Excited-state Intramolecular Proton Transfer Emission Research Articles

Exploring electron donor and acceptor effects: DFT analysis of ESIPT/GSIPT in 2-(oxazolinyl)-phenols for photophysical and luminophore enhancement.

Understanding excited-state intramolecular proton transfer (ESIPT) is essential for designing organic molecules to enhance photophysical and luminophore properties in the development of optoelectronic devices. In this context, an attempt has been made to understand the impact of substituents on the ESIPT process of 2-(oxazolinyl)-phenol. Electron donating (EDG: -NH2, -OCH3, and -CH3) and electron withdrawing (EWG: -Cl, -Br, -COOH, -CF3, -CN, and -NO2) substitutions have been computationally designed and screened through density functional theory (DFT) and time-dependent density-functional theory (TDDFT) calculations. Furthermore, the ground state intramolecular proton transfer and ESIPT mechanisms of these designed luminophores are explored using the transition state theory. The results reveal that molecules with EDG show higher absorption and emission peaks than molecules with EWG and also indicate that the mobility of charge carriers in 2-(oxazolinyl)-phenol derivatives is significantly influenced by substituents. We found that the EWGs decrease the reorganization energy and increase the vertical ionization potential and electron affinity values, as well as the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, compared to the EDG substituted molecules. Significantly, the excited state (S1) of the keto emission (K) form shows notably larger values for the EDG substitutions. The intersystem crossing pathway efficiency weakens with reduced spin-orbit coupling matrix element in the enol form with electron-donating substituents and vice versa in the keto form during S1-T3 transitions. Our research links intramolecular proton transfers and triplet generation, making these substituted molecules appealing for optoelectronic devices. Introducing EDGs, such as -NH2, boosts the ESIPT reaction in 2-(oxazolinyl)-phenol. This study guides designing ESIPT emitters with unique photophysical properties.

Cooperativity of ESPT and Aggregation-Induced Emission Effects-An Experimental and Theoretical Analysis of a 1,3,4-Thiadiazole Derivative.

4-[5-(Naphthalen-1-ylmethyl)-1,3,4-thiadiazol-2-yl]benzene-1,3-diol (NTBD) was extensively studied through stationary UV-vis absorption and fluorescence measurements in various solvents and solvent mixtures and by first-principles quantum chemical calculations. It was observed that while in polar solvents (e.g., methanol) only a single emission band emerged; the analyzed 1,3,4-thiadiazole derivative was capable of producing dual fluorescence signals in low polarity solvents (e.g., n-hexane) and certain solvent mixtures (e.g., methanol/water). As clearly follows from the experimental spectroscopic studies and theoretical modeling, the specific emission characteristic of NTBD is triggered by the effect of enol → keto excited-state intramolecular proton transfer (ESIPT) that in the case of solvent mixture is reinforced by aggregation of thiadiazole molecules. Specifically, the restriction of intramolecular rotation (RIR) due to environmental hindrance suppresses the formation of non-emissive twisted intramolecular charge transfer (TICT) excited keto* states. As a result, this particular thiadiazole derivative is capable of simultaneously producing both ESIPT and aggregation-induced emission (AIE).

Experimental and theoretical comprehension of ESIPT fluorophores based on a 2-(2'-hydroxyphenyl)-3,3'-dimethylindole (HDMI) scaffold.

Excited-State Intramolecular Proton Transfer (ESIPT) emission is associated with intense single or multiple fluorescence in the solid-state, along with enhanced photostability and sensitivity to the close environment. As a result, ESIPT probes are attractive candidates for ratiometric sensing of a variety of substrates. A new family of ESIPT fluorophores is described herein, inspired by the well-known 2-(2'hydroxyphenyl)benzazole (HBX) organic scaffold. The connection of 3,3'-dimethylindole (or 3H-indole) derivatives with phenol rings triggers the formation of novel 2-(2'-hydroxyphenyl)-3,3'-dimethylindole (HDMI) fluorophores, capable of stimuli-responsive ESIPT emission. This brand new family of dyes displays redshifted emission, as compared to HBX, along with an unprecedented acid/base-mediated stabilization of different rotamers, owing to supramolecular interactions with methyl groups. These compounds are therefore highly sensitive to external stimuli, such as the presence of acid or base, where protonated and deprotonated species have specific optical signatures. Moreover, a new pyridine-functionalized HDMI dye displays acid-sensitive AIE properties. The photophysical properties of all compounds have also been studied using ab initio calculations to support experiments in deciphering the nature of the various radiative transitions observed and the related excited rotameric species.

