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Molecular photoswitches provide interesting tools to reversibly control various biological functions with light. Thanks to its small size and easy introduction into the biomolecules, azobenzene derivatives have been widely employed in the field of photopharmacology. All visible‐light switchable azobenzenes with controllable thermostability are highly demanded. Based on the reported tetra‐o‐chloroazobenzenes, we synthesized push‐pull systems, by introducing dialkyl amine and nitro groups as strong electron‐donating and electron‐withdrawing groups on the para‐positions, and then transformed to push‐push systems by a simple reduction step. The developed push‐pull and push‐push tetra‐o‐chloroazobenzene derivatives displayed excellent photoswitching properties, as previously reported. The half‐life of the Z‐isomers can be tuned from milliseconds for the push‐pull system to several hours for the push‐push system. The n‐π* and π‐π* transitions have better resolution in the push‐push molecules, and excitation at different wavelengths can tune the E/Z ratio at the photostationary state. For one push‐pull molecule, structure and absorption spectra obtained from theoretical calculations are compared with experimental data, along with data on the push‐push counterpart. [ABSTRACT FROM AUTHOR]

Azo compounds are efficient electron acceptors. Upon one‐electron reduction they generally isomerize forming the thermodynamically most stable radical anion. Herein we show that the size of the central ring in 1,2‐diazocines and diazonines has a ruling influence on the configuration of the one‐electron reduced species. Markedly, diazonines, which bear a central nine membered heterocycle, show light‐induced E/Z isomerization, but retain the configuration of the diazene N=N moiety upon one‐electron reduction. Accordingly, E/Z isomerization is not induced by reduction. [ABSTRACT FROM AUTHOR]

Azobenzene photoswitches are valuable tools for controlling properties of molecular systems with light. We have been investigating azobenzene glycoconjugates to probe carbohydrate‐protein interactions and to design glycoazobenzene macrocycles with chiroptical and physicochemical properties modulated by light irradiation. To date, direct conjugation of glycosides to azobenzenes was performed by reactions providing target compounds in limited yields. We therefore sought a more effective and reliable coupling method. In this paper, we report on a straightforward thioarylation of azobenzene derivatives with glycosyl thiols as well as other thiols, thereby increasing the scope of azobenzene conjugation. Even challenging unsymmetrical conjugates can be achieved in good yields via sequential or one‐pot procedures. Importantly, red‐shifted azoswitches, which are addressed with visible light, were easily functionalized. Additionally, by oxidation of the sulfide bridge to the respective sulfones, both the photochromic and the thermal relaxation properties of the core azobenzene can be tuned. Utilizing this option, we realized orthogonal three‐state photoswitching in mixtures containing two distinct azobenzene thioglycosides. [ABSTRACT FROM AUTHOR]

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

To explore new cavity functions, we herein employed cis‐trans stereoisomers with a N=N, C=C, or C=N unit as guest indicators for a polyaromatic capsule. Thanks to the rigid, spherical cavity with a diameter of ∼1 nm, azobenzene and stilbene derivatives are quantitatively encapsulated by the capsule with 100 % cis‐selectivity in water. The isomerization of the cis‐azo compound is suppressed against heat and light in the cavity, due to the confinement effect. Furthermore, C,N‐diphenyl imine derivatives are quantitatively encapsulated by the capsule in water and adopt an otherwise unstable cis‐form. The polyaromatic cavity suppresses the hydrolysis of the imines in water, even at elevated temperature, due to the shielding effect. Accordingly, the properties of the cis‐trans isomers could be largely altered through supramolecular manipulation. [ABSTRACT FROM AUTHOR]

