Coherent electron transport through an azobenzene molecule: a light-driven molecular switch

Conductance Evolution of Photoisomeric Single-Molecule Junctions under Ultraviolet Irradiation and Mechanical Stretching

A comprehensive understanding of carrier transport in photoisomeric molecular junctions is crucial for the rational design and delicate fabrication of single-molecule functional devices. It has been widely recognized that the conductance of azobenzene (a class of photoisomeric molecules) based molecular junctions is mainly determined by photoinduced conformational changes. In this study, it is demonstrated that the most probable conductance of amine-anchored azobenzene-based molecular junctions increases continuously upon UV irradiation. In contrast, the conductance of pyridyl-anchored molecular junctions with an identical azobenzene core exhibits a contrasting trend, highlighting the pivotal role that anchoring groups play, potentially overriding (even reversing) the effects of photoinduced conformational changes. It is further demonstrated that the molecule with cis-conformation cannot be fully mechanically stretched into the trans-conformation, clarifying that it is a great challenge to realize a reversible molecular switch by purely mechanical operation. Additionally, it is revealed that the coupling strength of pyridyl-anchored molecules is dramatically weakened when the UV irradiation time is prolonged, whereas it is not observed for amine-anchored molecules. The mechanisms for these observations are elucidated with the assistance of density functional theory calculations and UV-Vis spectra combined with flicker noise measurements which confirm the photoinduced conformational changes, providing insight into understanding the charge transport in photoisomeric molecular junctions and offering a routine for logical designing synchro opto-mechanical molecular switches.

Photomobile Polymer-Piezoelectric Composite for Enhanced Actuation and Energy Generation

In this study, we present an innovative approach to increase the quantum yield and wavelength sensitivity of photomobile polymer (PMP) films based on azobenzene by doping the polymer matrix with noble metal nanoparticles. These doped PMP films showed faster and more significant bending under both UV as well as visible and near-infrared light regardless of whether it was coherent, incoherent, polarized, or unpolarized irradiation, expanding the potential of PMP-based actuators. To illustrate their practical implications, we created a proof-of-concept model of power generation by coupling it to flexible piezoelectric materials under simulated sunlight. This model has been tested under real operating conditions, thus demonstrating the possibility of generating electricity with variable light exposure. Additionally, our synthetic protocol is solvent-free, which is another benefit of environmental relevance. Our research lays the groundwork for the development of sunlight-sensitive devices, such as photomechanical actuators and advanced photovoltaic modules, which may break ground in the thriving field of smart materials. We are confident that the presented findings will contribute to the ongoing discourse in the field and inspire additional advances in renewable energy applications.

Light-driven modulation of electrical conductance with photochromic switches: bridging photochemistry with optoelectronics

Photochromic conducting molecules have emerged because of their unique capacity to modulate electrical conductivity upon exposure to light, toggling between high and low conductive states. This unique amalgamation has unlocked novel avenues for the application of these materials across diverse areas in optoelectronics and smart materials. The fundamental mechanism underpinning this phenomenon is based on the light-driven isomerization of conjugated π-systems which influences the extent of conjugation. The photoisomerization process discussed here involves photochromic switches such as azobenzenes, diarylethenes, spiropyrans, dimethyldihydropyrenes, and norbornadiene. The change in the degree of conjugation alters the charge transport in both single molecules and bulk states in solid samples or solutions. This article discusses a number of recent examples of photochromic conducting systems and the challenges and potentials of the field.

Retaining Hückel Aromaticity in the Triplet Excited State of Azobenzene

The implication of the potential concept of aromaticity in the relaxed lowest triplet state of azobenzene, an efficient molecular switch, using elementary aromaticity indices based on magnetic, electronic, and geometric criteria has been discussed. Azobenzene exhibits a major Hückel aromatic character retained in the diradical lowest relaxed triplet state (T₁ ) by virtue of a twisted geometry with partial delocalization of unpaired electrons in the perpendicular p-orbitals of two nitrogen atoms to the corresponding phenyl rings. The computational analysis has been expanded further to stilbene and N-diphenylmethanimine for an extensive understanding of the effect of closed-shell Hückel aromaticity in double-bond-linked phenyl rings. Our analysis concluded that stilbene has Hückel aromatic character in the relaxed T₁ state and N-diphenylmethanimine has a considerable Hückel aromaticity in the phenyl ring near the carbon atom while a paramount Baird aromaticity in the phenyl ring near the nitrogen atom of the C=N double bond. The results reveal the application of excited-state aromaticity as a general tool for the design of molecular switches.

