Carboxylated Photoswitchable Diarylethenes for Biolabeling and Super-Resolution RESOLFT Microscopy

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.

Amplifying dual-visible-light photoswitching in aqueous media via confinement promoted triplet-triplet energy transfer

Achieving visible-light photochromism is a long-term goal of chemists keen to exploit the opportunities of molecular photoswitches in multi-disciplinary research studies. Triplet-sensitization offers a flexible approach to building diverse visible-light photoswitches using existing photochromic scaffolds, circumventing the need for sophisticated molecular design and synthesis. Unfortunately, distance-dependence and environment-sensitivity of triplet-excited species remain as key challenges that severely impair sensitization efficiency and limit their practical availability. We present herein a nature-inspired nanoconfinement strategy in which a triplet-sensitized visible-light photoswitch/sensitizer system is assembled into nanoconfined micelles (d ∼ 40 nm). A ca. 10-fold efficiency increase of triplet-triplet energy transfer for photochromism as well as an amplified fluorescence on/off contrast upon bi-directional visible-light excitation (470/560 nm) was achieved in full aqueous media. By virtue of this, the hybrid photoswitchable system is successfully applied for both flash information encryption and multiple dynamic cell imaging assays, further proving its versatility in materials and life science.

Theranostic Fluorescent Probes

The ability to gain spatiotemporal information, and in some cases achieve spatiotemporal control, in the context of drug delivery makes theranostic fluorescent probes an attractive and intensely investigated research topic. This interest is reflected in the steep rise in publications on the topic that have appeared over the past decade. Theranostic fluorescent probes, in their various incarnations, generally comprise a fluorophore linked to a masked drug, in which the drug is released as the result of certain stimuli, with both intrinsic and extrinsic stimuli being reported. This release is then signaled by the emergence of a fluorescent signal. Importantly, the use of appropriate fluorophores has enabled not only this emerging fluorescence as a spatiotemporal marker for drug delivery but also has provided modalities useful in photodynamic, photothermal, and sonodynamic therapeutic applications. In this review we highlight recent work on theranostic fluorescent probes with a particular focus on probes that are activated in tumor microenvironments. We also summarize efforts to develop probes for other applications, such as neurodegenerative diseases and antibacterials. This review celebrates the diversity of designs reported to date, from discrete small-molecule systems to nanomaterials. Our aim is to provide insights into the potential clinical impact of this still-emerging research direction.

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.

No-wash fluorogenic labeling of proteins for reversible photoswitching in live cells

Photoswitchable fluorescent molecules (PSFMs) are positioned as valuable tools for biomolecule localization tracking and super-resolution imaging technologies due to their unique ability to reversibly control fluorescence intensity upon light irradiation. Despite the high demand for PSFMs that are suitable for live-cell imaging, no general method has been reported that enables reversible fluorescence control on proteins of interest in living cells. Herein, we have established a platform to realize reversible fluorescence switching in living cells by adapting a protein labeling system. We have developed a new PSFM, named HTL-Trp-BODIPY-FF, which exhibits strong fluorogenicity upon recognition of Halo-tag protein and reversible fluorescence photoswitching in living cells. This is the first example of a PSFM that can be applicable to a general-purpose Halo-tag protein labeling system for no-wash live-cell imaging.

2,3-Diarylmaleate salts as a versatile class of diarylethenes with a full spectrum of photoactivity in water

There is incessant interest in the transfer of common chemical processes from organic solvents to water, which is vital for the development of bioinspired and green chemical technologies. Diarylethenes feature a rich photochemistry, including both irreversible and reversible reactions that are in demand in organic synthesis, materials chemistry, and photopharmacology. Herein, we introduce the first versatile class of diarylethenes, namely, potassium 2,3-diarylmaleates (DAMs), that show excellent solubility in water. DAMs obtained from highly available precursors feature a full spectrum of photoactivity in water and undergo irreversible reactions (oxidative cyclization or rearrangement) or reversible photocyclization (switching), depending on their structure. This finding paves a way towards wider application of diarylethenes in photopharmacology and bioinspired technologies that require aqueous media for photochemical reactions.