Azobenzene and its derivatives are one of the most widespread molecular scaffolds used in a range of modern applications, as well as in fundamental research. After photoexcitation, azo‐based photoswitches revert back to the most stable isomer on a timescale (t1/2) that determines the range of potential applications. Attempts to bring t1/2 to extreme values prompted the development of azobenzene and azoheteroarene derivatives that either rebalance the E‐ and Z‐isomer stabilities, or exploit unconventional thermal isomerization mechanisms. In the former case, one successful strategy has been the creation of macrocycle strain, which tends to impact the E/Z stability asymmetrically, and thus significantly modifyt1/2. On the bright side, bridged derivatives have shown an improved optical switching owing to the higher quantum yields and absence of degradation. However, in most (if not all) cases, bridged derivatives display a reversed thermal stability (more stable Z‐isomer), and smaller t1/2 than the acyclic counterparts, which restricts their potential interest to applications requiring a fast forward and backwards switch. In this paper, the impact of alkyl bridges on the thermal stability of phenyl‐azoheteroarenes is investigated by using computational methods, and it is revealed that it is indeed possible to combine such improved photoswitching characteristics while preserving the regular thermal stability (more stable E‐isomer), and increased t1/2 values under the appropriate connectivity and bridge length. [ABSTRACT FROM AUTHOR]

Polystannanes with azobenzene moieties designed to protect the Sn-Sn backbone from light- and moisture-induced degradation are described. The azo-stannyl precursor 3 (70 %) is converted in good yields (88-91 %) to the mono- ( 4), and dichlorostannanes ( 5), by sequential chlorination, followed by further reduction of 5 to the dihydride ( 6) using NaBH4 (78 %). All stannanes were characterised by NMR (1H, 13C, 119Sn) spectroscopy and HRMS; in addition, 3, 4 and 5 were structurally elucidated using X-ray diffraction analysis. Metal-free dehydrocoupling of 6 at RT leads exclusively to homopolymer ( 7- i) displaying an initial solution 119Sn NMR signal ( δ=−196 ppm) that migrates to −235 ppm after 10 days ( 7- f). In contrast, metal-catalyzed dehydrocoupling of 6 in toluene at RT leads directly 7- f. Random co-polymers formed from 6 and ( nBu)2SnH2 at 4:1 ( 8 a) and 1:1 ( 8 b) ratios were compared to the alternating polystannane ( 9) prepared by the reaction of 6 with ( nBu)2Sn(NEt2)2. DFT calculations of 3- 6 indicate that hypercoordination at Sn is influenced by substituents and by solvation. Homopolymer 7 was found to have unprecedented moisture and light stability in the solid state for >6 months. [ABSTRACT FROM AUTHOR]

This report delineates the design and synthesis of negatively charged azobenzene derivatives that form photo-responsive ion-pairing assemblies. The azobenzene carboxylates possessing aliphatic chains were prepared as photo-responsive anions that promote the formation of ion-pairing dimension-controlled assemblies, including mesophases, when used in conjunction with a tetrabutylammonium (TBA) cation. The photo-responsive properties of the ion pairs and the precursory carboxylic acids in the bulk state were examined by polarized optical microscopy (POM) and X-ray diffraction (XRD), demonstrating that liquid crystal (LC)-liquid and crystal-liquid phase transitions occurred, depending on the number and lengths of the aliphatic chains of each assembly. An ion pair exhibited photo-induced crystal-crystal phase transitions upon switching between two irradiation wavelengths (365/436 nm). [ABSTRACT FROM AUTHOR]

We synthesized a novel green-light-responsive tetra- ortho-isopropoxy-substituted azobenzene (ipAzo). Cis-ipAzo forms a strong host-guest complex with γ-cyclo dextrin (γ-CD) whereas trans-ipAzo binds weakly. This new photoresponsive host-guest interaction is reverse to the well-known azobenzene (Azo)/α-cyclodextrin (α-CD) complex, which is strong only between trans-Azo and α-CD. By combining the UV-light-responsive Azo/α-CD and green-light-responsive ipAzo/γ-CD host-guest complexes, a photoresponsive orthogonal supramolecular system is developed. [ABSTRACT FROM AUTHOR]

The photomechanical motion of chiral crystals of trans-azobenzene derivatives with an (S)- and (R)-phenylethylamide group was investigated and compared with a racemic crystal. Changes in the UV/Vis absorption spectra of the powdered crystals before and after UV irradiation were measured by using an optical waveguide spectrometer, showing that the lifetime of the cis-to-trans thermal back-isomerization of the chiral crystals was faster than that of the racemic crystals. Upon UV irradiation, a long plate-like chiral microcrystal bent away from the light source with a twisting motion. A square-like chiral microcrystal curled toward the light with some twisting. Reversible bending of a rod-like chiral microcrystal was repeatable over twenty-five cycles. In contrast, bending of a plate-like racemic microcrystal was small. A possible mechanism for the bending and twisting motion was discussed based on the optimized cis conformer determined by using calculations, showing that the bending motion with twisting is caused by elongation along the b axis and shrinkage along the a axis. [ABSTRACT FROM AUTHOR]

