Synthesis,Photochromic, and Computational Studies of Dithienylethene‐Containing β‐Diketonate Derivatives and Their Near‐Infrared Photochromic Behavior Upon Coordination of a Boron(III) Center

A series of dithienylethene‐containing 1‐thienyl‐3‐aryl‐propane‐1,3‐diones (aryl=phenyl (Ph), thienyl (Th), and 4,5‐bis(2,5‐dimethylthiophen‐3‐yl)thiophen‐2‐yl (DTE‐Th)) and the corresponding boron(III) diketonates, (O^O)BR₂ (R=F, C₆F₅, and Ph), have been designed and synthesized. Their photophysical, electrochemical, and photochromic properties have been studied. Upon coordination of a boron(III) center, the closed forms of the dithienylethene‐containing β‐diketonates show near‐infrared response and the photochromic behavior was also found to be affected by the aryl substituents at the 3‐position of the β‐diketonates. Moreover, computational studies have been performed that help to provide an understanding of the effect of substituents on the photophysical and photochromic properties.     

Tunable Photochromism in the Robust Dithienylethene‐Containing Phospholes: Design,Synthesis, Characterization,Electrochemistry, Photophysics,and Photochromic Studies

A series of dithienylethene‐containing phosphole derivatives has been designed, synthesized and characterized. One of the compounds has been characterized by X‐ray crystallography. Upon photoexcitation, the compounds exhibit drastic color changes, ascribed to the reversible photochromic behavior. Their photochromic, photophysical and electrochemical properties have been studied. They show photochromic reactivities with high photocyclization quantum yields. Their photophysical and photochromic properties are found to be facilely tuned in this system by substitution at the phosphole backbone, as well as variation on the extent of π‐conjugation of the phosphole backbone. Some selected compounds have been demonstrated to exhibit photochromic properties in polymethylmethacrylate (PMMA) films.     

Syntheses and Photophysical Properties of N‐Pyridylimidazol‐2‐ylidene Tetracyanoruthenates(II) and Photochromic Studies of Their Dithienylethene‐Containing Derivatives

A series of tetracyanoruthenate(II) with chelating pyridyl N‐heterocyclic carbene ligands (NHC‐py) was synthesized and characterized. Their photophysical and electrochemical properties as well as the photochromic behavior of their dithienylethene‐containing complexes were studied. Photocyclization was found to take place upon irradiation into the metal‐to‐ligand charge transfer (MLCT) absorption bands of these complexes, and evidence is provided to support the triplet‐sensitizing reaction pathway.     

Photochromic Dithienylethene‐Containing Boron(III) Ketoiminates: Modulation of Photo‐Responsive Behavior through Variation of Intramolecular Motion

A series of dithienylethene‐containing boron(III) ketoiminates, as well as their corresponding β‐ketoimine ligands, have been synthesized and characterized. The photophysical, electrochemical, and photochromic properties of the compounds have been studied. Photocyclization has been found to be suppressed upon introduction of a phenyl substituent on the nitrogen atom of the β‐ketoiminate core, whereas photochromism could be observed by replacement of the phenyl substituent with a bulky mesityl group. It is believed that the steric effect of the mesityl unit restricts molecular rotation, resulting in such a prominent difference in the photochromic properties.     

Preparation and Acid‐Responsive Photophysical Properties of T‐Shaped π‐Conjugated Molecules Containing a Benzimidazole Junction

T‐shaped π‐conjugated molecules with an N‐methyl‐benzimidazole junction have been synthesized and their acid‐responsive photophysical properties owing to the change in the π‐conjugation system are discussed. T‐shaped π‐conjugated molecules consist of two orthogonal π‐conjugated systems including a phenyl thiophene extended from the 2‐position and alkyl phenylenes connected through various π‐spacers from the 4,7‐positions of the N‐methyl‐benzimidazole junction. The π‐spacers, such as thiophene, ethyne, and ethane, have an effect on the acid response of photophysical properties in terms of changes in conformation, excited‐state energy and charge‐transfer (CT) characteristics. In particular, the π‐conjugated molecule with ethynyl spacers exhibited a marked redshift in the fluorescence spectrum with a large Stokes shift upon the addition of acid, whereas the other molecules showed substantial quenching. The redshift in emission was studied in detail by temperature‐dependent fluorescence measurements, which indicated the transition to a CT state over the finite activation energy at the excited state. The change in the frontier molecular orbitals upon acid addition was further discussed by means of DFT calculations.     

