In situ preparation of highly fluorescent dyes upon photoirradiation

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene derivative

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S1 state, and (iii) relaxation into a lower state (S1' state) different from the S1 state. Time-resolved fluorescence measurements demonstrated that this S1' state is relaxed to the S1 state with the large emission probability. These findings obtained in the present work contribute to extension of the ON-OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner.

High-contrast multicolour photoswitching based on dithienylethene Schiff base with a hydrazinylquinoline moiety

A new asymmetrical photochromic diarylethene DTE-HQo composed of a 2-hydrazinoquinoline moiety as the binding unit for ions and dithenylethene as a photoswitching trigger was reported. DTE-HQo displayed favourable photochromism upon irradiation with UV/vis light. Its fluorescent behaviour could be efficiently modulated by light, Zn2+, Cd2+ and HSO4-. The binding of Zn2+ induced a strong fluorescence peak at 510 nm in DTE-HQo due to the formation of a 1:2 complex [Zn2+ + 2DTE-HQo], resulting in a notable colour change from dark to intense white emission. Triggered by Cd2+, DTE-HQo formed a 1:1 complex [Cd2+ + DTE-HQo], leading to an enhanced emission intensity by 21-fold with an emission peak red-shifted from 461 nm to 514 nm. Unexpectedly, [Zn2+ + 2DTE-HQo] underwent hydrolysis when stimulated with water, generating a yellow-emitting complex [Zn2+ + DTE-HQo]. This color change easily distinguishes it from Cd2+ complex. Additionally, DTE-HQo showed high selectivity towards HSO4- and exhibited distinct "turn-on" fluorescence with a colour change from dark to bright blue upon stimulation. Moreover, the strong emission complexes of DTE-HQo with Zn2+, Cd2+ and HSO4- could be effectively quenched during the photocyclization process. Therefore, DTE-HQo can serve as an unimolecular multicolour photoswitching chemosensor, offering potential applications as a multifunctional probe for detecting Zn2+, Cd2+ and HSO4-.

Diaryltriazolium Photoswitch: Reaching a Millisecond Cycloreversion with High Stability and NIR Absorption

The exceptional thermal stability of diarylethene closed isomers enabled many applications but also prevented utilization in photochromic systems that require rapid thermal reversibility. Herein, we report the diaryltriazolium (DAT+ ) photoswitch undergoing thermal cycloreversion within a few milliseconds and absorption of the closed form in the near-infrared region above 900 nm. Click chemistry followed by alkylation offers modular and fast access to the electron-deficient DAT+ scaffold. In addition to excellent fatigue resistance, the introduced charge increases water solubility, rendering this photoswitch an ideal candidate for exploring biological applications.

Investigating the effects of the local environment on bottlebrush conformations using super-resolution microscopy

The single-chain physics of bottlebrush polymers plays a key role in their macroscopic properties. Although efforts have been made to understand the behavior of single isolated bottlebrushes, studies on their behavior in crowded, application-relevant environments have been insufficient due to limitations in characterization techniques. Here, we use single-molecule localization microscopy (SMLM) to study the conformations of individual bottlebrush polymers by direct imaging. Our previous work focused on bottlebrushes in a matrix of linear polymers, where our observations suggested that their behavior was largely influenced by an entropic incompatibility between the bottlebrush side chains and the linear matrix. Instead, here we focus on systems where this effect is reduced: in solvent-swollen polymer materials and in systems entirely composed of bottlebrushes. We measure chain conformations and rigidity using persistence length (lp) as side chain molecular weight (Msc) is varied. Compared to a system of linear polymers, we observe greater flexibility of the backbone in both systems. For bottlebrushes in bottlebrush matrices, we additionally observed a scaling relationship between lp and Msc that more closely follows theoretical predictions. For the more flexible chains in both systems, we reach the edge of our resolution limit and cannot visualize the entire contour of every chain. We bypass this limitation by discussing the aspect ratios of the features within the super-resolution images.

Photo-Induced Fluorochromism of a Star-Shaped Photochromic Dye with 2,4-Dimethylthiazole Attaching to Triangle Terthiophene

A photochrmic triangle terthiophene dye with 2,4-dimethylthiazole attached was synthesized and shows regular photochromic properties when irradiated with UV/Vis light alternately. It was found that the attaching of 2,4-dimethylthiazole has a significant effect on both the photochromism and fluorescence of triangle terthiophene. During the photocyclizatioin prcess, not only the color but also the fluorescence of the dye in THF can be toggled between ring-open and ring-closed forms of the dye. Additionally, the absolute quantum yields (AQY) of ring-open and ring-closed forms of the dye (0.32/0.58) were greatly larger than the literature report. Along with the 254 nm light irradiation, the fluorescence color changed from deep blue (428 nm) to sky blue (486 nm) in THF. A fluorochromism cycle could be established based on the UV/visible light irradiation cycle, which provides a strategy for the design of new type fluorescent diarylethene derivatives for biological application.