Proton-Coupled Photochromic Hemithioindigo: Toward Photoactivated Chemical Sensing and Imaging

We report photoswitchable fluorescent hemithioindigos (HTIs) where the metastable E isomers were stabilized by the proton-bridged intramolecular hydrogen bond. Titration experiments and computational analysis indicated that the E isomers were much more basic than the Z isomers, which enabled photoactivated colorimetric and fluorescent pH response in solvents and polypropylene films. The HTIs exhibited reversibly switchable fluorescence with the Z isomers being the most fluorescent. Moreover, the HTIs were lysosomotropic and the kinetic fluorescence evolution during photoswitching was able to differentiate subcellular compartments with different pH. The combination of photoenhanced basicity, switchable fluorescence, and proton-coupled photochromism lay the groundwork for a broad range of chemical and biological applications.

A Photoswitchable Solvatochromic Dye for Probing Membrane Ordering by RESOLFT Super-resolution Microscopy

A switchable solvatochromic fluorescent dyad can be used to map ordering of lipids in vesicle membranes at a resolution better than the diffraction limit. Combining a Nile Red fluorophore with a photochromic spironaphthoxazine quencher allows the fluorescence to be controlled using visible light, via photoswitching and FRET quenching. Synthetic lipid vesicles of varying composition were imaged with an average 2.5-fold resolution enhancement, compared to the confocal images. Ratiometric detection was used to probe the membrane polarity, and domains of different lipid ordering were distinguished within the same membrane.

Biotinylated Fluorescent Polymeric Nanoparticles for Enhanced Immunostaining

The performance of fluorescence immunostaining is physically limited by the brightness of organic dyes, whereas fluorescence labeling with multiple dyes per antibody can lead to dye self-quenching. The present work reports a methodology of antibody labeling by biotinylated zwitterionic dye-loaded polymeric nanoparticles (NPs). A rationally designed hydrophobic polymer, poly(ethyl methacrylate) bearing charged, zwitterionic and biotin groups (PEMA-ZI-biotin), enables preparation of small (14 nm) and bright fluorescent biotinylated NPs loaded with large quantities of cationic rhodamine dye with bulky hydrophobic counterion (fluorinated tetraphenylborate). The biotin exposure at the particle surface is confirmed by Förster resonance energy transfer with dye-streptavidin conjugate. Single-particle microscopy validates specific binding to biotinylated surfaces, with particle brightness 21-fold higher than quantum dot-585 (QD-585) at 550 nm excitation. The nanoimmunostaining method, which couples biotinylated antibody (cetuximab) with bright biotinylated zwitterionic NPs through streptavidin, significantly improves fluorescence imaging of target epidermal growth factor receptors (EGFR) on the cell surface compared to a dye-based labeling. Importantly, cetuximab labeled with PEMA-ZI-biotin NPs can differentiate cells with distinct expression levels of EGFR cancer marker. The developed nanoprobes can greatly amplify the signal from labeled antibodies, and thus become a useful tool in the high-sensitivity detection of disease biomarkers.

All Visible Light Photoswitch Based on the Dimethyldihydropyrene Unit Operating in Aqueous Solutions with High Quantum Yields

Molecular systems and devices whose properties can be modulated using light as an external stimulus are the subject of numerous research studies in the fields of materials and life sciences. In this context, the use of photochromic compounds that reversibly switch upon light irradiation is particularly attractive. However, for many envisioned applications, and in particular for biological purposes, illumination with harmful UV light must be avoided and these photoactivable systems must operate in aqueous media. In this context, we have designed a benzo[e]-fused dimethyldihydropyrene compound bearing a methyl-pyridinium electroacceptor group that meets these requirements. This compound (closed state) is able to reversibly isomerize under aerobic conditions into its corresponding cyclophanediene form (open isomer) through the opening of its central carbon-carbon bond. Both the photo-opening and the reverse photoclosing processes are triggered by visible light illumination and proceed with high quantum yields (respectively 14.5% yield at λ = 680 nm and quantitative quantum yield at λ = 470 nm, in water). This system has been investigated by nuclear magnetic resonance and absorption spectroscopy, and the efficient photoswitching behavior was rationalized by spin-flip time-dependent density functional theory calculations. In addition, it is demonstrated that the isomerization from the open to the closed form can be electrocatalytically triggered.

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.

