Fast Negative Photochromism of 1,1'-Binaphthyl-Bridged Phenoxyl-Imidazolyl Radical Complex

Stepwise Photochromism of Large Macrocycles Incorporating Two Negative Photochromic Units

Macrocyclic photochromic molecules incorporating multiple photochromic units are known to exhibit cooperative and nonlinear photochromic reactions among distinct photochromic components. While extensive research has concentrated on positive photochromic molecules, this study presents a pioneering attempt in synthesizing macrocyclic photochromic molecules that integrate negative photochromic units. Binaphthyl-bridged phenoxyl imidazolyl radical complex, BN-PIC, exhibits unique negative photochromism in which the thermally stable colored isomer converts to the metastable colorless isomer via a short-lived biradical upon visible-light irradiation. Macrocyclic biphotochromic molecules incorporating two BN-PIC units were synthesized and the effects of ring strain on the photochromic properties including the photoconversion efficiencies and the rates of the thermal reverse reaction were investigated. The photokinetic study of these macrocyclic biphotochromic molecules demonstrated that the structural distortion of the ring caused by the isomerization of one photochromic unit significantly influenced the photoisomerization efficiency and the rate of the thermal reverse reaction of the other photochromic unit.

Optical Phenomena in Molecule-Based Magnetic Materials

Since the last century, we have witnessed the development of molecular magnetism which deals with magnetic materials based on molecular species, i.e., organic radicals and metal complexes. Among them, the broadest attention was devoted to molecule-based ferro-/ferrimagnets, spin transition materials, including those exploring electron transfer, molecular nanomagnets, such as single-molecule magnets (SMMs), molecular qubits, and stimuli-responsive magnetic materials. Their physical properties open the application horizons in sensors, data storage, spintronics, and quantum computation. It was found that various optical phenomena, such as thermochromism, photoswitching of magnetic and optical characteristics, luminescence, nonlinear optical and chiroptical effects, as well as optical responsivity to external stimuli, can be implemented into molecule-based magnetic materials. Moreover, the fruitful interactions of these optical effects with magnetism in molecule-based materials can provide new physical cross-effects and multifunctionality, enriching the applications in optical, electronic, and magnetic devices. This Review aims to show the scope of optical phenomena generated in molecule-based magnetic materials, including the recent advances in such areas as high-temperature photomagnetism, optical thermometry utilizing SMMs, optical addressability of molecular qubits, magneto-chiral dichroism, and opto-magneto-electric multifunctionality. These findings are discussed in the context of the types of optical phenomena accessible for various classes of molecule-based magnetic materials.

Photoinduced fluorescence modulation through controllable intramolecular [2+2] photocycloaddition in single molecules and molecular aggregates

We report a general molecular design strategy of spatial proximity, which allows intramolecular [2+2] photocycloaddition reaction to take place in both single molecules and molecular aggregates. Sharply contrasting photoinduced fluorescence changes in solution and in the solid state were found and attributed to the aggregation-induced quenching property of the monomers and the aggregation-induced emission nature of the photodimers.

From Visible to Near-Infrared Light-Triggered Photochromism: Negative Photochromism

Photochromic compounds, whose key molecular properties can be effectively modulated by light irradiation, have attracted significant attention for their potential applications in various research fields. The restriction of photoisomerization coloration induced by ultraviolet light limits their applications in the biomedical field and some other fields. Negative photochromism, wherein a relatively stable colored isomer transforms to a colorless metastable isomer under low-energy light irradiation, offers advantages in applications within materials science and life science. This review provides a summary of negatively photochromic compounds based on different molecular skeletons. Their corresponding design strategies and photochromic properties are presented to provide practical guidelines for future investigations. Negatively photochromic compounds can effectively expand the range of photochromic switches for future applications, offering unique properties such as responsiveness to visible to near-infrared light.

