Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators

Why Are Dithienylethene-Linked Biscobaltocenes so Hard to Photoswitch?

Attaching an organometallic unit to a dithienylethene (DTE) molecular switch can allow one to vary its switching and spectroscopic properties, and to create switchable magnetic properties. In this work, two different dithienylethene molecular switches are used as a bridge between two cobalt sandwich units. The only difference between the switching cores is in the size of the cycloalkene ring connecting both thiophene rings. The complexes present different oxidation states for the cobalt atoms, which are demonstrated to determine the switching reaction. The UV/Vis measurements show that while the Co(I) complexes undergo the switching reaction, the Co(II,III) complexes switch poorly. Kohn-Sham density functional theory calculations indicate diabatic ring-closure mechanisms and a large number of excited states hindering the cyclization reaction and favoring the relaxation to the open form of the molecular switch.

Taking Orders from Light: Photo-Switchable Working/Inactive Smart Surfaces for Protein and Cell Adhesion

Photoresponsive smart surfaces are promising candidates for a variety of applications in optoelectronics and sensing devices. The use of light as an order signal provides advantages of remote and noninvasive control with high temporal and spatial resolutions. Modification of the photoswitches with target biomacromolecules, such as peptides, DNA, and small molecules including folic acid derivatives and sugars, has recently become a popular strategy to empower the smart surfaces with an improved detection efficiency and specificity. Herein, we report the construction of photoswitchable self-assembled monolayers (SAMs) based on sugar (galactose/mannose)-decorated azobenzene derivatives and determine their photoswitchable, selective protein/cell adhesion performances via electrochemistry. Under alternate UV/vis irradiation, interconvertible high/low recognition and binding affinity toward selective lectins (proteins that recognize sugars) and cells that highly express sugar receptors are achieved. Furthermore, the cis-SAMs with a low binding affinity toward selective proteins and cells also exhibit minimal response toward unselective protein and cell samples, which offers the possibility in avoiding unwanted contamination and consumption of probes prior to functioning for practical applications. Besides, the electrochemical technique used facilitates the development of portable devices based on the smart surfaces for on-demand disease diagnosis.

Chirality controlled responsive self-assembled nanotubes in water

The concept of using chirality to dictate dimensions and to store chiral information in self-assembled nanotubes in a fully controlled manner is presented. We report a photoresponsive amphiphile that co-assembles with its chiral counterpart to form nanotubes and demonstrate how chirality can be used to effect the formation of either micrometer long, achiral nanotubes or shorter (∼300 nm) chiral nanotubes that are bundled. The nature of these assemblies is studied using a variety of spectroscopic and microscopic techniques and it is shown that the tubes can be disassembled with light, thereby allowing the chiral information to be erased.

Arylazoindazole Photoswitches: Facile Synthesis and Functionalization via SNAr Substitution

A straightforward synthetic route to arylazoindazoles via nucleophilic aromatic substitution is presented. Upon deprotonation of the NH group, a C₆F₅-substituted formazan undergoes facile cyclization as a result of intermolecular nucleophilic substitution (S_NAr). This new class of azo photoswitches containing an indazole five-membered heterocycle shows photochemical isomerization with high fatigue resistance. In addition, the Z-isomers have long thermal half-lives in the dark of up to several days at room temperature. The fluorinated indazole group offers a handle for further functionalization and tuning of its properties, as it is shown to be susceptible to a subsequent, highly selective nucleophilic displacement reaction.