Photo-triggered full-color circularly polarized luminescence based on photonic capsules for multilevel information encryption

Materials with phototunable full-color circularly polarized luminescence (CPL) have a large storage density, high-security level, and enormous prospects in the field of information encryption and decryption. In this work, device-friendly solid films with color tunability are prepared by constructing Förster resonance energy transfer (FRET) platforms with chiral donors and achiral molecular switches in liquid crystal photonic capsules (LCPCs). These LCPCs exhibit photoswitchable CPL from initial blue emission to RGB trichromatic signals under UV irradiation due to the synergistic effect of energy and chirality transfer and show strong time dependence because of the different FRET efficiencies at each time node. Based on these phototunable CPL and time response characteristics, the concept of multilevel data encryption by using LCPC films is demonstrated.

Diarylethene Isomerization by Using Triplet-Triplet Annihilation Photon Upconversion

Green-to-blue triplet-triplet annihilation photon upconversion with the well-studied upconversion pair 9,10-diphenylanthracene (DPA)/platinum octaethylporphyrin (PtOEP) was used to reversibly drive the photoisomerization of diarylethene (DAE) photoswitches by using visible light. By carefully selecting the kinetic and spectral properties of the molecular system as well as the experimental geometry, a single green light source can be used to selectively trigger both the ring-opening and the ring-closing reactions, whilst also inducing fluorescence from the colored closed isomer that can be used as a readout to monitor the isomerization process in situ. The upconversion solution and the DAE solution are kept physically separated, allowing them to be characterized both concomitantly and individually without further separation processes. The ring-closing reaction using upconverted photons was quantified and compared to the efficiency of direct isomerization with ultraviolet light.

Photoresponsive Supramolecular Polymers: From Light-Controlled Small Molecules to Smart Materials

Photoresponsive supramolecular polymers are well-organized assemblies based on highly oriented and reversible noncovalent interactions containing photosensitive molecules as (co-)monomers. They have attracted increasing interest in smart materials and dynamic systems with precisely controllable functions, such as light-driven soft actuators, photoresponsive fluorescent anticounterfeiting and light-triggered electronic devices. The present review discusses light-activated molecules used in photoresponsive supramolecular polymers with their main photo-induced changes, e.g., geometry, dipole moment, and chirality. Based on these distinct changes, supramolecular polymers formed by light-activated molecules exhibit photoresponsive disassembly and reassembly. As a consequence, photo-induced supramolecular polymerization, "depolymerization," and regulation of the lengths and topologies are observed. Moreover, the light-controlled functions of supramolecular polymers, such as actuation, emission, and chirality transfer along length scales, are highlighted. Furthermore, a perspective on challenges and future opportunities is presented. Besides the challenge of moving from harmful UV light to visible/near IR light avoiding fatigue, and enabling biomedical applications, future opportunities include light-controlled supramolecular actuators with helical motion, light-modulated information transmission, optically recyclable materials, and multi-stimuli-responsive supramolecular systems.

Small-molecule photoswitches for fluorescence bioimaging: engineering and applications

Fluorescence microscopy has revolutionised our understanding of biological systems, enabling the visualisation of biomolecular structures and dynamics in complex systems. The possibility to reversibly control the optical or biochemical properties of fluorophores can unlock advanced applications ranging from super-resolution microscopy to the design of multi-stimuli responsive and functional biosensors. In this Highlight, we review recent progress in small-molecule photoswitches applied to biological imaging with an emphasis on molecular engineering strategies and promising applications, while underlining the main challenges in their design and implementation.

Emission modulation of fluorescent turn-on mode dibenzothienyl sulfonyl ethene photoswitches embedded in a polymer film

For decades photochromic molecules have attracted attention for their potential in using light as an external stimulus to change their photophysical properties. Here we report the spectroscopic characterization of two emissive photochromic molecules that are intrinsically fluorescent and that undergo a photocyclization/cycloreversion reaction upon illumination with light in the UV and VIS spectral ranges. For appropriately adjusted illumination intensities the emission can be modulated between the high- and the low-level with a contrast ratio exceeding 80%. The data are in reasonable agreement with the predictions from a simple kinetic model.