Heterocyclic molecules with ESIPT emission: synthetic approaches, molecular diversities, and application strategies.

Excited-state intramolecular proton transfer (ESIPT) is one of the most essential emission processes in most circumstances because of its dual emission band in most cases and its high Stokes shifts. These distinguishing properties make ESIPT-based probes more suitable for a variety of applications, including analyte sensors, solid-state sensing mechanisms, optical technologies, and biomarkers for endogenous or exogenous compounds in various settings. As a result, researchers around the world are working on ESIPT emissions and developing different scaffolds for various applications or industry demands. This field of study is rapidly expanding and there is a need for an up-to-date review of synthesis methodologies and applications. This paper provides the highlights of ESIPT-based heterocyclic scaffolds, synthesis strategies, and application scenarios in the literature from 2017 to 2023.

Manipulation of excited-state intramolecular proton transfer by electron-donor substitution for high performance fluoride ions sensing

Excited-state intramolecular proton transfer (ESIPT) molecules has been using as a variety of functionalityled molecular systems. To investigate the relationship between the electron-donor substitution and luminescent properties of ESIPT luminogens, four 2-(2-hydroxyphenyl) benzothiazole derivatives with donor-π-acceptor (D-π-A)-structured were synthesized. The distinct fluorescence properties of them were found to be highly dependent on the electron-donor moiety (triphenylamine and anthracenyl), its substituent position (para and meta position) and solvent polarity. The M−TPA, P-En, and M−En showed ESIPT emission in organic solvents, while the P-TPA showed intramolecular charge transfer process (ICT) emission. It is due to the synergistic effect of the aggregation-induced emission (AIE) and ESIPT, that M−TPA and M−En exhibited high solid-state quantum yields and large Stokes shifts. They were used as a probe for detecting F−, which resulted in rapid colorimetric, high sensitivity and good selectivity. The M−TPA was a turn-on fluorescent probe, which had the best detection property, and the limit of detection was as low as 11 nM. Because M−TPA displayed phenol anion emission in DMSO and F− causes the deprotonation of the M−TPA, which led to significant red shift of the absorption band and enhancement of fluorescence emission. This work provides a reliable strategy for designing high-performance fluorescent sensor via ESIPT manipulation.

Reproducible and High-Performance WOLEDs Based on Independent High-Efficiency Triplet Harvesting of Yellow Hot-Exciton ESIPT and Blue TADF Emitters.

Hot exciton organic light-emitting diode (OLED) emitters can balance the high performance of a device and reduce efficiency roll-off by fast reverse intersystem crossing from high-lying triplets (hRISC). In this study, an excited-state intramolecular proton transfer (ESIPT) fluorophore of 2-(benzo[d]thiazol-2-yl)-4-(pyren-1-yl)phenol (PyHBT) with the typical characteristic properties of a hot exciton is developed. With high efficiency of utilization of the exciton (91%), its yellow OLED exhibited high external quantum efficiency (EQE) of 5.6%, current efficiency (CE) of 16.8cdA-1 , and power efficiency (PE) of 17.3lmW-1 . The performance of the yellow emissive "hot exciton" ESIPT fluorophores is among the highest recorded. Due to the large Stokes shift of the ESIPT emitter, non-energy-transferred high-performance white OLEDs (WOLEDs) are developed, which are reproducible and highly efficient. This is possible because of the independent harvesting of most of the triplets in both complementary-color emitters without the interference of energy transfer. The PyHBT-based WOLEDs exhibit a maximum EQE of 14.3% and CE of 41.1cdA-1 , which facilitates the high-yield mass production of inexpensive WOLEDs.

Excited-state intramolecular proton transfer emitter for efficient violet-blue organic light-emitting diodes with hybridized local/charge transfer channel

