Fast and stable photochromic oxazines for fluorescence switching

Targeted Photoconvertible BODIPYs Based on Directed Photooxidation-Induced Conversion for Applications in Photoconversion and Live Super-Resolution Imaging

Photomodulable fluorescent probes are drawing increasing attention due to their applications in advanced bioimaging. Whereas photoconvertible probes can be advantageously used in tracking, photoswitchable probes constitute key tools for single-molecule localization microscopy to perform super-resolution imaging. Herein, we shed light on a red and far-red BODIPY, namely, BDP-576 and BDP-650, which possess both properties of conversion and switching. Our study demonstrates that these pyrrolyl-BODIPYs convert into typical green- and red-emitting BODIPYs that are perfectly adapted to microscopy. We also showed that this pyrrolyl-BODIPYs undergo Directed Photooxidation Induced Conversion, a photoconversion mechanism that we recently introduced, where the pyrrole moiety plays a central role. These unique features were used to develop targeted photoconvertible probes toward different organelles or subcellular units (plasma membrane, mitochondria, nucleus, actin, Golgi apparatus, etc.) using chemical targeting moieties and a Halo tag. We notably showed that BDP-650 could be used to track intracellular vesicles over more than 20 min in two-color imagings with laser scanning confocal microscopy, demonstrating its robustness. The switching properties of these photoconverters were studied at the single-molecule level and were then successfully used in live single-molecule localization microscopy in epithelial cells and neurons. Both membrane- and mitochondria- targeted probes could be used to decipher membrane 3D architecture and mitochondrial dynamics at the nanoscale. This study builds a bridge between the photoconversion and photoswitching properties of probes undergoing directed photooxidation and shows the versatility and efficacy of this mechanism in advanced live imaging.

Enhanced Host-Guest Association and Fluorescence in Copolymers from Copper Salphen Complexes by Supramolecular Internalization of Anions

We describe the synthesis, crystallographic characterization of a new Cu-Salphen compound and its use as a host Lewis-acid against guest anions in two versions: a) free molecule, b) copolymerized with methyl methacrylate:n-butyl acrylate (1 : 4-wt.) as protective co-monomers. Higher contents in Cu-Salphen yielded larger and more homogeneous polymer sizes. Polymer size together with glass transitions, heat capacity, thermal degradation, guest-saturation degrees and host-guest species distribution profiles from spectrophotometric titrations explained growths of up to 630-fold in K₁₁ and 180000-fold in K₁₂ for the host's binding site attributable to a solvophobic protection from the macromolecular structure. Spectrofluorimetry revealed blue-shifted×13-16 larger luminescence for Cu-Salphen in the polymers (λ_em =488-498 nm) than that of the non-polymerized counterpart (λ_em =510-543 nm) and "turn-on" blue-shifted enhanced fluorescence upon guest association. We propose a cooperative incorporation of the guests occurring from the outer medium toward internally protected binding site pockets in the random coil polymer conformations.

Decoration of A-Ring of a Lupane-Type Triterpenoid with Different Oxygen and Nitrogen Heterocycles

Betulinic acid (BA) was used as starting building block to create a library of novel BA-derived compounds containing O- and N-heterocycles. Firstly, BA was converted into methyl betulonate (BoOMe), which was used as intermediate in the developed methodologies. 1,2-Oxazine-fused BoOMe compounds were obtained in 12–25% global yields through a Michael addition of nitromethane to methyl (E)-2-benzylidenebetulonate derivatives, followed by nitro group reduction and intramolecular cyclization. Remarkably, the triterpene acts as a diastereoselective inducer in the conjugate addition of nitromethane, originating only one diastereomer out of four possible ones. Furthermore, other oxygen and nitrogen-containing heterocycles were installed at the A-ring of BoOMe, affording 2-amino-3-cyano-4H-pyran-fused BoOMe, diarylpyridine-fused BoOMe and 1,2,3-triazole–BoOMe compounds, using simple and straightforward synthetic methodologies. Finally, BA was revealed to be a versatile starting material, allowing the creation of a molecular diversification of compounds containing a triterpenic scaffold and O- and N-heterocycles.

