Reversible Fluorescence Switching Research Articles

Substituent Controlled Tunable Fluorescence from Green to Red and pH Stimuli‐Induced Reversible Fluorescence Switching in Triphenylamine–Quinoxaline Derivatives

A series of triphenylamine‐quinoxaline donor‐acceptor (bipolar) compounds (TPA‐QH, TPA‐QMe, TPA‐QCOOH and TPA‐QNO2) with different substituents were synthesized and investigated their fluorescence properties, including stimuli‐induced fluorescence responses in solution and the solid‐state. Single crystal structural analysis revealed non‐planar molecular conformation, substituent controlled intermolecular interactions and molecular packing in the crystal lattice. TPA‐QH, TPA‐QMe, TPA‐QCOOH and TPA‐QNO2 showed tunable solid‐state emission from green to red. TPA‐QH showed strong fluorescence at 487 nm (quantum yield (Φf) = 28.3%) whereas NO2 substituted TPA‐QNO2 exhibited relatively weak fluorescence at 610 nm (Φf = 4.3%). Density functional theoretical (DFT) calculations also indicated reduction of optical bandgap with substituting electron withdrawing group. The donor‐acceptor structure with intramolecular charge transfer (ICT) resulted solvent polarity dependent fluorescence tuning. The presence of acid responsive quinoxaline group was utilized to demonstrate pH‐responsive fluorescence switching and dual state fluorescence was utilized for fabricating rewritable fluorescent platform. Thus, the present study provides a structural insight to develop dual state emissive triphenylamine–quinoxaline based bipolar materials for optoelectronic applications.

Parallel illumination for depletion microscopy through acousto-optic spatial light modulation

Several types of super-resolution microscopy techniques, such as Stimulated Emission Depletion (STED), Reversible Saturable Optical Fluorescence Transitions (RESOLFT) or Switching Laser Mode (SLAM) microscopies, employ Laguerre-Gaussian beams (also called vortex or doughnut beams) to obtain fluorescence information within a sub-wavelength region of the specimen under observation, thus breaking the diffraction limit and producing images of greatly improved quality. However, in general, these techniques operate on a point-by-point basis, so scanning the sample in order to build a full, meaningful image, takes time. Parallelization of the illumination is the only way to make these microscopy techniques more suitable for real-time live cell imaging applications. Here, we demonstrate the parallel production of arbitrary arrays of Gaussian and Laguerre-Gaussian lasers foci suitable for super-resolution microscopy, together with the possibility to fast scan through the sample, by means of acousto-optic spatial light modulation, a technique that we have pioneered in the past in several other fields. Parallelized illumination with both Gaussian and doughnut beams will be then used to acquire super-resolution images.

Reactive oxygen species-responsive nano-platform with dual-targeting and fluorescent lipid-specific imaging capabilities for the management of atherosclerotic plaques