Prominent nonequilibrium effects beyond the standard first-principles approach in nanoscale electronic devices

The standard density functional theory (DFT) based first-principles approach has been widely used for modeling nanoscale electronic devices. A recent experiment, however, reported surprising transport properties of thiol-terminated silane junctions that cannot be understood using the standard DFT approach, presenting a severe challenge for the current computational understanding of electron transport at the nanoscale. Using the recently proposed steady-state DFT (SS-DFT) for nonequilibrium quantum systems, we found that in silane junctions, underlying the puzzling experimental observations is a novel type of intriguing nonequilibrium effect that is beyond the framework of the standard DFT approach. Our calculations show that the standard DFT approach is a good approximation of SS-DFT when silane junctions are near equilibrium, but the aforementioned nonequilibrium effects could drive the thiol-terminated silanes far away from equilibrium even at low biases of around 0.2 V. Further analysis suggests that these nonequilibrium effects could generally exist in nanoscale devices in which there are conducting channels mainly residing at the source contact and close to the bias window. These findings significantly broaden our fundamental understanding of electron transport at the nanoscale.

Negative differential resistance, rectification, tunable peak-current position and switching effects in an alanine-based molecular device

The transport properties of a molecular bio-electronic device based on the alanine amino-acid are investigated. The considered device consists of an alanine molecule as the central potential-dot coupled to two zigzag graphene nanoribbon (ZGNR) conducting electrodes. The current-voltage characteristics of this dual tunnelling molecular junction are studied at two different optimised compositions of the central molecule. The proposed amino-acid based structure utilises the tunnelling coupling similar to that of semiconducting single-electron transistors (SETs) to avoid complications due to the atomic interfaces. The current-voltage characteristics show polarity-dependent behaviour making the device feasible of being applied as a molecular rectifier. Negative differential resistance (NDR) along with tuneable peak-current position has been also observed in the current-voltage characteristics. The device is also capable of being applied as a switch controllable by the central molecule orientation.

The influence of surface proximity on photoswitching activity of stilbene-functionalized N-heterocyclic carbene monolayers

Self-assembly of photo-responsive molecules is a robust technology for reversibly tuning the properties of functional materials. Herein, we probed the crucial role of surface-adsorbate interactions on the adsorption geometry of stilbene-functionalized N-heterocyclic carbenes (stilbene-NHCs) monolayers and its impact on surface potential. Stilbene-NHCs on Au film accumulated in a vertical orientation that enabled high photoisomerization efficiency and reversible changes in surface potential. Strong metal-adsorbate interactions led to flat-lying adsorption geometry of stilbene-NHCs on Pt film, which quenched the photo-isomerization influence on surface potential. It is identified that photo-induced response can be optimized by positioning the photo-active group in proximity to weakly-interacting surfaces.

Challenges for Incorporating Optical Switchability in Organic-Based Electronic Devices

Transistors operate by controlling the current flowing from a source to a drain electrode via a third electrode (gate), thus giving access to a binary treatment (ON/OFF or 0/1) of the signal currently exploited in microelectronics. Introducing a second independent lever to modulate the current would allow for more complex logic functions amenable to a single electronic component and hence to new opportunities for advanced electrical signal processing. One avenue is to add this second dimension with light by incorporating photochromic molecules in current organic-based electronic devices. In this Spotlight, we describe different concepts that have been implemented in organic thin films and in molecular junctions as well as some pitfalls that have been highlighted thanks to theoretical modeling.

