Photoswitching Properties Research Articles

Tunning of Optoelectronic Properties in Nanohybrids of Peptide-Appended Perylenebisimides and Carbon Nanodots

Two hybrid materials based on carbon dots (C-dots) and two perylenebisimide (PBI)-linked peptides with different functionality have been fabricated. These hybrids are designed to facilitate the electronic interaction between the electron donor C-dots and acceptor PBI-appended peptides, and the optoelectronic properties of these nanohybrids are compared. Amazingly, it is observed that the C-dots have exhibited favorable interactions with the PBI molecule (P1) bearing a similar charge (both negative) and have not shown any electronic interaction with the PBI molecule having opposite charges at their termini (P2). The pH of the medium has been found to be the limiting factor that has inhibited the electronic interaction between C-dots and P2. The interaction between C-dots and P1 has been attributed to the noncovalent complex formation. UV–visible and fluorescence spectroscopic study, field emission gun transmission electron microscopy, and time-correlated single-photon counting studies are made for this purpose. This nanohybrid material shows the emergence of good photoswitching behavior with the highest photocurrent gain of 5.14 (Ion/Ioff) compared to that of only P1 or C-dots. The increase in the photoswitching property is attributed to efficient charge (electron–hole) separation and holds a future promise for fabricating a new nanohybrid material with a high photocurrent conversion efficiency.

Improved Fluorescent Proteins for Dual-Colour Post-Embedding CLEM.

Post-embedding correlative light and electron microscopy (CLEM) has the advantage of high-precision registration and enables light and electron microscopy imaging of the same slice. However, its broad application has been hampered by the limited available fluorescent proteins (FPs) and a low signal-to-background ratio (SBR). Here, we developed a green photoswitchable FP, mEosEM-E with substantially high on/off contrast in EM samples embedded in Epon resin, which maximally preserves cellular structures but quenches the fluorescence of FPs. Taking advantage of the photoswitching property of mEosEM-E, the autofluorescence background from the resin was significantly reduced by a subtraction-based CLEM (sCLEM) method. Meanwhile, we identified a red fluorescent protein (RFP) mScarlet-H that exhibited higher brightness and SBR in resin than previously reported RFPs. With mEosEM-E and mScarlet-H, dual-colour post-Epon-embedding CLEM images with high SBR and no cross-talk signal were successfully performed to reveal the organization of nucleolar proteins. Moreover, a dissection of the influences of different EM sample preparation steps on the fluorescence preservation for several RFPs provides useful guidance for further probe development.

Voltage Mediated Control of Fluorescence Protein Emissions at the Metal-Solution Interface

