Mechanism Of Fluorescence Enhancement Research Articles

Fabrication of a novel mixed-valent copper-based coordination polymer as a fluorescent sensor for selective and efficient detection of multiple analytes

A novel three-dimensional fluorescent mixed-valent copper-based coordination polymer (CP), namely {[CuICuII4(triazol)6(bpta)2]Cl}n (Cu-CP), was synthesized hydrothermally in the presence of Cu(II) ion, 2,4,6-tri(1H-1,2,4-triazol-1-yl)-1,3,5-triazine (TTT), and 2,2′,4,4′-biphenyltetracarboxylic acid (H4bpta). The structure of the Cu-CP was studied using single-crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD). Interestingly, the crystallographic data revealed that a part of Cu(II) ions were reduced in situ to Cu(I) ions as well as the nitrogen-containing ligand TTT was converted to triazol, which possessed excellent fluorescence characteristics as a transition metal ion with a d10 electronic configuration, prompting further investigation into the fluorescence sensing properties of Cu-CP. The results confirmed that Cu-CP exhibited excellent fluorescence performance which an outstanding candidate as a fluorescence sensor for Fe3+, Cr2O72−, CrO42−, MnO4− and nitrobenzene (NB), with a low limit of detection. For ethylene glycol (EG) detection, Cu-CP showed high-efficiency fluorescence enhancement across a range of temperatures. The underlying mechanism of fluorescence enhancement was investigated through comprehensive experimental studies.

Green and real-time detection of GHB in soft drinks and alcoholic beverages using an eco-friendly cellulose paper-based fluorescent probe

Chemical submission, a nefarious tactic increasingly employed in criminal activities, has spurred urgent calls for innovative countermeasures. GHB, often dubbed “liquid ecstasy,” stands out as a favoured agent for its surreptitious nature and seamless solubility in water and alcoholic beverages. Addressing this menace head-on, a groundbreaking study delves into the development of advanced chemosensors, leveraging 2-aminonaphtoxazole- and benzoxazole-based compounds adorned with fluorescein, to construct a cellulose paper-based detection system. This ingenious setup not only detects GHB in water but extends its vigilance to real alcoholic and non-alcoholic beverages, illuminating a pathway to thwart potential assailants. With a fluorescence enhancement mechanism at play, the system boasts a dynamic range from 0 to 125 mM GHB in water, exhibiting a commendable limit of detection (LOD) at 7.3 mM. Crucially, its eco-friendly nature, devoid of solvent residuals, underscores its suitability as a proactive shield against chemical submission, embodying a beacon of hope in the fight against such insidious threats to public safety.

Insights into the Mechanisms of Single-Photon and Two-Photon Excited Surface Enhanced Fluorescence by Submicrometer Silver Particles.

Surface enhanced fluorescence (SEF) based on noble metal nanoparticles is an effective means to achieve high sensitivity in fluorescence detection. Currently, the physical mechanism behind enhanced fluorescence is not fully understood. This paper measures the fluorescence signals of Dihydroporphyrin f methyl ether (CPD4) under both single-photon and two-photon excitation based on submicrometer silver particles with rough morphologies, achieving enhancement factors of 34 and 45 times, respectively. On this basis, by combining the radiative field characteristics produced by the silver particles, a stimulated radiation model of molecules is established to elucidate the changes in the molecular photophysical process when influenced by silver particles. Moreover, the fluorescence lifetime of the molecules was measured, showing that the presence of silver particles induces an increase in the molecular radiative decay rate, causing the fluorescence lifetime to decay from 3.8 ns to 3 ns. The results indicate that the fluorescence enhancement primarily originates from the submicrometer silver particles' enhancement effect on the excitation light. Additionally, the fluorescence signal emitted by the molecules couples with the silver particles, causing the local surface plasmon resonances generated by the silver particles to also emit light signals of the same frequency. Under the combined effect, the fluorescence of the molecules is significantly enhanced. The findings provide a theoretical foundation for understanding the fluorescence enhancement mechanism of silver particles, adjusting the enhancement effect, and developing enhanced fluorescence detection devices based on submicrometer silver particles, holding significant practical importance.

