Photoswitchable Catalysts: Correlating Structure and Conformational Dynamics with Reactivity by a Combined Experimental and Computational Approach

Solvatochromism and cis-trans isomerism in azobenzene-4-sulfonyl chloride

Solvatochromism exhibited by azobenzene-4-sulfonyl chloride (here abbreviated as Azo-SCl) has been investigated in a series of non-polar, polar-aprotic and polar-protic solvents. The UV-vis spectra of Azo-SCl exhibit two long-wavelength bands, observed at 321-330 nm (band-I) and 435-461 nm (band-II), which are ascribed to the π*-π (S2 ← S0) and π*-n (S1 ← S0) transitions, respectively. The shorter wavelength band indicates a reversal in solvatochromism, from negative to positive solvatochromism, for a solvent with a dielectric constant of 32.66 (which is characteristic of methanol), while the longer wavelength band signposts negative solvatochromism in all range of solvent's dielectric constant investigated, demonstrating different interactions with the solvents in the S2 and S1 excited states. Using Catalán and Kamlet-Taft solvation energy models, we found that the shift in the solvatochromic behavior of band-I (S2 ← S0) happens because solvent dipolarity/polarizability and hydrogen bonding affect the S2 state in opposite ways. Dipolarity/polarizability stabilizes the S2 state compared to the ground state, while hydrogen bonding destabilizes it. In contrast, for S1, both effects work together to destabilize the excited state. For all studied solvents, UV irradiation (λ ≥ 311 nm; room temperature) was found to lead to fast trans-cis azo photoisomerization. In the absence of light, the photogenerated cis form quickly converts back to the trans form. Interpretation of the experimental data is supported by quantum chemical calculations undertaken within the Density Functional Theory (DFT) framework, including Time Dependent DFT calculations for excited states.

Azobenzene-Attached (NHC)Gold(I) and (NHC)Copper(I) Complexes as Photoswitchable Catalysts

Photoswitchable (pre)catalysts, N,N'-bis-azobenzene-based (NHC)gold(I) and N,N'-bis-azobenzene-derived (NHC)copper(I) complexes are reported. Trans to cis isomerization of the attached photoswitchable moieties in the Au(I) complex enables four-fold decrement in the rate of oxazoline formation reaction. Whereas the progress of the copper(I) catalyzed, azide-alkyne cycloaddition reaction gets reduced by at least threefold. Alternate exposure to UV and blue light could easily toggle the rate of reactions remotely. The catalytic activity of thermodynamically stable trans-trans isomers is found to be similar to the common N-aryl substituted NHC-Au/Cu(I) complexes. NHC-Au(I) and -Cu(I) compounds bearing (trans)azobenzene moieties were characterized by X-ray diffraction. Photoswitching, recyclability studies, and the metastable isomer's thermal half-life in both complexes were studied via UV-visible spectroscopy. Whereas the extent of photoswitching and concomitant formation of geometrical isomers were investigated by using 1H-NMR spectroscopic study. Calculated percentage buried volumes of the three geometrical isomers show the trend trans-trans Collapse

Key Words

• Azobenzene
• Photochromic (NHC)gold(I) complex
• Photoswitchable (NHC)copper(I) complex
• Photoswitchable catalyst
• Rate-controlled reaction

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MESH Headings Collapse

Grants

• CRG/2021/001247 Science and Engineering Research Board

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Affiliation(s)

Aminul Islam Sk Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, 400019, India
Ayan Ghosh Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
Kshama Kundu Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai, 400085, India
Indukumari Murugan Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, 400019, India
Pintu K Kundu Department of Oils, Oleochemicals and Surfactants Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai, 400019, India

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Enantiodiscrimination of Inherently Chiral Thiacalixarenes by Residual Dipolar Couplings

Inherently chiral compounds, such as calixarenes, are chiral due to a nonplanar three-dimensional (3D) structure. Determining their conformation is essential to understand their properties, with nuclear magnetic resonance (NMR) spectroscopy being one applicable method. Using alignment media to measure residual dipolar couplings (RDCs) to obtain structural information is advantageous when classical NMR parameters like the nuclear Overhauser effect (NOE) or J-couplings fail. Besides providing more accurate structural information, the alignment media can induce different orientations of enantiomers. In this study, we examined the ability of polyglutamates with different side-chain moieties─poly-γ-benzyl-l-glutamate (PBLG) and poly-γ-p-biphenylmethyl-l-glutamate (PBPMLG) ─to enantiodifferentiate the inherently chiral phenoxathiin-based thiacalix[4]arenes. Both media, in combination with two solvents, allowed for enantiodiscrimination, which was, to the best of our knowledge, proved for the first time on inherently chiral compounds. Moreover, using the experimental RDCs, we investigated the calix[4]arenes conformational preferences in solution, quantitatively analyzed the differences in the alignment of the enantiomers, and discussed the pitfalls of the use of the RDC analysis.

