Photochromic Acceptor Research Articles

Coupling of hydrophobic graphene quantum dots with photochromic molecule for fabrication of transparent photo-responsive polymeric films manifesting FRET functioning

In this study, a novel sensor on the basis of polymeric film comprising hydrophobic graphene quantum dots (GQDs-Amine) as fluorescent donor and spiropyran (SP) as photochromic acceptor was developed. The polymeric fluorescence sensor was synthesized through a four-step approach, beginning with the synthesize of photochromic component (SP) having hydroxyl functional group. Then, the amine-functionalized GQDs denoted as GQDs-Amine were synthesized through direct carbonization of citric acid in the presence of benzyl amine. GQDs-Amine was then decorated with SP to achieve GQDs-Amine-SP. In the final step, by using either polylactic acid (PLA) or polymethyl meta acrylate (PMMA) as polymeric precursors and via a simple solvent casting methodology, the related polymeric films comprising photoactive elements were prepared. All synthesized compounds were characterized using various methods such as nuclearmagnetic resonance (NMR) spectroscopy, elemental analysis (CHN), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, and field-emission scanning electron microscopy (FE-SEM). According to the related experiments, 0.582 mmol SP was successfully immobilized per gram of GQDs-Amine-SP. Also, after grafting of SP onto the surface of GQDs-Amine, the contact angle remarkably increased from 19° to 64° due to the difference hydrophobic nature between SP and its merocyanine (MC) form. The fluorescence resonance energy transfer (FRET) phenomenon occurred via exposure of GQDs-Amine-SP to alternating cycles of UV–vis irradiations. To confirm the FRET phenomenon and existing a good matching between the UV–vis absorption region of SP and the fluorescence emission spectra of GQDs-Amine, the photoluminescence activity of material was also performed in different mediums as well as in solid state. These solutions were exposed to UV light(365 nm) at various time intervals and their fluorescence properties were investigated under excitation at 400 nm. With increasing the time of UV light exposure up to 220 s, the fluorescence intensity increased at 620 nm, and inversely, the fluorescence intensity decreased at 471 nm. A similar pattern but with higher intensity were also observed for the polymeric films fabricated using phase inversion method. According to the UV–vis and fluorometric analyses, the GQDs-Amine-SP and the corresponding films showed excellent photoreversibility, photostability, and prominentfatigue resistance.

An all-photonic full color RGB system based on molecular photoswitches

On-command changes in the emission color of functional materials is a sought-after property in many contexts. Of particular interest are systems using light as the external trigger to induce the color changes. Here we report on a tri-component cocktail consisting of a fluorescent donor molecule and two photochromic acceptor molecules encapsulated in polymer micelles and we show that the color of the emitted fluorescence can be continuously changed from blue-to-green and from blue-to-red upon selective light-induced isomerization of the photochromic acceptors to the fluorescent forms. Interestingly, isomerization of both acceptors to different degrees allows for the generation of all emission colors within the red-green-blue (RGB) color system. The function relies on orthogonally controlled FRET reactions between the blue emitting donor and the green and red emitting acceptors, respectively.

Photoswitchable J-Aggregated Processable Organogel by Integrating a Photochromic Acceptor.

A novel π-chromophoric 1,4-bis(anthracenylethynyl)benzene (BAB)-based highly emissive J-aggregated organogel has been synthesized and characterized. Single-crystal structure determination of asymmetric π-chromophoric bola-amphiphilic BAB1 (dodecyl and triethyleneglycolmonomethylether containing side chains of bis(anthracenylethynyl)benzene) supports J-aggregation. Further, a photochromic acceptor chromophore, 4,4'-(perfluorocyclopent-1-ene-1,2-diyl)bis(5-methylthiophene-2-carbaldehyde), is noncovalently encapsulated in the gel and photoswitching studies have been performed based on photochromic Förster resonance energy transfer. The modulated emission of the processable soft material is further exploited for rewritable display. However, BAB2 (dodecyl side chain on both sides) does not show gelation property due to its low solubility.