The fluorescence protein technologies have made remarkable contributions to the advancement of life science. Accordingly, the physicochemical properties of fluorescence proteins have been deeply investigated in the bulk solution that mimics the cellular environment, but those at the less common environment such as surface and interface have not been deeply investigated. We recently found the phenomenon that the fluorescence protein immobilized at the metal-solution interface exhibits voltage-dependent photoluminescence. Upon the blue light photoexcitation of Venus, a yellow-emitting variant of green fluorescence protein, immobilized on the gold electrode surface, robust enhancement or decrease of fluorescence was induced by applying negative or positive bias, respectively. This previously unappreciated phenomenon was then implemented as a protein-based microdisplay. We then sought to solve the mechanism for the cathodic enhancement utilizing the characteristic optical properties in the three different fluorescence proteins. From the simultaneous electrochemical and fluorescence measurements in Venus, we found a strong correlation between the fluorescence modulation and the current reflecting cathodic hydrogen evolution, which led to a hypothesis that shift in the protonation-deprotonation equilibrium of the chromophore driven by hydrogen evolution at the metal surface underlies the phenomena. The hypothesis predicted that voltage dependency should be also found in the green-to-red photoconversion of fluorescence protein which is known as a protonation-dependent process. The hypothesis was verified by observing clear voltage dependency for the photoconversion in KikGR, an engineered photoconvertible fluorescence protein, at the interface. We then addressed how the shift in protonation equilibrium is driven by hydrogen evolution. The analysis using iR-phuruolin, a fluorescence protein variant with the inverse pH-sensitivity revealed the existence of an interface-specific mode of protonation-deprotonation reactions, where the protonation equilibrium is directly coupled to the cathodic hydrogen evolution. The interface-specific mode is distinct from that conventionally seen in protein in the bulk solution where the protonation patterns of the constituent titratable residues are determined through the local environmental acid-base equilibrium. The potential applications based on this interface-specific mechanism are then discussed, including the spatially resolved monitoring of hydrogen evolution reactions at the near-neutral condition. Fluorescent Protein The original Green Fluorescent Protein (GFP) was discovered by the researchers back in the early 1960s when studying the bioluminescent properties of a blue light emitting bioluminescent protein called aequorin together with another protein isolated from Aequorea victoria jellyfish and that another protein was eventually named the Green Fluorescent Protein [1]. GFP like fluorescent proteins have been discovered in other organisms including corals, copepods, sea anemones, lancelets, zoanithids [2]. GFP can be modified because the genetic code and amino acid code is well known [3]. Modifications allow for GFP to fluoresce with different colors such as blue (BFP), yellow (YFP), cyan (CFP), red (RFP) [4]. Their physiochemical phenomena can be used for various biological and medical research [5,6]. Advances such as, Förster resonance energy transfer (FRET), a fluorescence microscopy application was developed with GFP and permitting the researchers to use even more specific and powerful applications of fluorescence for their imaging [7]. last ten years, many new RSFPs (Reversibly photoswitchable fluorescent proteins) have been developed and novel applications in cell imaging discovered that rely on their photoswitching properties [8,9].

A Highly Efficient Phosphorescence/Fluorescence Supramolecular Switch Based on a Bromoisoquinoline Cascaded Assembly in Aqueous Solution.

Despite ongoing research into photocontrolled supramolecular switches, reversible photoswitching between room‐temperature phosphorescence (RTP) and delayed fluorescence is rare in the aqueous phase. Herein, an efficient RTP‐fluorescence switch based on a cascaded supramolecular assembly is reported, which is constructed using a 6‐bromoisoquinoline derivative (G3), cucurbit[7]uril (CB[7]), sulfonatocalix[4]arene (SC4A4), and a photochromic spiropyran (SP) derivative. Benefiting from the confinement effect of CB[7], initial complexation with CB[7] arouses an emerging RTP signal at 540 nm for G3. This structure subsequently coassembles with amphiphilic SC4A4 to form tight spherical nanoparticles, thereby further facilitating RTP emission (≈12 times) in addition to a prolonged lifetime (i.e., 1.80 ms c.f., 50.1 µs). Interestingly, following cascaded assembly with a photocontrolled energy acceptor (i.e., SP), the efficient light‐driven RTP energy transfer occurs when SP is transformed to its fluorescent merocyanine (MC) state. Ultimately, this endows the final system with an excellent RTP–fluorescence photoswitching property accompanied by multicolor tunable long‐lived emission. Moreover, this switching process can be reversibly modulated over multiple cycles under alternating UV and visible photoirradiation. Finally, the prepared switch is successfully applied to photocontrolled multicolor cell labeling to offer a new approach for the design and fabrication of novel advanced light‐responsive RTP materials in aqueous environments.

Design and Study of a Photo-Switchable Polymeric System in the Presence of ZnS Nanoparticles under the Influence of UV Light Irradiation.

Recent progress in the field of photosensitive materials has prompted a need to develop efficient methods to synthesize materials with basic intermolecular architectural designs and novel properties. Accordingly, in this work we design and study a photoactive polymer as a photo-switchable polymeric system in the presence and absence of ZnS nanoparticles (average size < 10 nm) at 5 wt.%. The influence of UV light irradiation on its properties were also studied. The photoactive block copolymer was obtained from styrene (S) and methyl methacrylate (MMA) as monomers and 1-(2-hydroxyethyl)-3,3-dimethylindoline-6-nitrobenzopyran (SP) was grafted to the block copolymer backbone as a photochromic agent. Furthermore, the incorporation of ZnS (NPs) as photo-optical switch component into the system enhances the purple colored photo-emission, with the open form of the spiropyran derivative (merocyanine, MC). The ZnS stabilize the isomeric equilibrium in the MC interconversion of the photochromic agent. The photo-switchable properties of the PS-b-PMMA-SP in the presence of ZnS (NPs) were examined using UV-VIS spectroscopy, Photoluminescence (PL) spectroscopy, optical fluorescence and scanning electronic microscopy (SEM-EDX.). The observed changes in the absorbance, fluorescence and morphology of the system were associated to the reversible interconversion of the two states of the photochromic agent which regulates the radiative deactivation of the luminescent ZnS NPs component. After UV irradiation the photoactive polymer becomes purple in color. Therefore, these basic studies can lead to the development of innovative functional and nanostructured materials with photosensitive character as photosensitive molecular switches.