Atherosclerosis (AS), a pathological cause of cardiovascular disease, results from endothelial injury, local progressive inflammation, and excessive lipid accumulation. AS plaques rich in foam cells are prone to rupture and form thrombus, which can cause life-threatening complications. Therefore, the assessment of atherosclerotic plaque vulnerability and early intervention are crucial in reducing the mortality rates associated with cardiovascular disease. In this work, A fluorescent probe FC-TPA was synthesized, which switches the fluorescence state between protonated and non-protonated, reducing background fluorescence and enhancing imaging signal-to-noise ratio. On this basis, FC-TPA is loaded into cyclodextrin (CD) modified with phosphatidylserine targeting peptide (PTP) and coated with hyaluronic acid (HA) to construct the intelligent responsive diagnostic nanoplatform (HA@PCFT). HA@PCFT effectively targets atherosclerotic plaques, utilizing dual targeting mechanisms. HA binds strongly to CD44, while PTP binds to phosphatidylserine, enabling nanoparticle aggregation at the lesion site. ROS acts as a smart release switch for probes. Both in vitro and in vivo evaluations confirm impressive lipid-specific fluorescence imaging capabilities of HA@PCFT nanoparticles (NPs). The detection of lipid load in atherosclerotic plaque by fluorescence imaging will aid in assessing the vulnerability of atherosclerotic plaque. Statement of significanceCurrently, numerous fluorescent probes have been developed for lipid imaging. However, some challenges including inadequate water solubility, nonspecific distribution patterns, and fluorescence background interference, have greatly limited their further applications in vivo. To overcome these limitations, a fluorescent molecule has been designed and synthesized, thoroughly investigating its photophysical properties through both theoretical and experimental approaches. Interestingly, this fluorescent molecule exhibits the reversible fluorescence switching capabilities, mediated by hydrogen bonds, which effectively mitigate background fluorescence interference. Additionally, the fluorescent molecules has been successfully loaded into nanocarriers functionalized with the active targeting abilities, which has significantly improved the solubility of the fluorescent molecules and reduced their nonspecific distribution in vivo for an efficient target imaging in atherosclerosis. This study provides a valuable reference for evaluating the performance of such fluorescent dyes, and offers a promising perspective on the design of the target delivery systems for atherosclerosis.

Deep blue emitting dual state fluorescent triphenylamine-dicyclohexylurea derivative: Multi-stimuli responsive fluorescence switching and methanol/water sensing

A new deep blue emissive organic fluorophore (N-cyclohexyl-N-(cyclohexylcarbamoyl)-4-(diphenylamino)benzamide (NCDPB)) was designed and synthesized, which showed strong fluorescence both in solution and solid-state. Solid-state structural analysis of NCDPB revealed non-planar twisted molecular conformation with extended hydrogen bonding between the amide functionalities. The propeller shaped triphenylamine (TPA) and non-planar cyclohexyl unit prevented close π…π stacking and produced strong deep blue emission in the solid state (λmax = 400 nm, quantum yield (Φf) = 12.6 %). NCDPB also exhibited strong solvent polarity dependent tunable emission in solution (λmax = 402–462 nm, Φf = 1.15 (compared to quinine sulphate)). NCDPB showed reversible fluorescence switching between two fluorescence states upon mechanical crushing and heating/solvent exposure. Mechanical crushing caused red shifting of fluorescence from 400 to 447 nm and heating/solvent exposure reversed the fluorescence. Further, NCDPB also displayed off–on reversible/self-reversible fluorescence switching upon exposure to trifluoracetic acid (TFA) and NH3. The repeated fluorescence switching cycles indicated high reversibility without any significant change of fluorescence intensity. The drastically different fluorescence of NCDPB in CH3OH and EtOH was utilized to distinguish them and monitor CH3OH contamination in ethanol and benzene. It showed limit of detection (LOD) of methanol up to 0.25 % and 7 % in benzene and ethanol, respectively. The water sensitive fluorescence modulation of NCDPB in organic solvents was used to sensing water contamination in common organic solvents. Thus, integration of twisted TPA with H-bonding urea produced dual state emitting organic fluorophore with multi-responsive fluorescence switching and solvent sensing.

Carbazole-Benzonitrile-Norbornadiene Conjugates for Photothermally Reversible Ambient Phosphorescence.

Organic photoswitches have attracted significant attention across various fields, such as sensing, bioimaging, photopharmacology, molecular machines, and solar energy storage. However, as a result of design complexities, achieving photothermally reversible ambient phosphorescence switching in the condensed state remains elusive. Herein, we explore the impact of norbornadiene (NBD)/quadricyclane (QC) substitution at position 5 of the benzonitrile acceptor covalently attached to the carbazole donor on photothermally reversible luminescence switching. Experimental investigations demonstrated that the CzN and TBCzN switches exhibited photothermally reversible fluorescence switching in solution. Moreover, in the condensed state, fluorescence and ambient phosphorescence switching were observed as a result of a low singlet-triplet (ΔEST) gap (CzN ⇆ CzQ, ΔESTCzN/CzQ = 0.05/0.28 eV; TBCzN ⇆ TBCzQ, ΔESTTBCzN/TBCzQ = 0.06/0.09 eV). Reversible ambient phosphorescence switching is primarily influenced by modulation of acceptor conjugation resulting from NBD ⇆ QC switching. This approach may provide important clues for the design of visible-light-absorbing photothermally reversible phosphorescent materials.

