Switching Diarylethenes Reliably in Both Directions with Visible Light

In vivo photopharmacological inhibition of hippocampal activity via multimodal probes - perspective and opening steps on experimental and computational challenges

Neurological conditions such as epilepsy can have a significant impact on people's lives. Here, we discuss a new perspective for the study/treatment of these conditions using photopharmacology. A multimodal, intracranial implant that incorporates fluidic channels for localised drug delivery, electrodes for recording and stimulation, and a light source for photoswitching is used for in vivo administration and deactivation of a photoresponsive AMPA antagonist. We review current advancements in the relevant disciplines and show experimentally that the inhibition of seizure-like events induced in the hippocampus by electrical stimulation can be altered upon switching the drug with light. We discuss the interconnection of the drug's photopharmacological properties with the design of the device by modelling light penetration into the rat brain with Monte Carlo simulations. This work delivers a new perspective, including initial experimental and computational efforts on in vivo photopharmacology to understand and eventually treat neurological conditions.

Triplet-Triplet Energy Transfer: A Simple Strategy for an Efficient Visible Light-Induced Photoclick Reaction

Photoclick reactions combine the advantages offered by light-driven processes and classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photo-crosslinking, and protein labeling. Despite these advances, the dependency of most of the photoclick reactions on UV light poses a severe obstacle for their general implementation, as this light can be absorbed by other molecules in the system resulting in their degradation or unwanted reactivity. However, the development of a simple and efficient system to achieve bathochromically shifted photoclick transformations remains challenging. Here, we introduce triplet-triplet energy transfer as a fast and selective way to enable visible light-induced photoclick reactions. Specifically, we show that 9,10-phenanthrenequinones (PQs) can efficiently react with electron-rich alkenes (ERAs) in the presence of a catalytic amount (as little as 5 mol %) of photosensitizers. The photocycloaddition reaction can be achieved under green (530 nm) or orange (590 nm) light irradiation, representing a bathochromic shift of over 100 nm as compared to the classical PQ-ERAs system. Furthermore, by combining appropriate reactants, we establish an orthogonal, blue and green light-induced photoclick reaction system in which the product distribution can be precisely controlled by the choice of the color of light.

All-visible-light-driven stiff-stilbene photoswitches

Molecular photoswitches are potent tools to construct dynamic functional systems and responsive materials that can be controlled in a non-invasive manner. As P-type photoswitches, stiff-stilbenes attract increasing interest, owing to their superiority in quantum yield, significant geometric differences between isomers, excellent thermostability and robust switching behavior. Nevertheless, the UV-light-triggered photoisomerization of stiff-stilbenes has been a main drawback for decades as UV light is potentially harmful and has low penetration depth. Here, we provided a series of para-formylated stiff-stilbenes by Rieche ortho-formylation to achieve all-visible-light-responsiveness. Additional phenolic groups provide access to late-stage chemical modification facilitating design of molecules responsive to visible light. Remarkably, the photoisomerization of aldehyde-appended stiff-stilbenes could be fully manipulated using visible light, accompanied by a high photostationary state (PSS) distribution. These features render them excellent candidates for future visible-light-controllable smart materials and dynamic systems.

Amplifying dual-visible-light photoswitching in aqueous media via confinement promoted triplet-triplet energy transfer

Achieving visible-light photochromism is a long-term goal of chemists keen to exploit the opportunities of molecular photoswitches in multi-disciplinary research studies. Triplet-sensitization offers a flexible approach to building diverse visible-light photoswitches using existing photochromic scaffolds, circumventing the need for sophisticated molecular design and synthesis. Unfortunately, distance-dependence and environment-sensitivity of triplet-excited species remain as key challenges that severely impair sensitization efficiency and limit their practical availability. We present herein a nature-inspired nanoconfinement strategy in which a triplet-sensitized visible-light photoswitch/sensitizer system is assembled into nanoconfined micelles (d ∼ 40 nm). A ca. 10-fold efficiency increase of triplet-triplet energy transfer for photochromism as well as an amplified fluorescence on/off contrast upon bi-directional visible-light excitation (470/560 nm) was achieved in full aqueous media. By virtue of this, the hybrid photoswitchable system is successfully applied for both flash information encryption and multiple dynamic cell imaging assays, further proving its versatility in materials and life science.