Photochemical Mechanisms of Fluorophores Employed in Single-Molecule Localization Microscopy

Decoding cellular processes requires visualization of the spatial distribution and dynamic interactions of biomolecules. It is therefore not surprising that innovations in imaging technologies have facilitated advances in biomedical research. The advent of super-resolution imaging technologies has empowered biomedical researchers with the ability to answer long-standing questions about cellular processes at an entirely new level. Fluorescent probes greatly enhance the specificity and resolution of super-resolution imaging experiments. Here, we introduce key super-resolution imaging technologies, with a brief discussion on single-molecule localization microscopy (SMLM). We evaluate the chemistry and photochemical mechanisms of fluorescent probes employed in SMLM. This Review provides guidance on the identification and adoption of fluorescent probes in single molecule localization microscopy to inspire the design of next-generation fluorescent probes amenable to single-molecule imaging.

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.

Light-Stimulated Luminescence Control of Lead Halide-Based Perovskite Nanocrystals Coupled with Photochromic Molecules via Electron and Energy Transfer

Photoswitchable nanomaterials are key materials in the development of advanced imaging techniques, such as super-resolution fluorescence microscopy. The combination of perovskite CsPbBr₃ nanocrystals (NCs) with bright photoluminescence (PL) emission and diarylethenes (DAEs) with structural changes in response to ultraviolet (UV) and visible light is a promising candidate system. Herein, CsPbBr₃ NCs are coupled with photochromic DAE molecules to control the PL emission from the NCs by light stimulation. The PL emission is successfully switched ON and OFF by alternating UV and visible light irradiation. Time-resolved PL emission studies suggest that Förster resonance energy transfer from CsPbBr₃ NCs to the closed-ring form of DAE occurs after UV irradiation, and the PL emission is quenched. Upon visible-light irradiation, DAE is converted to the open-ring isomer, and the PL emission is restored. Femtosecond pump-probe spectroscopy reveals that light stimulation induces not only energy transfer but also photoinduced electron transfer in the NC-DAE pair on the picosecond timescale to form DAE radicals. Thus, it is suggested that the holes residing in the NCs react with the NCs, degrading the PL emission. Stable PL switching is realized using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a hole scavenger to avoid the reaction between the holes and NCs.

Wide-range IR spectra of diarylethene derivatives and their simulation using the density functional theory

Diarylethenes (DAEs), promising photochromic molecular switches, undergo pericyclic reactions upon ultraviolet or visible light illumination. For this reason, most studies on DAEs employ UV–vis spectroscopies. However, also their infrared (IR) spectra are valuable, in particular, for understanding the vibrational dynamics which accompanies the relevant photoreactions. An accurate assignment of IR bands to molecular modes can be achieved through a comparison between experimental and computed theoretical spectra. Even though more sophisticated computational methods are available, the density functional theory (DFT) is usually employed for this task, because of its modest cost and versatility. Here, we have tested the ability of several DFT functionals to reproduce the wide-range, 400–3200 cm⁻¹, IR spectra of open and closed isomers of four representative DAE molecules. We find that global and range-separated, corrected for anharmonicity by scaling factors, hybrid DFT functionals are able to reproduce the IR spectra of DAEs, however, instead of the popular B3LYP functional we propose the use of the dispersion-corrected PBE0 functional. The paper also proposes a semi-automatic method of band assignment.

Unsymmetrical benzothiazole-based dithienylethene photoswitches

Herein, we investigate the structure-property relationships in a new series of benzothiazole based unsymmetrical hexafluorocyclopentene dithienylethenes (DTEs) and compare the results with the known facts for symmetric diarylethenes (DAEs). We reveal high photocyclization efficiency resulting from a significant shift of ground state equilibrium to the antiparallel conformation and a barrierless excited state pathway to conical intersection, which remains unperturbed even in polar solvents for most of the prepared DTEs. Furthermore, we uncover that the rate of back thermal cycloreversion correlates clearly more with the central C-C bond-length in the transition state than with the central C-C bond-length in the ground state of the cyclic form. Finally, our detailed vibrational spectral analysis of studied DTEs points out significant changes in Raman and infrared spectra during photoswitching cycles which pave the way for a non-destructive readout of stored information.

Stepping Out of the Blue: From Visible to Near-IR Triggered Photoswitches

Light offers unique opportunities for controlling the activity of materials and biosystems with high spatiotemporal resolution. Molecular photoswitches are chromophores that undergo reversible isomerization between different states upon irradiation with light, allowing a convenient means to control their influence over the system of interest. However, a significant limitation of classical photoswitches is the requirement to initiate the switching in one or both directions using deleterious UV light with poor tissue penetration. Red-shifted photoswitches are hence in high demand and have attracted keen recent research interest. In this Review, we highlight recent progress towards the development of visible- and NIR-activated photoswitches characterized by distinct photochromic reaction mechanisms. We hope to inspire further endeavors in this field, allowing the full potential of these tools in biotechnology and materials chemistry applications to be realized.