Development of highly sensitive metal-ion chemosensor and key-lock anticounterfeiting technology based on oxazolidine

Optical chemosensors and ionochromic cellulosic papers based on oxazolidine chromophores were developed for selective photosensing of metal ions and information encryption as security tags, respectively. The oxazolidine molecules have been displayed highly intense fluorescent emission and coloration characteristics that are usable in sensing and anticounterfeiting applications. Obtained results indicated that oxazolidine molecules can be used for selective detection of pb²⁺ (0.01 M), and photosensing of Fe³⁺, Co²⁺ and Ag⁺ metal ion solutions by colorimetric and fluorometric mechanisms with higher intensity and sensitivity. Also, oxazolidine derivatives were coated on cellulosic papers via layer-by-layer method to prepare ionochromic papers. Prepared ionochromic papers were used for printing and handwriting of optical security tags by using of metal ion solutions as a new class of anticounterfeiting inks with dual-mode fluorometric and colorimetric securities. The ionochromic cellulosic papers can be used for photodetection of metal ions in a fast and facile manner that presence of metal ions is detectable by naked eyes. Also, key-lock anticounterfeiting technology based on ionochromic papers and metal ion solution as ink is the most significant strategy for encryption of information to optical tags with higher security.

A pH-Sensitive Spirocyclization Strategy for Constructing a Single Fluorescent Probe Simultaneous Two-Color Visualizing of Lipid Droplets and Lysosomes and Monitoring of Lipophagy

Lipid droplets (LDs) and lysosomes are crucial for maintaining intracellular homeostasis. But single fluorescent probes (SFPs) capable of simultaneous and discriminative visualizing of two organelles above and their interaction in living cells are still challenging due to the lack of rational design strategies. To break this bottleneck, herein, we develop a reliable strategy based on a pH-sensitive intramolecular spirocyclization. As a proof of concept, an SFP CMHCH, which possesses a switchable hemicyanine/spiro-oxazine moiety induced by pH, has been designed and synthesized. In acidic environments, the ring-open form CMHCH exhibits red-shift emission and low logP value, whereas the ring-closed form CMHC displays blue-shift emission and high logP value in neutral or basic environments. Thus, the distinct different hydrophilicity/hydrophobicity and absorption/emission properties of these two forms enable targeting LDs and lysosomes simultaneously and discriminatingly. Very importantly, the dynamic process of lipophagy can be directly monitored with CMHCH. The success of CMHCH indicated that the spirocyclization strategy is efficient for constructing SFPs to LDs and lysosomes.

Synthesis and photolysis of new BODIPY derivatives with chelated boron centre

New borondipyromethene (BODIPY) derivatives chelated at the boron centre with different catecholate and salicylate ligands were synthesized via substituting the fluoride atoms with the aid of aluminum chloride that activates the boron-fluoride bond for substitution. The photophysical properties of the novel BODIPYs were investigated by normalized UV-vis absorption as well as the fluorescence emission spectra. Moreover, the fluorescence quantum yields of the chelated BODIPYs were also calculated and the ultraviolet irradiation of the salicylate derivatives was studied.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

Improving Fatigue Resistance of Dihydropyrene by Encapsulation within a Coordination Cage

Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a Pd^II₆L₄ coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.

Enhancement of Negative Photochromic Properties of Naphthalene-Bridged Phenoxyl-Imidazolyl Radical Complex

Negative photochromism has increased attention as a light-switch for functional materials. A development of fast photochromic molecules has been also expected because a rapid thermal back reaction within a millisecond time scale is useful for real-time switching. Herein, we synthesized the derivatives of the naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) showing the negative photochromism to demonstrate the efficient strategy to increase the visible light sensitivity and to control the thermal back reaction rates. The distances of the C-C bond of the transient 2,4'-isomer shows good agreement with the thermodynamic stability, leading to the control of the thermal back reaction rate. We revealed the cyclic voltammetry and the DFT calculations are efficient to predict the characters of the HOMO and LUMO. The introduction of the electron-withdrawing dicyanoquinodimethane group is efficient to induce the photochromic reaction with increased visible-light sensitivity by the expansion of the π-conjugation. The results will give an important insight for the future development of fast-responsive negative photochromic molecules.