Background-suppressed tumor-targeted photoacoustic imaging using bacterial carriers

Photoacoustic (PA) imaging offers promise for biomedical applications due to its ability to image deep within biological tissues while providing detailed molecular information; however, its detection sensitivity is limited by high background signals that arise from endogenous chromophores. Genetic reporter proteins with photoswitchable properties enable the removal of background signals through the subtraction of PA images for each light-absorbing form. Unfortunately, the application of photoswitchable chromoproteins for tumor-targeted imaging has been hampered by the lack of an effective targeted delivery scheme; that is, photoswitchable probes must be delivered invivo with high targeting efficiency and specificity. To overcome this limitation, we have developed a tumor-targeting delivery system in which tumor-homing bacteria (Escherichia coli) are exploited as carriers to affect the point-specific delivery of genetically encoded photochromic probes to the tumor area. To improve the efficiency of the desired background suppression, we engineered a phytochrome-based reporter protein (mDrBphP-PCMm/F469W) that displays higher photoswitching contrast than those in the current state of the art. Photoacoustic computed tomography was applied to achieve good depth and resolution in the context of invivo (mice) imaging. The present system effectively integrates a genetically encoded phytochrome-based reporter protein, PA imaging, and synthetic biology (GPS), to achieve essentially background-suppressed tumor-targeted PA monitoring in deep-seated tissues. The ability to image tumors at substantial depths may enable target-specific cancer diagnoses to be made with greater sensitivity, fidelity, and specificity.

Multiple Responsive CPL Switches in an Enantiomeric Pair of Perovskite Confined in Lanthanide MOFs.

Circularly polarized luminescence (CPL) switches have attracted widespread attention due to their potential applications in advanced information technologies. However, the design and fabrication of solid-state multiple-responsive CPL switches remain challenging. Here, through self-assembly of chiral metal-organic frameworks (MOFs) and perovskite nanocrystals (NCs), a pair of crystalline enantiomeric (P)-(+)/(M)-(-)-EuMOF⊃MAPbX3 (MA = CH3 NH3 + , X = Cl- , Br- , I- ) adducts is prepared, where the achiral MAPbBr3 perovskite NCs embedded into chiral MOFs inherit the chirality of host MOFs by host-guest EuBr and PbO coordination bonds, which is demonstrated by synchrotron-radiation-based X-ray absorption spectroscopy. The chiral adducts show enhanced photoluminescence quantum yield (PLQY), good thermal stability of CPL in air, and photoswitchable CPL properties upon altering different UV irradiation. Based on two chiral emission centers and their different characteristics, reversible CPL switches are realized upon a diversity of external stimuli, for example, chemicals (water /CH3 NH3 Br solution) or temperatures (room temperature/high temperature). Benefiting from the extraordinary stimuli-responsive and highly reversible switchable CPL, multiple information encryptions and decryptions integrated with CPL, together with a chiroptical logic gate are successfully designed. This work opens a new avenue to generally fabricate solid-state CPL composite materials and develops new applications based on switchable CPL.

Manipulating Selective Metal-to-Metal Electron Transfer to Achieve Multi-Phase Transitions in an Asymmetric [Fe2 Co]-Assembled Mixed-Valence Chain.