Reversible C═N Bond Formation Controls Charge-Separation in an Aza-Diarylethene Photoswitch

Diarylethenes belong to the most eminent photoswitches and have been studied for many decades. They are found in virtually every field of application and have become highly valuable molecular tools for instilling light-responsiveness into materials, catalysts, biological systems, or pharmacology. In this work, we present a novel and distinct type of pyrimidine-based aza-diarylethene, which undergoes a highly unusual zwitterion-forming photoreaction. During this fully reversible process, a CN double bond is established under concomitant aromatization and thiophene-ring opening. The metastable zwitterion thus possesses a positively charged extended aromatic structure and an independent conjugated thiolate function. It can further photoisomerize between a more stable Z and a less stable E isomer, resulting in effective four-state photoswitching. Unusual for diarylethenes, the metastable isomers show negative solvatochromism and red-shifted absorption in apolar solvents. With this behavior, aza-diarylethenes effectively bridge the properties of merocyanines and diarylethenes. Thermal stability of the zwitterions can be modulated from very labile to highly stable behavior in response to pH, again in a fully reversible manner. Pyrimidine-based aza-diarylethene thus establishes a unique photoreaction mechanism for diarylethenes, allowing control of charge separation, thermal stability, and color generation in a different way than hitherto possible.

Towards Optical Information Recording: A Robust Visible-Light-Driven Molecular Photoswitch with the Ring-Closure Reaction Yield Exceeding 96.3

Diarylethene molecular photoswitches hold great fascination as optical information materials due to their unique bistability and exceptional reversible photoswitching properties. Conventional diarylethenes, however, rely on UV light for ring-closure reactions, typically with modest yields. For practical application, diarylethenes driven by visible lights are preferred but achieving high ring-closure reaction yield remains a significant challenge. Herein, we synthesized a novel all-visible-light-driven photoswitch, TPAP-DTE, by facilely endcapping the dithienylethene (DTE) core with triphenylamine phenyl (TPAP) groups. Owing to the electron-donating conjugation effect of TPAP, the open-form TPAP-DTE responds strongly to short-wavelength visible lights with considerable photocyclization quantum yields and molar absorption coefficient. Upon 405 nm visible-light irradiation, TPAP-DTE achieves a ring-closure reaction yield exceeding 96.3 % (confirmed by both nuclear magnetic resonance spectroscopy and high-performance liquid chromatography). Its ring-opening reaction yield is 100 % upon irradiation with long-wavelength visible light. TPAP-DTE could be regarded as a bidirectional "quasi"-quantitative conversion molecular switch. Furthermore, TPAP-DTE exhibits robust fatigue resistance over 100 full photoswitching cycles and great anti-aging property under 85 °C and 85 % humidity for at least 1000 h. Consequently, its rewritable QR-code, multilevel data storage, and anti-counterfeiting/encryption applications are successfully demonstrated exclusively using visible lights, positioning TPAP-DTE as a highly promising medium for information recording.

Deducing the conformational space for an octa-proline helix

A molecular dyad, PY-P8-PER, comprising a proline octamer sandwiched between pyrene and perylene terminals has been synthesized in order to address the dynamics of electronic energy transfer (EET) along the oligo-proline chain. A simple pyrene-based control compound equipped with a bis-proline attachment serves as a reference for spectroscopic studies. The N-H NMR signal at the terminal pyrene allows distinction between cis and trans amides and, although the crystal structure for the control has the trans conformation, temperature-dependent NMR studies provide clear evidence for trans/cis isomerisation in D6-DMSO. Polar solvents tend to stabilise the trans structure for the pyrene amide group, even for longer oligo-proline units. Circular dichroism shows that the proline spacer for PY-P8-PER exists mainly in the all-trans geometry in methanol. Preferential excitation of the pyrene chromophore is possible at wavelengths in the 320-350 nm range and, for the dyad, is followed by efficacious EET to the perylene emitter. The probability for intramolecular EET, obtained from analysis of steady-state spectroscopic data, is ca. 80-90% in solvents of disparate polarity. Comparison with the Förster critical distance suggests the terminals are ca. 18 Å apart. Time-resolved fluorescence spectroscopy, in conjunction with DFT calculations, indicates the dyad exists as a handful of conformers displaying a narrow range of EET rates. Optimisation of a distributive model allows accurate simulation of the EET dynamics in terms of reasonable structures based on isomerisation of certain amide groups.