Supramolecular Complex of Photochromic Diarylethene and Cucurbit[7]uril: Fluorescent Photoswitching System for Biolabeling and Imaging

Photoswitchable fluorophores—proteins and synthetic dyes—whose emission is reversibly switched on and off upon illumination, are powerful probes for bioimaging, protein tracking, and super-resolution microscopy. Compared to proteins, synthetic dyes are smaller and brighter, but their photostability and the number of achievable switching cycles in aqueous solutions are lower. Inspired by the robust photoswitching system of natural proteins, we designed a supramolecular system based on a fluorescent diarylethene (DAE) and cucurbit[7]uril (CB7) (denoted as DAE@CB7). In this assembly, the photoswitchable DAE molecule is encapsulated by CB7 according to the host–guest principle, so that DAE is protected from the environment and its fluorescence brightness and fatigue resistance in pure water improved. The fluorescence quantum yield (Φ_fl) increased from 0.40 to 0.63 upon CB7 complexation. The photoswitching of the DAE@CB7 complex, upon alternating UV and visible light irradiations, can be repeated 2560 times in aqueous solution before half-bleaching occurs (comparable to fatigue resistance of the reversibly photoswitchable proteins), while free DAE can be switched on and off only 80 times. By incorporation of reactive groups [maleimide and N-hydroxysuccinimidyl (NHS) ester], we prepared bioconjugates of DAE@CB7 with antibodies and demonstrated both specific labeling of intracellular proteins in cells and the reversible on/off switching of the probes in cellular environments under irradiations with 355 nm/485 nm light. The bright emission and robust photoswitching of DAE-Male3@CB7 and DAE-NHS@CB7 complexes (without exclusion of air oxygen and addition of any stabilizing/antifading reagents) enabled confocal and super-resolution RESOLFT (reversible saturable optical fluorescence transitions) imaging with apparent 70–90 nm optical resolution.

Visualizing the Orientation of Single Polymers Induced by Spin-Coating

The orientation of chains within polymeric materials influences their electrical, mechanical, and thermal properties. While many techniques can infer the orientation distribution of a bulk ensemble, it is challenging to determine this information at the single-chain level, particularly in an environment of otherwise identical polymers. Here, we use single-molecule localization microscopy (SMLM) to visualize the directions of chains within spin-coated polymer films. We find a strong relationship between shear force and the degree and direction of orientation, and additionally, we reveal the effects of chain length and solvent evaporation rate. This work utilizes single-chain resolution to observe the important, though often overlooked, property of chain orientation in the common fabrication process of spin-coating.

Stepwise Photochromism of Bis(Imidazole Dimer) Bridged by a Sulfur Atom

We report the development of the stepwise photochromic imidazole dimer bridged by a sulfur atom. The one-photon absorption leads to the generation of the colored biradical species, which rapidly recombines to the initial imidazole dimer following first-order reaction kinetics. The further photochemical reaction of the biradical species produces the long-lived colored species, which shows intermolecular dimerization.

Turn-On Mode Circularly Polarized Luminescence in Self-Organized Cholesteric Superstructure for Active Photonic Applications

Developing circularly polarized luminescence (CPL)-active materials with a large luminescence dissymmetry factor (g_lum) or stimulus responses has evoked a lot of interest in the past few years; however, the light-controllable "on/off" CPL still remains a challenge. Here, a novel diarylethene-based chiral fluorescent photoswitch featuring "turn-on" CPL characteristic is developed, designated as (S,S)-switch 6, which can undergo reversible photocyclization/cycloreversion upon irradiation with UV and visible light. (S,S)-Switch 6 shows completely reversible "off-on-off"-responsive CPL behavior in solution. By doping (S,S)-switch 6 into nematic liquid crystals (LCs), the consequent luminescent cholesteric LCs (CLCs) exhibit a larger g_lum value enhanced 2 orders of magnitude when irradiated with UV light, which can be attributed to the highly ordered helical arrangement of CLCs. The potentials of this turn-on type CPL material for anticounterfeiting and information encryption are illustrated. Furthermore, the visualization of circularly polarized (CP) fluorescent patterns can be successfully achieved by constructing the double-layer CPL system consisting of a CP luminescent layer and a polymer cholesteric reflective layer. The proposed concept establishes a light-controlled off-on-off CPL platform that is of tremendous potential for applications in multi-informational data storage and encryption devices.

Future-Oriented Advanced Diarylethene Photoswitches: From Molecular Design to Spontaneous Assembly Systems

Diarylethene (DAE) photoswitch is a new and promising family of photochromic molecules and has shown superior performance as a smart trigger in stimulus-responsive materials. During the past few decades, the DAE family has achieved a leap from simple molecules to functional molecules and developed toward validity as a universal switching building block. In recent years, the introduction of DAE into an assembly system has been an attractive strategy that enables the photochromic behavior of the building blocks to be manifested at the level of the entire system, beyond the DAE unit itself. This assembly-based strategy will bring many unexpected results that promote the design and manufacture of a new generation of advanced materials. Here, recent advances in the design and fabrication of diarylethene as a trigger in materials science, chemistry, and biomedicine are reviewed.