Azobenzene-Integrated NHC Ligands: A Versatile Platform for Visible-Light-Switchable Metal Catalysis

A new class of photoswitchable NHC ligands, named azImBA, has been developed by integrating azobenzene into a previously unreported imidazobenzoxazol-1-ylidene framework. These rigid photochromic carbenes enable precise control over confinement around a metal's coordination sphere. As a model system, gold(I) complexes of these NHCs exhibit efficient bidirectional E-Z isomerization under visible light, offering a versatile platform for reversibly photomodulating the reactivity of organogold species. Comprehensive kinetic studies of the protodeauration reaction reveal rate differences of up to 2 orders of magnitude between the E and Z isomers of the NHCs, resulting in a quasi-complete visible-light-gated ON/OFF switchable system. Such a high level of photomodulation efficiency is unprecedented for gold complexes, challenging the current state-of-the-art in photoswitchable organometallics. Thorough investigations into the ligand properties paired with structure-reactivity correlations underscored the unique ligand's steric features as a key factor for reactivity. This effective photocontrol strategy was further validated in gold(I) catalysis, enabling in situ photoswitching of catalytic activity in the intramolecular hydroalkoxylation and -amination of alkynes. Given the significance of these findings and its potential as a widely applicable, easily customizable photoswitchable ancillary ligand platform, azImBA is poised to stimulate the development of adaptive, multifunctional metal complexes.

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.

From Organic Fragments to Photoswitchable Catalysts: The OFF-ON Structural Repository for Transferable Kernel-Based Potentials

Structurally and conformationally diverse databases are needed to train accurate neural networks or kernel-based potentials capable of exploring the complex free energy landscape of flexible functional organic molecules. Curating such databases for species beyond "simple" drug-like compounds or molecules composed of well-defined building blocks (e.g., peptides) is challenging as it requires thorough chemical space mapping and evaluation of both chemical and conformational diversities. Here, we introduce the OFF-ON (organic fragments from organocatalysts that are non-modular) database, a repository of 7869 equilibrium and 67,457 nonequilibrium geometries of organic compounds and dimers aimed at describing conformationally flexible functional organic molecules, with an emphasis on photoswitchable organocatalysts. The relevance of this database is then demonstrated by training a local kernel regression model on a low-cost semiempirical baseline and comparing it with a PBE0-D3 reference for several known catalysts, notably the free energy surfaces of exemplary photoswitchable organocatalysts. Our results demonstrate that the OFF-ON data set offers reliable predictions for simulating the conformational behavior of virtually any (photoswitchable) organocatalyst or organic compound composed of H, C, N, O, F, and S atoms, thereby opening a computationally feasible route to explore complex free energy surfaces in order to rationalize and predict catalytic behavior.

Exploring the impact of alignment media on RDC analysis of phosphorus-containing compounds: a molecular docking approach

Residual dipolar couplings (RDCs) are employed in NMR analysis when conventional methods, such as J-couplings and nuclear Overhauser effects (NOEs) fail. Low-energy (optimized) conformers are often used as input structures in RDC analysis programs. However, these low-energy structures do not necessarily resemble conformations found in anisotropic environments due to interactions with the alignment medium, especially if the analyte molecules are flexible. Considering interactions with alignment media in RDC analysis, we developed and evaluated a molecular docking-based approach to generate more accurate conformer ensembles for compounds in the presence of the poly-γ-benzyl-L-glutamate alignment medium. We designed chiral phosphorus-containing compounds that enabled us to utilize 31P NMR parameters for the stereochemical analysis. Using P3D/PALES software to evaluate diastereomer discrimination, we found that our conformer ensembles outperform moderately the standard, low-energy conformers in RDC analysis. To further improve our results, we (i) averaged the experimental values of the molecular docking-based conformers by applying the Boltzmann distribution and (ii) optimized the structures through normal mode relaxation, thereby enhancing the Pearson correlation factor R and even diastereomer discrimination in some cases. Nevertheless, we presume that significant differences between J-couplings in isotropic and in anisotropic environments may preclude RDC measurements for flexible molecules. Therefore, generating conformer ensembles based on molecular docking enhances RDC analysis for mildly flexible systems while flexible molecules may require applying more advanced approaches, in particular approaches including dynamical effects.

Near-Infrared, Organic Chiroptic Switch with High Dissymmetry Factors

Here we unveil a chiral molecular redox switch derived from PDI-based twistacenes─chPDI[2] that has the remarkable attributes of high-intensity and a broadband chiral response. This material exhibits facile, stable, and reversible multistate chiroptical switching behavior over a broad active wavelength range close to 700 nm, encompassing ultraviolet, visible, and near-infrared regions. Upon reduction, chPDI[2] exhibits a substantial increase in the amplitude of its circular dichroic response, with an outstanding |ΔΔε| > 300 M-1 cm-1 and a high dissymmetry factor of 3 × 10-2 at 960 nm. DFT calculations suggest that the long wavelength CD signal for doubly reduced chPDI[2] originates from excitation of the PDI backbone to the π* orbital of the bridging alkene. Importantly, the dimer's molecular contortion facilitates ionic diffusion, enabling chiral switching in solid state films. The high dissymmetry factors and near-infrared response establish chPDI[2] as a unique chiroptic switch.