Photoinduced Crystal-to-Liquid Transition of Acylhydrazone-Based Photoswitching Molecules

A photoinduced crystal-to-liquid transition (PCLT) behavior of new acylhydrazone derivatives (NCs) is reported. The photoswitching of the NCs was identified as a negative photochromism with a high E-to-Z conversion yield (>98%). A kinetic analysis shows a half-life of almost one month. Owing to these high photoswitching performances, we successfully isolated both E- and Z-forms, evaluated their crystal structures, and observed distinct thermal behaviors. The Z-form melts at a lower temperature than the E-form by several tens of degrees. The PCLT occurs at even lower temperatures. UV irradiation induces the E-to-Z conversion in the crystalline state, thereby inducing a eutectic melting. In addition to the PCLT, we observed a photomechanical behavior of the crystals, which suggests that the presented acylhydrazones can be new members of the photoresponsive crystalline materials.

Acceleration of the thermal back-reaction and the finding of a non-photochromic isomer for a negative photochromic binaphthyl-bridged imidazole dimer

The development of visible or near-infrared (NIR) light-responsive fast photoswitchable molecules for the real-time, non-contact control of physical and chemical properties has received increased attention because of the non-invasive features to materials and biological tissues. We report a new molecular design to accelerate the thermal back-reaction of the negative photochromic binaphthyl-bridged imidazole dimer, BN-ImD. We also found that irradiation of the BN-ImD derivative with methyl groups on the bridging binaphthyl unit with visible light produced an unprecedented photoreaction product with a unique eight-membered ring structure. These observations provide fascinating clues for the future development of fast negative photochromic molecules.

Photochromism of colloidal ZnO nanocrystal powders under ambient conditions

Zinc oxide (ZnO) nanocrystals (NCs) exhibit photochromic reactions under specific conditions upon ultraviolet light irradiation. Since the color is originated from the excited electrons at the conduction band of ZnO NCs, the photoinduced absorption is observed only in the solution with hole acceptors under inert conditions. ZnO is earth-abundant and less toxic than many other substances, and has been widely used in various industrial fields. If the photochromic reaction of ZnO can be observed consistently under ambient conditions, the material may pave the way for large-scale photochromic applications such as in pigments, windows, and building materials in addition to conventional photochromic applications. In this study, we synthesize hydrophilic ZnO NCs and observe the solid-state photochromic reactions in the visible to mid-infrared regions even in humid-air conditions. We reveal that the coloration of powders of ZnO NCs under ambient conditions originates mainly from two factors: (1) charge separation induced by hole trapping by water molecules adsorbed on the surface of NCs, and (2) deceleration of the reactions involving the electrons in the conduction band of ZnO NCs with molecular oxygen and the adsorbed water molecules.

A photochromic carbazolyl-imidazolyl radical complex

A carbazole-incorporated photochromic radical complex is synthesized. The long-wavelength photosensitivity of the photochromic reaction of the molecule is enhanced up to ∼580 nm by substituting a triphenylamine group into the 3-position of the carbazole moiety. These photochromic reactions are investigated by subpicosecond-to-microsecond transient absorption measurements.

Recent advances in low-power-threshold nonlinear photochromic materials

Incoherent nonlinear photophysical and photochemical processes based on stepwise two-photon absorption (2PA) processes have been recently used in materials science owing to their unique photoresponses beyond one-photon processes and lower power thresholds to induce the processes than those of coherent nonlinear optical processes. Among them, nonlinear photochromic materials have received considerable attention because they exhibit unconventional photoresponses compared with other incoherent nonlinear processes such as low-power-threshold nonlinear photoresponses with unimolecular systems, gated photochemical reactions and oxygen-insensitive nonlinear photoresponses. Nonlinear photochromic materials are important not only for colorimetric materials, but also for emergent materials that can enrich the next-generation society such as dynamic holographic materials, which are promising for three-dimensional displays. In this tutorial review, we introduce low-power-threshold nonlinear photochromic materials using stepwise 2PA processes. First, we explain the fundamental concepts of photochemistry as well as photochromic reactions. We attempt to provide an intuitive understanding of incoherent nonlinear optical processes using these fundamental concepts. Then, we introduce several recent examples and potential applications of nonlinear photochromic materials. This tutorial review is important for understanding the scientific progress related to these fields and provides a simple unified picture of the incoherent nonlinear optical properties of different types of photofunctional materials.