Excited-state intramolecular proton transfer (ESIPT) materials have attracted much interest, attributed to their metastable excited states which provide suitable platforms for efficient intersystem crossing (ISC) and reverse intersystem crossing (rISC). However, the studies on the utilization of ESIPT to achieve effective hybridized local/charge transfer (HLCT) emitters are rarely reported, due to their ubiquitous issue of low photoluminescence quantum yields (ΦPLs). In this research, a novel ESIPT molecule with HLCT characteristic, 4′-(diphenylamino)-4-(1,4,5- triphenyl-1H-imidazol-2-yl)-[1,1′-biphenyl]-3-ol (HIT-TPA), is designed and synthesized. The hydrogen atom in this molecule can transfer on the way of an intramolecular hydrogen bond from a proton-donating part (–OH/–NH–) to the accepting one (CO/CN–). The HLCT and /or color-tuned fluorescence are observed by the irradiation of specific wavelength, which followed the guidance of ESIPT. It is consistent with the results of transient absorption and time-resolved photoluminescent spectroscopy. By the synergistic interaction of ESIPT and HLCT, the film based on the target molecule presents a high ΦPL value of ∼75.5%. After employing HIT-TPA as the emitter for doped/undoped organic light-emitting diodes (OLEDs), a maximum external quantum efficiency of ∼10.08% is achieved, which proves the efficient triplet harvesting in the ESIPT-based HLCT material. Noticeably, the device exhibits a violet-blue emission with the CIE(x,y) of (0.152, 0.051). It is among the highest reported EQE values on the ESIPT-HLCT materials for violet-blue devices. These results not only open a novel pathway to design effective ESIPT attributed HLCT materials, but also provide a strategy to efficiently utilize ESIPT emitters in achieving high-performance OLEDs.

Dual-Mode Multiple Ion Sensing via Analyte-Specific Modulation of Keto-Enol Tautomerization of an ESIPT Active Pyrene Derivative: Experimental Findings and Computational Rationalization.

A pyrene-based e xcited - state intramolecular proton transfer (ESIPT) active probe PMHMP was synthesized, characterized, and employed for the ppb-level, dual-mode, and high-fidelity detection of Cu2+ (LOD: 7.8 ppb) and Zn2+ ions (LOD: 4.2 ppb) in acetonitrile medium. The colorless solution of PMHMP turned yellow upon the addition of Cu2+, suggesting its ratiometric, naked-eye sensing. On the contrary, Zn2+ ions displayed concentration-dependent fluorescence rise till a 0.5 mole fraction and subsequent quenching. Mechanistic investigations indicated the formation of a 1:2 exciplex (Zn2+:PMHMP) at a lower concentration of Zn2+, which eventually turned into a more stable 1:1 (Zn2+:PMHMP) complex with an additional amount of Zn2+ ions. However, in both cases, it was observed that the hydroxyl group and the nitrogen atom of the azomethine unit were involved in the metal ion coordination, which eventually altered the ESIPT emission. Furthermore, a green-fluorescent 2:1 PMHMP-Zn2+ complex was developed and additionally employed for the fluorimetric analysis of both Cu2+ and H2PO4 - ions. The Cu2+ ion, owing to its higher binding affinity for PMHMP, could replace the Zn2+ ion from the preformed complex. On the other hand, H2PO4 - formed a tertiary adduct with the Zn2+-complex, leading to a distinguishable optical signal. Furthermore, extensive and organized density functional theory calculations were performed to explore the ESIPT behavior of PMHMP and the geometrical and electronic properties of the metal complexes.

Heteroaryl-Substituted Bis-Anils: Aggregation-Induced Emission (AIE) Derivatives with Tunable ESIPT Emission Color and pH Sensitivity.

The two-step synthesis, structural, and photophysical properties of a series of heteroaryl-substituted bis-anil derivatives presenting aggregation-induced emission (AIE) coupled with an excited-state intramolecular proton transfer (ESIPT) process is described. The fluorescence color of the aggregates can be fine tuned by changing the electronic nature of the peripheral substitution, leading to a wide range of emission wavelengths (from green to the near infra-red). Moreover, upon introduction of strong electron-withdrawing groups such as cyano (CN), a competition between ESIPT and deprotonation is observed leading to the emission of the anionic species at low water percentage. This observation led to the synthesis of an additional mixed AIE fluorophore, functionalized by methoxy groups on one side and cyano groups on the other side. Upon addition of water, this dye displays first anionic emission, followed by typical AIE/ESIPT red fluorescence upon formation of the aggregates. TD-DFT calculations on selected AIE dyes were performed to rationalize the nature of the emissive transitions in these derivatives.

Selectively recognizing heptad-interfaced G-quadruplexes by a molecular rotor with an ESIPT emission.

Heptad-interfaced G-quadruplexes (G4s), formed with GGA repeats located in the nuclear proto-oncogene c-myb promoter, can be selectively targeted by a prenylated flavonol of sophoflavescenol (Sop) with restriction of molecular rotation to trigger strong excited state intramolecular proton transfer (ESIPT) emission.

Multistimulus Response of Two Tautomeric Zn(II) Complexes and Their White-Light Emission Based on Different Mechanisms.