Force-activated ratiometric fluorescence switching of tensile mechano-fluorophoric polyurethane elastomers with enhanced toughnesses improved by mechanically interlocked [c2] daisy chain rotaxanes

The extended and contracted conformations of [c2] daisy chain rotaxanes were introduced in mechano-fluorophoric polyurethane (PU) scaffolds via step-growth polymerizations to obtain distinct mechanical and optical features upon external forces. Particularly, the inherent ductile and stretchable capabilities along with notable toughnesses of PU films containing very low contents of [c2] daisy chain moieties with long-range sliding motions were obviously improved. Furthermore, appealing Förster resonance energy transfer (FRET) behaviors with the optimum energy transfer efficiency ca. 26.02% between blue-emitting tetraphenylethylene (TPE) and yellow-orange-emitting rhodamine derivatives could be acquired in the PU film under tensile conditions, which could be assigned to the incorporation of bi-fluorophoric daisy chain rotaxane into PU frameworks to gain reversible dual fluorescence switching behaviors upon stretching and relaxation processes. Additionally, the stretching deformations of PU films were inspected by X-ray diffraction (XRD) and FTIR techniques for the verification of correlated morphological properties of stretching states in the oriented daisy chain rotaxane-based PU films. Interestingly, impressive shape recovery and reversible ratiometric mechano-fluorophoric fluorescence switching properties of [c2] daisy chain rotaxane-based PU films could be attained upon heating, suggesting potential applications of PU films comprising artificial molecular muscles with glorious mechanical and optical behaviors in advanced polymer networks.

Redox regulation of electrofluorochromic behavior for an AIEgen-doped PVA hydrogel based on a ferrocene derivate

The fluorescent molecule FcMe-TPE with aggregation-induced emission properties was synthesized to prepare an EFC hydrogel material with high conductivity. The obtained hydrogel exhibited reversible fluorescent switching behavior on external stimulus.

Synthesis and Photochromic Properties of Diaryl [5]Helicene Derivatives.

Photoinduced tuning of aromaticity and correlative molecular properties has attracted enormous interest in recent years both for modulating photochromism properties and designing novel photochromic materials. Here, we report the synthesis and photochemical characterization of diaryl[5]helicene-based diarylethene molecular switches. 3,4-Bis(2,4-dimethyl-5-phenylthiophen-3-yl)dibenzo[c,g]phenanthrene derivative 1 a showed no photochromic reaction, whereas 3,4-bis(2-methyl-5-phenylthiophen-3-yl)dibenzo[c,g]phenanthrene derivative 2 a and 3,4-bis(5-methyl-2-phenylthiazol-4-yl)dibenzo[c,g]phenanthrene derivative 3 a exhibited reversible photochromism in different aprotic solvents with specific light irradiation. Meanwhile, the diarylethene compounds 2 a and 3 a underwent turn-off mode fluorescence photoswitching together with photoisomerization upon light irradiation. Remarkably, the photoinduced changes in the aromaticity of [5]helicene as a central ethene bridge along with the relative smaller activation energy and higher frequency factor facilitated the thermal bleaching rates of diarylethene switches 2 and 3 in solution. This research provides new insight for designing aromatic diarylethene photoswitches for reversible fluorescence switching, photoinduced changes in aromaticity and further fast thermal back reactions.