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.

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.

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...

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.

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.

On-Demand Chirality Transfer of Human Serum Albumin to Bis(thiophen-2-yl)hexafluorocyclopentenes through Their Photochromic Ring Closure

Photochromic 1,2-bis(5-carboxy-3-methyl-2-thienyl)hexafluorocyclopentene and its dimethyl ester incorporated in human serum albumin (HSA) showed highly enantioselective photochromic ring-closing reactions upon 366 nm light irradiation. The absolute stereochemistry of the major ring-closed form of the dicarboxylic acid at the newly formed sp³ carbon atoms was determined to be (S,S) by the process of docking simulation of the diarylethene molecule and HSA followed by molecular dynamics calculations and comparison of the measured and calculated CD spectra. Esterification of the major closed form of the diacid gave the minor closed form of the diester. The absolute stereochemistry of the major diester was thus determined to be (R,R).

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.

Long-term switching of single photochromic triads based on dithienylcyclopentene and fluorophores at cryogenic temperatures

Photochromic molecules can be reversibly converted between two bistable forms by light. These systems have been intensively studied for applications as molecular memories, sensing devices, or super-resolution optical microscopy. Here, we study the long-term switching behavior of single photochromic triads under oxygen-free conditions at 10 K. The triads consist of a photochromic unit that is covalently linked to two strong fluorophores that were employed for monitoring the light-induced conversions of the switch via changes in the fluorescence intensity from the fluorophores. As dyes we use either perylene bisimide or boron-dipyrromethen, and as photochromic switch we use dithienylcyclopentene (DCP). Both types of triads showed high fatigue resistance allowing for up to 6000 switching cycles of a single triad corresponding to time durations in the order of 80 min without deterioration. Long-term analysis of the switching cycles reveals that the probability that an intensity change in the emission from the dyes can be assigned to an externally stimulated conversion of the DCP (rather than to stochastic blinking of the dye molecules) amounts to 0.7 ± 0.1 for both types of triads. This number is far too low for optical data storage using single triads and implications concerning the miniaturization of optical memories based on such systems will be discussed. Yet, together with the high fatigue resistance, this number is encouraging for applications in super-resolution optical microscopy on frozen biological samples.

Photoswitchable stimulated Raman scattering spectroscopy and microscopy

Photoswitchable fluorescence is a powerful technique to realize super-resolution imaging, highlighting, and optical storage, while its multiplexing capability is limited. Raman scattering is attracting attention because it generates narrowband vibrational signatures, which are potentially useful for highly multiplexed detection of different constituents. Here, we demonstrate photoswitchable stimulated Raman scattering (SRS) spectroscopy and microscopy where narrowband vibrational signatures are switched with full reversibility at high speed. The demonstration of live-cell photoswitchable SRS imaging shows good sensitivity and compatibility with biological living systems.

GlycoBODIPYs: Sugars Serving as a Natural Stock for Water-soluble Fluorescent Probes of Complex Chiral Morphology

A range of unprocessed, reducing sugar substrates (mono-, di-, and trisaccharides) is shown to take part in a straightforward four-step synthetic route to water-soluble, uncharged BODIPY derivatives with unimpaired chiral integrity and high fluorescence efficiency. A wide compatibility with several postfunctionalizations is demonstrated, thus suggesting a universal utility of the multifunctional glycoconjugates, which we call GlycoBODIPYs. Knoevenagel condensations are able to promote a red-shift in the spectra, thereby furnishing strongly fluorescent red and far-red glycoconjugates of high hydrophilicity. The synthetic outcome was studied by X-ray crystallography and by comprehensive photophysical investigations in several solvent systems. Furthermore, cell experiments illustrate efficient cell uptake and demonstrate differential cell targeting as a function of the integrated chiral information.

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.

Photoswitchable Fluorescent Self-Assembled Metallacycles with High Photostability

In this study, photoswitchable fluorescent supramolecular metallacycles with high fatigue-resistance have been constructed by coordination-driven self-assembly by using bithienylethene with dipyridyl units (BTE) as a coordination donor and a fluorescent di-platinum(II) (Pt-F) as a coordination acceptor. The photo-triggered reversible transformation between the ring-open and ring-closed form of the metallacycles was confirmed by ¹ H NMR, ³¹ P NMR, and UV/Vis spectroscopy. This unique property enabled a reversible noninvasive "off-on" switching of fluorescence through efficient Förster resonance energy transfer (FRET). Importantly, the metallacycles remained structurally intact after up to 10 photoswitching cycles. The photoresponsive property and exceptional photostability of the metallacycles posit their potential promising application in optical switching, image storage, and super-resolution microscopy.