Manipulation of multi-functions in molecular materials is promising for future switching and memory devices, although it is currently difficult. Herein, we assembled the asymmetric {Fe2 Co} unit into a cyanide-bridged mixed-valence chain {[(Tp)Fe(CN)3 ]2 Co(BIT)} ⋅ 2CH3 OH (1) (Tp=hydrotris(pyrazolyl)borate and BIT=3,4-bis-(1H-imidazol-1-yl)thiophene), which showed reversible multi-phase transitions accompanied by photo-switchable single-chain magnet properties and a dielectric anomaly. Variable-temperature X-ray structural studies revealed thermo- and photo-induced selective electron transfer (ET) between the Co and one of the Fe ions. Alternating-current magnetic susceptibility studies revealed that 1 displayed on and off single-chain magnet behavior by alternating 946-nm and 532-nm light irradiation. A substantial anomaly in the dielectric constant was discovered during the electron transfer process, which is uncommon in similar ET complexes. These findings illustrate that 1 provided a new platform for multi-phase transitions and multi-switches adjusted by selective metal-to-metal ET.

Role of Material Composition in Photothermal Actuation of DASA-Based Polymers

We investigate the influence of the host matrix on the photothermally driven actuation performance of negatively photochromic, donor–acceptor Stenhouse adduct (DASA)-based polymers. Using a modular Diels–Alder “click” platform, we designed polymeric materials with varying DASA incorporation and investigated the relationships between the material composition and the resulting physical, mechanical, and photoswitching properties. We demonstrate that increasing the DASA concentration in polymer conjugates has a dramatic effect on the material’s physical and mechanical properties, such as the glass transition temperature (Tg) and elastic modulus, as well as the photoswitching properties, which are found to be highly dependent on Tg. We establish using a simple photoresponsive bilayer that actuation performance is controlled by the bilayer stiffness rather than the photochrome incorporation of DASA. Finally, we report and compare the light-induced property changes in Tg and the elastic modulus between the materials comprising the open or closed forms of DASAs. Our results demonstrate the importance of designing a material that is stiff enough to provide the mechanical strength required for actuation under load, but soft enough to reversibly switch at the operational temperature and provide key considerations for the development of application-geared photoswitchable materials.

Cross-conjugation controls the stabilities and photophysical properties of heteroazoarene photoswitches.

Azoarene photoswitches are versatile molecules that interconvert from their E-isomer to their Z-isomer with light. Azobenzene is a prototypical photoswitch but its derivatives can be poorly suited for in vivo applications such as photopharmacology due to undesired photochemical reactions promoted by ultraviolet light and the relatively short half-life (t1/2) of the Z-isomer (2 days). Experimental and computational studies suggest that these properties (λmax of the E isomer and t1/2 of the Z-isomer) are inversely related. We identified isomeric azobisthiophenes and azobisfurans from a high-throughput screening study of 1540 azoarenes as photoswitch candidates with improved λmax and t1/2 values relative to azobenzene. We used density functional theory to predict the activation free energies and vertical excitation energies of the E- and Z-isomers of 2,2- and 3,3-substituted azobisthiophenes and azobisfurans. The half-lives depend on whether the heterocycles are π-conjugated or cross-conjugated with the diazo π-bond. The 2,2-substituted azoarenes both have t1/2 values on the scale of 1 hour, while the 3,3-analogues have computed half-lives of 40 and 230 years (thiophene and furan, respectively). The 2,2-substituted heteroazoarenes have significantly higher λmax absorptions than their 3,3-substituted analogues: 76 nm for azofuran and 77 nm for azothiophene.

A viologen-derived host-guest MOF material: Photochromism, photoswitchable luminescence, and inkless and erasable printing

The encapsulation of photoactive guests into a metal-organic framework is advantageous in endowing extra functionalities such as photochromism and photoswitchable luminescence. In this paper, a host-guest MOF material, [Zn3(IPA)4 ​MV] (1) (H2IPA ​= ​isophthalic acid, MV·2NO3 ​= ​1,1'-dimethyl-(4,4'-bipyridine)-1,1'-diium dinitrate) has been constructed by encapsulating simplest methyl viologen guest into the anionic MOF framework. This material exhibits reversible naked-eye detectable photochromic properties varying from colorless to blue upon UV–Vis light irradiation. The MV2+ and the short O⋯N distance between the oxygen atom and pyridine ring of MV2+ play a crucial role in the photochromism of this compound. This host-guest MOF compound shows photoswitchable luminescent properties in the solid state. More interestingly, this host-guest MOF material can be deposited on filter paper and can be employed as inkless print material by controlling the UV–Vis light irradiation with the stencil.