Light-Controlled Reversible Modulation of Frontier Molecular Orbital Energy Levels in Trifluoromethylated Diarylethenes

Among bistable photochromic molecules, diarylethenes (DAEs) possess the distinct feature that upon photoisomerization they undergo a large modulation of their π-electronic system, accompanied by a marked shift of the HOMO/LUMO energies and hence oxidation/reduction potentials. The electronic modulation can be utilized to remote-control charge- as well as energy-transfer processes and it can be transduced to functional entities adjacent to the DAE core, thereby regulating their properties. In order to exploit such photoswitchable systems it is important to precisely adjust the absolute position of their HOMO and LUMO levels and to maximize the extent of the photoinduced shifts of these energy levels. Here, we present a comprehensive study detailing how variation of the substitution pattern of DAE compounds, in particular using strongly electron-accepting and chemically stable trifluoromethyl groups either in the periphery or at the reactive carbon atoms, allows for the precise tuning of frontier molecular orbital levels over a broad energy range and the generation of photoinduced shifts of more than 1 eV. Furthermore, the effect of different DAE architectures on the transduction of these shifts to an adjacent functional group is discussed. Whereas substitution in the periphery of the DAE motif has only minor implications on the photochemistry, trifluoromethylation at the reactive carbon atoms strongly disturbs the isomerization efficiency. However, this can be overcome by using a nonsymmetrical substitution pattern or by combination with donor groups, rendering the resulting photoswitches attractive candidates for the construction of remote-controlled functional systems.

4,7-Bis[3-(dimesitylboryl)thien-2-yl]benzothiadiazole: Solvato-, Thermo-, and Mechanochromism Based on the Reversible Formation of an Intramolecular B-N Bond

4,7-Bis-[3-(dimesitylboryl)thien-2-yl]benzothiadiazole (1) and monoborylated derivative 2 were synthesized and their chromic behavior was investigated. Photophysical measurements, single-crystal XRD analysis, and theoretical calculations revealed that an intramolecular B-N coordination bond formed reversibly. The equilibrium of this reversible bond formation depends on the solid-state structure, solvent, temperature, and mechanical forces, and leads to significant changes in the electronic structure and chromic behavior of these molecules. The responsiveness toward external stimuli, resulting in the reversible formation of open and closed forms of this system, is achieved through weak intramolecular B-N coordination bonds induced by the steric bulk of the mesityl groups on the boron centers.

Substituent effect in the photochromism of two isomeric asymmetric diarylethenes having pyrrole and thiophene units

Two new asymmetric isomeric diarylethenes having pyrrole and thiophene units have been synthesized by one-pot reaction and characterized by single crystal X-ray diffraction analysis. The two prepared diarylethenes had disparate crystal structures, and they exhibited distinctly different photochromic behavior, both in solution and in the solid state. Their photochromism, fatigue resistance, and fluorescence were investigated systematically. The methyl group at the reactive carbon atom could significantly enhance the quantum yield of cyclization step and decrease the quantum yield of cycloreversion step, whereas a cyano group at the same position could notably suppress the photocyclization reaction and promote the photocycloreversion reaction. The results indicated that the substituent at the reactive carbon atom could readily modulate the optoelectronic and physical properties for these diarylethenes.

Photochromic benzo[b]phosphole oxide with excellent thermal irreversibility and fatigue resistance in the thin film solid state via direct attachment of dithienyl units to the weakly aromatic heterocycle

A novel photochromic benzo[b]phosphole oxide has been demonstrated to display photochromic properties with excellent fatigue resistance and thermal irreversibility in polymethylmethacrylate (PMMA) thin film under ambient conditions. The remarkable photochromic behaviour can be achieved by rational molecular design, in which the weakly aromatic phosphole oxide is directly incorporated into the photo-responsive dithienylethene units. Photopatterning via photoinduced colouration and decolouration has been performed to demonstrate the repeatable and distinct transformation between the bistable states, making it a promising candidate with photoswitching properties for optoelectronic applications.

Deliberate Switching of Single Photochromic Triads

Photochromic molecules can be reversibly converted between two bistable conformations by light, and are considered as promising building blocks in novel macromolecular structures for sensing and imaging techniques. We have studied individual molecular triads consisting of two strong fluorophores (perylene bisimide) that are covalently linked via a photochromic unit (dithienylcyclopentene) and distinguished between deliberate switching and spontaneous blinking. It was verified that the probability for observing deliberate light-induced switching of a single triad (rather than stochastic blinking) amounts to 0.8 ± 0.1. In a few exceptional cases this probability can exceed 0.95. These numbers are sufficiently large for application in sensitive biosensing, and super-resolution imaging. This opens the possibility to develop devices that can be controlled by an external optical stimulus on a truly molecular length scale.