A highly stable, rapid and sensitive fluorescent probe for Al3+ and pefloxacin based on enhanced fluorescence of a Co(II) coordination polymer

Fabricating functional coordination polymer (CP) architectures and exploiting it as fluorescence sensors for detecting diverse metal ions and antibiotics, have attracted extensive attention. Based on a rarely-used benzimidazole-derived ligand 2,2′-bis-(2-pyridin-3-yl-vinyl)-3H,3′H-[5,5]bibenzoimidazole (L), a new CP {[Co(L)0.5(tbip)]·H2O}n (H2tbip =5‑tert-butylisophthalic acid) (1) was synthesized under solvothermal condition and characterized. Complex 1 reveals a two-dimensional network with the type of topology of 3,4,5L7. Complex 1 possesses excellent thermal and chemical stability. The fluorescence detection ability of complex 1 was explored and demonstrated that complex 1 can be used as the first fluorescent turn-on sensor to sense Al3+ and pefloxacin (PEF) with a low detection limit and fast response. The fluorescence enhancement mechanism was investigated in detail.

A novel fluorescence sensor based on Al3+-mediated aggregation of gold nanoclusters for determination of citric acid in beverages.

This paper revealed a new strategy for citric acid (CA) detection using aggregation-induced emission (AIE)-based fluorescent gold nanoclusters (AuNCs). AuNCs was synthesized using glutathione (GSH) as the template and reducing agent and used as the fluorescent probe to detect CA under aluminum ion (Al3+) mediation. The fluorescence intensity of AuNCs increased about 4 times with the addition of Al3+, but the enhanced fluorescence was quenched after the addition of CA. Based on this fluorescence phenomenon, an "on-off" fluorescence strategy was designed for the sensitive determination of CA and a linear detection range for CA was achieved within 0-80.0 μM. In addition, the developed probe exhibited high selectivity and accuracy for determination of CA. The mechanism of fluorescence enhancement and quenching of AuNCs was explored in detail. The established probe was used successfully for CA detection in beverages. The spiked recoveries from 97.50% to 103.67% were gratifying, which indicated the probe had potential prospects for detecting CA in food.

Fluorescein@NH2-UiO-66 Probe for Ratiometric Fluorescence Sensing of Trace Phosphate in Water

Phosphate pollution leads to deterioration in water quality, posing a serious threat to human health. Therefore, it is important to develop a highly selective and sensitive fluorescent probe for phosphate detection. Here, we report a novel ratiometric fluorescent probe, Fluorescein@NH2-UiO-66 (denoted as Flu@NH2-UiO-66), for the trace detection of phosphate in water. Specifically, during the in situ solvothermal synthesis of Flu@NH2-UiO-66, fluorescein molecules were encapsulated into the cavities of the metal-organic framework. Furthermore, the encapsulation amount of fluorescein was controlled by adjusting the acidity of the system. The Flu@NH2-UiO-66 (60 H+) sample, prepared with a 60:1 molar ratio of acetic acid to the metal center exhibited distinct dual fluorescence signal peaks. The probe showed a highly selective fluorescence response to phosphate. Within a range of 0–20 μM phosphate concentration, the probe demonstrated excellent linear detection capability with a detection limit of 0.37 μM. Moreover, the mechanism of fluorescence enhancement can be attributed to the addition of phosphate, which greatly increases the UV absorbance of the probe. This study developed a novel ratiometric fluorescent probe capable of rapid, sensitive, and stable detection of trace phosphate, which is of great significance for environmental management.