Controllable Spin Switching in a Single-Molecule Magnetic Tunneling Junction

A new type of spin-current filter is proposed that consists of a single-molecule magnet (SMM) coupled to two normal metal electrodes. It is shown that this tunneling junction can generate a highly spin-polarized current, whose spin polarization can be switched by means of magnetic fields and gate voltages applied to the SMM. This spin switching in the SMM tunnel junction arises from spin-selective single-electron resonant tunneling via the lowest unoccupied molecular orbit of the SMM. The electron current spectrum is still spin polarized in the absence of an external magnetic field, which can help to judge whether the molecule's spin state has reached the ground-state doublet [Formula: see text]. This device can be realized with current technologies and may have practical use in spintronics and quantum information.

Implementation of biomolecular logic gate using DNA and electrically doped GaAs nano-pore: a first principle paradigm

One of the emerging areas of today's research arena is molecular modeling and molecular computing. The molecular logic gate can be theoretically implemented from single-strand DNA which consists of four basic nucleobases. In this study, the electronic transmission characteristics of DNA chain are investigated to form the logic gate. This biomolecular single-strand DNA chain is passed through an electrically doped gallium-arsenide nano-pore to achieve reasonably improved transmission along <1 1 1> direction. Current-voltage characteristic and device density of states with HOMO-LUMO plot of the device are explained along with the conductivity of the device to confirm the characteristics of some important logic gates like a universal gate. Ultimately the property of resistivity proves the law of Boolean logic of AND gate and universal logic gate, viz., NAND and NOR gate. All the electronic properties of the Boolean logic gate are explored based on the first principle approach by non-equilibrium Green's function coupled with density functional theory in room temperature.

Spectral Tuning and Photoisomerization Efficiency in Push-Pull Azobenzenes: Designing Principles

This work demonstrates how push–pull substitution can induce spectral tuning toward the visible range and improve the photoisomerization efficiency of azobenzene-based photoswitches, making them good candidates for technological and biological applications. The red-shifted bright ππ* state (S₂) behaves like the lower and more productive dark nπ* (S₁) state because less potential energy along the planar bending mode is available to reach higher energy unproductive nπ*/S₀ crossing regions, which are responsible for the lower quantum yield of the parent compound. The stabilization of the bright ππ* state and the consequent increase in isomerization efficiency may be regulated via the strength of push–pull substituents. Finally, the torsional mechanism is recognized here as the unique productive route because structures with bending values attributable to the inversion mechanism were never detected, out of the 280 ππ* time-dependent density functional theory (RASPT2-validated) dynamics simulations.

Synthesis and Photochromism of Novel Pyridyl-Substituted Naphthopyrans

Multitarget synthetic strategies to access novel photochromic 3H-naphtho[2,1-b]pyrans decorated with pyridyl units are described. The new pyridyl-substituted 3H-naphtho[2,1-b]pyrans display good photochromic properties with reversible generation of photomerocyanines, which exhibit mainly orange/red hues. Photochromic parameters including photocolorability and persistence of color vary tremendously on structural modification of the naphthopyran core.

Switchable Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization with the Assistance of Azobenzenes

Modulating controlled radical polymerization is an interesting and important issue. Herein, modulating RAFT polymerization employing photosensitive azobenzenes is achieved. In the presence of azobenzenes and with visible light off, RAFT polymerization runs smoothly and follows a pseudo-first-order kinetics. In contrast, with light on, RAFT polymerization is greatly decelerated or quenched depending on the type and concentration of azobenzenes. Switchable RAFT polymerization of different (meth)acrylate monomers alternatively with light off and on is demonstrated. A mechanism of photoregulating RAFT polymerization involving radical quenching by azobenzenes is proposed.

Illuminating the photoisomerization of a modified azobenzene single crystal by femtosecond absorption spectroscopy

The mechanism of isomerization for azobenzene is a topic still to be completely elucidated. Here, we describe the ultrafast dynamics of a brominated dioxane-methoxy-azobenzene under single crystal conditions by means of femtosecond transient absorption (TA) spectroscopy. Upon excitation with 400 nm light, spectral components with decays of 0.72, 2.9, and >10 ps are observed. The fast components of the system correspond to vibrational cooling of the population on the S1 excited state, with a decay to a local minimum in the reaction coordinate, followed by a longer evolution to a dark intermediate state prior to relaxing to the ground state, S0. The long time constant can be used to describe the isomerization process, returning excited population to the ground state. Spectral frequencies observed at 33 and 82 cm−1 suggest that both rotation and inversion occur in the system, with a stronger contribution coming from the latter due to a weakened N–N double bond in the excited state. This information provides insight into the structural nature of modified azobenzene systems and sets the stage for future structural studies of the molecule’s isomerization dynamics.