Development of pyrene-based MOFs probe for water content and investigations on their mechanochromism and acidochromism

A smart fluorescent pyrene-based MOFs (HPU-26) was synthesized by the solvothermal method, which possessed multi-stimuli-responsive properties and could detect water content in common organic solvents. The reversible multicolor fluorescent switching between yellowish green and cyan were observed under the stimulation of force, heating, and acid-base vapors. PXRD pattern, DSC curve, FI-IR spectra and the solid-state UV–vis absorption spectra were conducted to prove that the changes of π - conjugation induced the transformations between excimer and monomer formation under external stimulation, thus resulting in different fluorescent properties. In addition, HPU-26 could realize the detection of water content by remarkable change of fluorescent color from yellowish green to blue with increasing the water content in common organic solvents, which was attributed to the decomposition of crystal structure. The limit of detection (LOD) values of HPU-26 for detecting the water content in EtOH, DMA, Acetone and ACN were determined as 0.0260 %, 0.0578 %, 0.1978 %, and 0.5637 % (v/v), respectively. Furthermore, HPU-26 could be applied in advanced anti-counterfeiting and convenient visual detection of water content in common organic solvents by smartphone-based software.

Benzofuran derived pH-responsive AIEgens: Design, synthesis and applications in monitoring the urea content in soil and food quality evaluation

The stimuli-responsive luminescent materials have received special attention because of the modulable physical or chemical properties under the influence of external stimuli such as pH, temperature, light, heat, pressure, and so on. Therefore, such materials have potential applications in mechano-sensing devices, security inks, memory devices, data storage, deformation detectors, and so on. We present here the design and synthesis of three pH-responsive luminogens L1-L3 with aggregation-induced emissive properties built over the benzofuran scaffold. The AIE active luminogens L1-L3 demonstrated reversible fluorescence switching when exposed to Trifluoroacetic acid (TFA) fumes followed by Triethylamine (TEA) or NH3 fumes. The pH-sensitive fluorescence behavior of L2 has been successfully used in developing fluorescent inks for anti-counterfeit applications, as well as to construct paper strip-based detecting tools for qualitatively detecting the urea content in the soil and evaluating the ripeness/maturity of the packed fruits (grapes) in a non-destructive manner. Furthermore, both L1 and L2 were found to be highly biocompatible and well internalized within the cytoplasm of MDA-MB-231 cancer cells, with very high fluorescence intensity.

Fabrication of diarylethene-based reversible fluorescent switch encapsulated in silicone material and its photostimulus response mechanism

Photoresponsive materials, altering their properties such as color, luminescence, and luminescence lifetime under light as external stimulus, would be ideal candidates for application in fluorescent anti-counterfeiting and information encryption. In this paper, we develop a water-dispersive photo-stimuli response hybrid by encapsulating lanthanide complexes and photochromic molecules with an organosilicon material (OSM) via the reprecipitation-encapsulation method. Interestingly, repeated cycles of UV/Vis light irradiation switch the color and structure of photochromic molecules, which in turn influence the Förster resonance energy transfer (FRET) between photochromic molecules and lanthanide complexes, resulting in reversible luminescence ON/OFF switches, accompanied by reduction and recovery of luminescence lifetime of Eu3+ complexes. Considering the above-mentioned interesting characteristics, we realized the application of Eu(DBM)3·phen/BTHFC@OSM suspension as a light-driven fluorescent ink for fingerprint depth encryption. We expect our findings can expand the application scope of photochromic luminescence materials and provide a powerful approach for information encryption and anti-counterfeiting.

Biodegradable and switchable near-infrared fluorescent probes for hypoxia detection.

Aims: Among solid tumors, hypoxia is a common characteristic and responsible for chemotherapeutic resistance. Hypoxia-sensitive imaging probes are therefore essential for early tumor detection, growth monitoringand drug-response evaluation. Despite significant efforts, detecting hypoxic oxygen levels remains challenging. Materials & methods: This paper demonstrates the use of an amine-rich carbon dot probe functionalized with an imidazole group that exhibits reversible fluorescence switching in normoxic and hypoxic environments. Results & conclusion: We demonstrate the ability to emit near-infrared light only under hypoxic conditions. The probes are found to be biodegradable in the presence of human digestive enzymes such as lipase. Exvivo tissue imaging experiments revealed promising near-infrared signals even at a depth of 5mm for the probe under exvivo imaging conditions.