Determining the Photoisomerization Quantum Yield of Photoswitchable Molecules in Solution and in the Solid State

Photoswitchable molecules are able to isomerize between two metastable forms through light stimuli. Originally being studied by photochemists, this type of molecule has now found a wide range of applications within physics, chemistry and biology. The extensive usage of photochromic molecules is due to the two isomers having fundamentally different physical and chemical properties. The most important attribute of a photoswitch is the photoisomerization quantum yield, which defines the efficiency of the photoisomerization event. Here we show how to determine the photoisomerization quantum yield in the solid state and in solution when taking thermal processes into account. The described method together with provided software allows for rapid and accurate determination of the isomerization process for this important class of molecules.

"Though It Be but Little, It Is Fierce": Excited State Engineering of Conjugated Organic Materials by Fluorination

Fluorination is frequently used to significantly change the properties of conjugated organic materials due to fluorine's exceptional properties; well-known is its impact on electronic structure, but it also impacts the geometry despite fluorine's small size. Less known, the changes in the electronic and geometrical properties may provoke drastic changes of the excited state properties like batho- and hypsochromic shifts of absorption and emission bands (inter alia leading to excited state switching), hypo- and hyperchromic effects, spectral broadening, and changes of the nonradiative deactivation pathways. The state of the art on these issues is summarized in the current Perspective to stimulate further discussions on this intriguing subject.

Photochromism and molecular logic gate operation of a water-compatible bis-glycosyl diarylethene

The synthesis of a bis-glycosyl diarylethene derivative by click chemistry for the water-compatible photochromism and operation of molecular logic gates is reported.

A fluorescent photochromic diarylethene based on naphthalic anhydride with strong solvatochromism

A diarylethene molecule consisting of an 1,8-naphthalic anhydride and two 2,5 dimethylthiophene which exhibits reversible fluorescence switching capacity as well as solvatochromism with red shift of the fluorescence maximum by more than 150 nm.

Fluorescent detection of multiple ions by two related chemosensors: structural elucidations and logic gate applications

Two related chemosensors L1 and L2 display selective detection of multiple ions (Cu²⁺, Al³⁺, Cd²⁺ and S²⁻) as a result of minor variation of functional groups at a remote arene ring.

Photochromic histone deacetylase inhibitors based on dithienylethenes and fulgimides

The synthesis, photochromic properties, inhibition of different HDACs and corresponding molecular dockings of photochromic inhibitors are described.

A photochromic prototype based on difurylperhydrocyclopentene with remarkable photoswitching behavior and in vivo application

A furan-based photochromic prototype with remarkable photoswitching behavior was facilely synthesized and applied in vivo.

Switching of the π-electronic conjugations in the reduction of a dithienylethene-fused p-benzoquinone

Visible light unlocks the π-electronic conjugation of a dithienylethene-fused p-benzoquinone derivative to cause a light-driven oxidation reaction.

Ab initio molecular dynamics of thiophene: the interplay of internal conversion and intersystem crossing

Ab initio on-the-fly molecular dynamics reveals that excited thiophene decays via low lying conical intersections and via intersystem crossing. Open-ring structures are responsible for the observed long life times.

Color change of alternating copolymers with phenyl vinylethylene carbonate and N-phenylmaleimide in a solution and in the solid-state, depending on their structure

The alternating PVEC and PMI copolymers with various composition ratios exhibited reversible color changes such as halochromism in solution and in the solid-state.