Optical modulation of cell nucleus penetration and singlet oxygen release of a switchable platinum complex

The activation of anticancer molecules with visible light constitutes an elegant strategy to target tumors and to improve the selectivity of treatments. In this context, we report here a visible-light activatable bis-platinum complex (DHP-Pt2) incorporating an organic photo-switchable ligand based on the dimethyldihydropyrene moiety. Illumination of this metal complex with red light (660 nm) under air readily produces the corresponding endoperoxide form (CPDO2-Pt2). These two metal complexes exhibit different DNA binding properties and, more importantly, we show that only the photogenerated CPDO2-Pt2 is able to penetrate into cancer cell nuclei, where it is then capable of releasing cytotoxic singlet oxygen. This study represents the first proof-of-concept showing that dimethyldihydropyrene derivatives can be used to transport and deliver singlet oxygen into cancer cell nuclei upon visible-light activation.

Internal Heavy-Atom Effect on Visible-Light-Induced Cyclization Reaction in Diarylethene-Perylenebisimide Dyads

All-visible-light switchable diarylethene-perylenebisimide (DAE-PBI) dyads having bromine heavy atoms in the molecule were designed and synthesized. Very recently, we found a unique visible-light-induced cyclization reaction in a DAE-PBI dyad. The dyad exhibited reversible cyclization and cycloreversion reactions upon alternate irradiation with green (500-550 nm) and red (>600 nm) light. From the experimental results, it was suggested that the triplet state of DAE unit was generated via multiplicity conversion based on intramolecular energy transfer from the singlet excited state of PBI unit and that the cyclization reaction of DAE unit proceeded from the triplet state. In addition, it was revealed that the reactivity remarkably increased in a solvent containing heavy atoms such as carbon tetrachloride and iodoethane (i.e., external heavy-atom effect). Based on such results, in this study, we attempted to design and synthesize novel DAE-PBI dyads introducing bromine heavy atoms at different positions in the molecule. The synthesized dyads exhibited higher quantum yields of photocyclization reaction under visible-light irradiation even in a heavy-atom-free solvent compared to the previous dyad having no heavy atoms. The magnitude of enhancement well correlated to the contribution ratio of atomic orbital of bromine to the molecular orbital in LUMOs. These results indicated that the internal heavy atom effectively contributed to the visible-light-induced cyclization reaction in DAE-PBI dyads. Such an internal heavy-atom effect will pave the way for new molecular design to develop all-visible-light-activatable molecular switches.

Cu(OTf)2-Phosphoric Acid-Catalyzed Tandem Oxa-Nazarov Cyclization and Dibromination of Conjugated 1,2-Diketones

A facile tandem oxa-Nazarov cyclization and dibromination has been developed. The combination of Cu(OTf)2 and diphenyl phosphate (DPP-H) was found to synergistically promote the coupling of conjugated 1,2-diketones and N-bromosuccinimide to form 2,4-dibromo-3(2H)-furanones in good yields.

Triplet sensitization enables bidirectional isomerization of diazocine with 130 nm redshift in excitation wavelengths

Diazocines are bridged azobenzenes with phenyl rings connected by a CH2-CH2 group. Despite this rather small structural difference, diazocine exhibits improved properties over azobenzene as a photoswitch and most importantly, its Z configuration is more stable than the E isomer. Herein, we reveal yet another unique feature of this emerging class of photoswitches. In striking contrast to azobenzenes and other photochromes, diazocine can be selectively switched in E → Z direction and most intriguingly from its thermodynamically stable Z to metastable E isomer upon successive excitation of two different triplet sensitizers present in solution at the same time. This approach leads to extraordinary large redshift of excitation wavelengths to perform isomerization i.e. from 400 nm blue to 530 nm green light (Z → E) and from 530 nm green to 740 nm far-red one (E → Z), which falls in the near-infrared window in biological tissue. Therefore, this work opens up of potential avenues for utilizing diazocines for example in photopharmacology, smart materials, light energy harvesting/storage devices, and out-of-equilibrium systems.

Diarylethene Isomerization by Using Triplet-Triplet Annihilation Photon Upconversion

Green-to-blue triplet-triplet annihilation photon upconversion with the well-studied upconversion pair 9,10-diphenylanthracene (DPA)/platinum octaethylporphyrin (PtOEP) was used to reversibly drive the photoisomerization of diarylethene (DAE) photoswitches by using visible light. By carefully selecting the kinetic and spectral properties of the molecular system as well as the experimental geometry, a single green light source can be used to selectively trigger both the ring-opening and the ring-closing reactions, whilst also inducing fluorescence from the colored closed isomer that can be used as a readout to monitor the isomerization process in situ. The upconversion solution and the DAE solution are kept physically separated, allowing them to be characterized both concomitantly and individually without further separation processes. The ring-closing reaction using upconverted photons was quantified and compared to the efficiency of direct isomerization with ultraviolet light.