First principle approach towards logic design using hydrogen-doped single-strand DNA

Molecular logic gate has been proposed using single-strand DNA (ssDNA) consisting of basic four nucleobases. In this study, density functional theory and non-equilibrium Green's function based first principle approach is applied to investigate the electronic transmission characteristics of ssDNA chain. The heavily hydrogen-doped-ssDNA (H-ssDNA) chain is connected with gold electrode to achieve enhanced quantum-ballistic transmission along 〈1 1 1〉 direction. Logic gates OR, Ex-OR, NXOR have been implemented using this analytical model of H-ssDNA device. Enhanced logic properties have been observed for ssDNA after H adsorption due to improved electronic transmission. Dense electron cloud is considered as logic 'high' (1) output in presence of hydrogen molecule and on the contrary sparse cloud indicate logic 'low' (0) in the absence of hydrogen molecule. Device current is significantly increased from 0.2 nA to 2.4 µA (approx.) when ssDNA chain is heavily doped with hydrogen molecule. The current-voltage characteristics confirm the formation of various Boolean logic gate operations.

Low-Threshold Reversible Electron-Induced and Selective Photoinduced Switching of Azobenzene Derivatives under Ambient Conditions

Mono-carboxyl-functionalized azobenzene and arylazopyrazole have been employed for electron-induced and photoinduced switching under ambient conditions. The microscopic structure and the switching behavior is understood using scanning tunneling microscopy. The carboxyl functional group in these molecules offers low threshold energy for the electron-induced reversible switching compared with nonfunctionalized azobenzene. The low threshold is understood using charged intermediate states during the switching. A selectivity has been observed for the photoinduced switching. Because of strong hydrogen bonding, only the free phenyl groups in the molecules change their configuration.

Mapping reversible photoswitching of molecular resistance fluctuations during the conformational transformation of azobenzene-terminated molecular switches

We report a direct mapping and analysis of electrical noise in azobenzene-terminated molecular monolayers, revealing reversible photoswitching of the molecular resistance fluctuations in the layers. In this work, a conducting atomic force microscope combined with a homemade spectrum analyzer was used to image electrical current and noise at patterned self-assembled monolayers (SAMs) of azobenzene-terminated molecular wires on a gold substrate. We analyzed the current and noise imaging data to obtain maps of molecular resistances and amount of mean-square fluctuations in the resistances of the regions of trans-azobenzene and a cis/trans-azobenzene mixture. We revealed that the fluctuations in the molecular resistances in the SAMs were enhanced after the trans-to-cis isomerization, while the resistances were reduced. This result could be attributed to enhanced disorders in the molecular arrangements in the cis-SAMs. Furthermore, we observed that the changes in the resistance fluctuations were reversible with respect to repeated trans-to-cis and cis-to-trans isomerizations, indicating that the effects originated from reversible photoswitching of the molecular structures rather than irreversible damages of the molecules. These findings provide valuable insights into the electrical fluctuations in photoswitchable molecules, which could be utilized in further studies on molecular switches and molecular electronics in general.

Principles of Design for Substrate-Supported Molecular Switches Based on Physisorbed and Chemisorbed States

The physisorbed (precursor) and chemisorbed states of a molecule on metal surfaces can be utilized to build a logic switch at the single-molecule level, enabling further microminiaturization of electronic devices beyond the silicon limits. However, a serious drawback of this design is easy lateral diffusion of the molecule in the physisorbed state, which may destroy the normal switch operation. Here, we demonstrate that anchoring engineering can be an effective way to enhance the stability of molecular switches without degrading switching functionality. As exemplified by trans-ADT on Cu(111), we show that the lateral diffusion of such molecular switch can be obstructed by the anchoring of the ending thiophene groups, along with a rotation of the adsorbate during the switching process. More general, our results also suggest that when searching for molecular switches with reversible physisorbed and chemisorbed states with excellent bistability and lateral stability, the focus should be on finding molecules with a moderate HOMO-LUMO energy gap and anchoring atoms with positive charge that can then be deposited on substrates with which they interact moderately. This allows further improvement of the lateral and vertical stability of such a molecular switch by substituting the thiophene groups with selenophene, thus establishing trans-ADS on Cu(111) as a promising switch.