Sensitive fluorescence detection of glyphosate and glufosinate ammonium pesticides by purine-hydrazone-Cu2+ complex

Organophosphorus pesticides play an important role as broad-spectrum inactivating herbicides in agriculture. Developing a method for rapid and efficient organophosphorus pesticides detection is still urgent due to the increasing concern on food safety. An organo-probe (ZDA), synthesized by purine hydrazone derivative and 2,2′-dipyridylamine derivative, was applied in sensitive recognition of Cu2+ with detection limit of 300 nM. Mechanism study via density functional theory (DFT) and job’s plot experiment revealed that ZDA and Cu2+ ions form a 1:2 complex quenching the fluorescence emission. Moreover, this fluorescent complex ZDA-Cu2+ was applicable for detecting glyphosate and glufosinate ammonium following fluorescence enhancement mechanism, with detection limits of 11.26 nM and 11.5 nM, respectively. Meanwhile, ZDA-Cu2+ was effective and sensitive when it is used for pesticide detection, reaching the maximum value and stabilizing in 1 min. Finally, the ZDA-Cu2+ probe could also be tolerated in cell assay environment, implying potential bio-application.

Dual-emission ciprofloxacin-gold nanoclusters enable ratiometric sensing of Cu2+, Al3+, and Hg2.

An innovative triple optical sensor is presentedthat utilizes gold nanoclusters (GNCs) stabilized with ciprofloxacin (CIP) and bovine serum albumin (BSA). The sensor is designed to identify three critical metal ions, namely Cu2+, Al3+, and Hg2+. Under 360nm excitation, the synthesized CIP-BSA-GNCs demonstrate dual fluorescence emission with peaks at 448nm (blue) and 612nm (red). The red emission is associated with the interior of the CIP-BSA-GNCs, whereas the blue emission results from the surface-bound CIP molecules. The sensitive and selective fluorescent nanosensor CIP-BSA-GNCs were employed to detect Cu2+, Al3+, and Hg2+ ions. Cu2+ effectively quenched the fluorescence intensity of the CIP-BSA-GNCs at both peaks via the internal charge transfer mechanism (ICT). Cu2+ could be detected within the concentration range 1.13 × 10-3 to 0.05µM, with a detection limit of 0.34nM. Al3+ increased the intensity of CIP fluorescence at 448nm via the chelation-induced fluorescence enhancement mechanism. The fluorescence intensity of the core CIP-BSA-GNCs at 612nm was utilized as a reference signal. Thus, the ratiometric detection of Al3+ succeeded with a limit of detection of 0.21nM within the dynamic range 0.69 × 10-3 to 0.07µM. Hg2+ effectively quenched the fluorescence intensity of the CIP-BSA-GNCs at 612nm via the metallophilic interaction mechanism. The fluorescence intensity of CIP molecules at 448nm was utilized as a reference signal. This allowed for the ratiometric detection of Hg2+ with a detection limit of 0.7nM within the concentration range 2.3 × 10-3 to 0.1µM.

Surface-enhanced photoluminescence and Raman spectroscopy of single molecule confined in coupled Au bowtie nanoantenna

Optical nanoantennas possess broad applications in the fields of photodetection, environmental science, biosensing and nonlinear optics, owing to their remarkable ability to enhance and confine the optical field at the nanoscale. In this article, we present a theoretical investigation of surface-enhanced photoluminescence spectroscopy for single molecules confined within novel Au bowtie nanoantenna, covering a wavelength range from the visible to near-infrared spectral regions. We employ the finite element method to quantitatively study the optical enhancement properties of the plasmonic field, quantum yield, Raman scattering and fluorescence. Additionally, we systematically examine the contribution of nonlocal dielectric response in the gap mode to the quantum yield, aiming to gain a better understanding of the fluorescence enhancement mechanism. Our results demonstrate that altering the configuration of the nanoantenna has a significant impact on plasmonic sensitivity. The nonlocal dielectric response plays a crucial role in reducing the quantum yield and corresponding fluorescence intensity when the gap distance is less than 3 nm. However, a substantial excitation field can effectively overcome fluorescence quenching and enhance the fluorescence intensity. By optimizing nanoantenna configuration, the maximum enhancement of surface-enhanced Raman can be turned to 9 and 10 magnitude orders in the visible and near-infrared regions, and 3 and 4 magnitude orders for fluorescence enhancement, respectively. The maximum spatial resolutions of 0.8 nm and 1.5 nm for Raman and fluorescence are also achieved, respectively. Our calculated results not only provide theoretical guidance for the design and application of new nanoantennas, but also contribute to expanding the range of surface-enhanced Raman and fluorescence technology from the visible to the near-infrared region.