Photomanipulation of Minimal Synthetic Cells: Area Increase, Softening, and Interleaflet Coupling of Membrane Models Doped with Azobenzene-Lipid Photoswitches

Light can effectively interrogate biological systems in a reversible and physiologically compatible manner with high spatiotemporal precision. Understanding the biophysics of photo-induced processes in bio-systems is crucial for achieving relevant clinical applications. Employing membranes doped with the photolipid azobenzene-phosphatidylcholine (azo-PC), a holistic picture of light-triggered changes in membrane kinetics, morphology, and material properties obtained from correlative studies on cell-sized vesicles, Langmuir monolayers, supported lipid bilayers, and molecular dynamics simulations is provided. Light-induced membrane area increases as high as ≈25% and a ten-fold decrease in the membrane bending rigidity is observed upon trans-to-cis azo-PC isomerization associated with membrane leaflet coupling and molecular curvature changes. Vesicle electrodeformation measurements and atomic force microscopy reveal that trans azo-PC bilayers are thicker than palmitoyl-oleoyl phosphatidylcholine (POPC) bilayers but have higher specific membrane capacitance and dielectric constant suggesting an increased ability to store electric charges across the membrane. Lastly, incubating POPC vesicles with azo-PC solutions results in the insertion of azo-PC in the membrane enabling them to become photoresponsive. All these results demonstrate that light can be used to finely manipulate the shape, mechanical and electric properties of photolipid-doped minimal cell models, and liposomal drug carriers, thus, presenting a promising therapeutic alternative for the repair of cellular disorders.

Mechanistic Basis for Red Light Switching of Azonium Ions

Azonium ions formed by the protonation of tetra-ortho-methoxy-substituted aminoazobenzenes photoisomerize with red light under physiological conditions. This property makes them attractive as molecular tools for the photocontrol of physiological processes, for example, in photopharmacology. However, a mechanistic understanding of the photoisomerization process and subsequent thermal relaxation is necessary for the rational application of these compounds as well as for guiding the design of derivatives with improved properties. Using a combination of sub-ps/ns transient absorption measurements and quantum chemical calculations, we show that the absorption of a photon by the protonated E-H+ form of the photoswitch causes rapid (ps) isomerization to the protonated Z-H+ form, which can also absorb red light. Proton transfer to solvent then occurs on a microsecond time scale, leading to an equilibrium between Z and Z-H+ species, the position of which depends on the solution pH. Whereas thermal isomerization of the neutral Z form to the neutral E form is slow (∼0.001 s-1), thermal isomerization of Z-H+ to E-H+ is rapid (∼100 s-1), so the solution pH also governs the rate at which E/E-H+ concentrations are restored after a light pulse. This analysis provides the first complete mechanistic picture that explains the observed intricate photoswitching behavior of azonium ions at a range of pH values. It further suggests features of azonium ions that could be targeted for improvement to enhance the applicability of these compounds for the photocontrol of biomolecules.

Pd(II)-catalyzed selective β-C-H functionalization of azobenzene carboxamides

We report a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided, site-selective β-C-H functionalization protocol for assembling modified azobenzene carboxamides. Considering the importance of azobenzenes in chemical sciences, this paper reports a new route for enriching the library of modified azobenzene motifs.

Advances in the Structural Strategies of the Self-Assembly of Photoresponsive Supramolecular Systems

Photosensitive supramolecular systems have garnered attention due to their potential to catalyze highly specific tasks through structural changes triggered by a light stimulus. The tunability of their chemical structure and charge transfer properties provides opportunities for designing and developing smart materials for multidisciplinary applications. This review focuses on the approaches reported in the literature for tailoring properties of the photosensitive supramolecular systems, including MOFs, MOPs, and HOFs. We discuss relevant aspects regarding their chemical structure, action mechanisms, design principles, applications, and future perspectives.

How Robust Is the Reversible Steric Shielding Strategy for Photoswitchable Organocatalysts?

A highly appealing strategy to modulate a catalyst's activity and/or selectivity in a dynamic and noninvasive way is to incorporate a photoresponsive unit into a catalytically competent molecule. However, the description of the photoinduced conformational or structural changes that alter the catalyst's intrinsic reactivity is often reduced to a handful of intuitive static representations, which can struggle to capture the complexity of flexible organocatalysts. Here, we show how a comprehensive exploration of the free energy landscape of N-alkylated azobenzene-tethered piperidine catalysts is essential to unravel the conformational characteristics of each configurational state and explain the experimentally observed reactivity trends. Mapping the catalysts' conformational space highlights the existence of false ON or OFF states that lower their switching ability. Our findings expose the challenges associated with the realization of a reversible steric shielding for the photocontrol of Brønsted basicity of piperidine photoswitchable organocatalysts.