Unique mechanofluorochromism and reversible multicolor switching of triphenylamine-functionalized sulfonyl hydrazone derivatives

Two new simple triphenylamine-based sulfonyl hydrazone derivatives DBBS ((E)-N'-(4-(diphenylamino)benzylidene)benzenesulfonohydrazide) and DBMS ((E)-N'-(4-(diphenylamino)benzylidene)-4-methylbenzenesulfonohydrazide) were designed and synthesized via one-step aldimine condensation reaction to investigate the influence of terminal methyl group on their photophysical, crystal stacking and mechanofluorochromic (MFC) properties. Interestingly, reversible multi-color fluorescence tuning and switching mechanofluorochromism was found for DBBS due to the nature of polymorphs, and DBMS exhibited reversible bicolor switching mechanofluorochromism. Additionally, the reversible MFC behavior was ascribed to the phase transition between the crystalline and amorphous states, which have been confirmed by powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FESEM) and differential scanning calorimetry (DSC) results. The obtained results showed that terminal methyl group did not only regulate molecular conformations and/or intermolecular interactions in crystals, but also affect the force-stimuli responsiveness. The present study provided insight into the development of multicolor switching materials by rational modification of classic luminogens.

Electrochemical fluorescence switching of enhanced green fluorescent protein

Switchable fluorescent proteins, for which fluorescence can be switched ON and OFF, are widely used for molecule tracking and super resolution imaging. However, the robust use of the switchable fluorescent proteins is still limited as either the switching is not repeatable, or such switching requires irradiation with coupled lasers of different wavelengths. Herein, we report an electrochemical approach to reversible fluorescence switching for enhanced green fluorescent proteins (EGFP) on indium tin oxide coated glass. Our results demonstrate that negative and positive electrochemical potentials can efficiently switch the fluorescent proteins between the dim (OFF) and bright (ON) states at the single molecule level. The electrochemical fluorescence switching is fast, reversible, and may be performed up to hundreds of cycles before photobleaching occurs. These findings highlight that this method of electrochemical fluorescence switching can be incorporated into advanced fluorescence microscopy.

Multi-stimuli responsive carbazole-imidazole derivatives: Effect of phenyl substitution and UV light-oxygen induced photochemical transformation

New carbazole-imidazole derivatives, (3-(4,5-diphenyl-1H-imidazole-2-yl)-9-(3-methoxyphenyl)-9H-carbazole (Cz-DPIM)) and 9-(3-methoxyphenyl)-3-(1,4,5-triphenyl-1H-imidazole-2-yl)-9H-carbazole (Cz-TPIM) are synthesized and investigated the effect of phenyl substitution on the fluorescence and stimuli-induced fluorescence properties. Cz-DPIM exhibited relatively strong fluorescence in solution compared to Cz-TPIM (Quantum yield (Φf, compared to quinine sulphate standard) = 0.28 (Cz-DPIM) and 0.09 (Cz-TPIM)). In contrast, Cz-DPIM is weakly fluorescence in solid-state but Cz-TPIM showed strong fluorescence (Φf = 1.3% (Cz-DPIM) and 18.2% (Cz-TPIM)). Cz-TPIM displayed mechanoresponsive reversible fluorescence switching upon crushing and solvent fuming. Both Cz-DPIM and Cz-TPIM exhibited reversible halofluorochromsim upon exposure of trifluoroacetic acid (TFA) and ammonia (NH3). Interestingly, Cz-DPIM and Cz-TPIM exhibited photocleavage upon UV light irradiation in presence of oxygen in CH3CN and produced green emissive fluorescence product. The controlled studies revealed that the presence of oxygen is important for exhibiting photocleavage by UV light exposure and suggest potential use for oxygen sensing application. The UV light and oxygen induced photocleavage of Cz-DPIM and Cz-TPIM also demonstrated in thin films.