Photochromism and Fluorescence Switch of Furan-Containing Tetraarylethene Luminogens with Aggregation-Induced Emission for Photocontrolled Interface-Involved Applications

It is extremely challenging to design photocontrolled molecular switches with absorption and fluorescence dual-mode outputs that are suited for a solid surface and interface. Herein, we report a group of furan-containing tetraarylethene derivatives with unique photophysical behavior of aggregation-induced emission (AIE) and distinct photochemical reaction-triggered photochromic behaviors by combining a photoactive furan or benzofuran group and an AIE-active triphenylethene molecule. The introduction of a furyl or benzofuryl group into the AIE luminogen endows the molecules with significant reversible photochromism and solid-state fluorescence. The coloration and decoloration of these molecules can be switched by respective irradiation of UV and visible light in a reversible way, and the photochromic changes are accompanied by a switch-on and switch-off of the solid-state fluorescence. It is revealed that the photocontrolled cyclization and cycloreversion reactions are responsible for the reversible photochromism and fluorescence switching based on experimental data and theoretical analysis. Both the position and conjugation of the introduced photoactive units have significant influence on the color and strength of the photochromism, and the simultaneous occurrence of photoinduced fluorescence change in the solid state is perfectly suited for surface-involved applications. The demonstrations of dual-mode signaling in photoswitchable patterning on a filter paper and anti-counterfeiting of an anti-falsification paper strongly highlight the unique advantage of these photochromic molecules with an aggregation-induced emission characteristic in various practical applications. This work proposes a general strategy to design photochromic molecules with AIE activity by introducing photoactive functionals into an AIEgen and demonstrates incomparable advantage in dual-mode signaling and multifunctional applications of these molecules.

Super-resolution RESOLFT microscopy of lipid bilayers using a fluorophore-switch dyad

Dyads consisting of a photochromic switch covalently linked to a fluorescent dye allow the emission from the dye to be controlled by reversible photoisomerization of the switch; one form of the switch quenches fluorescence by accepting energy from the dye. Here we investigate the use of dyads of this type for super-resolution imaging of lipid bilayers. Giant unilamellar vesicles stained with the dyads were imaged with about a two-fold resolution-enhancement compared with conventional confocal microscopy. This was achieved by exciting the fluorophore at 594 nm, using a switch activated by violet and red light (405/640 nm).

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.

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.

A Suzuki Approach to Quinone-Based Diarylethene Photochromes

Diarylethene photochromes show promise for use in advanced organic electronic and photonic materials with burgeoning considerations for biological applications; however, these compounds typically require UV light for photoswitching in at least one direction, thus limiting their appeal. We here introduce a naphthoquinone-based diarylethene that switches between open and closed forms with visible light. The synthesis of this quinone diarylethene relies on Suzuki methodology, allowing for the inclusion of functional groups not otherwise accessible with current synthetic routes.

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.

Targeted photoswitchable imaging of intracellular glutathione by a photochromic glycosheet sensor

The development of photochromic fluorescence sensors with dynamic and multiple-signaling is beneficial to the improvement of biosensing/imaging precision. However, elaborate designs with complicated molecular structures are always required to integrate these functions into one molecule. By taking advantages of both redox-active/high loading features of two-dimensional (2D) manganese dioxide (MnO₂) and dynamic fluorescence photoswitching of photochromic sensors, we here design a hybrid photochromic MnO₂ glycosheet (Glyco-DTE@MnO₂) to achieve the photoswitchable imaging of intracellular glutathione (GSH). The photochromic glycosheet manifests significantly turn-on fluorescence and dynamic ON/OFF fluorescence signals in response to GSH, which makes it favorable for intracellular GSH double-check in targeted human hepatoma cell line (HepG2) through the recognition between β-D-galactoside and asialoglycoprotein receptor (ASGPr) on cell membranes. The dynamic fluorescence signals and excellent selectivity for detection and imaging of GSH ensure the precise determination of cell states, promoting its potential applications in future disease diagnosis and therapy.