Photocyclization of photoswitches with high enantioselectivity in human serum albumin in an artificial environment

Three photochromic bisthienylethenes exhibited 56 to >99% enantiomeric excess in photochemical ring closure upon UV irradiation when incorporated in human serum albumin dissolved in 15% acetonitrile-phosphate buffer solution and incubated for 24 h at −4 °C.

Wavelength-selective and high-contrast multicolour fluorescence photoswitching in a mixture of photochromic nanoparticles

Wavelength-selective and high-contrast multicolour fluorescence photoswitching was successfully demonstrated based on the nonlinear fluorescence quenching in photochromic nanoparticles.

Lone pair-π interaction-induced generation of photochromic coordination networks with photoswitchable conductance

Lone pair-π interaction-induced variation of the degree of charge-transfer was successfully used for switching the conductance of a photochromic coordination network.

Reconfigurable photonic crystals with optical bistability enabled by “cold” programming and thermo-recoverable shape memory polymers

A type of reconfigurable photonic crystals with optically bistable states enabled by capillary pressure-induced programming and heat-caused recoverable shape memory polymers was reported.

Electrochemical control of the switching process of photochromic dimethyldihydropyrene derivatives

The electrochemical control of the isomerization process of a photo switchable dimethyldihydropyrene (DHP) derivative has been investigated.

A fluorescent chemosensor for Sn2+ and Cu2+ based on a carbazole-containing diarylethene

A carbazole-containing diarylethene was synthesized. Its fluorescence was enhanced and quenched, respectively, upon addition of Sn²⁺ and Cu²⁺ over other cations. Moreover, the compound can be applied to detect Sn²⁺ and Cu²⁺ in natural water sample.

Switchings of dielectric constant, second harmonic generation and polarization in a polar hybrid cyanometallate crystal

A polar-to-polar phase transition in a cyanometallate crystal (TA)₂{(H₃O)[Co(CN)₆]} (TA = thiazolium) triggers multiple switchings of the dielectric constant, second harmonic generation and polarization.

Acid-catalysed thermal cycloreversion of a diarylethene: a potential way for triggered release of stored light energy?

The light energy stored in a closed diarylethene can efficiently be released upon addition of catalytic amounts of acid.

Laser triggered phase transition in photothermal liquid crystals

Laser irradiation allows rapid high temperature phase transition directly from room temperature in pure liquid crystalline materials built around photothermal cores with a high spatial control and responsivness.

Visible-light responsive hydrogen-bonded supramolecular polymers based on ortho-tetrafluorinated azobenzene

Visible-light responsive hydrogen-bonded supramolecular polymers have been fabricated, whose properties could be regulated through the visible-light-triggered photoisomerization of the ortho-tetrafluorinated azobenzene.

Exploring the diethylaminoflavylium derivatives multistate system of chemical reactions in the presence of CTAB micelles: thermodynamic reversibility achieved through different kinetic pathways

The flavylium cation and trans-chalcone species are thermodynamically stable at pH 0.5 and 6.0, respectively, switching reversibly between these two pH values. However, the thermodynamic stability is achieved through different pathways depending on whether pH is increasing or decreasing.

Laser Patterning of Optically Reconfigurable Transistor Channels in a Photochromic Diarylethene Layer

Optical switching organic field-effect transistors (OFETs) provide a new direction for optoelectronics based on photochromic molecules. However, the patterning of OFETs is difficult because conventional fabrication processes, including lithography and ion etching, inevitably cause severe damage to organic molecules. Here, we demonstrate laser patterning of one-dimensional (1D) channels on an OFET with a photochromic diarylethene (DAE) layer. The main findings are (i) a number of 1D channels can be repeatedly written and erased in the DAE layer by scanning focused ultraviolet and visible light laser beams alternately between the source and drain electrodes, (ii) the conductivity (or resistivity) of the 1D channel can be controlled by the illumination conditions, such as the laser power density and the scan speed, and (iii) it is possible to draw an analogue adder circuit by optically writing 1D channels so that a portion of the channels overlaps and to perform optical summing operations by local laser illumination of the respective channels. These findings will open new possibilities for realizing various optically reconfigurable, low-dimensional organic transistor circuits, which are not possible with conventional thin film OFETs.