Construction of a Reversible Solid-state Fluorescence Switching Via Photochromic Diarylethene and Si-ZnO Quantum Dots.

Developing fluorescence switching as functional system is highly desirable for potential applications in the fields of light-responsive materials or devices. Attempt to construct fluorescence switching system tend to focus on the high fluorescence modulation efficiency, especially in solid state. Herein, a photo-controlled fluorescence switching system was constructed with photochromic diarylethene and trimethoxysilane modified zinc oxide quantum dots (Si-ZnO QDs) successfully. It was verified by the measurement of modulation efficiency, fatigue resistance as well as theoretical calculation. Upon irradiation with UV/Vis lights, the system exhibited excellent photochromic property and photo-controlled fluorescence switching performance. Furthermore, the excellent fluorescence switching characters could also be realized in solid state and the fluorescence modulation efficiency was determined to be 87.4%. The results will provide new strategies to the construction of reversible solid-state photo-controlled fluorescence switching for the application in the fields of optical data storage and security labels.

White light emissive organic fluorophores and halogen atom-controlled tunable and stimuli-induced reversible fluorescence switching

White light emissive organic fluorophores and halogen atom-controlled tunable and stimuli-induced reversible fluorescence switching

Polarity-Independent Temperature-Induced Reversible Fluorescence Switching of Organic Nanoparticles: Application to Intracellular Temperature Imaging

Polarity-Independent Temperature-Induced Reversible Fluorescence Switching of Organic Nanoparticles: Application to Intracellular Temperature Imaging

Synthesis of ESIPT fluorophores with two intramolecular H-bonding functionalities: Reversible mechanofluorochromism and conformation controlled solid state fluorescence efficiency

We have synthesized two new ESIPT fluorophores (1-((E)-((E)-((2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl) naphthalen-2-ol (Naph-2-OH) and 5-(diphenylamino)-2-((E)-((E)-((2-hydroxyphenyl)(phenyl) methylene)hydrazono)methyl) phenol (TPA-2-OH)) with two unsymmetrical intramolecular H-bonding functionalities and investigated the solid-state structural assembly and fluorescence properties. The crystal structure analysis confirmed the formation of strong intramolecular H-bonding and twisted molecular conformation. The nonplanar twisted molecular conformation produced well separated molecules in the crystal lattice that lead to strongly enhanced emission ((λmax = 538 nm, Φf = 5.2% (Naph-2-OH), λmax = 562 nm, Φf = 13.4% (TPA-2-OH)). Density functional theoretical (DFT) calculation supported the fluorescence tuning and intramolecular charge transfer (ICT) from TPA donor to imine acceptor. Both Naph-2-OH and TPA-2-OH showed mechanical crushing and heating induced reversible fluorescence switching due to the reversible phase transformation between crystalline and amorphous state.

Solid-state fluorescent pH sensors based on pyrimidine/purine-cored covalent organic framework

Fluorescent covalent organic frameworks have been increasingly used as chemical sensors due to their inherent outstanding nature. Here we report two cases of β-ketoenamine typed fluorescent COFs, pyrimidine-cored covalent organic framework (TpApy-COF) and purine-cored covalent organic framework (TpApu-COF). The emission intensities of TpApy-COF and TpApu-COF in aqueous dispersion descend gradually with the ascending of pH value in the range of from 1 to 13. The reversible fluorescence transformation along with the pH change indicates the favorable reusabilities of TpApy-COF and TpApu-COF. The successful assembly of fluorescent liquid glue and fluorescent crayon, as well as their fluorescent turn on and turn off sensing of HCl and NH3, suggest positive practicability and processability. The formation and disintegration of extra intramolecular H-bonding during protonation and deprotonation play crucial roles in the reversible fluorescence switching.