The photomechanical motion of chiral crystals of trans-azobenzene derivatives with an ( S)- and ( R)-phenylethylamide group was investigated and compared with a racemic crystal. Changes in the UV/Vis absorption spectra of the powdered crystals before and after UV irradiation were measured by using an optical waveguide spectrometer, showing that the lifetime of the cis-to- trans thermal back-isomerization of the chiral crystals was faster than that of the racemic crystals. Upon UV irradiation, a long plate-like chiral microcrystal bent away from the light source with a twisting motion. A square-like chiral microcrystal curled toward the light with some twisting. Reversible bending of a rod-like chiral microcrystal was repeatable over twenty-five cycles. In contrast, bending of a plate-like racemic microcrystal was small. A possible mechanism for the bending and twisting motion was discussed based on the optimized cis conformer determined by using calculations, showing that the bending motion with twisting is caused by elongation along the b axis and shrinkage along the a axis. [ABSTRACT FROM AUTHOR]

Transition-metal complexes containing stimuli-responsive systems are attractive for applications in optical devices, photonic memory, photosensing, as well as luminescence imaging. Amongst them, photochromic metal complexes offer the possibility of combining the specific properties of the metal centre and the optical response of the photochromic group. The synthesis, the electrochemical properties and the photophysical characterisation of a series of donor-acceptor azobenzene derivatives that possess bipyridine groups connected to a 4-dialkylaminoazobenzene moiety through various linkers are presented. DFT and TD-DFT calculations were performed to complement the experimental findings and contribute to their interpretation. The position and nature of the linker (ethynyl, triazolyl, none) were engineered and shown to induce different electronic coupling between donor and acceptor in ligands and complexes. This in turn led to strong modulations in terms of photoisomerisation of the ligands and complexes. [ABSTRACT FROM AUTHOR]

Two novel cyclic azobenzenophanes (SC, RC) with functional handles have been synthesized efficiently by a Glaser coupling reaction. Through a Suzuki coupling reaction, alternating ring/linear polymers with rigid (conjugated)/flexible (unconjugated) bridges were obtained from the resultant cyclic azobenzenophanes. The optical activities of linear, cyclic, and macromolecular binaphethyl-azobenzene derivatives were investigated by UV/Vis and circular dichroism (CD) spectra and the time-dependent (TD)-DFT method. Experimental results and theoretical analyses indicated that the cyclic configurations exhibited better chiroptical features than the others, and the reverse conformation and difference of dextro-/levo-rotation of azobenzenophanes were detected by comparing linear and cyclic structures, which provides an opportunity for the optical-rotation-controlled 'smart' materials systems in future. [ABSTRACT FROM AUTHOR]

Improving the photochemical properties of molecular photoswitches is crucial for the development of light-responsive systems in materials and life sciences. ortho-Fluoroazobenzenes are a new class of rationally designed photochromic azo compounds with optimized properties, such as the ability to isomerize with visible light only, high photoconversions, and unprecedented robust bistable character. Introducing σ-electron-withdrawing F atoms ortho to the NN unit leads to both an effective separation of the n→π* bands of the E and Z isomers, thus offering the possibility of using these two transitions for selectively inducing E/ Z isomerizations, and greatly enhanced thermal stability of the Z isomers. Additional para-electron-withdrawing groups (EWGs) work in concert with ortho-F atoms, giving rise to enhanced separation of the n→π* transitions. A comprehensive study of the effect of substitution on the key photochemical properties of ortho-fluoroazobenzenes is reported herein. In particular, the position, number, and nature of the EWGs have been varied, and the visible light photoconversions, quantum yields of isomerization, and thermal stabilities have been measured and rationalized by DFT calculations. [ABSTRACT FROM AUTHOR]