ortho‐Fluoroazobenzenes: Visible Light Switches with Very Long‐Lived Z Isomers

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.     

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

Light is a nearly ideal stimulus for molecular systems. It delivers information encoded in the form of wavelengths and their intensities with high precision in space and time. Light is a mild trigger that does not permanently contaminate targeted samples. Its energy can be reversibly transformed into molecular motion, polarity, or flexibility changes. This leads to sophisticated functions at the supramolecular and macroscopic levels, from light-triggered nanomaterials to photocontrol over biological systems. New methods and molecular adapters of light are reported almost daily. Recently reported applications of photoresponsive systems, particularly azobenzenes, spiropyrans, diarylethenes, and indigoids, for smart materials and photocontrol of biological setups are described herein with the aim to demonstrate that the 21st century has become the Age of Enlightenment—“Le siècle des Lumières”—in molecular sciences.     

DNA‐Binding Properties of Amidine‐Substituted Spiropyran Photoswitches

Two amidine‐substituted spiropyran derivatives have been characterized with respect to the DNA‐binding properties over a broad pH interval. The two derivatives differ in the number of positive charges. By varying the pH, the protonation state of the derivatives is also changed, allowing for additional variations in the charge distribution. We show that the closed spiro isomer does not bind for either of the two derivatives, whereas the open merocyanine forms bind both in the protonated and in the nonprotonated state, but with dramatically different binding constants. Flow‐oriented linear dichroism (LD) measurements also show that there are differences in the binding modes between the various forms. We rationalize these differences in terms of structure and charge distribution.     

Fused Bis(hemi-indigo): Broad-Range Wavelength-Independent Photoswitches

Wavelength-independent conversion of organic photoswitches in the photostationary state is a rare phenomenon that opens up a way for many practical applications. In this work, three fused bis(hemi-indigo) derivatives with different substitution patterns were synthesized and their photoswitching was investigated by optical spectroscopy, real-time NMR spectroscopy and TD-DFT calculations. We disclosed that the Z-E photoisomerization of the meta-bis(hemi-indigo) derivative was remarkably independent of the irradiation wavelength from UV up to yellow light. The wavelength-independent forward photoswitching together with the inhibited backward photoisomerization, high thermal stability of the photoinduced isomers as well as significant overlap between the photoswitch absorption and the solar spectrum allows to suggest bis(hemi-indigo) derivatives as promising candidates for molecular solar thermal energy storage (MOST) systems.     

Visible-Light Photoswitching by Azobenzazoles

Three visible-light responsive photoswitches are reported, azobis(1-methyl-benzimidazole) ( 1 ), azobis(benzoxazole) ( 2 ) and azobis(benzothiazole) ( 3 ). Photostationary distributions are obtained upon irradiation with visible light comprising approximately 80 % of the thermally unstable isomer, with thermal half-lives up to 8 min and are mostly invariant to solvent. On protonation, compound 1 H⁺ has absorption extending beyond 600 nm, allowing switching with yellow light, and a thermal half-life just under 5 minutes. The two isomers have significantly different pK_a values, offering potential as a pH switch. The absorption spectra of 2 and 3 are insensitive to acid, although changes in the thermal half-life of 3 indicate more basic intermediates that significantly influence the thermal barrier to isomerization. These findings are supported by high-level ab initio calculations, which validate that protonation occurs on the ring nitrogen and that the Z isomer is more basic in all cases.     

Arylazo-3,5-dimethylisoxazoles: Azoheteroarene Photoswitches Exhibiting High Z-Isomer Stability,Solid-State Photochromism,and Reversible Light-Induced Phase Transition

Reversibly photoswitchable phenylazo-3,5-dimethylisoxazole and 37 aryl-substituted derivatives were synthesized. Excellent photoswitching ability of these compounds in solution and the solid state was demonstrated. Through kinetics studies by means of NMR spectroscopy, high Z-isomer stability was demonstrated. Interestingly, the majority of the derivatives showed light-induced contrasting color changes in solution and the solid state. Besides, many of the derivatives exhibit partial phase transition upon UV irradiation. The highlight of this class of photoswitches is the reversible light-induced phase transition between solid and liquid phases in the parent compound, which can be used in patterned crystallization. These results show that this new class of azoheteroarene based photoswitches has opportunities to be useful in various domains.     