A triphenylamine (TPA)-based 2H-quinazoline Zn(II) complex (Q-TPA-Zn) exhibiting dual fluorescence and phosphorescence emission in the solid state was designed and prepared. It possesses mechanochromic luminescence and thermochromic luminescence properties. In the solid state, the white afterglow luminescence could be observed at 77 K (CIExy: 0.27, 0.33) while cyan luminescence could be observed at 297 K. After thermolysis at 300 °C, Q-TPA-Zn could be transformed into Schiff base complex S-TPA-Zn with white fluorescence in the powder state (CIExy: 0.32, 0.38), in methanol (CIExy: 0.32, 0.39), and in dimethylformamide (CIExy: 0.26, 0.32) at room temperature. This arises from dual emission of normal* emission and tautomeric* emission induced by excited-state intramolecular proton transfer (ESIPT) from the benzimidazole NH group to the Schiff base N atom. Q-TPA-Zn could also be transformed into its isomeric form, S-TPA-Zn, through photochemical ring-opening reaction upon irradiation under 365 nm in the solution, exhibiting high-contrast photochromic luminescence. Interestingly, S-TPA-Zn could further be transformed into its zwitterionic isomer after continuous irradiation. The same ring-opening reaction could also take place for the orgainc compound Q-TPA via heating or 365 nm irradiation. The ring-opening reaction mechanism and ESIPT emission were interpreted via theoretical calculation.

Photophysical properties of anti‐inflammatory piroxicam and its Cu(II) complex

AbstractPiroxicam (PRX), a nonsteroidal anti‐inflammatory drug, exhibits a large Stokes‐shift emission owing to the excited‐state intramolecular proton transfer (ESIPT) from enol‐PRX to keto‐PRX. In contrast, the Cu‐coordinated PRX complex (CuPRX) does not show any emission at 300 K. In the time‐resolved transient absorption (TA) spectra, PRX showed two positive TA bands at 430 and 700 nm, corresponding to the S1–Sn transitions of keto‐PRX* and a negative bleaching band at 540 nm due to ESIPT emission. These TA bands decayed with a lifetime of approximately 50 ps. CuPRX showed similar TA features, but no bleaching band was observed for CuPRX and decayed rapidly within 3 ps, indicating a rapid intersystem crossing process owing to the heavy atom effect. Based on the experimental results and theoretical calculations, the fate for excited keto‐PRX is discussed in terms of the ESIPT and reverse ESIPT processes.

Luminescent excited-state intramolecular proton-transfer dyes based on 4-functionalized 6,6′-dimethyl-3,3′-dihydroxy-2,2′-bipyridine (BP(OH)2-Rs); DFT simulation study

The 4-functionalized 6,6′-dimethyl-3,3′-dihydroxy-2,2′-bipyridine dyes (BP(OH)2-Rs) have exhibited dienol and diketo emissions. The optimum geometrical structures for ground, singlet and triplet excited states are computed by DFT/B3LYP/6–31++G that showed the planarity of BP(OH)2-Rs structure. The emission spectra of the molecules are determined in the gas-phase at singlet and triplet excited states using CIS/6–31++G. The theoretical calculations are carried out for BP(OH)2-Rs to understand the impact of different substituents (R = –H (I), –Br (II), –TMS (III), –C2H (IV), –terpyridine (V) and –bodipy (diazaboraindacene) (VI)) on excited-state intramolecular proton transfer (ESIPT) in singlet and triplet excited states. Based on the calculations, the concerted diproton transfer proceeds in the triplet excited state, in which nπ* state has a significant participation in ESIPT. The spectral variation at ESIPT emission of BP(OH)2-Rs is influenced by the electron-acceptor ability of the substituents. The compound V revealed a higher spectral intensity compared to the others. From the comparison with the experimental data, the molecule V is almost planar agreed with the X-ray structure and trend variation of wavelengths. The molecule VI contains bodipy chromophore that excitation energy transfers completely from BP(OH)2 core to a bodipy substituent, leading to emission from the lowest-lying bodipy substituent, and consequently, ESIPT does not occur for this dye.