Fluorescence enhancement mechanism of new 4-Cyanobiphenyl-based schiff base probe by coordination interaction with cadmium ion

In this research, we focus on the theoretical exploration of the fluorescent process and sensing mechanism for Cd2+ of a novel Schiff base probe BPC based on 4-Cyanobiphenyl [Journal of Luminescence, 249, (2022), 119,017.]. The proton transfer process of BPC is investigated by constructing potential energy curves of the ground (S0) and excited singlet (S1) states with the extended O1–H2 distance. In S1 state, the probe BPC, in addition to the excited state intramolecular proton transfer (ESIPT) process, is found to undergo a CN isomerization process by rotating the dihedral angle C2–C3N3–C4. Meanwhile, a non-emissive state in BPC is found in our work. The weak fluorescence phenomenon observed in experiment is caused by the CN isomerization product of BPC. Upon the addition of Cd2+, BPC can coordinate with Cd2+, which prevents CN isomerization and ESIPT processes from occurring and turns on the intense fluorescence emission. This study clarifies sensing mechanism of BPC for detecting Cd2+ based on the inhibition of ESIPT and CN isomerization processes, which paves the way for the further development of new fluorescent probes for metal ions in the future.

ZnS QDs Stabilized Concurrently with Glutathione and L-cysteine for Highly Sensitive Determining Adriamycin Based on the Fluorescence Enhancement Mechanism.

In this work, a facile and fast aqueous-phase synthetic method is proposed to prepare water-soluble ZnS quantum dots stabilized simultaneously with glutathione and L-cysteine (ZnS QDs-GSH/L-Cys). As-synthesized ZnS QDs-GSH/L-Cys were monodispersed spherical nanocrystals with a mean diameter of 5.0 ± 0.7nm. Besides, the obtained ZnS QDs-GSH/L-Cys emitted more intensive blue fluorescence and exhibited an improved stability in aqueous solution compared with ZnS quantum dots merely stabilized with GSH (ZnS QDs-GSH). Interestingly, Adriamycin, a representative anticancer drug, was added into the solution of ZnS QDs-GSH/L-Cys, the blue fluorescence of ZnS QDs-GSH/L-Cys was greatly enhanced instead of being quenched, which indicated that ZnS QDs-GSH/L-Cys can be used as an enhanced-fluorescence nanoprobe for determining Adriamycin. The observed fluorescent enhancement could be attributed to the blocking of photoinduced electron transfer (PET) in ZnS QDs-GSH/L-Cys due to the electrostatic interaction between the -COO- groups on the surface of quantum dots and the -NH3+ groups in Adriamycin, followed by the coordination interaction among ZnS QDs-GSH/L-Cys and Adriamycin. The fluorescence intensity of ZnS QDs-GSH/L-Cys presented a good linear response with the concentration of Adriamycin ranging from 2.0 to 20µg•mL-1. The proposed fluorescent nanoprobe exhibited an excellent sensitivity with the LOD of 0.1µg•mL-1 and a good accuracy for detecting Adriamycin.

Enhancement of solubility and dissolution rate of dipyridamole by salifying: Preparation, characterization, and theoretical calculation

Dipyridamole (DIP) is a medication utilized to treat cardiovascular diseases, belonging to the BCS II class of drugs with low solubility and high permeability. As the effectiveness of oral preparations is influenced by the solubility and dissolution rate of the active ingredients, this study developed eight salts for solid modification of DIP to overcome poor water solubility. These salts were characterized using single crystal X-ray diffraction, powder X-ray diffraction and thermal analysis. Dipyridamole-oxalate (DIP-OLA) and dipyridamole-malonate (DIP-MNA) showed significant increase in apparent solubility and dissolution rate in buffer compared to DIP. The crystal structure analysis showed that the improved solubility is due to the change of the dipyridamole conjugate system and the molecular arrangement in the crystal after the introduction of counterions. It is noteworthy that the fluorescence emission of some salts in the solid state is significantly stronger than that of DIP. According to the crystal structure, the destruction of π-π stacking of DIP molecules after salification is the main mechanism of fluorescence enhancement, and Hirshfeld analysis and HOMO-LUMO analysis are also used to further explain the fluorescence changes.