Design and Tuning of Photoswitches for Solar Energy Storage

Current energy demand makes it compulsory to explore alternative energy sources beyond fossil fuels. Molecular solar thermal (MOST) systems have been proposed as a suitable technology for the use and storage of solar energy. Compounds used for this application need to fulfil a long series of requirements, being the absorption of sunlight and the energy stored some of the most critical. In this paper, we study different families of well-known molecular photoswitches from the point of view of their potential use as MOST. Starting from basic structures, we use density functional theory (DFT) computational modelling to propose two different strategies to increase the energy difference between isomers and to tune the absorption spectrum. The inclusion of a mechanical lock in the structure, via an alkyl chain and the presence of a hydrogen bonding are shown to directly influence the energy difference and the absorption spectra. Results shown here prove that these two approaches could be relevant for the design of new compounds with improved performance for MOST applications.     

Spectral Tuning of Azobenzene Photoswitches for Biological Applications

Longer switching wavelengths and good photochemical yields and stabilities of the cis isomers in reducing aqueous environments are achieved by introducing 2,2′‐aminoalkyl substituents into 4,4′‐diamido‐substituted azobenzenes. The products are thus suitable for photocontrol of biomolecular structures in intracellular environments, such as switching between two peptide configurations (see picture).

     

Hydrazone Photoswitches for Structural Modulation of Short Peptides

Molecules that undergo light-driven structural transformations constitute the core components in photoswitchable molecular systems and materials. Among various families of photoswitches, photochromic hydrazones have recently emerged as a novel class of photoswitches with superb properties, such as high photochemical conversion, spectral tunability, thermal stability, and fatigue resistance. Hydrazone photoswitches have been adopted in various adaptive materials at different length scales, however, their utilization for modulating biomolecules still has not been explored. Herein, we present new hydrazone switches that can photomodulate the structures of short peptides. Systematic investigation on a set of hydrazone derivatives revealed that installation of the amide group does not significantly alter the photoswitching behaviors. Importantly, a hydrazone switch comprising an upper phenyl ring and a lower quinolinyl ring was effective for structural control of peptides. We anticipate that this work, as a new milestone in the research of hydrazone switches, will open a new avenue for structural and functional control of biomolecules.     

锡掺杂Bi2O3可见光响应光催化剂的制备及性能

以硝酸铋和四氯化锡为原料,采用浸渍法制备了纯Bi2O3和Sn掺杂Bi2O3光催化剂。利用X射线光电子能谱、X射线荧光光谱、X射线衍射、扫描电子显微镜、紫外-可见吸收光谱和光致发光光谱对样品进行了表征。在可见光下,利用2,4-二氯苯酚水溶液的光催化降解作为探针反应,考察了样品的可见光催化活性。结果表明,浸渍法能较好地实现Sn的掺杂,催化剂中掺杂剂Sn的价态为+4价,以锡的氧化物形式存在于Bi2O3晶格间隙或晶粒表面。并且掺杂适量的Sn,可有效抑制Bi2O3晶相由四方相向单斜相的转变,拓宽了Bi2O3的可见光响应范围,有效阻止了光生电子和空穴的复合,从而提高了Bi2O3的可见光催化活性。当Sn的掺杂量为2%时(物质的量的分数),Bi2O3具有最好的可见光催化活性。     

Photoisomerization of Arylazopyrazole Photoswitches: Stereospecific Excited‐State Relaxation

Electronic structure calculations and nonadiabatic dynamics simulations (more than 2000 trajectories) are used to explore the Z–E photoisomerization mechanism and excited‐state decay dynamics of two arylazopyrazole photoswitches. Two chiral S₁/S₀ conical intersections with associated enantiomeric S₁ relaxation paths that are barrierless and efficient (timescale of ca. 50 fs) were found. For the parent arylazopyrazole (Z8) both paths contribute evenly to the S₁ excited‐state decay, whereas for the dimethyl derivative (Z11) each of the two chiral cis minima decays almost exclusively through one specific enantiomeric S₁ relaxation path. To our knowledge, the Z11 arylazopyrazole is thus the first example for nearly stereospecific unidirectional excited‐state relaxation.     