Thermo‐ and Acid‐Responsive Photochromic Spironaphthoxazine‐Containing Organogelators

A series of photochromic spironaphthoxazine derivatives has been designed, synthesized, and characterized by using ¹H NMR spectroscopy, FAB mass spectrometry, and elemental analysis. Their photophysical and photochromic behavior have been investigated. Two of the compounds (G¹²‐en‐SA‐SO and G¹⁶‐en‐SA‐SO) have been shown to be capable of forming stable thermoreversible organogels in organic solvents, tested by the “stable‐to‐inversion of a test tube” method. Addition of p‐toluenesulfonic acid was found to induce the formation of stable organogels at concentrations below that of the critical gelation concentration (c.g.c.), with a concomitant change in color from colorless to purple. Transmission electron microscopy and scanning electron microscopy of the xerogels showed typical fibrous structures in the micrometer scale. The activation parameters for the bleaching reaction of G⁸‐en‐SA‐SO in the solution state and G¹⁶‐en‐SA‐SO in the gel state have been determined in ethanol through kinetic studies at various temperatures. The results showed that the rate of the bleaching reaction in the gel state was much slower than that in the solution state.     

A Versatile Dithienylethene‐Functionalized Ph‐Diazabutadiene (DAB) Ligand: From Photoswitchable Main‐Group Molecules to Photochromic Polymers

Within the past decade photochromic materials, specifically dithienylethenes (DTEs), have received increased interest because of their ability to function as potential photoswitchable molecular devices and optical memory storage systems. Current research in this area has focused on incorporating organic architectures to functionalize the DTE framework and alter the resulting photophysical properties; however, their syntheses are often not trivial. In this context, we have designed a simple and versatile diimine ( 2 ) containing adjacent 2,5‐dimethyl(thienyl) rings in the backbone. This redox active diimine ( 2 ) acts as a precursor to a novel photochromic ligand and has been used to coordinate to both boron and phosphorus elements, along with the synthesis of a phosphorane‐side‐chain functionalized polymer without further functionalization to the parent DTE framework. A study of the resulting photochromic properties of these compounds revealed that 1) the UV‐visible absorption spectra of the closed‐ring isomer were dependent of the element present in the N,N′‐chelating pocket and 2) incorporating the dithienylethene into a side‐functionalized phosphorane polymer greatly increased the closed‐/open‐ring reversibility and decreased the formation of by‐products.     

Creating Coordination‐Based Cavities in a Multiresponsive Supramolecular Gel

Creating cavities in varying levels, from molecular containers to macroscopic materials of porosity, have long been motivated for biomimetic or practical applications. Herein, we report an assembly approach to multiresponsive supramolecular gels by integrating photochromic metal–organic cages as predefined building units into the supramolecular gel skeleton, providing a new approach to create cavities in gels. Formation of discrete O‐Pd₂L₄ cages is driven by coordination between Pd²⁺ and a photochromic dithienylethene bispyridine ligand (O‐PyFDTE). In the presence of suitable solvents (DMSO or MeCN/DMSO), the O‐Pd₂L₄ cage molecules aggregate to form nanoparticles, which are further interconnected through supramolecular interactions to form a three‐dimensional (3D) gel matrix to trap a large amount of solvent molecules. Light‐induced phase and structural transformations readily occur owing to the reversible photochromic open‐ring/closed‐ring isomeric conversion of the cage units upon UV/visible light radiation. Furthermore, such Pd₂L₄ cage‐based gels show multiple reversible gel–solution transitions when thermal‐, photo‐, or mechanical stimuli are applied. Such supramolecular gels consisting of porous molecules may be developed as a new type of porous materials with different features from porous solids.     