Encryption and authentication of security patterns by ecofriendly multi-color photoluminescent inks containing oxazolidine-functionalized nanoparticles

Counterfeiting of confidential documents has been a costly challenge for banks, companies, and customers. Encryption of invisible security marks, such as barcodes, quick response codes, and logos, in national or international confidential documents by high-security anticounterfeiting inks is the most significant solution for counterfeiting problems. Ecofriendly multi-color photoluminescent anticounterfeiting inks based on highly-fluorescent polymer nanoparticles functionalized with new oxazolidine derivatives were developed for the fast and facile encryption of security labels on cellulosic documents, such as paper currency, passport, and certificate. Depending on the polarity of functionalized polymer nanoparticles, a wide range of colors and fluorescence emissions were observed as a result of polar-polar interactions between the oxazolidine molecules and surface functional groups of the nanoparticles. The fluorescent polymer nanoparticles showed spherical, vesicular, and cauliflower-like morphologies resulted from different surface functional groups. Functional polymer nanoparticles displayed high stability and printability on cellulosic substrates due to hydrogen bonding interactions. The highly-fluorescent polymer nanoparticles were also used to prepare anticounterfeiting inks with different colors and fluorescence emissions. All the ecofriendly polymeric anticounterfeiting inks were loaded to stamps with specific marks, and then applied to different confidential documents. Printed labels displayed highly intense fluorescence emission in different colors (green, orange, pink, and purple depending on the matrix polarity) under UV irradiation (365 nm). These water-based multi-color fluorescent anticounterfeiting inks with highly intense, bright, and sensitive fluorescence emission have potential applications in encryption and authentication of security patterns.

Red or Near-Infrared Light Operating Negative Photochromism of a Binaphthyl-Bridged Imidazole Dimer

The development of red or near-infrared light (NIR) switchable photochromic molecules is required for an efficient utilization of sunlight and regulation of biological activities. While the photosensitization of photochromic molecules to red or NIR light has been achieved by a two-photon absorption process, the development of a molecule itself having sensitivity to red or NIR light has been now a challenging study. Herein, we developed an efficient molecular design for realizing red or NIR-light-responsive negative photochromism based on binaphthyl-bridged imidazole dimers. The introduction of electron-donating substituents shows the red shift of the absorption band at the visible-light region because of the contribution of a charge-transfer transition. Especially, the introduction of a di(4-methoxyphenyl)amino group (TPAOMe) and a perylenyl group largely shifts the absorption edge of the stable colored form to 900 nm. In addition, because the absorption band of one of the derivatives substituted with TPAOMe covers the whole visible-light region, the colored form shows a neutral gray color. Upon red (660 nm) or NIR-light (790 nm) irradiation, we observed the negative photochromic reaction from the stable colored form to the metastable colorless form. Therefore, the substituted binaphthyl-bridged imidazole dimers constitute the attractive photoswitches within a biological window.

Nanostructured and Photochromic Material for Environmental Detection of Metal Ions

Compared to conventional spectroscopy or chromatography analysis, chemical sensing based on colorimetric changes offers an alternative to monitor potential metal hazards in aqueous environment through rapid and low-cost colorimetric changes which can be easily interpreted. In this work poly(ethylene glycol) (PEG 2000) was modified with a carboxylic acid spiropyran (SPCOOH) derivate by Steglich esterification (PEGSP2). PEGSP2 was incorporated into a poly(ε-caprolactone) (PCL) polymer matrix by electrospinning technique to produce nanofibers with photochromic properties. Spectroscopic analysis, thermal gravimetric analysis (TGA), and differential scanning calorimetry (DSC) were used to characterize PEGSP2. Drop shape analysis (DSA) and scanning electronic microscopy (SEM) were used to characterize the electrospun (ES) nanofibers morphology. Several metal ions solutions relevant to environmental hazards were prepared to be spotted on the surface of ES nanofibers for photochromatic sensing. Among them, Mg²⁺, Ca²⁺, Zn²⁺, Cd²⁺, La³⁺, and Er³⁺ demonstrated orange fluorescence when exposed to UV light. ES nanofibers also presented higher wettability when compared to a pure PCL polymer matrix, which is critical for sensitivity. Eighteen metals ions could be detected on the electrospun material. Additionally, among all metal ions Fe³⁺ was the most sensitive one in solution, in a µmol L⁻¹ range.