Solvent dielectric delimited nitro–nitrito photorearrangement in a perylenediimide derivative

The discovery of vibrant excited-state dynamics and distinctive photochemistry has established nitrated polycyclic aromatic hydrocarbons as an exhilarating class of organic compounds. Herein, we report the atypical photorearrangement of nitro-perylenediimide (NO₂-PDI) to nitrito-perylenediimide (ONO-PDI), triggered by visible-light excitation and giving rise to linkage isomers in the polar aprotic solvent acetonitrile. ONO-PDI has been isolated and unambiguously characterized using standard spectroscopic, spectrometric, and elemental composition techniques. Although nitritoaromatic compounds are conventionally considered to be crucial intermediates in the photodissociation of nitroaromatics, experimental evidence for this has not been observed heretofore. Ultrafast transient absorption spectroscopy combined with computational investigations revealed the prominence of a conformationally relaxed singlet excited-state (S^CR₁) of NO₂-PDI in the photoisomerization pathway. Theoretical transition state (TS) analysis indicated the presence of a six-membered cyclic TS, which is pivotal in connecting the S^CR₁ state to the photoproduct state. This article addresses prevailing knowledge gaps in the field of organic linkage isomers and provides a comprehensive understanding of the unprecedented photoisomerization mechanism operating in the case of NO₂-PDI.

The unprecedented photorearrangement of nitro-perylenediimide (NO₂-PDI) to nitrito-perylenediimide (ONO-PDI) is shown to occur through a cyclic six-membered transition state triggered by visible-light excitation.

Origin of the Anomalous Temperature Dependence of the Photochromic Reaction of Cu-Doped ZnS Nanocrystals

The temperature dependence of the color fading process of thermally reversible photochromic reactions is one of the most important challenges for their industrial applications. Generally, photochromic reactions of organic molecules have a strong temperature dependence due to the occurrence of large conformational changes during the reactions. In contrast, we recently reported that the photochromic reaction of Cu-doped ZnS nanocrystals (NCs) exhibits a very small temperature dependence around room temperature. However, the mechanism underlying this phenomenon has not been clarified yet. Here, we reveal that the anomalous temperature dependence of Cu-doped ZnS NCs originates from the balance between the temperature dependence of the charge recombination and that of the adsorption/desorption of water molecules on the surface of the NCs, which act as hole acceptors. Exploring temperature-insensitive photochromic reactions is important not only for gaining fundamental insight into nanomaterials but also for developing novel photochromic materials for outdoor applications.

Efficient Negative Photochromism by the Photoinduced Migration of Photochromic Merocyanine/Spiropyran in the Solid State

Efficient negative photochromism was achieved by the photoinduced migration of merocyanine in mesoporous silica to an organophilic clay as spiropyran. Depending on the nature of the organophilic clays (dioctadecyldimethylammonium and dioleyldimethylammonium clays), important differences in the negative photochromisms and the thermal coloration were observed; the dioleyldimethylammonium clay gave a higher yield (98%) and faster reaction (half-life t_1/2 = 2.8 h) than the dioctadecyldimethylammonium clay (94% and t_1/2 = 3.2 h) of the negative photochromism, indicating the important role of the surfactant assembly to control the molecular diffusion.

Carbon Spacer Strategy: Control the Photoswitching Behavior of Donor-Acceptor Stenhouse Adducts

Understanding the relationship between chemical structure and photoswitching property of donor-acceptor Stenhouse adducts (DASAs) is necessary for developments and applications of the novel photoresponsive molecule. In the current work, we demonstrated a close relationship between the length of carbon spacer and photoswitching property of DASAs. A series of DASAs with barbituric acid substituted electron-withdrawing part and N-methylaniline substituted electron-donating part were synthesized. With shortening the carbon spacer between the phenyl and amine groups in the electron-donating part, the efficiency and rate of the light-induced linear-to-cyclic isomerization are improved in all the test solvents. The molecular energy variation during the isomerization process was investigated by density functional theory calculation to further understand the mechanism. This work provides a reliable carbon spacer strategy to control the photoswitching behavior of DASAs using chemical methods.