Reversible Near-Infrared Fluorescence Photoswitching in Aqueous Media by Diarylethene: Toward High-Accuracy Live Optical Imaging

Fluorescence imaging is an indispensable tool in modern biological research, allowing simple and inexpensive color-coded visualizations of real-time events in living cells and animals, as well as of fixed states of ex vivo specimens. The accuracy of fluorescence imaging in living systems is, however, impeded by autofluorescence, light scattering, and limited penetration depth of light. Nevertheless, the clinical use of fluorescence imaging is expected to grow along with advances in imaging equipment, and will increasingly demand high-accuracy probes to avoid false-positive results in disease detection. To this end, a water-soluble and relatively safe diarylethene (DAE)-based reversible near-infrared (NIR) fluorescence photoswitch for living systems is prepared here. Furthermore, to facilitate excellent switching performance, the photoirradiation results obtained is compared using three different visible light sources to turn on NIR fluorescence through cycloreversion of DAE. While photoswitching using 589 nm light leads to slightly higher cell viability, fluorescence quenching efficiency and fatigue resistance are higher when 532 nm light with low photobleaching is used in both aqueous solution and living systems. The authors anticipate that their reversible NIR fluorescence photoswitch mediated by DAE can be beneficial for fluorescence imaging in aqueous media requiring accurate detection, such as in the autofluorescence-rich living environment.

Synthesis, structure–property relationships and absorbance modulation of highly asymmetric photochromes with variable oxidation and substitution patterns

Asymmetric 1,2-diarylperfluorocyclopentenes with “inverse” photochromism (due to shorter conjugation path in the closed-ring isomer), rapid switching and large absorption modulation are reported.

Green-/NIR-Light-Controlled Rapid Photochromism Featuring Reversible Thermally Activated Delayed Fluorescence and Photoelectronic Switching

Fluorescent dithienylethene based photochromic materials have been attracting considerable attention owing to their wide applications in biological and material sciences. However, limitations of detrimental UV irradiation for photocyclization, short emission...

Visualization of the microstructure and the position-dependent diffusion coefficient in a blended polymer solid using photo-activation localization microscopy combined with single-molecule tracking based on one-color fluorescence-switching of diarylethene

Using one-color fluorescence-switching of diarylethene derivatives (DAEs), thousands of single-molecule translational trajectories in a blended polymer solid were measured with nanometric accuracy.

Turn-on mode fluorescent diarylethene having neopentyl substituents that undergoes all-visible-light switching

This paper presents a strategy for improving all-visible-light switching response of turn-on mode fluorescent diarylethene derivatives. Introduction of neopentyl or isobutyl substituents at the reactive carbons (2- and 2’-positions) of...

A Universal Strategy for Tunable Persistent Luminescent Materials via Radiative Energy Transfer

In this work, a universal strategy for solid, solution, or gel state organic persistent luminescent materials via radiative energy transfer is proposed. The persistent luminescence (τ > 0.7 s) could be remotely regulated between different colors by controlling the isomerization of the energy acceptor. The function is relying on the simple radiative energy transfer (reabsorption) mechanism, rather than the complicated communication between the excited state of the molecules such as Förster resonance energy transfer or Dexter energy transfer. And the "apparent lifetime" for the energy acceptor is the same as the lifetime of the energy donor, which was different with traditional radiative energy transfer process. The simple working principle endows this strategy with huge universality, flexibility, and operability. This work offers a simple, feasible, and universal way to construct various persistent luminescent materials in solid, solution, and gel states.In this work, a universal strategy for solid, solution, or gel state organic persistent luminescent materials via radiative energy transfer is proposed. The persistent luminescence (τ > 0.7 s) could be remotely regulated between different colors by controlling the isomerization of the energy acceptor. The function is relying on the simple radiative energy transfer (reabsorption) mechanism, rather than the complicated communication between the excited state of the molecules such as Förster resonance energy transfer or Dexter energy transfer. And the "apparent lifetime" for the energy acceptor is the same as the lifetime of the energy donor, which was different with traditional radiative energy transfer process. The simple working principle endows this strategy with huge universality, flexibility, and operability. This work offers a simple, feasible, and universal way to construct various persistent luminescent materials in solid, solution, and gel states.