Photochromic Benzo[b]phosphole Alkynylgold(I) Complexes with Mechanochromic Property to Serve as Multistimuli‐Responsive Materials

A series of novel benzo[b]phosphole alkynylgold(I) complexes has been demonstrated to display photochromic and mechanochromic properties upon applying the respective stimuli of light and mechanical force. Promising multistimuli‐responsive properties of this series of gold(I) complexes have been successfully achieved through judicious molecular design, which involves incorporation of the photochromic dithienylethene‐containing benzo[b]phosphole into the triphenylamine‐containing arylethynyl ligand that is susceptible to mechanical force‐induced color changes via gold(I) complexation. With excellent thermal irreversibility and robust fatigue resistance of this series of gold(I) complexes, multicolor states controlled by the photochromism and mechanochromism have been realized. Repeatable photochromic and mechanochromic cycles without apparent loss of reactivity have also been observed under ambient conditions. The present work provides important insight and an alternative strategy for the molecular design of multistimuli‐responsive materials, paving the way for further development of the underexplored photoresponsive gold(I) complexes and the multistate photocontrolled system.     

Bifunctional Indenyl‐Derived Receptors for Fluoride Chelation and Detection

Anion receptors based on a [CpFe(indenyl)] scaffold offer the possibility for the incorporation of adjacent Lewis acidic functions onto a six‐membered carbocyclic framework, while at the same time retaining the colorimetric/electrochemical reporter mechanisms available to synthetically simpler ferrocene systems. Thus, [CpFe(indenyl)] systems featuring mutually ortho BMes₂ and PPh₂Me⁺ substituents (with either 4,5 or 5,6 regiochemistry) are accessible which are capable of cooperative fluoride ion fixation. Simultaneous binding at the borane and phosphonium centres can be established by spectroscopic, structural and computational approaches, and is responsible for the favourable thermodynamics associated with F^? uptake. Thus, in contrast to simple BMes₂ systems, the binding of fluoride is found to be more favourable than the uptake of cyanide (which interacts only with the borane Lewis acid). Moreover, in the case of a 4‐(MePh₂P)‐5‐(Mes₂B)‐7‐Me‐indenyl derivative, fluoride chelation is signalled not only by a large cathodic shift in the Fe^II/Fe^III potential (>500 mV in THF), but also by a distinct colour change from green (for the free receptor) to maroon for the adduct.     

Regulation of Charge Delocalization in a Heteronuclear Fe2Ru System by a Stepwise Photochromic Process

Heteronuclear complexes FeCp₂?DTE?C?C?Ru(dppe)₂Cl ( 1 o ; dppe=1,2‐bis(diphenylphosphino)ethane, Cp=cyclopentadienyl, DTE=dithienylethene) and FeCp₂?DTE?C?C?Ru(dppe)₂?C?C?DTE?FeCp₂ ( 2 oo ), with redox‐active ferrocenyl and ruthenium centers separated by a photochromic DTE moiety, were prepared to achieve photoswitchable charge delocalization and Fe???Ru electronic communication. Upon UV‐light irradiation of 2 oo , the Fe???Ru heterometallic electronic interaction is increasingly facilitated with stepwise photocyclization, 2 oo → 2 co → 2 cc ; this is ascribed to the gradual increase in π‐conjugated systems. The near‐infrared absorptions in mixed‐valence species [ 2 oo ]⁺/[ 2 co ]⁺/[ 2 cc ]⁺ are gradually intensified following the conversion of [ 2 oo ]⁺→[ 2 co ]⁺→[ 2 cc ]⁺, which demonstrates that the extent of charge delocalization shows progressive enhancement with stepwise photocyclization. As revealed by electrochemical, spectroscopic, and theoretical studies, complex 2 exhibits nine switchable states through stepwise photochromic and reversible redox processes.     