Heterocyclic Hemipiperazines: Water-Compatible Peptide-Derived Photoswitches

Hemipiperazines are a recently discovered class of peptide-derived molecular photoswitches with high biocompatibility and therapeutic potential. Here, for the first time we describe photochromism of heterocyclic hemipiperazines. They demonstrate long thermal lifetimes, and enlarged band separation between photoisomers. Efficient photoisomerization occurs under aqueous conditions, although with a need for organic co-solvent. Bidirectional switching with visible light is observed for an extended aromatic system.

All Visible Light Photoswitch Based on the Dimethyldihydropyrene Unit Operating in Aqueous Solutions with High Quantum Yields

Molecular systems and devices whose properties can be modulated using light as an external stimulus are the subject of numerous research studies in the fields of materials and life sciences. In this context, the use of photochromic compounds that reversibly switch upon light irradiation is particularly attractive. However, for many envisioned applications, and in particular for biological purposes, illumination with harmful UV light must be avoided and these photoactivable systems must operate in aqueous media. In this context, we have designed a benzo[e]-fused dimethyldihydropyrene compound bearing a methyl-pyridinium electroacceptor group that meets these requirements. This compound (closed state) is able to reversibly isomerize under aerobic conditions into its corresponding cyclophanediene form (open isomer) through the opening of its central carbon-carbon bond. Both the photo-opening and the reverse photoclosing processes are triggered by visible light illumination and proceed with high quantum yields (respectively 14.5% yield at λ = 680 nm and quantitative quantum yield at λ = 470 nm, in water). This system has been investigated by nuclear magnetic resonance and absorption spectroscopy, and the efficient photoswitching behavior was rationalized by spin-flip time-dependent density functional theory calculations. In addition, it is demonstrated that the isomerization from the open to the closed form can be electrocatalytically triggered.

Fast Screening of Minimum Energy Crossing Points with Semiempirical Tight-Binding Methods

The investigation of photochemical processes is a highly active field in computational chemistry. One research direction is the automated exploration and identification of minimum energy conical intersection (MECI) geometries. However, due to the immense technical effort required to calculate nonadiabatic potential energy landscapes, the routine application of such computational protocols is severely limited. In this study, we will discuss the prospect of combining adiabatic potential energy surfaces from semiempirical quantum mechanical calculations with specialized confinement potential and metadynamics simulations to identify S₀/T₁ minimum energy crossing point (MECP) geometries. It is shown that MECPs calculated at the GFN2-xTB level can provide suitable approximations to high-level S₀/S₁ab initio conical intersection geometries at a fraction of the computational cost. Reference MECIs of benzene are studied to illustrate the basic concept. An example application of the presented protocol is demonstrated for a set of photoswitch molecules.

All-Visible (>500 nm)-Light-Induced Diarylethene Photochromism Based on Multiplicity Conversion via Intramolecular Energy Transfer

Photoswitching molecules that reversibly switch upon visible-light irradiation are some of the most attractive targets for biological and imaging applications. In this study, we found a diarylethene (DAE) derivative having a covalently attached perylenebisimide (PBI) unit (DAE-PBI dyad) underwent an unexpected cyclization reaction upon irradiation with green (500-550 nm) light, where the DAE unit has no absorbance. The photoreactivity was enhanced in solvents containing heavy atoms and in the presence of oxygen. As inferred from the solvent dependence and the calculated excited-state energies of DAE and PBI units, it was suggested that the probable mechanism for this unique visible-light-induced cyclization reaction is multiplicity conversion based on intramolecular energy transfer from the excited singlet state of the PBI unit to the triplet state of DAE units (i.e., DAE-¹[PBI]* → ³[DAE]*-PBI). Such a unique photoreaction mechanism with the assistance of oxygen will pave the way for new molecular design for the development of visible-light switching molecules.

