Optical modulation of nano-gap tunnelling junctions comprising self-assembled monolayers of hemicyanine dyes

Light-driven conductance switching in molecular tunnelling junctions that relies on photoisomerization is constrained by the limitations of kinetic traps and either by the sterics of rearranging atoms in a densely packed monolayer or the small absorbance of individual molecules. Here we demonstrate light-driven conductance gating; devices comprising monolayers of hemicyanine dyes trapped between two metallic nanowires exhibit higher conductance under irradiation than in the dark. The modulation of the tunnelling current occurs faster than the timescale of the measurement (∼1 min). We propose a mechanism in which a fraction of molecules enters an excited state that brings the conjugated portion of the monolayer into resonance with the electrodes. This mechanism is supported by calculations showing the delocalization of molecular orbitals near the Fermi energy in the excited and cationic states, but not the ground state and a reasonable change in conductance with respect to the effective barrier width

Tuning microstructures in organogels: gelation and spectroscopic properties of mono- and bis-cholesterol-linked diphenylbutadiene derivatives

The gelation and photophysical properties of mono- and bis-cholesterol derivatives linked to diphenylbutadiene have been investigated. Scanning electron microscopy of xerogels of the monocholesterol derivatives indicated that these molecules self-assemble into 3D networks consisting of helically twisted fibers. In contrast, the morphology of xerogels of the bis-cholesterol derivatives indicated agglomerated spheres. In concentrated solutions (&gt;10<SUP>-4</SUP> M), the self-assembled superstructure of the monocholesterol derivatives consists of helically twisted fibers whereas that of the bis-cholesterol derivatives indicated clustered spheres. An investigation of spectroscopic properties suggests that the morphology of the superstructures formed in these systems may be correlated to the nature of the molecular aggregates formed. Absorption and emission spectral studies as a function of concentration and temperature suggested the formation of predominantly J-type aggregates in the monocholesterol and H-type aggregates in the bis-cholesterol derivatives. It is proposed that the slipped stack arrangement within the J aggregates of the monocholesterol derivatives resulted in the formation of helically twisted fibers, whereas the cofacial arrangement in the H aggregates of the bis-cholesterol derivatives could prevent such fiber formation, resulting in the formation of the agglomerated spheres

Photovoltaic Effect in Self-Assembled Molecular Monolayers on Gold: Influence of Orbital Energy Level Alignment on Short-Circuit Current Generation

Photoinduced current generation at metal–organic monolayer interfaces is observed upon photoexcitation of a monolayer of hemicyanine molecules chemically adsorbed onto a gold electrode. A series of hemicyanines is investigated that bind to the gold via a thiol moiety, in an orientation such that the acceptor moiety of the hemicyanine is closer to the metal than its donor part. The quantum yield of short-circuit photocurrent generation in a diode using a liquid electrolyte as second contact, correlates with the strength of the donor moiety of the dyes. Modeling of the photocurrent generation using Marcus theory indicates that the net photocurrent results from asymmetry in the electron transfer rates of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) with the electrodes. The quantum efficiency of short-circuit photocurrent generation decreases for the HOMO levels of the hemicyanine going deeper below the Fermi-level of the metal. The deeper HOMO level provides a larger driving force for back electron transfer from the metal to the photo-oxidized molecule and suppresses current generation in favor of quenching of the excited state

Reversible thermal and photochemical switching of liquid crystalline phases and luminescence in diphenylbutadiene-based mesogenic dimers

The synthesis and study of the photo- and thermoresponsive behavior of a series of novel asymmetric mesogenic dimers, consisting of a cholesterol moiety linked to a diphenylbutadiene chromophore via flexible alkyl chains are reported. These mesogenic dimers possess the combined glass forming properties of the cholesterol moiety and the photochromic and luminescent properties of the butadiene moiety. Photoinduced cis/trans isomerization of the butadiene chromophore in these materials could be utilized to bring about an isothermal phase transition from the smectic to the cholesteric state. By photochemically controlling the cis/trans isomer ratio, the pitch of the cholesteric could be continuously varied making it possible to tune the color of the film over the entire visible region, and the color images thus generated could be stabilized by converting them to N glasses. These materials were also polymorphic, exhibiting two crystalline forms possessing distinctly different fluorescence properties. The ability to thermally switch these materials from one crystalline form to the other in a reversible manner also makes them useful for recording fluorescent images

Photoresponsive glass-forming butadiene-based chiral liquid crystals with circularly polarized photoluminescence

The synthesis and study of the photo- and thermoresponsive behavior of a series of novel trimesogens consisting of a diphenylbutadiene core linked to cholesterol moieties on either side via flexible alkyl chains are reported. These molecules possess the combined glass-forming properties of bischolesterols and the photochromic and luminescent properties of the butadiene moiety. The pitch of the cholesteric phase of these materials could be continuously varied thermally and photochemically, making it possible to tune the color of the film over the entire visible region. The color images thus generated could be stabilized by converting them to N<SUP>&#8727;</SUP> glasses. These materials were also highly photoluminescent, exhibiting circularly polarized characteristics in the glassy liquid-crystalline state even by linearly polarized excitation

Indane-1,3-dione and cholesterol containing butadiene derivatives: photoresponsive liquid crystalline glasses for imaging applications

Synthesis, thermo-optical and glass forming properties of a series of photoresponsive luminescent mesogenic materials containing a butadiene moiety linked to a cholesterol group (CBIN, CBIN8 and CBIN12) as well as colour imaging devices based on them are described. These molecules exhibited relatively high fluorescence quantum efficiency in the solid and liquid crystalline states. The cholesteric phase was observed to be the major phase in these molecules and the pitch of cholesteric helix was sensitive to external stimuli such as temperature and light, making it possible to tune their iridescent colour in the visible region. The colour reflected by the cholesteric films could be stabilized by converting them to the glassy state by sudden cooling from their cholesteric phase to ~0 °C. Selective exposure of the material in its liquid crystalline phase to light resulted in trans-cis photoisomerization of the butadiene chromophore. Consequently the pitch of the irradiated portion changed depending upon the intensity of the illuminating light. The coloured images thus formed could be stored for long periods by converting the films to glasses by sudden cooling. The images stored in the glassy state were stable over long periods of time (>one year)

Aromatic Ring Overlap Pedals the Nature of Exciton Coupling and Carrier Transport in a Series of Electron-Deficient Anthracenes

Planar aromatic semiconductors are excellent active layer materials for charge transport (mostly p-type) and light emission in various organic electronic devices. The exciton coupling is well known for controlling the optical traits of such materials in the bulk state. In terms of the carrier mobility, to achieve ambipolar transport in these classes of materials, the frontier molecular orbital energy levels should be appropriately engineered. Core functionalization using a moderately strong electron-withdrawing group (such as −CF3) could be a potential strategy to obtain appropriate energy levels. Herein, we have selected positional isomers of the bis–CF3 substituted derivatives as the possible group of molecules. In the bulk film state, depending on the spatial overlap of the aromatic core unit, distinct optical traits of the J- and H-type excitonically coupled dimeric states were evident in these series. Prominent enhancement of the radiative rate constant (and photoluminescence quantum yield) in J-type dimers (first pair of molecules) and substantially quenched excimer state emission for the H-type dimers was evident for the other pair in the solid-state samples. At the same time, in contradiction to the general perception of significant aromatic overlap for higher carrier mobility, the derivative with the highest spatial overlap had the least among the four products. An analogy could be drawn between the aromatic overlap in the solid-state dimers with their optical and the charge carrier mobility. Detailed experimental outcomes and the analysis are discussed in the present article