pH Tuning of Water-Soluble Arylazopyrazole Photoswitches

Arylazopyrazoles are an emerging class of photoswitches with redshifted switching wavelength, high photostationary states, long thermal half-lives and facile synthetic access. Understanding pathways for a simple modulation of the thermal half-lives, while keeping other parameters of interest constant, is an important aspect for out-of-equilibrium systems design and applications. Here, it is demonstrated that the thermal half-life of a water-soluble PEG-tethered arylazo-bis(o-methylated)pyrazole (AAP) can be tuned by more than five orders of magnitude using simple pH adjustment, which is beyond the tunability of azobenzenes. The mechanism of thermal relaxation is investigated by thorough spectroscopic analyses and density functional theory (DFT) calculations. Finally, the concepts of a tunable half-life are transferred from the molecular scale to the material scale. Based on the photochromic characteristics of E- and Z-AAP, transient information storage is showcased in form of light-written patterns inside films cast from different pH, which in turn leads to different times of storage. With respect to prospective precisely tunable materials and time-programmed out-of-equilibrium systems, an externally tunable half-life is likely advantageous over changing the entire system by the replacement of the photoswitch.     

Azobenzene: a Visible-Light Chemical Actinometer for the Characterization of Fluidic Photosystems

(E)-Azobenzene is introduced as a suitable chemical actinometer in the visible spectral range (440–540 nm) for photon flux determination of fluidic microphotoreactors or for assessing efficiency of visible light photo-induced reactions, its evaluation is straightforward without tedious analytics. Photoisomerization quantum yields (Φ_E→_Z) of (E)-azobenzene were accurately determined upon irradiation at several wavelengths and in different solvents based on well-known diarylethene.     

不同胺介质中一步合成具有可见光响应的TiO2

 以TiCl3为钛源,在140 ℃下采用溶剂热法在不同有机胺体系中一步合成二氧化钛晶体. 采用X射线衍射、紫外-可见吸收光谱、元素分析、傅里叶变换红外光谱以及X射线光电子能谱对所得粉体进行了表征. 结果表明,该方法制备的二氧化钛晶体吸收带边明显红移至可见光区,其中,在乙二胺体系中制备的二氧化钛晶体对可见光的吸收明显优于三乙胺和二乙胺体系. 在乙二胺体系中制备的二氧化钛对可见光响应的原因可归结为部分氮原子取代氧原子进入了二氧化钛晶体结构中. 苯酚的光催化降解实验表明,该掺氮型二氧化钛在可见光照射下具有较好的光催化活性.     

Modulating Guest Uptake in Core–Shell MOFs with Visible Light

A two‐component core–shell UiO‐68 type metal–organic framework (MOF) with a nonfunctionalized interior for efficient guest uptake and storage and a thin light‐responsive outer shell was prepared by initial solvothermal MOF synthesis followed by solvent‐assisted linker exchange. The bulky shell linker features two tetra‐ortho‐fluorinated azobenzene moieties to exploit their advantageous photoisomerization properties. The obtained perfect octahedral MOF single crystals can be switched repeatedly and with an unprecedented efficiency between E‐ and Z‐rich states using visible light only. Due to the high photoswitch density per pore of the shell layer, its steric demand and thus molecular uptake (and release) can be conveniently modulated upon green and blue light irradiation. Therefore, the “smart” shell acts as a light‐controlled kinetic barrier or “gate” for the diffusion of cargo molecules in and out of the MOF crystals.     

Sulfur and Azobenzenes,a Profitable Liaison: Straightforward Synthesis of Photoswitchable Thioglycosides with Tunable Properties

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.     

Bismuth Gadolinium Oxychloride with a Remarkable Visible-Light-Responsive O2 Evolution Activity Promoted by Iodine Doping

Visible-light-responsive bismuth-based oxyhalide has recently attracted extensive attention, however, the promotion of its charge separation is still challenging. Herein, we introduce iodine into Bi₂GdO₄Cl to synthetize I-doped Bi₂GdO₄Cl (denoted as yI−Bi₂GdO₄Cl, 0≤y≤2). The incorporation of I⁻ ions is found to enhance light absorption and to accelerate charge separation by combining various characterizations such as density functional theory calculation, photoelectrochemical test, electrochemical impedance spectroscopy, photoluminescence spectrum, and open-circuit voltage decay. The O₂-evolving performances of 1I−Bi₂GdO₄Cl with optimized dopant concentration of I⁻ ion and IrO₂ loaded 1I−Bi₂GdO₄Cl are tremendously enhanced by ca. 4 and 45 times compared to pristine Bi₂GdO₄Cl. Notably, The O₂ evolution rate reaches as high as 154.8 μmol ⋅ h⁻¹ with an apparent quantum efficiency of ∼1.1 % at 420 nm. The synthetic iodine-doped photocatalyst remains stable after long-term photoreaction, demonstrating its potential in the field of photocatalysis.