The rapid and sensitive detection of trace copper ions by L-cysteine capped ZnS nanoparticle fluorescent probe and the insight into micro-mechanism: Experiments and DFT study.

L-cysteine (L-Cys) capped ZnS fluorescent probe (L-ZnS) were synthesized by binding ZnS nanoparticles in situ with L-Cys, the fluorescence intensity of L-ZnS increased more than 3.5 times than that of ZnS due to the cleavage of S-H bonds and the formation of Zn-S bonds between the thiol group of L-Cys and ZnS. The addition of copper ions (Cu2+) can effectively quench the fluorescence of L-ZnS to realize the rapid detection of trace Cu2+. The L-ZnS showed high sensitivity and selectivity to Cu2+. The LOD (limit of detection) of Cu2+ was as low as 7.28nM and linearity in the concentration range of 3.5-25.5μM. Meanwhile, for the first time, electron localization function (ELF), bond order density (BOD), and natural adaptive orbital (NAdO) analysis in the Multiwfn wavefunction program based on density functional theory were carried out to probe the binding sites and binding mode of L-Cys with Cu2+, it indicated that the deprotonated carboxyl oxygen atoms of L-Cys had the lowest electrostatic potential (ESP) and provided lone pair electrons to coordinate with Cu2+ to form non-luminescent ground state complexes, which led to fluorescence quenching of L-ZnS. From the microscopic point of view of atoms, the mechanism of fluorescence enhancement of L-Cys capped ZnS and the mechanism of fluorescence quenching after adding Cu2+ were revealed in depth, the theoretical analysis results were accordance with the experiments.

Insights of Life Molecules' Dynamic Distribution in Live Cells via Sequence-Structure Bispecific Fluorescent RNA.

Life molecules' distributions in live systems construct the complex dynamic reaction networks, whereas it is still challenging to demonstrate the dynamic distributions of biomolecules in live systems. Herein, we proposed a dynamic analysis strategy via sequence-structure bispecific RNA with state-adjustable molecules to monitor the dynamic concentration and spatiotemporal localization of these biomolecules in live cells based on the new insight of fluorescent RNA (FLRNA) interactions and their mechanism of fluorescence enhancement. Typically, computer-based nucleic acid-molecular docking simulation and molecular theoretical calculation have been proposed to provide a simple and straightforward method for guiding the custom-design of FLRNA. Impressively, a novel FLRNA with sequence and structure bispecific RNA named as a structure-switching aptamer (SSA) was introduced to monitor the real-time concentration and spatiotemporal localization of biomolecules, contributing to a deeper insight of the dynamic monitoring and visualization of biomolecules in live systems.

Binding of CsPbBr3 Nanocrystals to MOF-5 for the Detection of Cadmium Ions in Aqueous Media

CsPbBr3 perovskite nanocrystals (CNCs) have emerged as promising candidates for optoelectronics, although instability issues have retarded their progress toward commercialization. As a solution to this core issue, herein CNCs are bound to the metal–organic framework MOF-5 to form a stable composite MOF:CNC for sensing applications in aqueous media. The electrostatic interaction of Pb2+ and Br– ions of CNCs with the terephthalate and Zn2+ ions of MOF-5 do not disturb the structural integrity of the MOFs and the photoluminescence (PL) properties of the CNCs but renders more stability to both the MOFs and CNCs in a water medium. As a proof-of-concept experiment, this water-resistant MOF:CNC probe was successfully validated for the detection of Cd2+ in a water medium. The mechanism of fluorescence enhancement during the sensing of Cd2+ is elucidated using X-ray photoelectron spectroscopy, temperature-dependent, and time-resolved PL studies. The high stability and photoefficiency exhibited by this MOF:CNC composite proves that it is a viable system for applications in aqueous media.

