Reversible Red Fluorescent Molecular Switches

A Visible-Light-Responsive Fluorescent Diarylethene Having a Betaine Structure

As a new molecular scaffold of photoswitchable fluorophore, we developed a photochromic diarylethene containing a betaine structure based on pyridinium N-enolate. A facile reaction of a pyridyl-substituted dithienylperfluorocyclopentene derivative with octafluorocyclopentene constructed the betaine structure. The introduction of the betaine moiety provided the diarylethene molecule with bathochromically shifted optical absorption and fluorescing ability, thus enabling the molecule to function as a visible-light-responsive turn-off mode photoswitchable fluorophore. The molecule in the open-ring form emitted bright bluish green fluorescence. Upon irradiation with 405 nm light, the molecule underwent cyclization isomerization to form the closed-ring isomer and the fluorescence intensity significantly decreased. The turn-off mode fluorescence photoswitching was observed not only in solution but also in polymer films.

Recent Progress of Photoswitchable Fluorescent Diarylethenes for Bioimaging

Photochromic diarylethene has attracted broad research interest in optical applications owing to its excellent fatigue resistance and unique bistability. Photoswitchable fluorescent diarylethene become a powerful molecular tool for fluorescence imaging recently. Herein, the recent progress on photoswitchable fluorescent diarylethenes in bioimaging is reviewed. We summarize the structures and properties of diarylethene fluorescence probes and emphatically introduce their applications in bioimaging as well as super-resolution imaging. Additionally, we highlight the current challenges in practical applications and provide the prospects of the future development directions of photoswitchable fluorescent diarylethene in the field of bioimaging. This comprehensive review aims to stimulate further research into higher-performance photoswitchable fluorescent molecules and advance their progress in biological application.

Kinetic Analysis of the Photochemical Paths in Asymmetric Diarylethene Dimer

An asymmetric diarylethene dimer composed of 2- and 3-thienylethene units linked by m-phenylene developed various colors upon UV irradiation via an independent photochromic reaction on each unit. The change in contents and the other photoresponses of the photogenerated four isomers were analyzed using quantum yield for all the possible photochemical paths, i. e., photoisomerization, fluorescence, energy transfer, and the other non-radiative paths. Almost all the rate constants of photochemical paths were calculated using measurable quantum yields and lifetimes. It was found that a significant contribution on photoresponse was the competition between photoisomerization and intramolecular energy transfer. The clear difference was observed in the photoresponses of the dimer and the 1 : 1 mixture solution of the model compounds. The m-phenylene spacer appropriately regulated the rate of energy transfer in the asymmetric dimer, and the spacer enabled isolation of the excited state of the dimer, making the above quantitative analysis possible.

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.

Long-Wavelength Photoconvertible Dimeric BODIPYs for Super-Resolution Single-Molecule Localization Imaging in Near-Infrared Emission

Sulfoxide-bridged dimeric BODIPYs were developed as a new class of long-wavelength photoconvertible fluorophores. Upon visible-light irradiation, a sulfoxide moiety was released to generate the corresponding α,α-directly linked dimeric BODIPYs. The extrusion of SO from sulfoxides was mainly through an intramolecular fashion involving reactive triplet states. By this photoconversion, not only were more than 100 nm red shifts of absorption and emission maxima (up to 648/714 nm) achieved but also stable products with bright fluorescence were produced with high efficiency. The combination of photoactivation and red-shifted excitation/emission offered optimal contrast and eliminated the interference from biological autofluorescence. More importantly, the in situ products of these visible-light-induced reactions demonstrated ideal single-molecule fluorescence properties in the near-infrared region. Therefore, this new photoconversion could be a powerful photoactivation method achieving super-resolution single-molecule localization imaging in a living cell without using UV illumination and cell-toxic additives.

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.

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...

A simple fluorescent switch with four states based on benzothiazole-spiropyran for reversible multicolor displays and anti-counterfeiting

The multi-state fluorescent switched with significant visible and fluorescent color variations in solution and the solid state upon multi stimuli.

