Conjugated Schiff Base Research Articles

Versatile fluorenone sensor for swift identification of nerve agent mimic DCP and hazardous Cu2+/Hg2+ ions

A fluorenone and indole conjugated Schiff base (1) has been designed and synthesized through a one-pot reaction, and utilized as a multifunctional sensor for recognition of various analytes, DCP, Cu2+, Hg2+, and Cysteine. The UV–vis absorption spectra demonstrated the detection of the nerve agent mimic, DCP, indicated by a blue shift in the spectra in the solution of CH3CN. The sensor 1 showed high efficiency in detecting DCP among other phosphates and ions in absorption spectrum, with a detection limit of 1.16 µM. Additionally, when various anions and metal ions were introduced to the CH3CN solution of sensor 1 (λex 260 nm), the fluorescent probe was quenched completely in the presence of Cu2+ and Hg2+ ions. The association constants for Cu2+ and Hg2+ ions with sensor 1 were calculated to be 6.68 × 104 and 9.10 × 103 M−1, respectively, with a 1:1 binding stoichiometry. Moreover, the in-situ formed complex of sensor 1 and Cu2+ (1 + Cu2+) has been further utilized to synergistically recognize cysteine. The practical applications of sensor 1 were also investigated for detecting Cu2+ and Hg2+ ions in real water samples collected from different sources.

Investigation of the anticancer effect of newly synthesized palladium conjugate Schiff base metal complexes on non-small cell lung cancer cell line and mouse embryonic fibroblast cell line

Lung cancer remains one of the leading causes of death worldwide. Due to the side effects of chemotherapeutic agents on normal cells and the development of resistance by cancer cells, there is an urgent need for alternative new pharmacological agents. Palladium (Pd)-conjugated Schiff base (SB) compounds represent an alternative approach with promising potential applications in cancer treatment. This study aims to identify novel therapeutic agents on A549 cells through the synthesis and characterization of Schiff base conjugated-Palladium complexes (Pd-L1 and Pd-L2). Additionally, it seeks to elucidate the mechanism of action of these compounds on both the A549 and NIH/3T3 cell lines. In the present study, two new Pd-L1 and Pd-L2 were synthesized for the first time and characterized mainly by single crystal X-ray diffraction and 1H, 13C, 31P NMR techniques. The cytotoxic effect of the compounds was evaluated by MTT assay on A549 and NIH/3T3 cell lines for 24 and 48 h. Cisplatin was used as a positive control group. Based on the cytotoxicity results, the complexes were evaluated for their anticancer activities against A549 cell lines for 48 h through reactive oxygen species (ROS), cell cycle, apoptotic, and necrotic cell analyses. The most potent cytotoxic effects were determined for Pd-L1 (IC50: 23.33 μM), Pd-L2 (IC50: 3.19 μM), and cisplatin (IC50: 33.27 μM) on A549 cells (p < 0.05). The compounds exhibited a significant cytotoxic effect at lower concentrations on A549 cells compared to NIH/3T3 cells (p < 0.05). All compounds showed a significant increase in ROS levels in A549 cells compared to the control group (p < 0.05). While necrosis and apoptosis was observed in A549 cells treated with cisplatin, induction of apoptosis was effective in cell death for A549 cells treated with Pd-L1 and Pd-L2 (p < 0.05). Additionally, it was observed that the compounds inhibited cell proliferation in the G0/G1 and G2/M cell cycle phases (p < 0.05). All compounds induced cell cycle arrest and cell death in A549 cells by increasing ROS levels. The results obtained in the present study could advance the utilization of the compounds as anticancer agents.

A Highly Effective Self-Healing Waterborne Polyurethane Vitrimer Containing Conjugated Schiff Base Bonds Driven by Photothermal

A Highly Effective Self-Healing Waterborne Polyurethane Vitrimer Containing Conjugated Schiff Base Bonds Driven by Photothermal

Pyrrole‐Based Schiff‐Bases: Synthesis, Fluorescent Properties, Molecular Docking and in silico ADME/Tox Profiling Studies

AbstractCondensation reactions of methyl 5‐formyl‐1H‐pyrrole‐2‐carboxylate with 4‐aminobenzonitrile and N, N‐dimethyl‐p‐phenylenediamine yielded two conjugated Schiff bases with pyrrole rings. Fluorescence spectroscopy was used to examine the molecules′ selectivity for different metal salts, and it was discovered that the PL1 molecule was selective for the Pb2+ ion. In addition, the interactions of the same molecules with the polo‐like kinase 1 (PLK1) enzyme were investigated by molecular docking with the Autock Tools 1.5.6 program, and the synthesized Schiff bases formed stable complexes with the PLK1 with binding affinity values of −6.5 and −6.4 kcal/mol for PL1 and PL2 respectively. The SwissADME and ADMET web tools were used to calculate in silico ADME/Tox profiles of the synthesized molecules.