A dual-functional chemical sensor for the detection of Cu2+ and Cd2+ based on the photochromic diarylethene

A dual-functional sensor DTF-o with a photochromic diarylethene moiety has been designed and synthesized. Its structure was confirmed and analyzed by NMR and MS. The photochromism and fluorescence photoswitching properties of DTF-o were studied in acetonitrile/water (9:1, v/v) solution. DTF-o could selectively recognize Cu2+ by changing its color from colorless to yellow which was attributed to the metal-to-ligand charge transfer (MLCT) absorption band of metal-ligand cooperation. Meanwhile, DTF-o could also be used to detect Cd2+ through obvious emission intensity enhancement by inhibition of CN isomerization with chelation enhanced fluorescence (CHEF). Finally, DTF-o was successfully used in practical samples and made into test strips to detect Cu2+/Cd2+ with high accuracy.

Dithienylethene metallodendrimers with high photochromic efficiency

It has been challenging to achieve multi-photochromic systems without affecting the individual photoswitching properties of the constituent units. Herein, we present the design and synthesis of a new family of platinum-acetylide dendrimers containing up to twenty-one photochromic dithienylethene (DTE) units that exhibit both high photochromic efficiency and individual switching properties. Upon irradiation with ultraviolet (UV) and visible (vis) light, the resultant metallodendrimers display high conversion yield and good fatigue resistance. More interestingly, cyclization-cycloreversion kinetics revealed that the photochromic property of each DTE unit in these metallodendrimers is unaffected by its neighbor and the full ring-closure of up to twenty-one DTE units in one single dendrimer has been achieved.

Photofluorochromic water-dispersible nanoparticles for single-photon-absorption upconversion cell imaging

Photofluorochromic diarylethene (DAE) molecules have been widely investigated due to their excellent fatigue resistance and thermal stability. However, the poor water solubility of DAEs limits their biological applications to some extent. Herein, we reported two kinds of water-dispersible DAE nanoparticles (DAEI-NPs and DAEB-NPs), in which DAE molecules were stabilized by the amphiphilic polymer DSPE-mPEG2000 using the nanoprecipitation approach. The fabricated nanoparticles retain well-controlled luminescence and fluorescence photoswitching properties in aqueous solution, which could be reversibly switched on and off under the alternating irradiation of ultraviolet (UV) and visible light. In addition, the closed-ring isomers of DAEB-NPs performed hot-band-absorption-based photon upconversion when excited by a 593.5 nm laser. Bearing excellent photophysical properties and low cytotoxicity, DAEB-NPs were applicable for upconversion cell imaging without high-excitation power density and free from oxygen removal. Additionally, the imaging process could be switched on by regulating the photofluorochromic nanoparticles.

Azobenzene-based polycatenars: Investigation on photo switching properties and optical storage devices

We report the photo switching behaviour of azobenzene-based rod-like polycatenar molecules having three hexyloxy chains at one terminus and a variable chain at the other terminus. The latter was varied with OC6H13, SC6H13, C7H13, OC10H21 and O(CH2)4C6F13 chains. All molecules are liquid crystalline exhibiting wide ranges of nematic phases with cybotactic clusters in addition to Smectic C phase. The effect of variation of terminal chains on the photo-switching properties of the above series of azo compounds upon light irradiation and their kinetic reactions are presented. Surprisingly, trans-cis conversion in solutions takes around 85 s, while relaxing back i.e. cis-trans conversion takes ~30–60 min depending on the type of the terminal chain. On the other hand, irrespective of the nature of the terminal chain all of them exhibit fast thermal back relaxation time. A possible reason for the observed phenomena is presented based on a caterpillar model. Moreover, the potential of one selected example in optical storage device is elucidated. Therefore, this work shows the structure-property relationship in such type of polycatenar molecules related to their photo switching properties.