An inclusion complex of cucurbit[7]uril with benzimidazolyl benzyl viologen exhibits fluorescence and photochromic properties

A self-assembled supramolecular inclusion complex of Q[7] with benzimidazolyl benzyl viologen exhibits interesting fluorescence emission and reversible photochromism.

Late-Stage Functionalization of the Periphery of Oligophenylene Dendrimers with Various Arene Units via Fourfold C-H Borylation

Late-stage functionalization of the periphery of oligophenylene dendrimers was efficiently achieved via site-selective C-H activation of a preconstructed, readily accessible dendron. By fourfold iridium-catalyzed C-H borylation followed by Suzuki-Miyaura cross-coupling, various arene units were introduced into the end points of the 1,3,5-phenylene-based hydrocarbon dendron. Coupling of the modified dendrons with a core unit, such as 2,6-dibromobenzoic acid derivatives, afforded the periphery-functionalized dendrimers that also have an endohedral functionality at the core position.

Experimental and Theoretical Characterization of Ultrafast Water-Soluble Photochromic Photoacids

UV-visible transient absorption spectroscopy and quantum mechanical simulations are combined to elucidate the photochemical mechanism of two metastable merocyanine/spiropyran photoacids, 2-[(E)-2-(2-hydroxyphenyl)ethenyl]-3,3-dimethyl-1-(3-sulfopropyl)-3H-indol-1-ium (phenylhydroxy-MCH) and 2-[(E)-2-(1H-indazol-7-yl)ethenyl]-3-(3-sulfopropyl)-1,3-benzothiazol-3-ium (indazole-MCH). Transient absorption spectra demonstrate that trans-acid isomerization to the cis form results in deprotonation on a picosecond time scale. Ring closure to form spiropyran follows promptly from the appropriate conformation or follows at longer time delays (≫3.5 ns) following a barrier crossing for single-bond isomerization to the appropriate conformation. Consistent with the results of Berton et al. [ Chem. Sci. 2020, 11, 8457-8468] , we find that cis-phenylhydroxy-MCH is a stronger acid than trans-phenylhydroxy-MCH. The decrease in pK_a upon isomerization is further investigated to benchmark quantum chemical methods for their accuracy. Calculations were performed with nine levels of theory including continuum solvent models and explicit water. The calculations are not sufficient to describe the ΔpK_a following isomerization of these photoacids, and more work is necessary to properly evaluate the physical basis for the acidity of the cis photoacids.

Electron transfer photochromism of solid-state supramolecules constructed by cucurbit[n]uril (n = 5–8) and 1-(4-carboxybenzyl)-4-[2-(4-pyridyl)-vinyl]-pyridinium chloride

Solid-state supramolecular complexes of a bispyridinium ethylene derivative with cucurbit[n = 5–8]uril exhibit fast responsive photochromic properties, which are caused by the electron transfer from the host to the guest.

Construction of a viologen-derived 2D material with photochromism, photoswitchable luminescence and inkless and erasable prints via sunlight irradiation

A Zn(ii) material based on thiophenedicarboxylate and viologen derivative mixed ligands is synthesized. Thanks to pyridinium unit embedded in its structure, the material exhibits a rapid photochromic response via sunlight irradiation.

Photonic Switching Porous Organic Polymers toward Reversible Control of Heterogeneous Photocatalysis

Sonogashira-Hagihara coupling reaction of photoswitchable dithienylethene (AEDTE) with metal-free 5,10,15,20-tetrakis(4-iodophenyl)porphyrin and its metal derivatives (MTIPP, M = H₂, Zn(II), Fe(II)) results in three porous organic polymers (POPs) including AEDTE-H₂TIPP-POP, AEDTE-ZnTIPP-POP, and AEDTE-FeTIPP-POP. The morphology, components, and structures of newly obtained POPs have been examined by a range of spectroscopic and microscopic techniques including infrared spectroscopy (IR), solid-state UV-vis diffuse reflectance spectroscopy, thermogravimetric analysis (TGA), powder X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The porous structures have been estimated by nitrogen and carbon dioxide sorption isotherms at 77 and 196 K, respectively. The open-AEDTE-H₂TIPP-POP with AEDTE in an open form was revealed to be an effective and stable heterogeneous photocatalyst for visible light-driven oxidation of N-methylpyridinium salts possibly because of its relatively large specific surface area. In particular, a proof-of-concept of photoswitchable POP photocatalysts has been established using different light irradiation upon open-AEDTE-H₂TIPP-POP to control its heterogeneous photocatalytic behaviors because of the adjustment over the electron transfer process and porous structures through photoisomerization of AEDTE. The present result highlights the bright perspective of photoswitching POPs in the field of materials chemistry and catalysis community.