A Magneto‐optical Molecular Device: Interplay of Spin Crossover,Luminescence, Photomagnetism,and Photochromism

A bis(pyrazolylpyridyl) ligand, L, containing a central photochromic dithienylethene spacer predictably forms a ferrous [Fe₂L₃]⁴⁺ helicate exhibiting spin crossover (SCO). In solution, the compound [Fe₂L₃](ClO₄)₄ ( 1 ) preserves the magnetic properties and is fluorescent. The structure of 1 is photo‐switchable following the reversible ring closure/opening of the central dithienylethene via irradiation with UV/visible light. This photoisomerization switches on and off some emission bands of 1 and provides a means of externally manipulating the magnetic properties of the assembly.     

Controlled Generation of 9‐Boratriptycene by Lewis Adduct Dissociation: Accessing a Non‐Planar Triarylborane

A highly bent triarylborane, 9‐boratriptycene, was generated in solution by selective protodeboronation of the corresponding tetra‐aryl boron ate complex with the strong Brønsted acid HNTf₂. The iptycene core confers enhanced Lewis acidity to 9‐boratriptycene, making it unique in terms of structure and reactivity. We studied the stereoelectronic properties of 9‐boratriptycene by quantifying its association with small N‐ and O‐centered Lewis bases, as well as with sterically hindered phosphines. The resultant Lewis adducts exhibited unique structural, spectroscopic, and photophysical properties. Beyond the high pyramidalization of the 9‐boratriptycene scaffold and its low reorganization energy upon Lewis base coordination, quantum chemical calculations revealed that the absence of π donation from the triptycene aryl rings to the boron vacant p_z orbital is one of the main reasons for its high Lewis acidity.     

Photochromic Metal–Organic Frameworks: Reversible Control of Singlet Oxygen Generation

The controlled generation of singlet oxygen is of great interest owing to its potential applications including industrial wastewater treatment, photochemistry, and photodynamic therapy. Two photochromic metal–organic frameworks, PC‐PCN and SO‐PCN, have been developed. A photochromic reaction has been successfully realized in PC‐PCN while maintaining its single crystallinity. In particular, as a solid‐state material which inherently integrates the photochromic switch and photosensitizer, SO‐PCN has demonstrated reversible control of ¹O₂ generation. Additionally, SO‐PCN shows catalytic activity towards photooxidation of 1,5‐dihydroxynaphthalene.     

A Magneto‐optical Molecular Device: Interplay of Spin Crossover,Luminescence, Photomagnetism,and Photochromism

A bis(pyrazolylpyridyl) ligand, L, containing a central photochromic dithienylethene spacer predictably forms a ferrous [Fe₂L₃]⁴⁺ helicate exhibiting spin crossover (SCO). In solution, the compound [Fe₂L₃](ClO₄)₄ ( 1 ) preserves the magnetic properties and is fluorescent. The structure of 1 is photo‐switchable following the reversible ring closure/opening of the central dithienylethene via irradiation with UV/visible light. This photoisomerization switches on and off some emission bands of 1 and provides a means of externally manipulating the magnetic properties of the assembly.     

Aromatic Sulfonium Polyoxomolybdates: Solid‐State Photochromic Materials with Tunable Properties

A new aromatic sulfonium counter‐ion motif for polyoxometalate (POM) clusters with potential for structural and electronic fine‐tuning has been designed. Its two derivatives 4‐hydroxyphenyl dimethylsulfonium triflate (HPDST) and 4‐(allyloxy)phenyl dimethylsulfonium triflate (APDST) exhibit ionic liquid behaviors under ambient conditions. HPDST and APDST are used to develop a series of aromatic sulfonium POM hybrids (HPDS/APDS)_n[XMo₁₂O₄₀] (HPDS and APDS are the cations of HPDST and APDST, respectively; X=P or Si; n=3 or 4), which are tested for photochromic behavior. On exposure to UV light, these POM hybrids undergo color change from yellow to green/blue. The coloration kinetics half lives (t_1/2) are less for APDS‐based hybrids than for HPDS‐based hybrids, suggesting that alkyl substitution on the phenolic group helps to fine‐tune the electron availability on the sulfonium moiety and hence to control the photochromic behavior of the POM hybrids. The t_1/2 values of these hybrids are considerably lower than those of the reported aliphatic sulfonium POM hybrids. We have also demonstrated the application of photoreduced POM hybrids as catalysts for the reduction of 4‐nitrophenol to 4‐aminophenol.     