Emission color tuning and white-light generation based on photochromic control of energy transfer reactions in polymer micelles.

We encapsulate a fluorescent donor molecule and a photochromic acceptor unit (photoswitch) in polymer micelles and show that the color of the emitted fluorescence is continuously changed from blue to yellow upon light-induced isomerization of the acceptor. Interestingly, white-light generation is achieved in between. With the photoswitch in the colorless form, intense blue emission from the donor is observed, while UV-induced isomerization to the colored form induces an energy transfer reaction that quenches the donor emission and sensitizes the yellow emission from the colored photoswitch. The process is reversed by exposure to visible light, triggering isomerization to the colorless form.

Red Fluorescent Protein with Reversibly Photoswitchable Absorbance for Photochromic FRET

We have developed the first red fluorescent protein, named rsTagRFP, which possesses reversibly photoswitchable absorbance spectra. Illumination with blue and yellow light switches rsTagRFP into a red fluorescent state (ON state) or nonfluorescent state (OFF state), respectively. The ON and OFF states exhibit absorbance maxima at 567 and 440 nm, respectively. Due to the photoswitchable absorbance, rsTagRFP can be used as an acceptor for a photochromic Förster resonance energy transfer (pcFRET). The photochromic acceptor facilitates determination of a protein-protein interaction by providing an internal control for FRET. Using pcFRET with EYFP as a donor, we observed an interaction between epidermal growth factor receptor and growth factor receptor-binding protein 2 in live cells by detecting the modulation of both the fluorescence intensity and lifetime of the EYFP donor upon the ON-OFF photoswitching of the rsTagRFP acceptor.

Spiropyrans as molecular optical switches

Optical microscopes use visible light and an arrangement of lenses to provide us with magnified images of small samples. Combined with efficient fluorescent probes and highly sensitive fluorescence detection techniques they allow the non-invasive 3D study of subcellular structures even in living cells or tissue. However, optical microscopes are subject to diffraction of light which limits optical resolution to approximately 200 nm in the imaging plane. In the recent past, powerful methods emerged that enable fluorescence microscopy with subdiffraction optical resolution. Since most of these methods are based on the temporal control of fluorescence emission of fluorophores, photochromic molecules that can be switched reversibly between a fluorescent on- and a non-fluorescent off-state are the key for super-resolution imaging methods. Here, we present our approach to use spiropyran-fluorophore conjugates as efficient molecular optical switches (photoswitches). In these photochromic conjugates fluorescence emission of the fluorophore is controlled by the state of the spiropyran, which can be switched reversibly between a colorless spiropyran and a colored merocyanine form upon irradiation with light. Thus, the efficiency of energy transfer from the fluorophore to the spiropyran can be modulated by the irradiation conditions. We present ensemble data of the switching process of various spiropyrans and spiropyran-fluorophore conjugates and demonstrate photoswitching at the single-molecule level. Our data suggest that spiropyrans have to be immobilized in polymers to stabilize the merocyanine form in order to be useful for super-resolution fluorescence imaging based on precise localization of individual emitters. Special emphasis is put on photobleaching of donor fluorophores due to UV irradiation, i.e. photoswitching of the photochromic acceptor. Furthermore, we present a water soluble switchable spiropyran derivative and demonstrate the first intermolecular single-molecule photoswitching experiments in polymers.

Photochromic Relaxation Kinetics

ABSTRACT A new method for evaluating the rate constants of chemical reactions has been developed based on the differential photoconversion efficiencies of photochromic FRET acceptors incorporated into molecules on both sides of a chemical equilibrium. In the application of Photochromic Relaxation Kinetics (pcRelKin) to a bimolecular binding reaction, one of the two interacting molecules bears a FRET donor and the second a photochromic acceptor. The close proximity required for FRET is achieved only in the complex. After photoconversion, the system exists in a thermodynamically asymmetric state, which relaxes according to first order kinetics. PcRelKin offers unique advantages over classical relaxation kinetics.