Synthesis and Transformations of NH-Sulfoximines

Recent years have seen a marked increase in the occurrence of sulfoximines in the chemical sciences, often presented as valuable motifs for medicinal chemistry. This has been prompted by both pioneering works taking sulfoximine containing compounds into clinical trials and the concurrent development of powerful synthetic methods. This review covers recent developments in the synthesis of sulfoximines concentrating on developments since 2015. This includes extensive developments in both S-N and S-C bond formations. Flow chemistry processes for sulfoximine synthesis are also covered. Finally, subsequent transformations of sulfoximines, particularly in N-functionalization are reviewed, including N-S, N-P, N-C bond forming processes and cyclization reactions.

Achieving Enhanced Photochromic Properties of Diarylethene through Host-Guest Interaction in Aqueous Solution

Diarylethene (DTE) has been widely used in fluorescence probes, molecular logic gates, optical data-storage devices owing to the excellent photochromic property, while constructing high-performance photochromic DTE in aqueous media remains a big challenge. Herein we present several host-guest systems formed between cucurbit[n]uril (CB[n], n=7, 8, 10) and two water-soluble DTE derivatives 1 and 2. It was found that host-guest interactions not only affect the photophysical properties of photochromic guests, but also make great differences on the photoreaction process. Different host-guest binding behaviors also lead to different effects on the photochromic properties of guests. In the presence of CB[n], both 1 and 2 showed enhanced emission and higher fluorescence quenching ratio at photostationary state. Besides, CB[10]⋅1 exhibited faster response rate in cyclization reaction and better photofatigue resistance than free 1 in aqueous solution, while the supramolecular assembly of (CB[8])_n ⋅(2)_n showed slower response rate in both directions of the reversible photoreaction. Besides, the photofatigue resistance of 2 can be greatly improved through binding with CB[7]. Our results suggest that host-guest interactions could be an efficient way to improve photochromic properties of DTE in aqueous solution.

Low-Dose High-Resolution TOF-PET Using Ionization-activated Multi-State Low-Z Detector Media

We propose PET scanners using low atomic number media that undergo a persistent local change of state along the paths of the Compton recoil electrons. Measurement of the individual scattering locations and angles, deposited energies, and recoil electron directions allows using the kinematical constraints of the 2-body Compton scattering process to perform a statistical time-ordering of the scatterings, with a high probability of precisely identifying where the gamma first interacted in the detector. In these cases the Line-of-Response is measured with high resolution, determined by the underlying physics processes and not the detector segmentation. There are multiple such media that act through different mechanisms. As an example in which the change of state is quantum-mechanical through a change in molecular configuration, rather than thermodynamic, as in a bubble chamber, we present simulations of a two-state photoswitchable organic dye, a 'Switchillator', that is activated to a fluorescent-capable state by the ionization of the recoil electrons. The activated state is persistent, and can be optically excited multiple times to image individual activated molecules. Energy resolution is provided by counting the activated molecules. Location along the LOR is implemented by large-area time-of-flight MCP-PMT photodetectors with single photon time resolution in the tens of ps and sub-mm spatial resolution. Simulations indicate a large reduction of dose.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Luminescent lanthanide-macrocycle supramolecular assembly

A series of macrocyclic compounds, including crown ether, cyclodextrin, cucurbituril and pillararene, bound to various specific organic/inorganic/biological guest molecules and ions through various non-covalent interactions, can not only make a single system multifunctional but also endow the system with intelligence, especially for luminescent materials. Due to their excellent luminescence properties, such as long-lived excited states, sharp linear emission bands and large Stokes shift, lanthanides have shown great advantages in luminescence, and have been more and more applied in the design of advanced functional luminescent materials. Based on reported research, we summarize the progress of lanthanide luminescent materials based on different macrocyclic compounds from ion or molecule recognition to functional nano-supramolecular assembly of the lanthanide-macrocycle supramolecular system including photo-reaction mediated switch of lanthanide luminescent molecules, multicolor luminescence, ion detection and cell imaging of rare-earth up-conversion of macrocyclic supramolecular assembly. Finally, we put forward the prospects of future development of lanthanide luminescent macrocyclic supramolecular materials.

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 (g_lum ) 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.

A light-activatable photosensitizer for photodynamic therapy based on a diarylethene derivative

Herein, a light-activatable photosensitizer based on a diarylethene derivative, DAE-TPE, was developed for photodynamic therapy. Upon UV exposure, the "opened" form (OF) of DAE-TPE NPs was converted to the "closed" form (CF), and photosensitization was activated. The CF of DAE-TPE NPs exhibited sufficient photodynamic therapy effects upon HeLa cells.