Solid-state photoswitching in arylazopyrazole-embedded polydimethylsiloxane composite thin films

Arylazopyrazoles (AAPs) show significant potential as a new family of molecular photoswitches owing to their efficient reversible trans -to- cis photoisomerization behavior and the high thermal stability of their metastable cis -isomer. In this study, AAPs have been used for the fabrication of solid-state photoswitchable polydimethylsiloxane (PDMS) based composite thin films. The thin films were prepared by using PDMS as a polymer matrix and different concentrations of AAPs as chromophores at 150 ​°C via spin-coating. The photoswitching behavior of the AAPPDMS composite films and the pristine AAP samples induced by irradiation with specific wavelength of light were investigated. We found that the as prepared AAPPDMS composite films showed rapid and near-quantitative (>98%) reversible trans -to- cis isomerization upon alternating irradiation with UV (λ ​= ​365 ​nm) and green (λ ​= ​525 ​nm) light which is comparable to the isomerization behavior of the pristine AAP chromophores in solution. This indicates that the excellent photoswitching property of the AAPs is preserved in the solid-state of the AAP-PDMS composite films. The results also show that the optical properties of the AAP-PDMS composite films can be tuned by using different ratios of the AAP chromophores and exposure to UV-light irradiation. Additionally, the thin films were tested for their photo-actuation behavior by UV–vis spectroscopy. Irradiation of the thin film with alternating 365 ​nm UV and 525 ​nm green light lead to a slight reversible bending behavior. This is presumably caused by the light-induced conformational change of the AAP moiety embedded within the PDMS matrix and the soft nature of the PDMS. This methodology provides a new approach for exploring the fabrication of polymers with enhanced mechanical behavior and solidstate photoswitching properties. Arylazopyrazoles (AAPs) show significant potential as a new family of molecular photoswitches owing to their efficient reversible trans -tocis photoisomerization behavior and the high thermal stability of their metastable cis - isomer. In this study, AAPs have been used for the fabrication of solid-state photoswitchable polydimethylsiloxane (PDMS) based composite thin films. The thin films were prepared by using PDMS as a polymer matrix and different concentrations of AAPs as chromophores at 150 ​°C via spin-coating. The photoswitching behavior of the AAP-PDMS composite films and the pristine AAP samples induced by irradiation with specific wavelength of light were investigated. We found that the as prepared AAP-PDMS composite films showed rapid and near-quantitative (>98%) reversible trans -to- cis isomerization upon alternating irradiation with UV (λ ​= ​365 ​nm) and green (λ= 525 ​nm) light which is comparable to the isomerization behavior of the pristine AAP chromophores in solution. This indicates that the excellent photoswitching property of the AAPs is preserved in the solid-state of the AAP-PDMS composite films. The results also show that the optical properties of the AAP-PDMS composite films can be tuned by using different ratios of the AAP chromophores and exposure to UV-light irradiation. Additionally, the thin films were tested for their photo-actuation behavior by UV–vis spectroscopy. Irradiation of the thin film with alternating 365 ​nm UV and 525 ​nm green light lead to a slight reversible bending behavior. This is presumably caused by the light-induced conformational change of the AAP moiety embedded within the PDMS matrix and the soft nature of the PDMS. This methodology provides a new approach for exploring the fabrication of polymers with enhanced mechanical behavior and solid-state photoswitching properties.

Multi-Light-Responsive Upconversion-and-Downshifting-Based Circularly Polarized Luminescent Switches in Chiral Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have attracted tremendous attention for several novel applications. However, functional MOFs with light-responsive circularly polarized luminescence (CPL) are not examined in detail. Therefore, a dual CPL switch exhibiting both upconversion (UC) and downshifting (DS) CPL in the solid state is constructed by loading a luminescent diarylethene derivative (DAEC) and UC nanoparticles (UCNPs) into chiral MOFs. The chiral MOF⊃DAEC composites exhibit both photoswitchable luminescence and DS-CPL properties under alternating UV and visible light irradiation. Additionally, a reversible UC-CPL switch is realized using near-infrared (NIR) and visible light irradiation by introducing energy-level-matched UCNPs and DAEC into the chiral MOFs. The dissymmetry factor (glum ) of UC-CPL is noted to be significantly amplified through energy transfer compared to that of DS-CPL, which indicates that the information on circular polarization can be manipulated by altering the incident light. A chiroptical logic circuit with a 2D information output is designed with UV, visible, and NIR light as inputs by setting a rational threshold.