Turn-On Mode Fluorescence Switch by Using Negative Photochromic Imidazole Dimer

The development of fluorescence switchable molecules in several polar and apolar environments has been required for fluorescence imaging of nanostructures. Photochromic molecules are an important class for the reversible light-triggered fluorescence switching. Although many studies of fluorescence switching by using photochromic reactions have been reported, the report of photochromic molecules reversibly showing turn-on mode fluorescence switching has been limited in spite of their importance. Herein, we report the photoactivatable fluorescence based on negative photochromism, where the absorption spectrum of the compound after irradiation is blue-shifted relative to that before irradiation. We introduced naphthalimide units as a green fluorophore to the negative photochromic binaphthyl-bridged imidazole dimer. The fluorescence of the naphthalimide unit is efficiently quenched in the initial colored isomer (fluorescence quantum yield: Φ_fluo. = 0.01) by Förster resonance energy transfer. In contrast, the fluorescence quantum yield increases up to 0.75 in the transient isomer formed by the negative photochromic reaction. The fluorescence intensity thermally decreases with the thermal back reaction to form the original stable colored form. These results indicate that the negative photochromic molecules are suitable for turn-on mode fluorescence switches and will give an attractive insight for the development of reversible fluorescence switching molecules.

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.

Ratiometric temperature sensing with fluorescent thermochromic switches

The connection of fluorescent chromophores to switchable heterocycles translates into molecular probes with ratiometric response to temperature. The opening and closing of their heterocyclic component equilibrates two emissive species with resolved fluorescence. Their relative emission intensities change monotonically with temperature to enable the visualization of thermal distributions at the microscale.

Reversible Surface Dual-Pattern with Simultaneously Dynamic Wrinkled Topography and Fluorescence

The reversible surface patterns with fluorescence and topography can possibly enable information recording and reading and provide an important alternative to realize the higher information security. We demonstrated a reversible dual-pattern with simultaneously responsive fluorescence and topography using an anthracene (AN) and naphthalene diimide (NDI) containing copolymer (PAN-NDI-BA) as the skin layer, in which the reversible photodimerization of AN can simultaneously control the cross-linking and CT interaction between AN and NDI. Upon irradiation with UV light and thermal treatment, the resulting pattern assumes a reversible change between smooth and wrinkled states, and its fluorescence changes reversibly from red to white to blue-green. The smart surfaces with dynamic hierarchical wrinkles and fluorescence were achieved by selective irradiation with photomasks and can be employed for potential applications in smart displays and anticounterfeiting.

Photochemical Barcodes

A photochemical strategy to encode fluorescence signals in vivo with spatial control was designed around the unique properties of a photoactivatable borondipyrromethene (BODIPY). The photoinduced disconnection of two oxazines, flanking a single BODIPY, in two consecutive steps produces a mixture of three emissive molecules with resolved fluorescence inside polymer beads. The relative amounts and emission intensities of the three fluorophores can be regulated precisely in each bead by adjusting the dose of activating photons to mark individual particles with distinct codes of fluorescence signals. The visible wavelengths and mild illumination sufficient to induce these transformations permit the photochemical barcoding of beads also in living nematodes. Different regions of the same animal can be labeled with distinct barcodes to allow the monitoring of their dynamics for long times with no toxic effects. Thus, our photochemical strategy for the generation of fluorescence barcodes can produce multiple and distinguishable labels in the same biological sample to enable the spatiotemporal tracking of, otherwise indistinguishable, targets.

Fluorescence activation with switchable oxazines

Activatable fluorophores allow the spatiotemporal control of fluorescence required to acquire subdiffraction images, highlight cancer cells and monitor dynamic events

Structural implications on the excitation dynamics of fluorescent 3H-indolium cations

Conformational changes in the excited state control the excitation dynamics of fluorescent 3H-indolium cations.