Visible Light-Gated Organocatalysis Using a RuII -Photocage

The light-gated organocatalysis via the release of 4-N,N-dimethylaminopyridine (DMAP) by irradiation of the [Ru(bpy)₂ (DMAP)₂ ]²⁺ complex with visible light was investigated. As model reaction the acetylation of benzyl alcohols with acetic anhydride was chosen. The pre-catalyst releases one DMAP molecule on irradiation at wavelengths longer than 455 nm. The photochemical process was characterized by steady-state irradiation and ultrafast transient absorption spectroscopy. The latter enabled the observation of the ³ MLCT state and the spectral features of the penta-coordinated intermediate [Ru(bpy)₂ (DMAP)]²⁺ . The released DMAP catalyzes the acetylation of a wide range of benzyl alcohols with chemical yields of up to 99 %. Control experiments revealed unequivocally that it is the released DMAP which takes the role of the catalyst.

Azo-dimethylaminopyridine-functionalized Ni(II)-porphyrin as a photoswitchable nucleophilic catalyst

We present the synthesis and the photochemical and catalytic switching properties of an azopyridine as a photoswitchable ligand, covalently attached to a Ni(II)-porphyrin. Upon irradiation with 530 nm (green light), the azopyridine switches to the cis configuration and coordinates with the Ni2+ ion. Light of 435 nm (violet) isomerizes the ligand back to the trans configuration, which decoordinates for steric reasons. This so-called record player design has been used previously to switch the spin state of Ni2+ between singlet and triplet. We now use the coordination/decoordination process to switch the catalytic activity of the dimethylaminopyridine (DMAP) unit. DMAP is a known catalyst in the nitroaldol (Henry) reaction. Upon coordination to the Ni2+ ion, the basicity of the pyridine lone pair is attenuated and hence the catalytic activity is reduced. Decoordination restores the catalytic activity. The rate constants in the two switching states differ by a factor of 2.2, and the catalytic switching is reversible.

Azobenzene Isomerization-Induced Photomodulation of Electronic Properties of N-Heterocyclic Carbenes

Azobenzene-based protonated N-heterocyclic carbenes (NHCs), N,N'-bis(azobenzene)imidazolium chlorides (IAz-X⋅HCl; X=OMe, Br, H) were successfully synthesized and switching abilities of the attached azobenzene units along with the concomitant photoinduced generation of geometric isomers were studied. Upon irradiation with 365 nm UV light, a p-methoxy-azobenzene derivative get transformed from all-trans isomer to nearly all-cis isomer at the photostationary state. The extent of photomodulation of electronic properties in the NHC ring of the p-methoxy-azobenzene derivative is determined through the Tolman Electronic Parameter (TEP). Iridium complex, [(IAz-OMe)IrCl(CO)₂ ] was synthesized and infrared spectra were recorded in dichloromethane solution as film in NaCl crystals and by drop-casting in an ATR crystal. Comparison in averaged carbonyl stretching frequency between all-trans isomer ( ν ˜ t t av ) and all-cis isomer ( ν ˜ c c a v ) indicates a significant decrement of Δ^tt-cc ν ˜ _av =2.7 cm^-1 (film) and 3.8 cm^-1 (ATR). Therefore, moderate to excellent enhancement of electron density (Δ^tt-cc TEP=2.3 cm^-1 [film] and 3.2 cm^-1 [ATR]) at the C-2 position of the NHC is achieved through trans→cis isomerization of the remotely placed azobenzene units. This fine phototuning of electron-donating ability at the catalytic center would pave the way to control NHC/NHC-metal catalyzed organic transformations through external stimuli.

Temporal control in tritylation reactions through light-driven variation in chloride ion binding catalysis – a proof of concept

A proof-of-concept on temporal control in the tritylation reactions has been demonstrated using a designed tripodal triazole-linked azo(hetero)arene-based photoswitchable catalyst.