Photoswitching of Local (Anti)Aromaticity in Biphenylene-Based Diarylethene Molecular Switches

Photoinduced tuning of (anti)aromaticity and associated molecular properties is currently in the focus of attention for both tailoring photochemical reactivity and designing new materials. Here, we report on the synthesis and spectroscopic characterization of diarylethene-based molecular switches embedded in a biphenylene structure composed of rings with different levels of local (anti)aromaticity. We show that it is possible to modulate and control the (anti)aromatic character of each ring through reversible photoswitching of the aryl units of the system between open and closed forms. Remarkably, it is shown that the irreversible formation of an annulated bis(dihydro-thiopyran) side-product that hampers the photoswitching can be efficiently suppressed when the aryl core formed by thienyl groups in one switch is replaced by thiazolyl groups in another.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Reversible Near-Infrared Fluorescence Photoswitching in Aqueous Media by Diarylethene: Toward High-Accuracy Live Optical Imaging

Fluorescence imaging is an indispensable tool in modern biological research, allowing simple and inexpensive color-coded visualizations of real-time events in living cells and animals, as well as of fixed states of ex vivo specimens. The accuracy of fluorescence imaging in living systems is, however, impeded by autofluorescence, light scattering, and limited penetration depth of light. Nevertheless, the clinical use of fluorescence imaging is expected to grow along with advances in imaging equipment, and will increasingly demand high-accuracy probes to avoid false-positive results in disease detection. To this end, a water-soluble and relatively safe diarylethene (DAE)-based reversible near-infrared (NIR) fluorescence photoswitch for living systems is prepared here. Furthermore, to facilitate excellent switching performance, the photoirradiation results obtained is compared using three different visible light sources to turn on NIR fluorescence through cycloreversion of DAE. While photoswitching using 589 nm light leads to slightly higher cell viability, fluorescence quenching efficiency and fatigue resistance are higher when 532 nm light with low photobleaching is used in both aqueous solution and living systems. The authors anticipate that their reversible NIR fluorescence photoswitch mediated by DAE can be beneficial for fluorescence imaging in aqueous media requiring accurate detection, such as in the autofluorescence-rich living environment.

Multi-photoresponsive triphenylethylene derivatives with photochromism, photodeformation and room temperature phosphorescence

A series of triphenylethylene derivatives exhibited multi-photoresponsive properties, including photochromism, photodeformation and room temperature phosphorescence (RTP), which are strongly related to molecular conformations and packing in the aggregated states. Accordingly, these properties can be subtly adjusted by substituents to the center double bond and/or peripheral phenyl moieties. The introduction of bromine atom was beneficial to photochromism and photodeformation properties as a result of the additional C-H⋯Br interactions and electron-withdrawing property. Also, it can promote the RTP effect via heavy atom effect, resulting in persistent afterglow lasting up to 150 min as detected by chemiluminescent imaging system.

Visible Light-Driven Molecular Switches and Motors: Recent Developments and Applications

Inspired by human vision, a diverse range of light-driven molecular switches and motors has been developed for fundamental understanding and application in material science and biology. Recently, the design and synthesis of visible light-driven molecular switches and motors have been actively pursued. This emerging trend is partly motivated to avoid the harmful effects of ultraviolet light, which was necessary to drive the classical molecular switches and motors at least in one direction, impeding their employment in biomedical and photopharmacology applications. Moreover, visible light-driven molecular switches and motors are demonstrated to enable benign optical materials for advanced photonic devices. Therefore, during the past several years, visible light-driven molecular switches based on azobenzene derivatives, diarylethenes, 1,2-dicyanodithienylethenes, hemithioindigo derivatives, iminothioindoxyls, donor-acceptor Stenhouse adducts, and overcrowded alkene based molecular motors have been judiciously designed, synthesized, and used in the development of functional materials and systems for a wide range of applications. In this Review, we present the recent developments toward the design of visible light-driven molecular switches and motors, with their applications in the fabrication of functional materials and systems in material science, bioscience, pharmacology, etc . The visible light-driven molecular switches and motors realized so far undoubtedly widen the scope of these interesting compounds for technological and biological applications. We hope this Review article could provide additional impetus and inspire further research interests for future exploration of visible light-driven advanced materials, systems, and devices.