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.

A Solid-State Fluorescence Switch Based on Triphenylethene-Functionalized Dithienylethene With Aggregation-Induced Emission

The development of novel dithienylethene-based fluorescence switches in the aggregated state, and the solid state is highly desirable for potential application in the fields of optoelectronics and photopharmacology. In this contribution, three novel triphenylethene-functionalized dithienylethenes (1-3) have been designed and prepared by appending triphenylethene moieties at one end of dithienylethene unit. Their chemical structures are confirmed by 1H NMR, 13C NMR, and HRMS (ESI). They display good photochromic behaviors with excellent fatigue resistance upon irradiation with UV or visible light in Tetrahydrofuran (THF) solution. Before irradiation with UV light, they exhibit Aggregation Induced Emission (AIE) properties and luminescence behaviors in the solid state. Moreover, upon alternating irradiation with UV/visible light, they display effective fluorescent switching behaviors in the aggregated state and the solid state. The experimental results have been validated by the Density Functional Theory (DFT) calculations. Thus, they can be utilized as novel fluorescence switches integrated in smart, solid-state optoelectronic materials and photopharmacology.

Rapid amplitude-modulation of a diarylethene photoswitch: en route to contrast-enhanced fluorescence imaging

A water soluble diarylethene (DAE) derivative that displays exceptionally intense fluorescence from the colorless open form has been synthesized and characterized using UV/vis spectroscopy and fluorescence microscopy. We show that the bright emission from the open form can be rapidly switched using amplitude modulated red light, that is, by light at wavelengths longer than those absorbed by the fluorescent species. This is highly appealing in any context where undesired background fluorescence disturbs the measurement, e.g., the autofluorescence commonly observed in fluorescence microscopy. We show that this scheme is conveniently applicable using lock-in detection, and that robust amplitude modulation of the probe fluorescence is indeed possible also in cell studies using fluorescence microscopy.

A water soluble diarylethene derivative displaying exceptionally bright fluorescence in the open isomeric form has been used for emission amplitude-modulation. We apply this scheme in fluorescence microscopy, aiming to suppress undesired background.

Turn-on mode diarylethenes for bioconjugation and fluorescence microscopy of cellular structures

In superresolution fluorescence microscopy, employing synthetic dyes that can be reversibly photoswitched between a nonfluorescent (“dark”) and a fluorescent (“bright”) state has been an attractive alternative to using photoswitchable fluorescent proteins. However, employing such synthetic dyes has been elusive because they have defied reliable attachment to proteins and required UV light for photoswitching. Here we prepared “turn-on mode” fluorescent diarylethenes (fDAEs) that are switchable with visible rather than UV light and blink between a bright fluorescent and a dark state in aqueous buffers. Moreover, our thienyl-substituted fDAEs effectively labeled two thiol groups on nanobodies bearing a single maleimide tag. With these small-sized probes, we acquired superresolution images of vimentin filaments in cells by applying just yellow (561 nm) light.

The use of photoswitchable fluorescent diarylethenes (fDAEs) as protein labels in fluorescence microscopy and nanoscopy has been limited by labeling inhomogeneity and the need for ultraviolet light for fluorescence activation (on-switching). To overcome these drawbacks, we prepared “turn-on mode” fDAEs featuring thienyl substituents, multiple polar residues, and a reactive maleimide group in the core structure. Conjugates with antibodies and nanobodies displayed complete on-switching and excitation with violet (405 nm) and yellow-green (<565 nm) light, respectively. Besides, they afforded high signal-to-noise ratios and low unspecific labeling in fluorescence imaging. Irradiation with visible light at 532 nm or 561 nm led to transient on-off switching (“blinking”) of the fDAEs of double-labeled nanobodies so that nanoscale superresolution images were readily attained through switching and localization of individual fluorophores.

Switching Photon Upconversion by Using Photofluorochromic Annihilator with Low-Lying Triplet

Photon upconversion based on triplet-triplet annihilation (TTA-UC) has attracted great attention due to its remarkable features including the high upconversion quantum yield, low threshold, and flexible combination of sensitizer and annihilator. Endowing TTA-UC with responsiveness will offer additional application dimensions; however, it is a challenge to develop annihilators with responsive features in the excited triplet state. Here we demonstrate the synthesis and photophysical behaviors of photofluorochromic annihilators derived from fluorescent diarylethenes. A series of turn-on mode fluorescent diarylethenes based on 1,2-bis(2-ethyl-1-benzothiophen-1,1-dioxide-3-yl)perfuorocyclopentene were synthesized, and their photochromism and photofluorochromism behaviors were thoroughly investigated. When sensitized by near-infrared ruthenium phthalocyanine, TTA-UC could be observed under excitation of 730 nm, accompanied by upconverted emission ranging from 500 to 700 nm. Because of the photoresponsive properties of the annihilators, TTA-UC can be switched between "on" and "off" by alternating irradiation of ultraviolet and visible light.