Two-dimensional Cu2Si sheet: a promising electrode material for nanoscale electronics

Building electronic devices on top of two-dimensional (2D) materials has recently become one of most interesting topics in nanoelectronics. Finding high-performance 2D electrode materials is one central issue in 2D nanoelectronics. In the current study, based on first-principles calculations, we compare the electronic and transport properties of two nanoscale devices. One device consists of two single-atom-thick planar Cu₂Si electrodes, and a nickel phthalocyanine (NiPc) molecule in the middle. The other device is made of often-used graphene electrodes and a NiPc molecule. Planer Cu₂Si is a new type of 2D material that was recently predicted to exist and be stable under room temperature [11]. We found that at low bias voltages, the electric current through the Cu₂Si-NiPc-Cu₂Si junction is about three orders higher than that through graphene-NiPc-graphene. Detailed analysis shows that the surprisingly high conductivity of Cu₂Si-NiPc-Cu₂Si originates from the mixing of the Cu₂Si state near Fermi energy and the highest occupied molecular orbital of NiPc. These results suggest that 2D Cu₂Si may be an excellent candidate for electrode materials for future nanoscale devices.

Light controllable catalytic activity of Au clusters decorated with photochromic molecules

By ab initio calculations, we show that when decorated with a photochromic molecule, the catalytic activity of an Au nanocluster can be reversibly controlled by light. The combination of a photochromic thiol-pentacarbonyl azobenzene (TPA) molecule and an Au₈ cluster is chosen as a model catalyst. The TPA molecule has two configurations (trans and cis) that can be reversibly converted to each other upon photo-excitation. Our calculations show that when the TPA takes the trans configuration, the combined system (trans-Au₈) is an excellent catalyst for CO oxidation. The reaction barrier of the catalyzed CO oxidation is less than 0.4 eV. While, the reaction barrier of CO oxidation catalyzed by cis-Au₈ is very high (>2.7 eV), indicating that the catalyst is inactive. These results pave the way for a new class of light controllable nanoscale catalysts.

Double Molecular Photoswitch Driven by Light and Collisions

The shapes of many molecules can be transformed by light or heat. Here we investigate collision- and photon-induced interconversions of EE, EZ, and ZZ isomers of the isolated Congo red (CR) dianion, a double molecular switch containing two ─N═N─ azo groups, each of which can have the E or Z configuration. We find that collisional activation of CR dianions drives a one-way ZZ→EZ→EE cascade towards the lowest-energy isomer, whereas the absorption of a single photon over the 270-600 nm range can switch either azo group from E to Z or Z to E, driving the CR dianion to lower- or higher-energy forms. The experimental results, which are interpreted with the aid of calculated statistical isomerization rates, indicate that photoisomerization of CR in the gas phase involves a passage through conical intersection seams linking the excited and ground state potential energy surfaces rather than through isomerization on the ground state potential energy surface following internal conversion.

Multicontrol Over Graphene-Molecule Hetereojunctions

The vertical configuration is a powerful tool recently developed experimentally to investigate field effects in quasi two-dimensional systems. Prototype graphene-based vertical tunneling transistors can achieve an extraordinary control over current density utilizing gate voltages. In this work, we study theoretically vertical tunneling junctions that consist of a monolayer of photoswitchable aryl azobenzene molecules sandwiched between two sheets of graphene. Azobenzene molecules transform between trans and cis conformations upon photoexcitation, thus adding a second knob that enhances the control over physical properties of the junction. Using first-principles methods within the density functional framework, we perform simulations with the inclusion of field effects for both trans and cis configurations. We find that the interference of interface states resulting from molecule-graphene interactions at the Fermi energy introduces a dual-peak pattern in the transmission functions and dominates the transport properties of gate junctions, shedding new light on interfacial processes.