A visible light-controllable Rho kinase inhibitor based on a photochromic phenylazothiazole

Rho-associated coiled-coil-containing protein kinase (ROCK) is a serine-threonine kinase whose inhibitors are useful for the regulation of the actomyosin system. Here, we developed a photoswitchable ROCK inhibitor based on a phenylazothiazole scaffold. The reversible trans-cis isomerization by visible light stimuli enabled us to manipulate ROCK activities in vitro and in cells.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Determining Factor of the Quantum Yield of the Cyclization Reaction via Triplet States for Dye-Attached Diarylethene

The difference in the quantum yields of the cyclization reaction of two isomers of dye-attached diarylethene via the triplet state observed in the experiment [J. Phys. Chem. C 2009, 113, 11623-11627] was theoretically examined by quantum chemical calculations. By evaluating the spin-orbit couplings, we found that the ratio of the rate constants from the S₁ state to the T₂ state between two isomers agreed with that of the experimental quantum yield of the cyclization reaction. The difference in the spin-orbit couplings is due to the difference in the delocalization of the orbitals between diarylethene and dye. We further found that after the intersystem crossing took place the cyclization reaction via the triplet state occurred in the experiment due to the low energy barrier (∼10 kcal/mol) for the reaction.

Encapsulation and Stabilization of a Donor-Acceptor Stenhouse Adduct Isomer in Water Inside the Blue Box: A Combined Experimental and Theoretical Approach

We synthesized two types of donor-acceptor Stenhouse adducts (DASAs), a new type of photochromic molecules showing dual color in two different isomeric forms in solution phase, using Meldrum acid (DASA-Mel) and barbituric acid (DASA-Bar), along with a naphthalimide derivative to obtain interesting fluorescence properties. DASA-Mel was found to have fast photochromic conversion in comparison to DASA-Bar, evident from ultraviolet-visible (UV-vis) and fluorescence spectroscopic studies. The colored form of DASA-Mel was encapsulated inside the water-soluble Stoddart's blue box and became soluble in water much faster than DASA-Bar. Interestingly, the competitive encapsulation experiment showed that DASA-Mel was selectively encapsulated inside the blue box in water whereas DASA-Bar was mostly separated out from the solution after centrifugation, and this phenomenon was confirmed by ¹H and DOSY NMR and mass spectroscopies. Moreover, we found through density functional theory (DFT) optimization that the open form of DASA-Mel was more stable during the encapsulation reaction in a water medium in comparison to DASA-Bar. The calculated binding energies of encapsulated DASA-Mel and DASA-Bar are -10.2 and -9.9 kcal/mol, respectively, clearly showing that the former is more stable by 0.3 kcal. Consequently, the organic macrocycle selectively separating one kind of DASA from a mixture by encapsulation in water is reported for the first time with experimental and theoretical support in the literature.

Azulene as an ingredient for visible-light- and stimuli-responsive photoswitches

The azulene molecule features a unique combination of optical, luminescence, and stimuli-responsive properties. This makes the azulene motif a promising functional group to be introduced in photoswitches. Recent challenges in the field of photochromic compounds require the development of new approaches to molecules that are switched by visible light (400-760 nm), are proton responsive and have advanced luminescent properties. Merging azulene with photoswitches opens prospects for fulfilling these requirements. Herein, we highlight recent results on the application of this hydrocarbon motif in various photochromic systems, such as stilbenes, diarylethenes, and azobenzenes.

Chloro- and BF2bdk-substituted dithienylethene: Synthesis, photophysical properties, and optical switching behavior

A novel chloro- and BF2bdk-substituted dithienylethene derivative, in which a chlorine atom and a difluoroboron β-diketonate (BF2bdk) group are appended at the termini of the dithienylethene core, is developed. The structure was confirmed by 1H NMR, 13C NMR, and high-resolution mass spectrometry (electrospray ionization). It displayed solvent-dependent photophysical properties, and blue/red light-triggered optical switching behavior in nonpolar or less polar solvents.

Circularly Polarized Luminescence in Nanoassemblies: Generation, Amplification, and Application

Currently, the development of circularly polarized luminescent (CPL) materials has drawn extensive attention due to the numerous potential applications in optical data storage, displays, backlights in 3D displays, and so on. While the fabrication of CPL-active materials generally requires chiral luminescent molecules, the introduction of the "self-assembly" concept offers a new perspective in obtaining the CPL-active materials. Following this approach, various self-assembled materials, including organic-, inorganic-, and hybrid systems can be endowed with CPL properties. Benefiting from the advantages of self-assembly, not only chiral molecules, but also achiral species, as well as inorganic nanoparticles have potential to be self-assembled into chiral nanoassemblies showing CPL activity. In addition, the dissymmetry factor, an important parameter of CPL materials, can be enhanced through various pathways of self-assembly. Here, the present status and progress of self-assembled nanomaterials with CPL activity are reviewed. An overview of the key factors in regulating chiral emission materials at the supramolecular level will largely boost their application in multidisciplinary fields.