Design, synthesis and properties of near-infrared molecular switches containing a fluorene ring

Three molecular switches containing a fluorene ring were designed and synthesized. The introduction of the amino group substituted fluorene ring resulted in the target molecular switches having some optical properties in the near-infrared region. It was demonstrated that the N-substituents on the fluorene rings and the switch units both had great influence on the molecular switch optical properties including the absorption maximum, absorption intensity and fluorescence quantum yield. The open-ring forms and showed obvious solvatochromic behaviour. The closed-ring forms and showed obvious hydrochromic behaviour in MeCN/water binary solvent systems and acidichromic behaviour in MeCN solution with high reversibility. Especially, the distinct off-on fluorescence signal in the near-infrared region using the stimuli of acid means that the designed compounds have great potential application value in the field of biological sensing.

Fast Negative Photochromism of 1,1'-Binaphthyl-Bridged Phenoxyl-Imidazolyl Radical Complex

Negative photochromism, in which a thermally stable colored form isomerizes to the transient colorless form by light irradiation and the back reaction occurs thermally, is advantageous in its applications for photoswitching materials because visible light can cause the photochromic color change of the materials. Moreover, the photochromic color change can be induced even on the inside of the materials due to the absence of the reabsorption of the visible excitation light by the photogenerated colorless species. While several negative photochromic compounds have been reported, the time scales of the back reaction are still slower than minutes, and no available fast responsive negative photochromic compounds have been reported. Here, we developed a negative photochromic 1,1'-binaphthyl-bridged phenoxyl-imidazolyl radical complex (BN-PIC) which enables fast photoswitching by visible light. The stable colored BN-PIC shows instantaneous decoloration by visible light irradiation, and the photogenerated colorless form thermally reverts to the initial colored form with a half-life of 1.9 s at room temperature. BN-PIC can also cause the drastic change in the chiroptical properties by the photochromic reaction, and the rate of the thermal back reaction is affected by the chirality of the solvent. Since the negative photochromic reaction can occur on the inside of the materials, the fast negative photochromism is expected to have an impact in the fields of photoresponsive materials of solid states and molecular aggregates.

Photoswitchable aggregation-induced emission polymer containing dithienylethene and tetraphenylethene moieties

Upon irradiation with UV and visible light, the fluorescence of the prepared AIE polymer could be photoswitched.

Thiophene-substituted phenoxyl-imidazolyl radical complexes with high photosensitivity

Fast photoswitch molecules became sensitive to visible light by using a thiophene ring as a radical linker unit.

Activation of BODIPY fluorescence by the photoinduced dealkylation of a pyridinium quencher

The photoinduced cleavage of a 2-nitrobenzyl group from a pyridinium quencher covalently attached to the meso position of a BODIPY fluorophore activates the emission of the latter. This photochemical transformation prevents the transfer of one electron from the BODIPY platform to its heterocyclic appendage upon excitation and, as a result, permits the radiative deactivation of the excited fluorophore. This versatile mechanism for fluorescence switching can translate into the realization of an entire family of photoactivatable fluorophores based on the outstanding photophysical properties of BODIPY chromophores.

Fluorescence Modulation and Photochromism in Azobismaleimide Derivatives

A fluorophore-photochrome system incorporating an aryleneimine type fluorophore and an azobismaleimide photochrome was developed and the photochemical properties of this system were investigated. The photoisomerization of trans-azoaromatic chromophore leads to the increase of the fluorescence intensity of fluorophore. The cis azobismaleimide isomers revert photochemically to the trans form and the emission intensity decreases. The fluorescence intensity of the imine fluorophore can be modulated under irradiation with UV and visible (436 nm) light due to reversible trans-cis-trans photoisomerization of azobismaleimide partner. The photoisomerization kinetics was obeyed a first-order relationship with a rate constant of 1.95 × 10(-2) s(-1) for azobismaleimide/imine fluorophore system and for polyazothioetherimide/imine derivative the kinetics was described by a biexponential equation.

Reversible photodynamic chloride-selective sensor based on photochromic spiropyran

We report here for the first time on a reversible photodynamic bulk optode sensor based on the photoswitching of a spiropyran derivative (Sp). The photoswitching of Sp induces a large basicity increase in the polymeric phase, which triggers the extraction of Cl^– and H⁺. Cl^– is stabilized by a lipophilic chloride-selective ionophore inside the membrane, while H⁺ binds with the open form of Sp and induces a spectral change, hence providing the sensor signal. The system was studied with spectroscopic and electrochemical methods.