Fluorescence sensors for imaging membrane lipid domains and cholesterol

Lipid membrane domains are supramolecular lateral heterogeneities of biological membranes. Of nanoscopic dimensions, they constitute specialized hubs used by the cell as transient signaling platforms for a great variety of biologically important mechanisms. Their property to form and dissolve in the bulk lipid bilayer endow them with the ability to engage in highly dynamic processes, and temporarily recruit subpopulations of membrane proteins in reduced nanometric compartments that can coalesce to form larger mesoscale assemblies. Cholesterol is an essential component of these lipid domains; its unique molecular structure is suitable for interacting intricately with crevices and cavities of transmembrane protein surfaces through its rough β face while "talking" to fatty acid acyl chains of glycerophospholipids and sphingolipids via its smooth α face. Progress in the field of membrane domains has been closely associated with innovative improvements in fluorescence microscopy and new fluorescence sensors. These advances enabled the exploration of the biophysical properties of lipids and their supramolecular platforms. Here I review the rationale behind the use of biosensors over the last few decades and their contributions towards elucidation of the in-plane and transbilayer topography of cholesterol-enriched lipid domains and their molecular constituents. The challenges introduced by super-resolution optical microscopy are discussed, as well as possible scenarios for future developments in the field, including virtual ("no staining") staining.

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.

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).

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.

Dual-color fluorescent nanoparticles showing perfect color-specific photoswitching for bioimaging and super-resolution microscopy

Dual-emissive systems showing color-specific photoswitching are promising in bioimaging and super-resolution microscopy. However, their switching efficiency has been limited because a delicate manipulation of all the energy transfer crosstalks in the systems is unfeasible. Here, we report a perfect color-specific photoswitching, which is rationally designed by combining the complete off-to-on fluorescence switching capability of a fluorescent photochromic diarylethene and the frustrated energy transfer to the other fluorescent dye based on the excited-state intramolecular proton transfer (ESIPT) process. Upon alternation of UV and visible light irradiations, the system achieves 100% switching on/off of blue emission from the diarylethene while orange emission from the ESIPT dye is unchanged in the polymer film. By fabricating this system into biocompatible polymer nanoparticles, we demonstrate microscopic imaging of RAW264.7 macrophage cells with reversible blue-color specific fluorescence switching that enables super-resolution imaging with a resolution of 70 nm.

Photoswitchable nanoparticles can be used for selective imaging in biological systems but usually have only one color. Here the authors develop a two-color fluorescent emissive system that allows full on-off switching of one component color of the system while the other color is unaffected, which has implications for super-resolution imaging.

A bifunctional sensor based on diarylethene for the colorimetric recognition of Cu2+ and fluorescence detection of Cd2+

A novel bifunctional sensor based on diarylethene with a benzyl carbazate unit was synthesized successfully. It not only served as a colorimetric sensor for the recognition of Cu²⁺ by showing changes in absorption spectra and solution color, but also acted as a fluorescent sensor for the detection of Cd²⁺ through obvious emission intensity enhancement and fluorescence color change. The sensor exhibited excellent selectivity and sensitivity towards Cu²⁺ and Cd²⁺, and the limits of detection for Cu²⁺ and Cd²⁺ were 8.36 × 10⁻⁸ mol L⁻¹ and 1.71 × 10⁻⁷ mol L⁻¹, respectively, which were much lower than those reported by the WHO and EPA in drinking water. Furthermore, its application in practical samples demonstrated that the sensor can be effectively applied for the detection of Cu²⁺ and Cd²⁺ in practical water samples.

A bifunctional sensor for colorimetric recognition of Cu²⁺ and fluorescent detection of Cd²⁺ was synthesized. It not only showed high selectivity and sensitivity to Cu²⁺ and Cd²⁺, but also could be used in practical water samples with high accuracy.

Regiocontrolled Halogen Dance of Bromothiophenes and Bromofurans

The LDA (lithium diisopropylamide)-promoted regiocontrolled halogen dance of α-bromothiophenes and α-bromofurans is described. Bromothiophenes bearing a diethyl acetal moiety undergo selective deprotonation at the β-position adjacent to the bromo group. In contrast, oxazoline, ester, and amide groups act as directing groups in the initial lithiation step to generate a carbanion at the β-position neighboring the directing group to exclusively give the other regioisomer. These results can be applied to the regiocontrolled halogen dance of bromofuran derivatives.

Near Infrared Light Sensitive Ultraviolet-Blue Nanophotoswitch for Imaging-Guided "Off-On" Therapy

Photoswitchable materials are important in broad applications. Recently appeared inorganic photoswitchable upconversion nanoparticles (PUCNPs) become a competitive candidate to surmount the widespread issue of the organic counterparts -photobleaching. However, current PUCNPs follow solely Yb³⁺/Nd³⁺ cosensitizing mode, which results in complex multilayer doping patterns and imperfectness of switching in UV-blue region. In this work, we have adopted a new strategy to construct Nd³⁺ free PUCNPs-NaErF₄@NaYF₄@NaYbF₄:0.5%Tm@NaYF₄. These PUCNPs demonstrate the superior property of photoswitching. A prominent UV-blue emission from Tm³⁺ is turned on upon 980 nm excitation, which can be completely turned off by 800 nm light. The quasi-monochromatic red upconversion emission upon 800 nm excitation-a distinct feature of undoping NaErF₄ upconversion system-endows the PUCNPs with promising image-guided photoinduced "off-on" therapy in biomedicine. As a proof-of-concept we have demonstrated the imaging-guided photodynamic therapy (PDT) of cancer, where 800 nm excitation turns off the UV-blue emission and leaves the emission at 660 nm for imaging. Once the tumor site is targeted, excitation switching to 980 nm results in UV-blue emission and the red emission. The former is used to induce PDT, whereas the latter is to monitor the therapeutic process. Our study implies that this upconversion photoswitching material is suitable for real-time imaging and image-guided therapy under temporal and spatial control.

