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A tripodal system for anchoring photochromic dithienylethenes on gold surfaces is reported. The self-assembled monolayers of a tripod-functionalized dithienylethene were characterized by cyclic voltammetry, surface-enhanced Raman spectroscopy (SERS), and X-ray photoelectron spectroscopy (XPS). These data are compared with solution studies, solid state Raman spectroscopy, and density functional theory (DFT) calculations. We show that the tripod-functionalized dithienylethene forms stable monolayers on gold in which all three legs of the tripod are adsorbed via the thiol units, thus providing a fixed position and orientation of the dithienylethene moiety with respect to the surface. Importantly, immobilization in this way allows for retention of both the photochemical and electrochemical functionality of the dithienylethene unit and reduces photochemical fatigue observed in solution.

In this review recent progress in combining metal carbonyl and bipyridyl transition metal complexes with dithienylethene photochromic switches is discussed. A key challenge in designing such systems is to allow for interaction between the various components, but without loss in photochemical activity, i.e. the cyclization/cycloreversion of the dithienylethene unit. Examples of where the properties of the switching unit can be fine-tuned either photochemically or electrochemically by the addition of either bipyridyl transition metals or organometallic moieties to the switch will be highlighted. (C) 2014 Elsevier B.V. All rights reserved.

In this review recent progress in combining metal carbonyl and bipyridyl transition metal complexes with dithienylethene photochromic switches is discussed. A key challenge in designing such systems is to allow for interaction between the various components, but without loss in photochemical activity, i.e. the cyclization/cycloreversion of the dithienylethene unit. Examples of where the properties of the switching unit can be fine-tuned either photochemically or electrochemically by the addition of either bipyridyl transition metals or organometallic moieties to the switch will be highlighted. (C) 2014 Elsevier B.V. All rights reserved.

Diarylethene photochromic switches use light to drive structural changes through reversible electrocyclization reactions. High efficiency in dynamic photoswitching is a prerequisite for applications, as is thermal stability and the selective addressability of both isomers ring-opened and -closed diarylethenes. These properties can be optimized readily through rational variation in molecular structure. The efficiency with regard to switching as a function of structural variation is much less understood, with the exception of geometric requirements placed on the reacting atoms. Ultimately, increasing the quantum efficiency of photochemical switching in diarylethenes requires a detailed understanding of the excited-state potential energy surface(s) and the mechanisms involved in switching. Through studies of the temperature dependence, photoswitching and theoretical studies demonstrate the occurrence or absence of thermal activation barriers in three constitutional isomers that bear distinct pi-conjugated systems. We found that a decrease in the thermal barriers correlates with an increase in switching efficiency. The origin of the barriers is assigned to the decrease in pi-conjugation that is concomitant with the progress of the photoreaction. Furthermore, we show that balanced molecular design can minimize the change in the extent of pi-conjugation during switching and lead to optimal bidirectional switching efficiencies. Our findings hold implications for future structural design of diarylethene photochromic switches.

Diarylethenes have proven to be versatile responsive components in many applications due to their photochromic properties. In recent years their potential use as redox switchable components has become increasingly apparent. Applying dithienylethenes as redox switchable components requires their immobilisation on conducting substrates and hence electropolymerisation is a promising, albeit, challenging approach to developing such systems. In this review the electrochemical properties of dithienylethenes will be discussed together with a consideration of mechanistic aspects of the switching processes observed followed by a review of recent efforts to develop dithienylethene based redox and photoswitchable redox polymers through electropolymerisation.

Bringing the functionality of molecular systems to interfaces while avoiding cross-talk and loss of function is essential to realize their full potential. The photochromic and -physical properties in polymer films formed from a terthiophene-diarylethene bifunctional monomer are lost due to aggregation but restored by solvent swelling, recovering electro- and photochromic switching as well as on/off control of fluorescence with memory.

Redox-driven molecular motion is an attractive alternative to light-driven processes. Here, the ability of an overcrowded alkene-based unimolecular light-driven rotary motor (A) to be driven by oxidation/reduction cycles is explored. We show that two-electron oxidation of A is followed by irreversible deprotonation and reduction to form a monocationic species D+, in which the stereogenic center is lost. This latter species was isolated through preparative electrolysis and its structure was confirmed by using single-crystal X-ray analysis. However, at short timescales and in the absence of Bronsted acids, these processes can be outrun and the oxidation of A to a dicationic species B2+ occurs, in which the central double bond (the axle of the molecular motor) becomes a single bond; when followed by rapid reduction, it results in the reformation of A, potentially in both its stable and unstable conformations. The latter conformation, if formed, undergoes thermal helix inversion, completing a rotary cycle. The data obtained regarding these reactions provide a window of opportunity for the motor to be driven electrochemically, without degradation from chemical reactions of the oxidized motor.

The direct synthesis of six-fold symmetric hexaphenylbenzenes with multiple photochromic dithienylethene units via a cobalt-catalyzed cyclotrimerization is reported. This approach allows for six photochromic units to be held in proximity with a well-defined spatial separation without affecting the photochromic properties of each unit.

The synthesis and characterisation of a series of chiral and achiral low molecular weight organogelators (LMWGs) based on bis-amide substituted dithienylethene photochromic switches is reported. The LMWGs gelate a range of solvents depending on the specific functionalisation of the hydrogen bonding amide groups. In mixtures Of Chiral and achiral LMWGs the Steteochemical outcome of the chiral aggregation is determined by the chiral LMWG molecules in most cases. However, for the first time we demonstrate that the stereochemical outcome of the aggregation can be influenced by the achiral LWMG molecules in some cases. Furthermore specific pi-pi (and/or van der Waals) interactions of chiral LMWGs 1-3o with the solvent allow the solvent to influence the control of chirality of aggregation. This influence of the solvent has a dramatic effect on whether four- or two-gel states are available. (C) 2008 Published by Elsevier Ltd.