X-ray and UV Dual Photochromism, Thermochromism, Electrochromism, and Amine-Selective Chemochromism in an Anderson-like Zn7 Cluster-Based 7-Fold Interpenetrated Framework

Smart materials are highly desirable over the recent decade due to the growing demand of complicated nature. Stable stimuli-responsive smart materials exhibit widespread potential for applications in smart windows, sensors, separators, chemical valves, and release platforms but are rare. Despite being good candidates, viologen-based multifunctional smart materials are still a challenging task for chemists. To obtain such materials, the judicious strategy is to introduce polynuclear metal-carboxylate clusters as electron donors into a stable framework to increase chromic sensitivity. Toward this endeavor, we have synthesized a novel viologen-based polymer with a unique Anderson-like metal-carboxylate cluster, [Zn₇(bpybc)₃(o-BDC)₆]·2NO₃·6H₂O (bpybc = 1,1'-bis(4-carboxyphenyl)-4,4'-bipyridinium, o-BDC = o-benzenedicarboylic acid) (1), which is a particular 7-fold interpenetrated framework with a 3D pcu network in which bpybc ligand as the linker and Zn₇O₃₀C₁₂ as the second building unit (Zn₇ SBU) were used as 6-connected nodes. More importantly, it shows excellent chromic behavior in response to multiple external stimuli especially soft X-ray and UV dual light, temperature, electricity, and organic amines, which stand out in the viologen-based polymers. Interestingly, the coloration process of 1 from "core" to "edge" is observed upon heating at the appropriate temperature, which has not yet been found in other reported thermochromic materials. Of particular interest for 1 is the couple of quaternary stimuli-sensitive abilities because it simultaneously meets the following conditions: (i) the capability of withstanding high light, higher temperature, extreme pH, and other harsh conditions; and (ii) the high sensitivity to external stimuli keeping away from photodegradation, thermal relaxation, side reactions, and so on. To be noted, 1 has high thermal stability and chemical stability, which are excellent advantages as smart materials. To further develop possible practical utilization, 1 has been doped into the polymer matrixes to construct a hybrid film, which not only keeps the response to external stimuli but also significantly improves the repeatability of the photochromic process, indicating that a new smart device with multi-stimuli-responsive functions will emerge successively in the future.

QuatCy: A Heptamethine Cyanine Modification With Improved Characteristics

A major restriction on optical imaging techniques is the range of available fluorophores that are compatible with aqueous media without aggregation, absorb light above 750 nm with high extinction coefficients, fluoresce with relatively high quantum yields, and resist photodecomposition. Indocyanine green (ICG or A in this paper) is an important example of a fluorophore that fits this description. Other dyes that are becoming increasingly prevalent are select heptamethine cyanine dyes (Cy7) which feature a cyclohexyl framework to rigidify the conjugated alkenes, and meso-chlorine substitution; MHI-148 (B) is one example. Methods: Research described here was initiated to uncover the consequences of a simple isoelectronic substitution to MHI-148 that replaces a cyclohexyl methylene with a dialkyl ammonium fragment. Solubility experiments were carried out in aqueous and cell culture media, photophysical properties including fluorescence quantum yields, brightness and stability were measured. Moreover, in vivo pharmacokinetics, distribution and tumor seeking properties were also explored. Results: Modification to incorporate dialkyl ammonium fragment leads to a brighter, more photostable fluorophore, with a decreased tendency to aggregation, complementary solubility characteristics, and a lower cytotoxicity. Conclusion: All the above-mentioned parameters are favorable for many anticipated applications of the new dye we now call quaternary cyanine-7 or QuatCy.

Photoswitchable catalysis based on the isomerisation of double bonds

Photoswitchable catalysis is a young but rapidly evolving field that offers great potential for non-invasive dynamic control of both activity and selectivity in catalysis. This Feature Article summarises the key developments accomplished over the past years through the incorporation of photoswitchable double bonds into the structure of catalytically competent molecules.

Photochromic dithienylethene based on porphyrin for a nondestructive information processing

A 1-[2-methyl-5-(4-tetraphenylporphyrinphenyl)-3-thienyl)-2-[2-methyl-3-thienyl]cyclopentene combined by 3,3[Formula: see text]-(1-cyclopentene-1,2-diyl)bis(5-chloro-2-methyl)thiophene and tetraphenylporphyrin can transform between the open isomer and the closed isomer upon the irradiation of UV or visible light. Thus they can be utilized to write binary data. Furthermore, the open form can emit luminescence but the closed cannot while irradiated by another light that cannot cause optical chemical reactions. Therefore, data can be read out without damage.

Synthesis of Hydrazines via Radical Generation and Addition of Azocarboxylic tert-Butyl Esters

A new chemistry of azo compounds that is a radical generation and addition in situ of azocarboxylic tert-butyl esters to synthesize hydrazines has been described. The protocol provides a novel strategy for the synthesis of various hydrazines. The advantages of the transformation include broad substrate scope, benign conditions, and convenient operation.