Spironaphthoxazine switchable dyes for biological imaging

Recent developments in super-resolution microscopy have significantly expanded the requirements for switchable dyes, leading to demand for specially designed molecular switches. We report the synthesis and characterization of a spironaphthoxazine photochromic switch (a derivative of palatinate purple) displaying high photoconversion (85-95%) under readily accessible 405 nm light, broad absorption in the visible, and excellent fatigue resistance. The indole substituent on this spironaphthoxazine is twisted out of conjugation with the naphthalene unit, yet it is crucial for activation with visible light. The open colored merocyanine form of the spironaphthoxazine reverts to the closed form with a lifetime of 4.7 s in dichloromethane at 20 °C; this thermal reversion is even faster in more polar solvents. The photochemical quantum yields for ring-opening and ring-closing are approximately 8% and 1%, respectively, in dichloromethane. The ring-opening and ring-closing reactions have been characterized by time-resolved infrared and transient absorption spectroscopies. Ring opening occurs rapidly (τ = 2.1 ns) and efficiently (∼90%) from the singlet excited state to form an intermediate (assigned as a cisoid merocyanine), which returns to the closed ground state (τ = 4.5 ns) in competition with relaxation to the transoid open form (τ = 40 ns). Photochemical ring closing is a faster and simpler process: the excited state proceeds to the closed spirooxazine with a time constant of 0.28 ns. This photochromic switch can be used in conjunction with commercial fluorescent dyes to create a small-molecule switchable fluorescent dyad that shows high contrast and good fatigue resistance in living cells. These properties make the dyads suitable for application in RESOLFT microscopy.

A solid-phase synthetic approach to pH-independent rhodamine-type fluorophores

An efficient methodology using the Fukuyama-Mitsunobu reaction was successfully applied to prepare various Rhodamine B-based amides with the locked possibility to form a lactam ring. The procedure was developed for solid-phase synthesis, which can be advantageously applied to the synthesis of chemical libraries in a combinatorial fashion. A series of derivatives including aliphatic as well as aromatic rhodamine amides alkylated via a reaction with various alcohols were synthesized, and their spectral properties were investigated. Blocking lactamization via N-alkylation enabled us to prepare rhodamine derivatives with an excellent fluorescence response. In comparison to their non-alkylated counterparts, these derivatives exhibited pH independence and higher quantum yields.

Styrylisoxazole-based fluorescent probes for the detection of hydrogen sulfide

Styrylisoxazoles bearing a nitro group were utilized for detection of H₂S through a reduction reaction.

Synthesis, photophysical, antibacterial and molecular docking studies on aromatic ring core-containing rhodamine B decorated triazole bridged dendrimers

Rhodamine B decorated dendrimers 1–6 were synthesized by a convergent approach using click chemistry. The zeroth generation dendrimer 1 (G0) and the first generation dendrimer 4 (G1) showed better antibacterial activity than the other dendrimers.

Organogels composed of trifluoromethyl anthryl cyanostyrenes: enhanced emission and self-assembly

CF₃ substituted anthryl cyanostyrenes were synthesized and examined for their self-assembly and organogel formation.

Temperature dependence of the conversion efficiency of photochromic perylene bisimide dithienylcyclopentene triads embedded in a polymer

Photochromic molecules that are covalently linked to a strong fluorophore combine the requirements of external control and strong fluorescence, which will become increasingly important for super-resolution microscopy techniques based on single molecules. However, given the bulky structure of such constructs, steric hindrance might affect their photoconversion efficiencies upon immobilising them for imaging purposes. In this study the efficiencies of the photochromic conversion processes of molecular triads that are embedded in a polymer have been studied as a function of temperature. The triads consist of two perylene bisimide dye molecules that are connected via a dithienylcyclopentene photochromic bridge that undergoes a cyclization/cycloreversion reaction upon appropriate illumination. It is found that photochromic switching remains active, even at 5 K, yet with reduced but finite efficiency for the cycloreversion reaction. This might even be an advantage for the achievement of high labelling densities in super-resolution microscopy.