Diheteroarylethenes as thermally stable photoswitchable acceptors in photochromic fluorescence resonance energy transfer (pcFRET).

We have employed diheteroarylethenes as acceptors for photochromic FRET (pcFRET), a technique introduced for the quantitative determination of fluorescence resonance energy transfer (FRET). In pcFRET, the fluorescent emission of the donor is modulated by cyclical transformations of a photochromic acceptor. Light induces a reversible change in the structure and, concomitantly, in the absorption properties of the acceptor. Only the closed forms of the selected diheteroarylethenes 2a and 2b have an absorption band overlapping the emission band of the donor, 1. The corresponding variation in the overlap integral (and thus critical transfer distance R(o)) between the two states provides the means for reversibly switching the process of FRET on and off, allowing direct and repeated evaluation of the relative changes in the donor fluorescence quantum yield. The diheteroarylethenes demonstrate excellent stability in aqueous media, an absence of thermal back reactions, and negligible fatigue. The equilibration of these systems after exposure to near-UV or visible light follows simple monoexponential kinetics. We developed a general conceptual scheme for such coupled photochromic-FRET reactions, allowing quantitative interpretations of the photostationary and kinetic data, from which the quantum yields for the cyclization and cycloreversion reactions of the photochromic acceptor were calculated.

A photochromic acceptor as a reversible light-driven switch in fluorescence resonance energy transfer (FRET)

A method has been developed for the quantitative determination of fluorescence resonance energy transfer (FRET) based on the modulation of donor fluorescence upon the reversible photoconversion of a photochromic acceptor. A model system was devised, consisting of Lucifer Yellow cadaverine (LYC, donor) conjugated to the photochromic molecule, 6-nitroBIPS (1′,3′-dihydro-1′-(2-carboxyethyl)-3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-(2H)-indoline]). Near-ultraviolet irradiation catalyzes the conversion of the colorless spiropyran (SP) to the colored merocyanine (MC) form of 6-nitroBIPS. Only the MC form absorbs at the emission wavelengths of the donor, thereby potentiating FRET, as demonstrated by quenching of the donor. Subsequent irradiation in the visible MC absorption band reverts 6-nitroBIPS to the SP form and FRET is inactivated. The acceptor exhibited high photostability under repeated cycles of alternating UV–Vis irradiation. In this model system, the intramolecular FRET efficiency was close to 100%. The observed maximal donor quenching of 34±3% was indicative of an equilibrium determined by the high quantum efficiency of forward conversion (SP→MC) induced by near-UV irradiation and a low but finite quantum efficiency of the back reaction resulting from excitation of the MC form directly as well as indirectly (by FRET via the donor). A quantitative formalism for the photokinetic scheme was developed. Photochromic FRET (pcFRET) permits repeated, quantitative, and non-destructive FRET determinations for arbitrary relative concentrations of donor and acceptor and thus offers great potential for monitoring dynamic molecular interactions in living cells over extended observation times by fluorescence microscopy.

Photochromism-Fret (phFRET): Modulation of Fluorescence Resonance Energy Transfer by A Photochromic Acceptor

We have designed a new method for the quantitative determination of fluorescence resonance energy transfer (FRET) based on the modulation of the fluorescence emission of a donor by a photochromic acceptor. Light induces a change in the structure and thus the absorption properties of the acceptor. Only one of the absorption spectra overlaps with the emission of the donor, thereby providing the means for switching the process of FRET on and off reversibly. A direct evaluation of the change in the donor quantum yield due to FRET is possible. A test molecule was devised consisting of a Lucifer Yellow donor bound covalently to a 6-nitroBIPS acceptor via a cysteine linker molecule. Photoisomerisation of the acceptor by near-UV irradiation from the spiropyran to the merocyanine form potentiated the FRET process, as evidenced by quenching of the donor. This effect disappeared upon thermal reversion to the initial spiropyran form. Photokinetic simulations were also carried out.