Azobispyrazole Family as Photoswitches Combining (Near-) Quantitative Bidirectional Isomerization and Widely Tunable Thermal Half-Lives from Hours to Years*.

Azobenzenes are classical molecular photoswitches that have been widely used. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatic rings has emerged as a strategy to expand molecular diversity and access improved photoswitching properties. Many mono-heteroaryl azo molecules with unique structures and/or properties have been developed, but the potential of bis-heteroaryl architectures is far from fully exploited. We report a family of azobispyrazoles, which combine (near-)quantitative bidirectional photoconversion and widely tunable Z-isomer thermal half-lives from hours to years. The two five-membered rings remarkably weaken the intramolecular steric hindrance, providing new possibilities for engineering the geometric and electronic structure of azo photoswitches. Azobispyrazoles generally exhibit twisted Z-isomers that facilitate complete Z→E photoisomerization, and their thermal stability can be broadly adjusted regardless of the twisted shape, overcoming the conflict between photoconversion (favored by the twisted shape) and Z-isomer stability (favored by the orthogonal shape) encountered by mono-heteroaryl azo switches.

Azobispyrazole Family as Photoswitches Combining (Near‐) Quantitative Bidirectional Isomerization and Widely Tunable Thermal Half‐Lives from Hours to Years**

AbstractAzobenzenes are classical molecular photoswitches that have been widely used. In recent endeavors of molecular design, replacing one or both phenyl rings with heteroaromatic rings has emerged as a strategy to expand molecular diversity and access improved photoswitching properties. Many mono‐heteroaryl azo molecules with unique structures and/or properties have been developed, but the potential of bis‐heteroaryl architectures is far from fully exploited. We report a family of azobispyrazoles, which combine (near‐)quantitative bidirectional photoconversion and widely tunable Z‐isomer thermal half‐lives from hours to years. The two five‐membered rings remarkably weaken the intramolecular steric hindrance, providing new possibilities for engineering the geometric and electronic structure of azo photoswitches. Azobispyrazoles generally exhibit twisted Z‐isomers that facilitate complete Z→E photoisomerization, and their thermal stability can be broadly adjusted regardless of the twisted shape, overcoming the conflict between photoconversion (favored by the twisted shape) and Z‐isomer stability (favored by the orthogonal shape) encountered by mono‐heteroaryl azo switches.

Bis-Pyrene Photo-Switch Open- and Closed-Form Differently Bind to ds-DNA, ds-RNA and Serum Albumin and Reveal Light-Induced Bioactivity.

Newly designed and synthesized diarylethene (DAE) derivatives with aliphatic amine sidearms and one with two pyrenes, revealed excellent photo-switching property of central DAE core in MeOH and water. The only exception was bis-pyrene analogue, its DAE core very readily photochemically closed, but reversible opening completely hampered by aromatic stacking interaction of pyrene(s) with cyclic DAE. In this process, pyrene fluorescence showed to be a reliable monitoring method, an open form characterized by strong emission at 480 nm (typical for pyrene-aggregate), while closed form emitted weakly at 400 nm (typical for pyrene-DAE quenching). Only open DAE-bis-pyrene form interacted measurably with ds-DNA/RNA by flexible insertion in polynucleotide grooves, while self-stacked closed form did not bind to DNA/RNA. For the same steric reasons, flexible open DAE-bis-pyrene form was bound to at least three different binding sites at bovine serum albumin (BSA), while rigid, self-stacked closed form interacted dominantly with only one BSA site. Preliminary screening of antiproliferative activity against human lung carcinoma cell line A549 revealed that all DAE-derivatives are non-toxic. However, bis-pyrene analogue efficiently entered cells and located in the cytoplasm, whereby irradiation by light (315–400 nm) resulted in a strong, photo-induced cytotoxic effect, typical for pyrene-related singlet oxygen species production.