Fluorescence Switchable Conjugated Polymer Microdisk Arrays by Cosolvent Vapor Annealing

Depositing minute light emitters into a regular array is a basic but essential technique in display technology. However, conventional lithographic methodologies involve multistep and energy-consuming processes. Here, we develop a facile method in which organic and polymeric fluorescent dyes spontaneously aggregate to form a patterned microarray. We find that a thin film of fluorescent π-conjugated polymer transforms into micrometer-sized aggregates when exposed to binary organic vapor at ambient temperature. The arrayed microaggregates can be formed over the whole substrate surface when using a quartz substrate that is prepatterned with regular hydrophilic boxes and hydrophobic grids. The resultant microarray is applicable to optical memories and displays when photoswitchable fluorophores are doped into the polymer matrix.

Getting DNA and RNA out of the dark with 2CNqA: a bright adenine analogue and interbase FRET donor

With the central role of nucleic acids there is a need for development of fluorophores that facilitate the visualization of processes involving nucleic acids without perturbing their natural properties and behaviour. Here, we incorporate a new analogue of adenine, 2CNqA, into both DNA and RNA, and evaluate its nucleobase-mimicking and internal fluorophore capacities. We find that 2CNqA displays excellent photophysical properties in both nucleic acids, is highly specific for thymine/uracil, and maintains and slightly stabilises the canonical conformations of DNA and RNA duplexes. Moreover, the 2CNqA fluorophore has a quantum yield in single-stranded and duplex DNA ranging from 10% to 44% and 22% to 32%, respectively, and a slightly lower one (average 12%) inside duplex RNA. In combination with a comparatively strong molar absorptivity for this class of compounds, the resulting brightness of 2CNqA inside double-stranded DNA is the highest reported for a fluorescent base analogue. The high, relatively sequence-independent quantum yield in duplexes makes 2CNqA promising as a nucleic acid label and as an interbase Förster resonance energy transfer (FRET) donor. Finally, we report its excellent spectral overlap with the interbase FRET acceptors qAnitro and tCnitro, and demonstrate that these FRET pairs enable conformation studies of DNA and RNA.

Di(benzothienyl)cyclobutenes: Toward Strained Photoswitchable Fluorophores

Bis(benzothienyl)ethene sulfones are very interesting molecules for super-resolution microscopy due to their photoswitching properties. However, functionalization of the 'classical' bis(benzothienyl)ethene sulfones with a five-membered central ring leads to significant decrease of quantum yields of photoconversion of the fluorescent closed form of the dye to the non-fluorescent open form that limits their application in microscopy. Here, we designed and synthesized diarylethenes with a fluorinated four-membered central ring that adds extra strain to the closed form of the dye. The reaction mechanism of their formation was studied, and byproducts formed upon structural rearrangement of the benzothiophene fragment were characterized. The photochromic properties of the new molecules were investigated by NMR and absorption spectroscopy. Some of these compounds show enhanced tendency to ring opening and have quantum yields of the ring-opening reaction in the range of 0.2-0.5.

Self-assembly of an alkynylpyrene derivative for multi-responsive fluorescence behavior and photoswitching performance

Highly emissive fluorophores based on polyaromatic hydrocarbons with tunable emission properties and aggregated structures play a very important role in relevant functional studies. In this study, a novel alkynylpyrene derivative 1 was synthesized, which exhibits unimolecular to excimer emission in methanol with an increasing concentration accompanied by the formation of nanovesicles via the π-π stacking, hydrogen bond and hydrophobic interaction. The self-assembly behavior as well as emission properties of 1 in aprotic polar solvents (ACN, acetone, DMF and DMSO) can also be adjusted by the volume fraction of the poor solvent H2O, which can induce 1 self-assembly to excimer state and could be applied in information transfer. Moreover, upon visible light irradiation, photoswitchable performance of nanovesicles of 1 was observed in which the emission markedly changes from yellow to blue; this is attributed to the cycloaddition reaction of alkynyl groups and singlet oxygen, which can be generated without the addition of external photosensitizers. The multi-responsive and fluorescence behavior of the alkynylpyrene derivative show that the self-assembly can be used to expand the development of this type of fluorophores, and the novel photoinduced tunability of the fluorescence emission provides an effective strategy to obtain high-performance transmitting and sensing materials.