Cyclization from Higher Excited States of Diarylethenes Having a Substituted Azulene Ring

The cyclization reaction of diarylethenes having an azulene ring occurs only via higher excited states. Novel diarylethenes having an azulene ring with a strong donor or acceptor were synthesized and examined in these reactions. A derivative having an electron-donating 1,3-benzodithiol-2-ylidenemethyl group at the 1-position of the azulene ring showed photochromism, whereas neither a derivative having a π-conjugated electron-donating group at the 3-position of the azulene ring nor derivatives having a π-conjugated electron-withdrawing group at the 1- or 3-position of the azulene ring showed any photochromism. The photoreactivities of these compounds were explained by calculating forces and bond orders on the excited states using density functional theory (DFT) and time-dependent (TD)-DFT.

Toward Two-Photon Absorbing Dyes with Unusually Potentiated Nonlinear Fluorescence Response

The combination of two two-photon-induced processes in a Förster resonance energy transfer (FRET)-operated photochromic fluorene-dithienylethene dyad lays the foundation for the observation of a quartic dependence of the fluorescence signal on the excitation light intensity. While this photophysical behavior is predicted for a four-photon absorbing dye, the herein proposed approach opens the way to use two-photon absorbing dyes, reaching the same performance. Hence, the spatial resolution limit, being a critical parameter for applications in fluorescence imaging or data storage with common two-photon absorbing dyes, is dramatically improved.

Photochromic organic solar cells based on diarylethenes

Photovoltaic devices that switch color depending on illumination conditions may find application in future smart window applications. Here a photochromic diarylethene molecule is used as sensitizer in a ternary bulk heterojunction blend, employing poly(4-butylphenyldiphenylamine) (poly-TPD) and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) for the transport of holes and electrons, respectively. Sandwiched between two electrodes, the blend creates a photochromic photovoltaic device that changes color, light absorption, and photon-to-electron conversion efficiency in the visible spectral range after having been illuminated with UV light.

High response photochromic films based on D-A diarylethenes and their application in holography

A set of photochromic dithienylethenes bearing amino and nitro groups are synthesised and embedded at high concentrations in a polymer matrix (Cellulose Acetate Butyrate, CAB) to produce films showing a large reversible modulation of the complex refractive index in the Vis-NIR spectral range, thanks to an interesting combination of remarkable response at the molecular level and very high load capability in the chosen matrix. The photochromic derivatives are characterized in solution and in CAB films by means of electronic and vibrational spectroscopy, complemented by DFT calculations. Both the real and imaginary part of the refractive index are determined by spectroscopic ellipsometry. The modulation of the refractive index in the near infrared is in the range 0.02–0.04. These are very large values for such kinds of systems and they are due to a favourable combination of very large solubility of the derivatives in CAB and a high polarisability change. As for the change in transparency in the visible, contrast values larger than 10³ are easily achieved. Based on such films, holograms are written and reconstructed with a very high fidelity and efficiency.

Photochromic diarylethenes with D–A structure are good candidates in holography thanks to their very large modulation of the complex refractive index.

Mechanistic Insights into the Triplet Sensitized Photochromism of Diarylethenes

Operating photoswitchable molecules repetitively and reliably is crucial for most of their applications, in particular in (opto)electronic devices, and related to reversibility and fatigue resistance, which both critically depend on the photoisomerization mechanism defined by the substitution pattern. Two diarylethene photoswitches bearing biacetyl triplet sensitizers either at the periphery or at the core were investigated using both stationary as well as transient UV/Vis absorption spectroscopy ranging from the femtosecond to the microsecond time scale. The diarylethene with two biacetyl moieties at the periphery is switching predominantly from the triplet excited state, giving rise to an enhanced fatigue resistance. In contrast, the diarylethene bearing one diketone at the photoreactive inner carbon atom cyclizes from the singlet excited state and shows significantly higher quantum yields for both cyclization and cycloreversion.