Synthesis, Photochromic Properties, and Crystal Structures of Salts Containing a Pyridinium-Fused Spiropyran: Positive and Negative Photochromism in the Solution and Solid State

Salts containing the merocyanine form of a pyridinium-fused spiropyran ([6'-MC]X; X = I and PF₆) were prepared, and their crystal structures were determined. In addition, the photochromic properties of the salts were spectroscopically and kinetically investigated. In the solution state, the salts exhibited negative photochromism. Theoretical calculations revealed that the negative photochromism of the salt originates from the drastic stabilization of the merocyanine structure by electron delocalization of the pyridinium ring. Furthermore, the salts containing the merocyanine and spiropyran forms ([6'-MC]I, [6'-MC]PF₆, and [6'-SP]PF₆) were obtained by recrystallization. The crystals of [6'-SP]PF₆ exhibited positive photochromism in the solid state; however, no photochromism was observed in the [6'-MC]X crystals.

Fast Photochromism of the Imidazole Dimers Bridged by Group 14 Atoms

We developed fast photochromic imidazole dimers bridged by group 14 atoms. These compounds reversibly break the C-N bond to generate the colored open-ring biradical form. The colored form thermally reproduces the initial colorless form in the microsecond time scales. Furthermore, the color of the biradical can be easily controlled by the introduction of two different types of the imidazolyl radicals. These results give attractive insights for the further development of fast photochromic imidazole dimers.

Enhancement of Negative Photochromic Properties of Naphthalene-Bridged Phenoxyl-Imidazolyl Radical Complex

Negative photochromism has increased attention as a light-switch for functional materials. A development of fast photochromic molecules has been also expected because a rapid thermal back reaction within a millisecond time scale is useful for real-time switching. Herein, we synthesized the derivatives of the naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) showing the negative photochromism to demonstrate the efficient strategy to increase the visible light sensitivity and to control the thermal back reaction rates. The distances of the C-C bond of the transient 2,4'-isomer shows good agreement with the thermodynamic stability, leading to the control of the thermal back reaction rate. We revealed the cyclic voltammetry and the DFT calculations are efficient to predict the characters of the HOMO and LUMO. The introduction of the electron-withdrawing dicyanoquinodimethane group is efficient to induce the photochromic reaction with increased visible-light sensitivity by the expansion of the π-conjugation. The results will give an important insight for the future development of fast-responsive negative photochromic molecules.

Photochromic Radical Complexes That Show Heterolytic Bond Dissociation

Photochromic materials have been widely used in various research fields because of their variety of photoswitching properties based on various molecular frameworks and bond breaking processes, such as homolysis and heterolysis. However, while a number of photochromic molecular frameworks have been reported so far, there are few reports on photochromic molecular frameworks that show both homolysis and heterolysis depending on the substituents with high durability. The biradicals and zwitterions generated by homolysis and heterolysis have different physical and chemical properties and different potential applications. Therefore, the rational photochromic molecular design to control the bond dissociation in the excited state on demand expands the versatility for photoswitch materials beyond the conventional photochromic molecular frameworks. In this study, we synthesized novel photochromic molecules based on the framework of a radical-dissociation-type photochromic molecule: phenoxyl-imidazolyl radical complex (PIC). While the conventional PIC shows the photoinduced homolysis, the substitution of a strong electron-donating moiety to the phenoxyl moiety enables the bond dissociation process to be switched from homolysis to heterolysis. This study gives a strategy for controlling the bond dissociation process of the excited state of photochromic systems, and the strategy enables us to develop further novel radical and zwitterionic photoswitches.

Red or Near-Infrared Light Operating Negative Photochromism of a Binaphthyl-Bridged Imidazole Dimer

The development of red or near-infrared light (NIR) switchable photochromic molecules is required for an efficient utilization of sunlight and regulation of biological activities. While the photosensitization of photochromic molecules to red or NIR light has been achieved by a two-photon absorption process, the development of a molecule itself having sensitivity to red or NIR light has been now a challenging study. Herein, we developed an efficient molecular design for realizing red or NIR-light-responsive negative photochromism based on binaphthyl-bridged imidazole dimers. The introduction of electron-donating substituents shows the red shift of the absorption band at the visible-light region because of the contribution of a charge-transfer transition. Especially, the introduction of a di(4-methoxyphenyl)amino group (TPAOMe) and a perylenyl group largely shifts the absorption edge of the stable colored form to 900 nm. In addition, because the absorption band of one of the derivatives substituted with TPAOMe covers the whole visible-light region, the colored form shows a neutral gray color. Upon red (660 nm) or NIR-light (790 nm) irradiation, we observed the negative photochromic reaction from the stable colored form to the metastable colorless form. Therefore, the substituted binaphthyl-bridged imidazole dimers constitute the attractive photoswitches within a biological window.