Photoswitchable Fluorescent Diarylethene Derivatives with Thiophene 1,1-Dioxide Groups: Effect of Alkyl Substituents at the Reactive Carbons

Photoswitching and fluorescent properties of sulfone derivatives of 1,2-bis(2-alkyl-4-methyl-5-phenyl-3-thienyl)perfluorocyclopentene, 1–5, having methyl, ethyl, n-propyl, i-propyl, and i-butyl substituents at the reactive carbons (2- and 2′-positions) of the thiophene 1,1-dioxide rings were studied. Diarylethenes 1–5 underwent isomerization reactions between open-ring and closed-ring forms upon alternate irradiation with ultraviolet (UV) and visible light and showed fluorescence in the closed-ring forms. The alkyl substitution at the reactive carbons affects the fluorescent property of the closed-ring isomers. The closed-ring isomers 2b–5b with ethyl, n-propyl, i-propyl, and i-butyl substituents show higher fluorescence quantum yields than 1b with methyl substituents. In polar solvents, the fluorescence quantum yield of 1b markedly decreases, while 2b–5b maintain the relatively high fluorescence quantum yields. Although the cycloreversion quantum yields of the derivatives with methyl, ethyl, n-propyl, and i-propyl substituents are quite low and in the order of 10⁻⁵, introduction of i-butyl substituents was found to increase the yield up to the order of 10⁻³. These results indicate that appropriate alkyl substitution at the reactive carbons is indispensable for properly controlling the photoswitching and fluorescent properties of the photoswitchable fluorescent diarylethenes, which are potentially applicable to super-resolution fluorescence microscopies.

Deliberate Switching of Single Photochromic Triads

Photochromic molecules can be reversibly converted between two bistable conformations by light, and are considered as promising building blocks in novel macromolecular structures for sensing and imaging techniques. We have studied individual molecular triads consisting of two strong fluorophores (perylene bisimide) that are covalently linked via a photochromic unit (dithienylcyclopentene) and distinguished between deliberate switching and spontaneous blinking. It was verified that the probability for observing deliberate light-induced switching of a single triad (rather than stochastic blinking) amounts to 0.8 ± 0.1. In a few exceptional cases this probability can exceed 0.95. These numbers are sufficiently large for application in sensitive biosensing, and super-resolution imaging. This opens the possibility to develop devices that can be controlled by an external optical stimulus on a truly molecular length scale.

Photostable and photoswitching fluorescent dyes for super-resolution imaging

Super-resolution fluorescence microscopy is a recently developed imaging tool for biological researches. Several methods have been developed for detection of fluorescence signals from molecules in a subdiffraction-limited area, breaking the diffraction limit of the conventional optical microscopies and allowing visualization of detailed macromolecular structures in cells. As objectives are exposed to intense laser in the optical systems, fluorophores for super-resolution microscopy must be tolerated even under severe light irradiation conditions. The fluorophores must also be photoactivatable and photoswitchable for single-molecule localization-based super-resolution microscopy, because the number of active fluorophores must be controlled by light irradiation. This has led to growing interest in these properties in the development of fluorophores. In this mini-review, we focus on the development of photostable and photoswitching fluorescent dyes for super-resolution microscopy. We introduce recent efforts, including improvement of fluorophore photostability and control of photoswitching behaviors of fluorophores based on photochemical and photophysical processes. Understanding and manipulation of chemical reactions in excited fluorophores can develop highly photostable and efficiently photoswitchable fluorophores that are suitable for super-resolution imaging applications.

A fluorescent photochromic diarylethene based on naphthalic anhydride with strong solvatochromism

A diarylethene molecule consisting of an 1,8-naphthalic anhydride and two 2,5 dimethylthiophene which exhibits reversible fluorescence switching capacity as well as solvatochromism with red shift of the fluorescence maximum by more than 150 nm.

Synthesis, mechanism and efficient modulation of a fluorescence dye by photochromic pyrazolone with energy transfer in the crystalline state

The emission intensity of DPA can be efficiently switched by fluorescence resonance energy transfer between the energy donor DPA and acceptor 1b.