Effect of Ring Strain on the Charge Transport of a Robust Norbornadiene-Quadricyclane-Based Molecular Photoswitch

Integrating functional molecules into single-molecule devices is a key step toward the realization of future computing machines based on the smallest possible components. In this context, photoswitching molecules that can make a transition between high and low conductivity in response to light are attractive candidates. Here we present the synthesis and conductance properties of a new type of robust molecular photothermal switch based on the norbornadiene (NB)-quadricyclane (QC) system. The transport through the molecule in the ON state is dominated by a pathway through the π-conjugated system, which is no longer available when the system is switched to the OFF state. Interestingly, in the OFF state we find that the same pathway contributes only 12% to the transport properties. We attribute this observation to the strained tetrahedral geometry of the QC. These results challenge the prevailing assumption that current will simply flow through the shortest through-bond path in a molecule.

Huge magnetoresistance induced by half-metal-semiconductor phase transition in a one-dimensional spin chain: a first-principles study

In experimental studies, magnetoresistance (MR) values of 10³ are hard to reach for conventional single-molecule spin-valves. Motivated by a recent experiment [Nano Lett., 2016, 16, 577-582], where tailored Co-salophene-based all-spin molecular devices are successfully realized, we demonstrate the functionality of a Co-salophene-based spin chain without magnetic electrodes. By using nonequilibrium Green's functions in combination with density functional theory, we find that the maximum MR ratio of this spin chain can reach 10⁶ by manipulating its spins in a controlled way, which is several orders of magnitude higher than previously reported experimental values. As the Co-salophene-based spin chain has been successfully synthesized, we are highly expectant of the experimental realization of huge MR ratios.

Sensitivity of photoelectron diffraction to conformational changes of adsorbed molecules: Tetra-tert-butyl-azobenzene/Au(111)

Electron diffraction is a standard tool to investigate the atomic structure of surfaces, interfaces, and adsorbate systems. In particular, photoelectron diffraction is a promising candidate for real-time studies of structural dynamics combining the ultimate time resolution of optical pulses and the high scattering cross-sections for electrons. In view of future time-resolved experiments from molecular layers, we studied the sensitivity of photoelectron diffraction to conformational changes of only a small fraction of molecules in a monolayer adsorbed on a metallic substrate. 3,3',5,5'-tetra-tert-butyl-azobenzene served as test case. This molecule can be switched between two isomers, trans and cis, by absorption of ultraviolet light. X-ray photoelectron diffraction patterns were recorded from tetra-tert-butyl-azobenzene/Au(111) in thermal equilibrium at room temperature and compared to patterns taken in the photostationary state obtained by exposing the surface to radiation from a high-intensity helium discharge lamp. Difference patterns were simulated by means of multiple-scattering calculations, which allowed us to determine the fraction of molecules that underwent isomerization.

Spin-dependent electron transport through a Mn-phthalocyanine molecule — A steady-state density functional theory (SS-DFT) study

We generalize the recently proposed steady-state density functional theory (SS-DFT) to spin-dependent cases and theoretically investigate the electronic and transport properties of a Mn-phthalocyanine molecule sandwiched between two graphene nanoribbon leads. The junction filters spin-up (minority spin) electrons while allowing spin-down (majority spin) electrons to pass with a filtering efficiency of about 99.5% at low biases. The spin-down electrons are found to tunnel through the junction via the HOMO orbital of the Mn-phthalocyanine molecule. Detailed analysis of the spin-dependent electron tunneling mechanism as well as the electronic/magnetic properties of the junction is presented.