Negative photochromism of a blue cyanine dye

We found a novel blue cyanine dye that exhibits negative photochromism caused by E–Z isomerisation upon photoirradiation.

Unveiling how intramolecular stacking modes of covalently linked dimers dictate photoswitching properties

Covalently linked π-stacked dimers represent the most significant platform for elucidating the relationship between molecular alignments and their properties. Here, we present the one-pot synthesis of two intramolecularly π-stacked dimers and disclose how intramolecular stacking modes dictate photoswitching properties. The dimer, which features cofacially stacked chromophores and geometrically favours intramolecular photochemical [2 + 2] cycloadditions, displays a nearly irreversible photoswitching behaviour. By contrast, the dimer, bearing crosswise stacked chromophores, is geometrically unfavourable for the cycloaddition and exhibits a highly reversible photoswitching process, in which the homolysis and reformation of carbon−carbon single bonds are involved. Moreover, the chiral carbon centres of both dimers endow these photoswitches with chirality and the separated enantiomers exhibit tuneable chiroptical properties by photoswitching. This work reveals that intramolecular stacking modes significantly influence the photochemical properties of π-stacked dimers and offers a design strategy toward chiral photoswitchable materials.

Covalently bridged π-stacked dimers are excellent molecular platforms for understanding the relationship between stacking orientation and properties. Here, the authors synthesize a pair of π-stacked dimers that are aligned either cofacially or crosswise, allowing them to compare how the intramolecular stacking mode affects each dimer’s photoswitching properties.

Excited state dynamics for visible-light sensitization of a photochromic benzil-subsituted phenoxyl-imidazolyl radical

Visible-light sensitized photoswitches have been paid particular attention in the fields of life sciences and materials science because long-wavelength light reduces photodegradation, transmits deep inside of matters, and achieves the selective excitation in condensed systems. Among various photoswitch molecules, the phenoxyl-imidazolyl radical complex (PIC) is a recently developed thermally reversible photochromic molecule whose thermal back reaction can be tuned from tens of nanoseconds to tens of seconds by rational design of the molecular structure. While the wide range of tunability of the switching speed of PIC opened up various potential applications, no photosensitivity to visible light limits its applications. In this study, we synthesized a visible-light sensitized PIC derivative conjugated with a benzil unit. Femtosecond transient absorption spectroscopy revealed that the benzil unit acts as a singlet photosensitizer for PIC by the Dexter-type energy transfer. Visible-light sensitized photochromic reactions of PIC are important for expanding the versatility of potential applications to life sciences and materials science.

Acceleration of photochromism and negative photochromism by the interactions with mesoporous silicas

The adsorption of merocyanine dye onto mesoporous silicas with varied pore sizes (5.5, 9.4 and 2.2 nm) from the toluene solution of 1,3,3-trimethylindolino-6'-nitrobenzopyrylospiran under UV irradiation was investigated quantitatively. The photoinduced adsorption of merocyanine onto SBA-15 with the pore diameter of 9.4 nm followed the pseudo-second order kinetics and the rate constant was larger than that observed for MCM-41 (pore size of 2.2 nm) owing to the efficient diffusion of merocyanine. The maximum adsorbed amounts of the merocyanine dye was 152 mg g-1 of SBA-15, which corresponded to the sufficiently high concentration of merocyanine in the pores (0.376 mol L-1 of pore). The resulting red-colored hybrids (SBA-15 containing merocyanine) showed decoloration in the solid-state by visible light irradiation (negative photochromism). The conversion was high (about 80% at the photostationary state) under visible light irradiation at room temperature using a solar simulator (100 W). The red color was re-generated by storing the photochemically formed colorless samples in the dark at room temperature. The half-lives of the thermal coloration process were 2.6, 1.9 and 1.3 h for the MCM-41, SBA-15s with the BJH pore sizes of 5.5 and 9.4 nm, respectively. Since the coloration was affected by the diffusion of the molecules in the pores, larger pores provided the efficient molecular diffusion, leading to faster reactions.