A hydroxypropiophenone-based fluorescent probe for the selective determination of Al(III) ions in aqueous ethanol

In this work, we have synthesized a novel dihydrazone-based fluorescent probe N'1,N'2-bis{1-(2-hydroxyphenyl)propylidene}oxalohydrazide (H2hpoh)for Al3+ ions by a simple condensation reaction. The prepared organic probe has been characterized by different physicochemical and spectroscopic techniques. The single-crystal structure of the receptor has also been reported. Crystal data for C20H22N4O4: monoclinic, space group P21/c (no. 14), a = 6.0747(15) Å, b = 11.621(5) Å, c = 13.453(4) Å, β = 94.61(3)°, V = 946.6(5) Å3, Z = 2, T = 293(2) K, μ(MoKα) = 0.096 mm-1, Dcalc = 1.342 g/cm3, 4046 reflections measured (6.076° ≤ 2Θ ≤ 58.05°), 2149 unique (Rint = 0.0876, Rsigma = 0.2223) which were used in all calculations. The final R1 was 0.0972 (I > 2σ(I)) and wR2 was 0.2316 (all data). The ethanolic aqueous solution of the probe shows enhanced fluorescence in the presence of Al3+ ions, whereas no appreciable change in the spectral pattern is observed in the presence of other cations, i.e., Na+, K+, Ca2+, Ba2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, Cd2+ and Hg2+. The binding mode of the receptor with Al3+ ions was studied using various spectral titration techniques such as UV-visible, fluorescence, and 1H NMR. The receptor acts as a dibasic hexadentate ligand and interacts with two Al3+ ions with a high binding constant KB = 8.99×1010 1/M. The lowest detection limit for the Al3+ complex of H2hpoh was determined to be 7.8×10−5 M. With the help of DFT calculations, the mechanism of fluorescence enhancement has been explained.

Polyethyleneimine-induced fluorescence enhancement strategy for AIEgen: the mechanism and application.

Aggregation-induced emission luminogens (AIEgens) are attracting extensive research attention in the biosensor fields. Herein, we report a new polyethyleneimine (PEI)-induced strategy for enhancing luminescence of TCBPE (an AIEgen) to promote its development in biosensor. The copolymer dots (TCBPE-PEI) with high quantum yield (39.7%) and outstanding stability were synthesized via a one-pot method. The fluorescence enhancement mechanism based on the PEI strategy originated from the restriction of intramolecular motions of TCBPEs and the form of donor-acceptor structures to decrease the inherent energy bandgap. Benefiting from chelating property of TCBPE-PEI by Cu2+, a fluorescence-quenching sensor for Cu2+ detection was developed based on the fluorescence quenching of the electron transfer effect. Especially, a good linear range of 10-250nM with a low limit of detection 1.1nM was achieved, and it was further applied in samples successfully. The current work provides a novel approach to fabricate AIEgen biosensors and shows great potential in Cu2+ detection.

Green synthetic carbon quantum dots based on waste tobacco leaves and its application to detecting borax content in flour and its products

In this study, carbon quantum dots (CQDs) were prepared using discarded tobacco leaves as the precursor material combined. And the method used in our experiment is a green, low-cost, and simple one-step hydrothermal method. The structural morphology, optical properties, and fundamental elemental composition of these CQDs were characterized via transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectrum (XRD), etc. A fluorescence probe was developed with these CQDs for borax determination. Influencing factors including solution salt concentration, interfering substances, and solution pH on the detection of borax were investigated. The fluorescence enhancement degree of these CQDs had a linear relationship with different concentrations of borax (0.025–5 mmol·L-1), r2 = 0.9961, and based on the formula 3σ/S, σ represents the standard deviation of the detection data for 11 blank samples, S refers to the slope of the standard curve, LOD = 8.0 µmol·L-1. Furthermore, we also initially elucidated the fluorescence enhancement mechanism of CQDs. Satisfactory results were obtained for borax determination in flour, bread, and instant noodle samples.