Computational and Crystallographic Examination of Naphthoquinone Based Diarylethene Photochromes

Photochromic compounds have a lengthy history of study and a profusion of applications that stand to gain from these studies. Among the classes of photochromic compounds, diarylethenes show desirable properties including high fatigue resistance and thermal stability, thus meeting some of the most important criteria necessary to enter the realm of practical applications. Recently, photochromic diarylethenes containing quinone functionalities have demonstrated interesting optical and solid-state properties. When properly interfaced with suitable electron withdrawing groups on the aryl component, both the ring-opening and ring-closing reactions can be achieved with visible light; this is in contrast to most other diarylethenes where UV light is required for ring closure. Unfortunately, quantitative conversion from open to closed forms is not possible. In this work, we examine the relative energies of conformations of solid-state structures observed by X-ray crystallography and evaluate their thermal stabilities based on density functional theory (DFT) calculations. Time-dependent DFT (TD–DFT) is used to model the UV-vis absorption spectra of these quinone diarylethenes. We show that spectral overlap between open and closed forms is a major hindrance to full photoconversion.

Diarylethene Dyes

This article introduces the general characteristics of the diarylethene class of photochromic dye and the structural features that make photochromism possible. It touches on the methodologies employed to synthesize these compounds as well as the influences that typical substitution patterns exert on photocoloration. A demonstration is then given of the great diversity pertaining to the potential applications in which researchers are seeking to exploit them as functional colorants.

All Visible Light Switch Based on the Dimethyldihydropyrene Photochromic Core

Two photoswitchable compounds that can operate under visible light irradiation are prepared and investigated using spectroscopic and computational studies. These all-visible systems are based on the dimethyldihydropyrene (DHP)/cyclophanediene (CPD) photochromic couple connected either to a bipyridine (bpy) unit or to a (tris(bpy)ruthenium(II)) complex through a pyridinium bridge. In these compounds, the DHP to CPD isomerization and the reverse CPD to DHP conversion can be triggered by illumination with red (>630 nm) and blue (460 nm) lights, respectively. The unambiguous and reversible response of these systems triggered by visible light make them potential candidates for biological purposes and electronic devices.

Photochromic Diarylethenes Designed for Surface Deposition: From Self-Assembled Monolayers to Single Molecules

The efficient switching that can occur between two stable isomers of diarylethenes makes them particularly promising targets for opto- and molecular electronics. To examine these classes of molecules for electronics applications, they have been subjected to a series of scanning tunneling microscopy (STM) experiments, which are the focus of this Review. A brief introduction to the chemical design of diarylethenes in terms of their switching capabilities along with the basics of STM are presented. Next, initial STM studies on these compounds under ambient conditions are discussed. An overview of how molecular design affects the isomerization and self-assembly of diarylethenes at the solid-liquid interface as investigated by STM is then presented, as well as single-molecule studies under ultrahigh vacuum. The last section presents further prospects for molecular design in the field.

Imidazole diarylethene switches: an alternative to acid-gated photochromism

Improving the photoactivity of imidazole diarylethenes by modifying their response sites in imidazole instead of appended aryl units is accomplished.

Dual-responsive "smart" window and visually attractive coating based on a diarylethene photochromic dye

Controlling the intensity and manipulating the spectral composition of sunlight are critical for many devices including "smart" windows, greenhouses, and photomicroreactors, but these are also important in more decorative applications. Here, we use a diarylethene dye incorporated in a liquid crystal host to create a dual-responsive "smart" window regulated both by an electrical trigger and by specific wavelengths of light. By incorporating the same diarylethene dye in a polymerizable host and using inkjet printing, coatings can be made with complete freedom in the applied pattern design, although the electrical response is lost. The color change of the diarylethene dye can be controlled in simulated sunlight by concurrent light exposure from an LED source, allowing a manual override for outdoor use. Photoluminescence of the closed isomer of the diarylethene from the light guide edges could be used for lighting or electricity generation in a luminescent solar concentrator architecture.

Aldehyde-substituted acceptor-DTE-acceptor-type dithienylethene as a versatile building block for near-infrared photochromic materials

A novel aldehyde-substituted acceptor-DTE-acceptor-type dithienylethene derivative, in which the difluoroboron β-diketonate and aldehyde groups at the termini of the dithienylethene function as electron acceptors, has been developed. The structure was confirmed by 1H NMR, 13C NMR, 19F NMR, high-resolution mass spectrometry (electrospray ionization) and infrared. The derivative displays solvent-dependent photophysical properties, visible light–triggered near-infrared photochromism, and fluorescent switching behavior in solvents of different polarity. Moreover, it can be utilized as a versatile building block to construct novel near-infrared photochromic materials.