Comparing Isomeric Tridentate Carbazole-Based Click Ligands: Metal Complexes and Redox Chemistry

Two novel bis(triazolyl)carbazole ligands Hbtc1 (3,6-di(tert-butyl)-1,8-bis[(1-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-4-yl)]-9H-carbazole) and Hbtc2 (3,6-di(tert-butyl)-1,8-bis[(4-(3,5-di(tert-butyl)phenyl)-1,2,3-triazol-1-yl)]-9H-carbazole), differing in the regiochemistry of triazole attachment, have been synthesized by Cu-catalyzed azide-alkyne cycloaddition, the so-called "click-reactions". Metalation with Ru, Zn, and Ni precursors led to the formation of M(btc)₂ complexes (M=Ru, Zn, Ni), with two deprotonated ligands coordinating to the metal center in tridentate fashion, forming almost perfectly octahedral coordination spheres. The redox properties of M(btc)₂ complexes have been investigated by cyclic voltammetry, UV/Vis spectroscopy, spectroelectrochemistry, and chemically. The CV of the ruthenium complexes revealed three quasi-reversible one-electron oxidation processes, one assigned as the Ru^II/III couple and two originating from ligand-based oxidations. The CVs of both Zn and Ni complexes contained only two oxidation waves corresponding to the oxidation of the two ligands. The oxidation potentials of complexes derived from Hbtc1 ligands were found to be 300-400 mV lower than those of the corresponding complexes derived from Hbtc2, reflecting the significant difference in donation through the N(2) or N(3) atom of the triazole moiety.

Engineered control of enzyme structural dynamics and function

Enzymes undergo a range of internal motions from local, active site fluctuations to large-scale, global conformational changes. These motions are often important for enzyme function, including in ligand binding and dissociation and even preparing the active site for chemical catalysis. Protein engineering efforts have been directed towards manipulating enzyme structural dynamics and conformational changes, including targeting specific amino acid interactions and creation of chimeric enzymes with new regulatory functions. Post-translational covalent modification can provide an additional level of enzyme control. These studies have not only provided insights into the functional role of protein motions, but they offer opportunities to create stimulus-responsive enzymes. These enzymes can be engineered to respond to a number of external stimuli, including light, pH, and the presence of novel allosteric modulators. Altogether, the ability to engineer and control enzyme structural dynamics can provide new tools for biotechnology and medicine.

Illumination of Nanoliter-NMR Spectroscopy Chips for Real-Time Photochemical Reaction Monitoring

We report the use of a small-volume nuclear-magnetic-resonance (NMR)-spectroscopy device with integrated fiber-optics for the real-time detection of UV-vis-light-assisted chemical reactions. An optical fiber is used to guide the light from LEDs or a laser diode positioned safely outside the magnet toward the 25 nL detection volume and placed right above the microfluidic channel, irradiating the transparent back of the NMR chip. The setup presented here overcomes the limitations of conventional NMR systems for in situ UV-vis illumination, with the microchannel permitting efficient light penetration even in highly concentrated solutions, requiring lower-power light intensities, and enabling high photon flux. The efficacy of the setup is illustrated with two model reactions activated at different wavelengths.

Chemical signal activation of an organocatalyst enables control over soft material formation

Cells can react to their environment by changing the activity of enzymes in response to specific chemical signals. Artificial catalysts capable of being activated by chemical signals are rare, but of interest for creating autonomously responsive materials. We present an organocatalyst that is activated by a chemical signal, enabling temporal control over reaction rates and the formation of materials. Using self-immolative chemistry, we design a deactivated aniline organocatalyst that is activated by the chemical signal hydrogen peroxide and catalyses hydrazone formation. Upon activation of the catalyst, the rate of hydrazone formation increases 10-fold almost instantly. The responsive organocatalyst enables temporal control over the formation of gels featuring hydrazone bonds. The generic design should enable the use of a large range of triggers and organocatalysts, and appears a promising method for the introduction of signal response in materials, constituting a first step towards achieving communication between artificial chemical systems.Enzymes regulated by chemical signals are common in biology, but few such artificial catalysts exist. Here, the authors design an aniline catalyst that, when activated by a chemical trigger, catalyses formation of hydrazone-based gels, demonstrating signal response in a soft material.

Influence of Intramolecular Charge Transfer and Nuclear Quantum Effects on Intramolecular Hydrogen Bonds in Azopyrimidines

Intramolecular hydrogen bonds (IMHBs) in 5-azopyrimidines are investigated by NMR spectroscopy and DFT computations that involve nuclear quantum effects. A series of substituted 5-phenylazopyrimidines with one or two hydrogen bond donors able to form IMHBs with the azo group is prepared by azo coupling. The barrier of interconversion between two rotamers of the compounds with two possible IMHBs is determined by variable temperature NMR spectroscopy and it is demonstrated that the barrier is significantly affected by intramolecular charge transfer. Through-hydrogen-bond scalar coupling is investigated in ¹⁵N labeled compounds and the stability of the IMHBs is correlated with experimental NMR parameters and rationalized by path integral molecular dynamics simulations that involve nuclear quantum effects. Detailed information on the hydrogen bond geometry upon hydrogen-to-deuterium isotope exchange is obtained from a comparison of experimental and calculated NMR data.