CuI-p-DPA coordination polymer isomers for "turn-on" fluorescence detection of thiophanate-methyl.

Three copper iodide coordination polymer (CuI-p-DPA) isomers were prepared from the fluorescent organic ligand p-DPA and cuprous iodide (CuI) under different solvothermal conditions, which exhibited quenched fluorescence behaviors after forming coordination polymers (CPs). These CuI-p-DPA isomers showed discrepant fluorescence responses to thiophanate-methyl (TM). Among these CuI-p-DPA isomers, α-CuI-p-DPA exhibited the maximum fluorescence enhancement after its incubation with TM in aqueous solution. The fluorescence enhancement mechanism was that TM competed with the ligand p-DPA to coordinate with CuI clusters, and then α-CuI-p-DPA released p-DPA into the solution and induced fluorescence enhancement. The present detection method possesses the advantages of good selectivity, high sensitivity, short response time, and strong anti-interference ability with a linear range of 0.5-100 μM and a detection limit of 0.01 μM. This study not only reveals that the spatial structures of CPs play an important role in the fluorescence response ability, but also provide a new fluorescence signal-on analysis method to rapidly and sensitively determine the pesticide residue for TM.

Mechanism of fluorescence enhancement of HClO detected by excited-state intramolecular proton transfer based HBT-OMe molecule

The molecule with excited-state intramolecular proton transfer (ESIPT) has wide applications in fluorescent probe, biology imaging, light-emitting materials, etc. Biologically active oxygen hypochlorite (HClO) exists widely in the biological and chemical environment, which can pose a great threat to human health. Design of HClO-sensitive molecules in solvents is very important. Recently, Wu et al. [Wu L L, Yang Q Y, Liu L Y, et al. <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1039/C8CC03717E">2018 <i>Chem. Commun.</i> <b>54</b> 8522</ext-link>] designed an ESIPT-based HBT-OMe probe molecule, which can detect HClO due to its methoxy-hydroxy-benzothiazole. They found that the fluorescence intensity of the system gradually increases with HClO increasing. However, the microscopic mechanism of this highly efficient fluorescent probe is not well understood. Therefore, in this work, we theoretically investigate the ESIPT mechanism of the HBT-Ome and its product molecule by using density functional theory and time-dependent density functional theory. Based on polarizable continuum model (PCM) with the integral equation formalism variant (IEFPCM) and Becke’s three-parameter hybrid exchange function with the Lee-Yang-Parr gradient-corrected functional (B3LYP) as well as the TZVP basis, the optimized structures are obtained. The structures show that the HBT-Ome product molecules tend to undergo proton transfer in the excited state but HBT-OMe molecules cannot undergo the proton transfer process. The analysis of frontier molecular orbitals not only explains the reason why the fluorescence of the HBT-Ome product is enhanced, but also demonstrates that the HBT-Ome fluorescence intensity is diminished owing to twisted intramolecular charge transfer in the excited state. It is twisted intramolecular charge transfer that leads smaller charge density to be overlapped and the fluorescence intensity of HBT-OMe molecule to be further weakened. Infrared vibrational spectrum shows the enhancement of intramolecular hydrogen bond of O—H, which indicates the tendency of proton transfer. The molecular covalent interaction analysis shows that the intramolecular interactions of HBT-OMe remain largely unchanged clearly. The intramolecular O—H bonding interaction is weakened, and the N—H bonding interaction is increased for HBT-OMe product molecule. The enhancement of intramolecular hydrogen bond of N—H further illustrates the trend of proton transfer. The calculated potential energy curve provides direct evidence for the occurrence of ESIPT in the HBT-Ome product molecule. Our work is of great significance in designing and synthesizing the HClO fluorescent probes based on ESIPT molecules.