Bidirectional Photomodulation of Surface Tension in Langmuir Films

Switching systems operating in a cooperative manner capable of converting light energy into mechanical motion are of great interest for optical devices, data storage, nanoscale energy converters and molecular sensing. Herein, photoswitchable monolayers were formed at the air-water interface from either a pure bis(thiaxanthylidene)-based photoswitchable amphiphile or from a mixture of the photoswitchable amphiphile with a conventional lipid dipalmitoylphosphatidylcholine (DPPC). Efficient photoisomerization of the anti-folded to syn-folded geometry of the amphiphile's central core induces changes in the surface pressure in either direction, depending on the initial molecular density. Additionally, the switching behavior can be regulated in the presence of DPPC, which influences the packing of the molecules, thereby controlling the transformation upon irradiation. Bis(thiaxanthylidene)-based photoswitchable monolayers provide a promising system to explore cooperativity and amplification of motion.

Suppressing photochemical reactions with quantized light fields

Photoisomerization, that is, a photochemical reaction leading to a change of molecular structure after absorption of a photon, can have detrimental effects such as leading to DNA damage under solar irradiation, or as a limiting factor for the efficiency of solar cells. Here, we show that strong coupling of organic molecules to a confined light mode can be used to strongly suppress photoisomerization, as well as other photochemical reactions, and thus convert molecules that normally show fast photodegradation into photostable forms. We find this to be especially efficient in the case of collective strong coupling, where the distribution of a single excitation over many molecules and the light mode leads to a collective protection effect that almost completely suppresses the photochemical reaction.

Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications

Light-driven phenomena both in living systems and nonliving materials have enabled truly fascinating and incredible dynamic architectures with terrific forms and functions. Recently, liquid crystalline materials endowed with photoresponsive capability have emerged as enticing systems. In this Review, we focus on the developments of light-driven liquid crystalline materials containing photochromic components over the past decade. Design and synthesis of photochromic liquid crystals (LCs), photoinduced phase transitions in LC, and photoalignment and photoorientation of LCs have been covered. Photomodulation of pitch, polarization, lattice constant and handedness inversion of chiral LCs is discussed. Light-driven phenomena and properties of liquid crystalline polymers, elastomers, and networks have also been analyzed. The applications of photoinduced phase transitions, photoalignment, photomodulation of chiral LCs, and photomobile polymers have been highlighted wherever appropriate. The combination of photochromism, liquid crystallinity, and fabrication techniques has enabled some fascinating functional materials which can be driven by ultraviolet, visible, and infrared light irradiation. Nanoscale particles have been incorporated to widen and diversify the scope of the light-driven liquid crystalline materials. The developed materials possess huge potential for applications in optics, photonics, adaptive materials, nanotechnology, etc. The challenges and opportunities in this area are discussed at the end of the Review.

Conditional repair by locally switching the thermal healing capability of dynamic covalent polymers with light

Healable materials could play an important role in reducing the environmental footprint of our modern technological society through extending the life cycles of consumer products and constructions. However, as most healing processes are carried out by heat alone, the ability to heal damage generally kills the parent material's thermal and mechanical properties. Here we present a dynamic covalent polymer network whose thermal healing ability can be switched 'on' and 'off' on demand by light, thereby providing local control over repair while retaining the advantageous macroscopic properties of static polymer networks. We employ a photoswitchable furan-based crosslinker, which reacts with short and mobile maleimide-substituted poly(lauryl methacrylate) chains forming strong covalent bonds while simultaneously allowing the reversible, spatiotemporally resolved control over thermally induced de- and re-crosslinking. We reason that our system can be adapted to more complex materials and has the potential to impact applications in responsive coatings, photolithography and microfabrication.