A thermoresponsive fluorophore based on a photochromic diarylethene having donor-acceptor moieties

A thermoresponsive fluorophore based on a photochromic diarylethene possessing donor and acceptor moieties in close proximity to each other has been synthesized. The fluorophore exhibits dual fluorescence, where photoresponsive turn-off switching as well as a thermoresponsive relative intensity change are observed.

Turn-on mode fluorescent diarylethenes: effect of electron-donating and electron-withdrawing substituents on photoswitching performance†

Diarylethene derivatives having benzothiophene S,S-dioxide groups undergo turn-on mode fluorescence photoswitching. For the practical application to super-resolution fluorescence microscopy, photoswitchable fluorescent molecules are desired to be resistant against photodegradation. Here we synthesized turn-on mode fluorescent diarylethenes having electron-withdrawing (trifluoromethyl or nitro) or electron-donating (methyl, methoxy, or dimethylamino) substituents on phenyl rings at 6- and 6'-positions of the benzothiophene S,S-dioxide groups and examined the effect of the substituents on the photoswitchiing performance. The derivatives having electron-donating substituents showed significant bathochromic shifts of the absorption and fluorescence spectra. The cycloreversion quantum yield was increased by introducing electron-withdrawing substituents, while it was decreased by the electron-donating ones. Introduction of electron-donating substituents was found to remarkably improve the fatigue resistance of the fluorescent diarylethene under continuous ultraviolet (UV) irradiation. Such highly fatigue-resistant fluorescent diarylethenes are useful for super-resolution fluorescence imaging or single-molecule fluorescence tracking.

Restriction of the conrotatory motion in photo-induced 6π electrocyclic reaction: formation of the excited state of the closed-ring isomer in the cyclization

The electrocyclic reaction dynamics of a photochromic dithiazolylarylene derivative, 2,3-dithiazolylbenzothiophene (DTA) was investigated by using time-resolved transient absorption and fluorescence spectroscopies. The closed-ring isomer of DTA undergoes cycloreversion through the conical intersection mediating the potential energy surfaces of the excited and ground states, which is in agreement with the Woodward–Hoffmann rules for the electrocyclic reactions of 6π electron systems. On the other hand, a large portion of the open-ring isomer undergoes cyclization along the distinct reaction scheme, in which the cyclization takes place in the excited state manifold leading to the formation of the excited state of the closed-ring isomer. The suppression of the geometrical motion of DTA due to the intramolecular interaction could open a new efficient reaction pathway resulting in the formation of the electronically excited state of the product.

Restriction of the molecular geometry opens up a novel pathway in the cyclization reaction of a photochromic dithiazolylarylene derivative.

A simplicity-guided cocktail approach toward multicolor fluorescent systems

A molecular cocktail containing two photochromic diarylethene derivatives that displays multicolor emission spanning blue-green to orange in a color-correlated fashion has been devised. The function does not rely on excited state communication such as energy transfer reactions, which is the typical case for similar systems. Instead, harnessing the intrinsic fluorescent and photochromic properties of the two individual diarylethene derivatives run in parallel is enough to realize the color changes. This offers an extremely flexible situation as for the choice of the fluorophores and their respective concentrations. The function is conveniently demonstrated in bulk solution at μM concentrations, where a single light source serves as the color changing stimulus.

Quinoline-containing diarylethenes: bridging between turn-on fluorescence, RGB switching and room temperature phosphorescence

Simple structural modifications using oxidation and methylation of a quinoline-containing diarylethene result in dramatic variation of photophysical properties. Turn-on fluorescence, room temperature phosphorescence (RTP) and red-green-blue (RGB) switching were achieved in three different related compounds. Photoswitchable diarylethenes (DAEs) that exhibit turn-on fluorescence are in high demand for super-resolution microscopy, and the development of purely organic phosphorescent materials in the amorphous state is attractive but challenging. The findings reported here provide a novel toolkit for designing turn-on fluorescence DAEs for super-resolution microscopy and extending the scope of amorphous RTP materials. More importantly, we bridge between these two fundamentally significant photochemical and photophysical phenomena, and reveal structure-property relationships between DAE photochromism and RTP.

Multicolour fluorescent “sulfide–sulfone” diarylethenes with high photo-fatigue resistance

Compact, photochromic, reversible and fluorescent: blue and red emission from the “open” and “closed” forms (see picture) and up to 20 000 cycles in organic solvents without exclusion of air–oxygen.

Multi-color emission with orthogonal input triggers from a diarylethene pyrene-OTHO organogelator cocktail

A cocktail combination of stimuli responsive materials, a photoswitch and gelator, is used for multicolored emission tuning.