A Multifunctional MnII Phosphonate for Rapid Separation of Methyl Orange and Electron‐Transfer Photochromism

A Mn^II phosphonate of the general formula [Mn(H₂L)₂(H₂O)₂(H₂bibp)] adopts a layered motif with protonated H₂bibp²⁺ cations embedded in the channels (H₄L=thiophene‐2‐phosphonic acid; bibp=4,4′‐bis(1‐imidazolyl)biphenyl). The title compound exhibits excellent adsorptive removal of methyl orange (MO) dye from aqueous solution. Its advantageous features include fast adsorption, high uptake capacity, selective removal, and reusability, which are of great significance for practical application in wastewater treatment. Meanwhile, the compound displays rapid photochromism upon irradiation with visible light at room temperature. Extensive research has demonstrated that such behavior is based on a ligand‐to‐ligand charge‐transfer (LLCT) mechanism. The irradiated sample possesses an ultra‐long‐lived charge‐separated state. Moreover, not only is the compound the first Mn‐based photochromic MOF, but it is also one of the very few examples showing LLCT with non‐photochromic components.     

Synthesis and Photochromic Behaviour of Novel 2H‐1‐Benzopyrans (=2H‐Chromenes) Derived from Carbazololes

The synthesis and photochromic properties of new 2,2‐diphenyl‐2H‐1‐benzopyrans, fused to an indole moiety, are described. All compounds exhibit photochromic behaviour in solution at room temperature. The heteroanellation effects are variable and depend on the position and geometry of the fused indole moiety. A general bathochromic shift in the spectra of the open forms is observed. The presence of a N‐methyl group prevents the broadening of the absorption spectra and promotes the instability of some photoinduced forms of compounds with the indole moiety fused at the 5,6 positions of the 2H‐1‐benzopyran skeleton. The enhanced photocolouration efficiency in the near‐UV and the kinetics of thermal bleaching indicate that the novel compounds with an indole moiety fused at the 6,7 positions, particularly those with a linked thiophene moiety, are very interesting molecules for applications in the field of variable optical absorption systems.     

Photochromic Free MOF‐Based Near‐Infrared Optical Switch

We demonstrate herein an all‐optical switch based on stimuli‐responsive and photochromic‐free metal–organic framework (HKUST‐1). Ultrafast near‐infrared laser pulses stimulate a reversible 0.4 eV blue shift of the absorption band with up to 200 s^?1 rate due to dehydration and concomitant shrinking of the structure‐forming [Cu₂C₄O₈] cages of HKUST‐1. Such light‐induced switching enables the remote modulation of intensities of photoluminescence of single crystals of HKUST‐1 as well visible radiation passing through the crystal by 2 order of magnitude. This opens up the possibility of utilyzing stimuli‐responsive MOFs for all‐optical data processing devices.     

An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules,Including Metal‐Free Dehydrogenation of Water

The first single‐component N‐heterocyclic silylene borane 1 (LSi‐R‐BMes₂; L=PhC(N^t Bu)₂; R=1,12‐xanthendiyl spacer; Mes=2,4,6‐Me₃C₆H₂), acting as a frustrated Lewis pair (FLP) in small‐molecule activation, can be synthesized in 65 % yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character. In small‐molecule activation 1 allows access to the intramolecular silanone–borane 3 featuring a Si=O→B interaction through reaction with O₂, N₂O, or CO₂, and formation of silanethione borane 4 from reaction with S₈. The Si^II center in 1 undergoes immediate hydrogenation if exposed to H₂ at 1 atm pressure in benzene, affording the silane borane 5‐H₂ , L(H₂)Si‐R‐BMes₂. Remarkably, no H₂ activation occurs if the single silylene LSiPh and Mes₃B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre‐organized Si–B separation in 1 enables a metal‐free dehydrogenation of H₂O to give the silanone–borane 3 as reactive intermediate.