A complementary pair of enantioselective switchable organocatalysts

A pair of enantioselective switchable bifunctional catalysts are shown to promote a range of conjugate addition reactions in up to 95 : 5 e.r. and 95% conversion. Each catalyst can be switched OFF using conditions that switch the other catalyst ON. Catalyst ON : OFF ratios of up to 98 : 2 and 1 : 99 were achieved, with a ratio of reaction rates of up to 16 : 1 between the ON and OFF states, maintained over complete ON-OFF-ON and OFF-ON-OFF cycles. However, simultaneous operation of the catalyst pair in the same reaction vessel, which in principle could allow product handedness to be switched by simple E-Z isomerisation of the catalyst pair, was unsuccessful. In this first generation complementary pair of enantioselective switchable organocatalysts, the OFF state of one catalyst inhibits the ON state of the other.

Photoswitchable Catalysis by a Nanozyme Mediated by a Light-Sensitive Cofactor

The activity of a gold nanoparticle-based catalyst can be reversibly up- and down-regulated by light. Light is used to switch a small molecule between cis- and trans-isomers, which inhibits the catalytic activity of the nanoparticles to different extent. The system is functional in aqueous buffer, which paves the way for integrating the system in biological networks.

Bifunctional Molecular Photoswitches Based on Overcrowded Alkenes for Dynamic Control of Catalytic Activity in Michael Addition Reactions

The emerging field of artificial photoswitchable catalysis has recently shown striking examples of functional light-responsive systems allowing for dynamic control of activity and selectivity in organocatalysis and metal-catalysed transformations. While our group has already disclosed systems featuring first generation molecular motors as the switchable central core, a design based on second generation molecular motors is lacking. Here, the syntheses of two bifunctionalised molecular switches based on a photoresponsive tetrasubstituted alkene core are reported. They feature a thiourea substituent as hydrogen-donor moiety in the upper half and a basic dimethylamine group in the lower half. This combination of functional groups offers the possibility for application of these molecules in photoswitchable catalytic processes. The light-responsive central cores were synthesized by a Barton-Kellogg coupling of the prefunctionalized upper and lower halves. Derivatization using Buchwald-Hartwig amination and subsequent introduction of the thiourea substituent afforded the target compounds. Control of catalytic activity in the Michael addition reaction between (E)-3-bromo-β-nitrostyrene and 2,4-pentanedione is achieved upon irradiation of stable-(E) and stable-(Z) isomers of the bifunctional catalyst 1. Both isomers display a decrease in catalytic activity upon irradiation to the metastable state, providing systems with the potential to be applied as ON/OFF catalytic photoswitches.

Photoregulated Living Supramolecular Polymerization Established by Combining Energy Landscapes of Photoisomerization and Nucleation-Elongation Processes

The significant contribution of conventional living polymerization to polymer science assures that living supramolecular polymerization will also lead to a variety of novel phenomena and applications. However, the monomer scope still remains limited in terms of the self-assembly energy landscape; a kinetic trap that retards spontaneous nucleation has to be coupled with a supramolecular polymerization pathway, which is challenging to achieve by molecular design. Herein, we report a rational approach to addressing this issue. We combined the supramolecular polymerization and photoisomerization processes to build the energy landscape, wherein the monomer can be activated/deactivated by light irradiation. In this way, the supramolecular polymerization and kinetic trap can be independently designed in the energy landscape. When the "dormant" monomer was activated by light in the presence of the seed of the supramolecular polymer, the "activated" free monomer was polymerized at the termini of the seed in a chain-growth manner. As a result, we achieved supramolecular polymers with controlled lengths and a narrow polydispersity. Although photoisomerization has been extensively employed in supramolecular polymer chemistry, most studies have focused on the stimuli responsiveness. In this respect, the present study would provoke supramolecular chemists to revisit stimuli-responsive supramolecular polymer systems as potential candidates for devising living supramolecular polymerization.

Controlling Proton Delivery through Catalyst Structural Dynamics

The fastest synthetic molecular catalysts for H₂ production and oxidation emulate components of the active site of hydrogenases. The critical role of controlled structural dynamics is recognized for many enzymes, including hydrogenases, but is largely neglected in designing synthetic catalysts. Our results demonstrate the impact of controlling structural dynamics on H₂ production rates for [Ni(P^Ph₂ N^C6H4R₂ )₂ ]²⁺ catalysts (R=n-hexyl, n-decyl, n-tetradecyl, n-octadecyl, phenyl, or cyclohexyl). The turnover frequencies correlate inversely with the rates of chair-boat ring inversion of the ligand, since this dynamic process governs protonation at either catalytically productive or non-productive sites. These results demonstrate that the dynamic processes involved in proton delivery can be controlled through modification of the outer coordination sphere, in a manner similar to the role of the protein architecture in many enzymes. As a design parameter, controlling structural dynamics can increase H₂ production rates by three orders of magnitude with a minimal increase in overpotential.