Photo/Thermal Dual Responses in Aqueous-Soluble Copolymers Containing 1-Naphthyl Methacrylate

Photoresponsive polymers capable of luminescence switching are attracting significant interest due to their potential application in fluorescence patterning, bioimaging, optical data storage, and anti-counterfeiting. In this work, we have developed aqueous-soluble copolymers of 1-naphthyl methacrylate and oligo(ethylene glycol) methyl ether methacrylate [P(1-NMA-co-OEGMA)] that undergo a significant shift in fluorescence emission wavelength after UV irradiation. Irradiation of the 1-naphthyl methacrylate moieties results in the photo-Fries rearrangement to form hydroxy aryl ketones, which exhibit strong emission at 475 nm through excited-state intramolecular proton transfer (ESIPT) and excited-state proton transfer (ESPT). The resultant shift in fluorescence emission maximum from 338 to 475 nm after rearrangement can potentially be exploited for fluorescence patterning. Furthermore, the copolymers are thermally sensitive in aqueous solutions. The lower critical solution temperature (LCST) of the copolymers depends on the content of hydrophobic 1-naphthyl methacrylate units; the photo-Fries rearrangement results in a more polar structure, shifting the LCST to a higher temperature. Of note, the temperature-triggered volume phase transition of copolymer hydrogels selectively ″switches off″ fluorescence arising from the ESPT mechanism, while the ESIPT emission is unaffected. We also demonstrate that films formed by coating the copolymers onto various substrates can be selectively patterned to form gradients in fluorescence intensity. These versatile P(1-NMA-co-OEGMA) copolymers are simple to prepare at low cost, demonstrate effective photoswitching, and have excellent water solubility, thus ensuring potential applications in a number of important areas.

A H2O-induced fluorescence turn-on diarylethene derivative and its fluorescent sensing Al3+

A diarylethene derivative has been synthesized by attaching a photochromic core to a pyrazine moiety through an aroylhydrazone linker. The compound showed weak excited state intramolecular proton transfer (ESIPT) based fluorescence in a series of polar solvents such as CH3CN, THF, DMSO, CH3CH2OH, CH3OH and their binary mixtures with water. However, when the water fraction in solutions was above 70%, a strong orange ESIPT emission at ~ 580 nm was found, which originated molecules aggregate and ESIPT. The active ESIPT process remains in the solid state, which exhibited orange fluorescence under 365 nm light illumination. The diarylethene derivative has been found to show fluorescence ‘‘turn-on’’ sensing Al3+ in CH3CN with the detection limit of 16.6 nM. Al3+ enhanced the fluorescence with 30 nm blue shifts and the fluorescence can be ON and OFF through interaction with Al3+ and EDTA. Moreover, the Al3+ in several water samples was detected using the sensor with high accuracy.

Enhanced solid-state photoluminescence and fluorescence spectral behaviors for an ESIPT molecule: An experimental and theoretical investigation

3-Dimethylamino-1-(2-hydroxy-phenyl)-propenone (DHP) molecule with excited-state intramolecular proton transfer (ESIPT), was good candidate as active organic laser dyes. In this work, the enhanced solid-state photoluminescence and the structure-property relationship for photophysical properties are investigated by time-dependent density functional theory (TD-DFT) method and the experimental spectra. The calculated potential energy surface (PES) curve of the ground-state S0 indicates that the planar DHP-cis-enol (α isomer) conformer with innate intramolecular H-bond is the global energy minimum. The PES of the lowest excited-state S1 indicate that the photoexcitated DHP-cis-enol (α* isomer) and DHP-cis-keto (β isomer) conformers are achieved as well, corresponding to dual-peak fluorescence emission in solution. Especially, in addition to the planar DHP-cis-keto isomer of DHP deriving from ESIPT, a barrierless twist process of phenyl hydroxide group brings out another stable excited-state isomer (γ isomer), which dramatically influences the photophysical properties. Furthermore, the low-frequency modes of phenyl hydroxide group rotation with large Huang-Rhys factors for the twisted isomer result in a noteworthy conical intersection of the S0 and S1 energy curves, which dissipate the excited-state energy nonradiatively for DHP in solution. However, in solid-state, the fluorescence spectrum of DHP features only ESIPT emission band which demonstrates the β isomer mainly exist in S1 state to promote the radiative decay processes with suppressed population of the α* and γ isomers, which can explain the phenomenon of the enhanced solid-state photoluminescence.