Density Functional Theory for Steady-State Nonequilibrium Molecular Junctions

We present a density functional theory (DFT) for steady-state nonequilibrium quantum systems such as molecular junctions under a finite bias. Based on the steady-state nonequilibrium statistics that maps nonequilibrium to an effective equilibrium, we show that ground-state DFT (GS-DFT) is not applicable in this case and two densities, the total electron density and the density of current-carrying electrons, are needed to uniquely determine the properties of the corresponding nonequilibrium system. A self-consistent mean-field approach based on two densities is then derived. The theory is implemented into SIESTA computational package and applied to study nonequilibrium electronic/transport properties of a realistic carbon-nanotube (CNT)/Benzene junction. Results obtained from our steady-state DFT (SS-DFT) are compared with those of conventional GS-DFT based transport calculations. We show that SS-DFT yields energetically more stable nonequilibrium steady state, predicts significantly lower electric current, and is able to produce correct electronic structures in local equilibrium under a limiting case.

Toward understanding tautomeric switching in hydroxynaphthaldehydes: Characterization of electronic absorption spectra

Long-wavelength electronic absorption spectra of 4-hydroxy-1-naphthaldehyde, its dimer complexes, and 4-hydroxy-3-(piperidine-1-ylmethyl)-1-naphthaldehyde are investigated using time-dependent density functional theory with the TPSSh functional within a continuum solvation model. The results are correlated to recent experimental findings on solvent-, pH- and concentration-dependent absorption. It is confirmed that with decreasing wavelength the spectrum is dominated by the deprotonated (360 nm–400 nm), the dimer (340 nm–370 nm) and the monomer (< 280 nm) species. The potential use of hydroxynaphthaldehydes for the design of tautomeric switches is discussed.

Instability-induced pattern formation of photoactivated functional polymers

Since the pioneering work of Turing on the formation principles of animal coat patterns [Turing AM (1952) Phil Trans R Soc Lond B 237(641):37-72], such as the stripes of a tiger, great effort has been made to understand and explain various phenomena of self-assembly and pattern formation. Prominent examples are the spontaneous demixing in emulsions, such as mixtures of water and oil [Herzig EM, et al. (2007) Nat Mater 6:966-971]; the distribution of matter in the universe [Kibble TWB (1976) J Phys A: Math Gen 9(8):1387]; surface reconstruction in ionic crystals [Clark KW, et al. (2012) Nanotechnol 23(18):185306]; and the pattern formation caused by phase transitions in metal alloys, polymer mixtures and binary Bose-Einstein condensates [Sabbatini J, et al. (2011) Phys Rev Lett 107:230402]. Photoactivated pattern formation in functional polymers has attracted major interest due to its potential applications in molecular electronics and photoresponsive systems. Here we demonstrate that photoactivated pattern formation on azobenzene-containing polymer films can be entirely explained by the physical concept of phase separation. Using experiments and simulations, we show that phase separation is caused by an instability created by the photoactivated transitions between two immiscible states of the polymer. In addition, we have shown in accordance with theory, that polarized light has a striking effect on pattern formation indicated by enhanced phase separation.

Reconfigurable and actuating structures from soft materials

The recent interest in reconfigurable soft materials may lead to the next paradigm in the development of adaptive and actuating materials and structures. Actuating soft materials eventually can be precisely designed to show stimuli-sensing, multi-length scale actuation, tunable transport, programmed shape control and multifunctional orthogonal responses. Herein, we discuss the various advances in the emerging field of reconfigurable soft materials with a focus on the various parameters that can be modulated to control a complex system behavior. In particular, we detail approaches that use either long-range fields (i.e. electrical, magnetic) or changes in local thermodynamic parameters (e.g., solvent quality) in order to elicit a precise dimensional and controlled response. The theoretical underpinnings and practical considerations for different approaches are briefly presented alongside several illustrative examples from the recent studies. In the end, we summarize recent accomplishments, critical issues to consider, and give perspectives on the developments of this exciting research field.

Voltage-controlled nonvolatile molecular memory of an azobenzene monolayer through solution-processed reduced graphene oxide contacts

The solution-processed fabrication of an azobenzene (ABC10) monolayer-based nonvolatile memory device on a reduced graphene oxide (rGO) electrode is successfully accomplished. Trans--cis isomerizations of ABC10 between two rGO electrodes in a crossbar device are controlled by applied voltage. An rGO soft-contact top electrode plays an important role in the conformational-change-dependent conductance switching process of an ABC10 monolayer.