Synthesis and Spectroscopic Investigation of Diketopyrrolopyrrole - Spiropyran Dyad for Fluorescent Switch Application

We report the synthesis and characterization of a new fluorescent dyad SP-DPP-SP(9) via efficient palladium-catalyzed Sonogashira coupling of prop-2-yn-1-yl 3-(3',3'dimethyl-6-nitrospiro[chromene-2,2'-indolin]-1'-yl)propanoatespiropyran, SP(8), a well known photochromic accepter, with 3,6-bis(5-bromothiophen-2-yl)-2,5-bis((R)-2-ethylhexyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione, DPP(4), a highly fluorescent donor. Under visible light exposure the SP unit is in a closed hydrophobic form, whereas under UV irradiation it converts to a polar, hydrophilic open form named Merocyanine (MC), which is responsible for functioning of photo-switch application. The photochemistry pertaining to fluorescence switch, 'on/off' behaviour, of model dyad SP-DPP-SP(9) is experimentally analyzed in solution as well as in solid state in polymer matrices by photoluminescence(PL) and absorption spectroscopy. After absorption of UV light the spiropyran unit of the dyad under goes the rupture of the spiro C-O bond leading to the formation of MC. The absorption band of MC fairly overlaps to the fluorescence of DPP unit resulting quenching of fluorescence via fluorescence resonance energy transfer from exited DPP unit to ground state MC. In contrary, the fluorescence of DPP is fully regained upon transformation of MC to SP by exposure to visible light or thermal stimuli. Hence, the fluorescence intensity of dyad 9 is regulated by reversible conversion among the two states of the photochromic spiropyran units and the fluorescence resonance energy transfer (FRET) between the MC form of SP and the DPP unit. Conversely, these scrutiny of the experiment express that the design of dyad 9 is viable as efficient fluorescent switch molecule in many probable commercial applications, such as, logic gates and photonic and optical communications.

Carboxylated Photoswitchable Diarylethenes for Biolabeling and Super-Resolution RESOLFT Microscopy

Reversibly photoswitchable 1,2‐bis(2‐ethyl‐6‐phenyl‐1‐benzothiophene‐1,1‐dioxide‐3‐yl)perfluorocyclopentenes (EBT) having fluorescent “closed” forms were decorated with four or eight carboxylic groups and attached to antibodies. Low aggregation, efficient photoswitching in aqueous buffers, specific staining of cellular structures, and good photophysical properties were demonstrated. Alternating light pulses of UV and blue light induce numerous reversible photochemical transformations between two stables states with distinct structures. Using relatively low light intensities, EBTs were applied in biology‐related super‐resolution microscopy based on the reversible saturable (switchable) optical linear fluorescence transitions (RESOLFT) and demonstrated optical resolution of 75 nm.

Photoswitchable Spiropyran Dyads for Biological Imaging

The synthesis of a small-molecule dyad consisting of a far-red-emitting silicon rhodamine dye that is covalently linked to a photochromic spironaphthothiopyran unit, which serves as a photoswitchable quencher, is reported. This system can be switched reversibly between the fluorescent and nonfluorescent states using visible light at wavelengths of 405 and 630 nm, respectively, and it works effectively in aqueous solution. Live-cell imaging demonstrates that this dyad has several desirable features, including excellent membrane permeability, fast and reversible modulation of fluorescence by visible light, and good contrast between the bright and dark states.

Novel triphenylamine based rhodamine derivatives: synthesis, characterization, photophysical properties and viscosity sensitivity

Five novel triphenylamine based deep red to NIR emitting rhodamine derivatives were synthesized and characterized by ¹H and ¹³C NMR spectroscopy and elemental analysis.

A new type of triphenylamine based coumarin–rhodamine hybrid compound: synthesis, photophysical properties, viscosity sensitivity and energy transfer

A series of novel core modified triphenylamine coumarin–rhodamine systems (compounds MCMR, MCDR and DCMR) was designed and synthesized by incorporating a coumarin moiety on one and a rhodamine moiety on the other phenyl ring of the triphenylamine molecular skeleton.

Light-triggered “on–off” switching of fluorescence based on a naphthopyran-containing compound polymer micelle

We report a compound micelle based on a naphthopyran-containing copolymer and a fluorophore-containing copolymer for light-triggered “on–off” switching of fluorescence.

Reversible Photomodulation of Electronic Communication in a π-Conjugated Photoswitch-Fluorophore Molecular Dyad

The extent of electronic coupling between a boron dipyrromethene (BODIPY) fluorophore and a diarylethene (DAE) photoswitch has been modulated in a covalently linked molecular dyad by irradiation with either UV or visible light. In the open isomer, both moieties can be regarded as individual chromophores, while in the closed form the lowest electronic (S0 →S1 ) transition of the dyad is slightly shifted, enabling photomodulation of its fluorescence. Transient spectroscopy confirms that the dyad behaves dramatically different in the two switching states: while in the open isomer it resembles an undisturbed BODIPY fluorophore, in the closed isomer no fluorescence occurs and instead a red-shifted DAE behavior prevails.