Picosecond Switchable Azo Dyes

Azo dyes that combine electron-withdrawing thiazole/benzothiazole heterocycles and electron-donating amino groups within the very same covalent skeleton exhibit relaxation times for their thermal isomerization kinetics within milli- and microsecond timescales at room temperature. Notably, the thermal back reaction of the corresponding benzothiazolium and thiazolium salts occurred much faster, within the picosecond temporal domain. In fact, these new light-sensitive platforms are the first molecular azo derivatives capable of reversible switching between their trans and cis isomers in a subnanosecond timescale under ambient conditions. In addition, theoretical calculations revealed very low activation energies for the isomerization process, in accordance with the fast subnanosecond kinetics that were observed experimentally.

Electrochromism of fast photochromic radical complexes forming light-unresponsive stable colored radical cation

We demonstrated the electrochromism of photochromic radical complexes containing triaryl imidazole: fast photoswitchable pentaarylbiimidazole (PABI) and the phenoxyl-imidazolyl radical complex (PIC). Cyclic voltammetry and spectroelectrochemistry revealed that PABI and PIC generate the highly stable radical cation by one-electron oxidation accompanied by a color change from colorless to green. The stability of the radical cation is strongly affected by the dihedral angle between the imidazole ring and the phenyl ring at the 2-position of the imidazole ring.

Turn-On Mode Fluorescence Switch by Using Negative Photochromic Imidazole Dimer

The development of fluorescence switchable molecules in several polar and apolar environments has been required for fluorescence imaging of nanostructures. Photochromic molecules are an important class for the reversible light-triggered fluorescence switching. Although many studies of fluorescence switching by using photochromic reactions have been reported, the report of photochromic molecules reversibly showing turn-on mode fluorescence switching has been limited in spite of their importance. Herein, we report the photoactivatable fluorescence based on negative photochromism, where the absorption spectrum of the compound after irradiation is blue-shifted relative to that before irradiation. We introduced naphthalimide units as a green fluorophore to the negative photochromic binaphthyl-bridged imidazole dimer. The fluorescence of the naphthalimide unit is efficiently quenched in the initial colored isomer (fluorescence quantum yield: Φ_fluo. = 0.01) by Förster resonance energy transfer. In contrast, the fluorescence quantum yield increases up to 0.75 in the transient isomer formed by the negative photochromic reaction. The fluorescence intensity thermally decreases with the thermal back reaction to form the original stable colored form. These results indicate that the negative photochromic molecules are suitable for turn-on mode fluorescence switches and will give an attractive insight for the development of reversible fluorescence switching molecules.

New Acylhydrazone Photoswitches with Quantitative Conversion and High Quantum Yield but without Hydrogen Bond Stabilizing ( Z)-Isomer

Hydrazones are recently attracting increasing interest because of their facile synthesis and high addressability, fatigue resistance, and modifiability as molecular switches. However, this new class of switches generally suffers from low conversion from E- to Z-configuration. Here, novel benzoylhydrazones were synthesized by condensation of 2-methoxynaphthaldhyde and benzoylhydrazine. In this hydrazone system, both sides of the imine double bond had large steric hindrance, so that the ( E)-isomer of the benzoylhydrazones was less stable and easily converted into the ( Z)-isomer even without an intramolecular hydrogen bond. Up to 99% conversion efficiency and 89% quantum yield were obtained, in addition to excellent addressability and high fatigue resistance. Outstandingly, the crystal structure of one ( Z)-isomer disclosed no intermolecular hydrogen bonds between the molecules of the ( Z)-isomer but strong and sequential hydrogen bonds between those of the ( E)-isomer. Therefore, the ( E)-isomer was less soluble in solvents than the ( Z)-isomer. This molecular switch system could be easily modified by both hydrophilic pentaethylene glycol chains and hydrophobic octyl chains. Under light irradiation, the resultant amphiphilic acylhydrazone could be transferred from ( E)-isomer to ( Z)-isomer in more than 90% yield even in water after light irradiation. Meanwhile, the self-assembled big nanospheres could rearrange into much smaller vesicles because of the solubility difference of ( Z)- and ( E)-isomers. After the anticancer drug procarbazine was loaded by this kind of acylhydrazone in water, it could be released by light irradiation, showing potential application in photocontrollable drug release.

