directly from pristine graphene through one-pot cycloaddition reactions. The hybrid materials were characterized by thermogravimetric analysis (TGA), by TEM, by UV/Vis, FTIR, Raman, and luminescence spectroscopy, and by fluorescence/phosphorescence lifetime measurements. The presence of the covalent linkages between graphene and porphyrin was confirmed by FTIR and Raman spectroscopy and further supported by control experiments. The presence of TPP (or PdTPP) in the hybrid material was demonstrated by UV/Vis spectroscopy, with TGA results indicating that the graphene-TPP and graphene-PdTPP hybrid materials contained approximately 18 % TPP and 20 % PdTPP. The quenching of fluorescence (or phosphorescence) and reduced lifetimes suggest excited state energy/electron transfer between graphene and the covalently attached TPP (or PdTPP) molecules.