Controlling Aggregation and Excited-State Intramolecular Proton Transfer in BODIPYs by Incorporation of 2-(2-Hydroxyphenyl)quinazoline and Variation of Substituents

A series of BODIPY-based AIEgens (QB1–QB5 and Bis-QB) containing 2-(2-hydroxyphenyl)quinazoline have been synthesized and thoroughly characterized. Photophysical properties of these compounds in solution and the aggregated state have been meticulously investigated and fine-tuned via structural modifications. These display green emission (∼530 nm) in solution and bright red emission (600–655 nm) in the aggregated/solid state with increased quantum yield. Crystal structure analyses and spectral studies revealed efficient J-type aggregation in these derivatives. Significant impact of 2-(2-hydroxyphenyl)quinazoline toward modulating intermolecular interactions and facilitating J-type stacking between BODIPY units has also been established. Moreover, the essential role of excited-state intramolecular proton transfer (ESIPT) in inducing emission in the aggregated state and tuning of ESIPT emission by variation of substituents have been supported by various studies.

Intrinsically ESIPT-exhibiting and enhanced emission in polymer nanoparticles as signaling for sensing nitrite.

A straightforward approach to the fabrication of intrinsically excited-state intramolecular proton transfer (ESIPT)-fluorescent polymer nanoparticles (e-PNPs) was developed. The e-PNPs were obtained by self-assembly of the homopolymers derived from 4-aminosalicylic acid in aqueous solution. By incorporating ESIPT modules into polymer nanoparticles, the ESIPT reaction can be endowed with moderate hydrophobic micro-environment by nanoparticle scaffolds, eliciting enhanced ESIPT emission. The newly developed e-PNPs exhibit strong tautomeric fluorescence(e-FL), good photostability, low-toxicity and favourable biocompatibility in aqueous solution. Upon the addition of NO2-, the e-FL can be significantly quenched owing to the reaction of NO2- with the amide groups on e-PNPs. From this basis, the fluorescence detection of NO2- was implemented, which showed a linear relationship between 0 nM and 110 nM with a detection limit of 2.3 nM. Furthermore, e-PNPs were used as nanoprobes to monitor the NO2- levels in HeLa cells by fluorescence imaging, demonstrating the ability of discrimination from different concentrations of NO2-. The proposed method can be applied to a wide range of other ESIPT modules to integrate into polymer nanoparticles and offer highly sensitive nanosensing platform for bioanalysis and molecular imaging.

A novel structure for ESIPT emission: Experimental and theoretical investigations

A number of substituted imidazole-based molecules were synthesized via a two-step one-pot reaction and their UV/vis and fluorescence spectra were analyzed. Emissions of the imidazole-based molecules were attributed to the excited state intramolecular proton transfer (ESIPT) process. Imidazole-based sensor molecules have high Stokes shifts which is good clue for the ESIPT mechanism. Furthermore, the ESIPT mechanism was confirmed by some control experiments in which acceptor and donor groups were functionalized, resulting in hampered ESIPT emission. Time-resolved experiments showed that the skeleton synthesized had different exponential decays depending on the functional groups (OMe and OH). According to the time-resolved experiments, the ESIPT reaction occurred between 1.0 and 3.10 ns. Furthermore, the lifetime of molecules in EtOH increased when the phenyl ring possessed OMe or OH groups. Dihedral angles between the NH and carbonyl groups were calculated and found to be important for a stable configuration. The methyl substituted NH group on the imidazole skeleton in which the ESIPT reaction should be interrupted, revealed a 0.3 ns lifetime in DMSO. Potential energy curves for all compounds were analyzed using TD-DFT. It was seen that there is no barrier for excited state proton transfer resulting in ultrafast ESIPT process.

A rationally designed flavone-based ESIPT fluorescent chemodosimeter for highly selective recognition towards fluoride and its application in live-cell imaging

Fluoride recognition and sensing via artificial receptors have attracted a great deal of attention since it plays an important and indispensable role in biological and environmental processes. Herein, a novel colorimetric and fluorescent chemodosimeter (3-triisopropylsilylflavone 1) based on 3-hydroxyflavone framework has been rationally designed and synthesized for selective recognition towards fluoride. As confirmed by UV–vis and fluorescence techniques, the fluorescent probe 1 exhibited a highly sensitive and selective as well as rapidly responsive (20 s) recognition towards fluoride with a large Stokes shift of 180 nm. The detection limit was deduced to be 0.68 μM, which is much lower than the limit concentration level in drinking water specified by USEPA. The distinctive sensing mechanism is attributed to fluoride-triggered desilylation reaction on the cleavage of SiO bonds, leading to the strong excited-state intramolecular proton transfer emission of 3-hydroxyflavone. In addition, test strips experiment based on the sensor demonstrated the excellently practical feasibility of 1 to sense fluoride even in ppm level. Moreover, the probe is cell permeable and can be capable of reliably detecting fluoride in living cells.