The photochemical and electrochemical properties of self-assembled monolayers (SAM) of three structurally distinct hexahydro- and hexafluoro-dithienylcyclopentene-based photochromic switches on gold electrodes are reported. The photochemical and electrochemical switching between the open and closed forms of the dithienylethene SAMs is examined and found to be sensitive to the molecular structure of the switch. For the three dithienylethenes, the electrochemical behavior with respect to electrochemical ring opening/closing is retained in the SAMs. In contrast, a marked dependence on the nature of the anchoring group is observed upon immobilization in terms of the retention of the photochemical properties observed in solution. For the meta-thiophenol anchor both photochemical ring opening and closing are observed in the SAM, while for the thienyl-thiol-anchored switches the photochemically properties are changed markedly compared with those observed in solution. The stability of the monolayers toward desorption following photochemical and electrochemical switching is examined through electrochemistry and X-ray photoelectron spectroscopy.

Light-controlled conductance switching of diarylethenes attached to Au(111) is reported (see figure). First, scanning tunneling microscopy is used to demonstrate reversible photoswitching for individual molecules. Second, reversible switching in self-assembled monolayers is established by means of optical spectroscopy.

The non-destructive readout of photochromic memory materials based on the dithienylethene unit both by IR spectroscopy and Raman scattering is explored. A representative series of C5-substituted thienyl hexahydro- and hexafluoro-cyclopentene based photochromes was investigated to explore the effect and potential usefulness of substitution for the development of multicomponent memory materials. The effect of the deposition method on the photochemistry of solid materials containing photochromic dithienylcyclopentene switches was also explored. Photoconversion in the solid state to the closed form was found to be low when starting from the open form, but, in contrast, ring opening to the open state from the closed form was found to be complete. The effect was found to be due to inner filter rather than conformational phenomena. Characteristic vibrational bands for the central dithienyl core are assigned and a comparison made of the vibrational spectroscopic properties of the perhydro- and perfluoro switches. The data enable the determination of the photoconversion achievable in the solid state as well as some assessment of the influence of the deposition method on the photoconversion. The potential of Raman spectroscopy as a method of achieving non-destructive optical readout is demonstrated through the large differences in absolute Raman scattering intensity between the open and closed states, when monitored at wavelengths which do not result in photochemical ring opening.

We report the properties and switching characteristics of a series of dithienylethene photochromic switches immobilized on gold. Self-assembled monolayers (SAMs) of three structurally related dithienylethenes were formed on roughened gold bead substrates and studied by surface-enhanced Raman spectroscopy (SERS). These data were compared to SERS spectra obtained by aggregation of colloidal gold, solid state Raman spectra, and Raman spectra calculated using density functional theory (DFT). Two of the dithienylethenes studied have an "asymmetric" design, which was demonstrated earlier to lower the thermal barrier for photochemical ring opening in solution. Herein, we show that, when immobilized on a gold surface, the asymmetric dithienylethenes in fact display a higher thermal barrier than that of their symmetric counterparts. In addition, we show that photochemical ring closing of asymmetric dithienylethenes is inhibited when immobilized on gold.

Reversible switching of conductance using redox triggered switching of a polymer-modified electrode is demonstrated. A bifunctional monomer comprising a central electroswitchable core and two bithiophene units enables formation of a film through anodic electropolymerization. The conductivity of the polymer can be switched electrochemically in a reversible manner by redox triggered opening and closing of the diarylethene unit. In the closed state, the conductivity of the modified electrode is higher than in the open state.

have been demonstrated. Photoswitching effects are based on the reorganization of the p-conjugated backbone of the molecule that consequently leads to different physical properties. Knowledge of the vital factors governing photochromic processes is essential for future utilization of diarylethenes as smart materials as well as for a fundamental understanding of the photoprocesses involved. Hence, temperature-dependent studies are necessary to reveal new aspects and limits of the switching process, as previously shown by measurements at elevated temperatures. [19] Additionally, determining possible switching restrictions at low temperatures is particularly important in view of carrying out conductance measurements at the single-molecular level, which are restricted at room temperature because of the high mobility of the gold atoms constituting the electrodes in this type of experiment. In order to achieve stability of the metal/molecule/metal system it is often necessary to perform measurements at low temperatures. [20] Herein, we explore the temperature dependence of the photochemical behavior of dithienylcyclopentenes. We report the suppression of the ring-opening process with decreasing temperature, leading to the complete absence of the photoreaction below a cutoff temperature. By contrast, we demonstrate that the reverse ring-closure process shows no significant temperature dependence above 115 K. These observations of the temperature dependence for the ring-opening process are essential to understand the behavior of these molecular switches before they can be used in advanced technological applications. The theoretical description of the ringopening process is not completely understood. Furthermore, the observed suppression of the ring opening might have important implications for gating of photochromic processes. Like for other photoreactions that have a photostationary state with less than 100% product, the ring-closure process for diarylethenes is accompanied by ring opening, because both forms have absorption peaks in the UV region, hence decreasing total conversions. [21] On the other hand, if ring opening can be blocked, total conversion increases. [22]