Visible light intensity dependent negative photochromism of a binaphthyl-bridged phenoxyl-imidazolyl radical complex

We herein report a novel biphotochromic molecule composed of two fast negative photochromic phenoxyl-imidazolyl radical complex units showing incident light intensity dependence of the photochemical reaction.

Structure-function relationships of donor-acceptor Stenhouse adduct photochromic switches

The first in-depth, systematic study of the photoswitching properties of Donor-Acceptor Stenhouse Adducts (DASAs) is reported. Barbituric acid derived DASAs functionalised with 14 different amines ranging from dimethylamine to 4-methoxy-N-methylaniline were structurally characterised in solution using 1H and 13C NMR spectroscopy and, in eight cases, in the solid state by single crystal X-ray diffraction. The distribution of coloured and colourless isomers in the dark, their photostationary states under irradiation, apparent thermal half-lives, and fatigue resistance are systematically compared. A simple kinetic model is used to characterise photoswitching behaviour and reveals that minor structural modifications can significantly improve the photoswitching properties of DASA photochromes. These modifications result in excellent photoswitching properties for '1st generation' DASAs in chloroform, including exceptional fatigue resistance, opening the door for these photochromic molecules to find widespread applications.

New molecular switch architectures

In this paper we elaborate on recently developed molecular switch architectures and how these new systems can help with the realization of new functions and advancement of artificial molecular machines. Progress in chemically and photoinduced switches and motors is summarized and contextualized such that the reader may gain an appreciation for the novel tools that have come about in the past decade. Many of these systems offer distinct advantages over commonly employed switches, including improved fidelity, addressability, and robustness. Thus, this paper serves as a jumping-off point for researchers seeking new switching motifs for specific applications, or ones that address the limitations of presently available systems.

Modern Spin on the Electrochemical Persistence of Heteroatom-Bridged Triphenylmethyl-Type Radicals

Herein we present a clarification of the ambiguous persistence of the 10-methyl-9-phenylacridanyl, 9-phenylxanthenyl, and 9-phenylthioxanthenyl radicals in electrochemical experiments. Each of these radicals has separately been the subject of conflicting literature results for decades with publications claiming both their chemical inertness and propensity to dimerize. We assert that each radical is persistent at conventional electrochemical time scales up to several minutes based on reversible redox couples and cyclic voltammogram simulations of the radicals and their respective cations. All three radicals are rapidly consumed by aerial O₂, which lends irreversibility to the redox couples after fewer than 20 s of exposure to air. With appreciation for the O₂ sensitivity of these radicals, their electrochemically generated UV-visible absorption spectra have been acquired and matched to predictions made by TD-DFT calculations. Further, we propose that previous claims to have electrochemically measured radical-radical dimerizations have only observed reaction of these radicals with dissolved O₂.

Reversible Valence Photoisomerization between Closed-Shell Quinoidal and Open-Shell Biradical Forms

We report here a kinetic study on the thermal equilibrium process between the biradical form and the quinoidal form starting from the singlet biradical form alone. A photochromic phenoxyl-imidazolyl radical complex repeatedly generates biradical species upon UV light irradiation, and the following thermal equilibrium process responsible for valence isomerization from the open-shell singlet biradical to the closed-shell quinoidal form is observed in the microsecond time region. The thermodynamic parameters for the equilibrium process were determined for the first time by nanosecond laser flash photolysis. We also found that visible-light excitation to the equilibrium state causes valence photoisomerization from the quinoidal to the biradical form, which returns thermally to the quinoidal form.