Naked-eye colorimetric anion probing and fluorescent switching features of conjugated Schiff Bases derived from 4-(Trifluoromethyl) benzenesulfonamide

In this work, synthesis, spectroscopic, density functional theory (DFT) and photophysical studies of the three conjugated benzenesulfonamide Schiff bases were reported. We also calculated the energies of the frontier molecular orbitals (FMOs) and UV–Vis spectra of Schiff bases theoretically using the DFT and TD-DFT methods. Additionally, photophysical properties were investigated by UV–Vis and fluorescence spectroscopy techniques. The ground state interactions show that the absorption band has been separated into two branches and bathochromatically tuned to the visible region, and fluorescence intensity decreased in the case of the Schiff base incorporated naphthalene unit. Furthermore, the charge transfer dynamics of the compounds were analyzed using femtosecond transient absorption spectroscopy measurements proving the triplet-triplet transitions via intersystem crossing mechanism. Based on the ultrafast pump-probe experimental results, as well as decay kinetics of the compounds, the Schiff base with naphthalene unit, showed a fast transition to the triplet state (1.2 ps) as compared to Schiff bases including methyl (4 ps) and chloro (2.7 ps) units. However, upon the addition of various anions, the molecule structure gains anion sensing and fluorescence switching capabilities under daylight and UV light. The obtained experimental results pointed out that the modification of Schiff base molecules with methyl, chlorine and naphthalene units affected its photophysical properties as well as excited-state dynamics significantly.

Fluorescent sensing (Cu2+ and pH) and visualization of latent fingerprints using an AIE-active naphthaldehyde-pyridoxal conjugated Schiff base

Tunable light-emissive materials have achieved an extensive research attention due to their potential applications in the environmental and biomedical fields. This work introduced an aggregation-induced emission (AIE) active naphthaldehyde-pyridoxal (NPPY) conjugated Schiff base for the fluorescence sensing of pH and Cu2+, and visualization of latent fingerprints (LFPs). The weakly emissive NPPY in freely soluble DMSO converted to a bright yellow fluorescent upon increasing the poor solvent water fraction above 50% in DMSO. The DLS and SEM analysis supported the formation of self-aggregates of NPPY, and conversion of NPPY into a bright yellow fluorescent Schiff base due to the restriction of intramolecular rotation. The yellow fluorescent NPPY between pH 2.7 to 10.3 became green fluorescent above pH 10.3 with an estimated pKa of 10.8. The metal ions recognition ability of the AIE active NPPY was explored in HEPES buffer (5% DMSO, 10 mM HEPES, pH = 7.4). The fluorescent intensity at 570 nm of NPPY was quenched upon complexation with Cu2+. The chelation enhanced fluorescence quenching was occurred due to the electron transfer from NPPY to the paramagnetic Cu2+. The detection limit reached the nanomolar level of 32.9 nM. Sensor NPPY was applied for the quantification of Cu2+ in real environmental and biological samples. Also, NPPY coated paper test strips and cotton buds were developed for the qualitative visual detection of Cu2+. Additionally, the aggregates of NPPY was applied for the visualization of LFPs and also fluorescent gelatine gel was developed for future applications in fabricating light-emitting devices.

Thermal aging resistance of natural rubber compounded with conjugated Schiff base

Schiff’s base iron salt can be used as a new and effective antioxidant for rubber compounds.The tensile strength after 96 h of aging was approx. 36% and 12% higher, respectively, than in the caseof the antioxidants 4010NA and BHT used. Natural rubber vulcanizate (NR) containing the iron salt of Schiff’s base showed excellent rheological and mechanical properties, as well as very good thermalstability. These properties were far superior to that of other commercially available antioxidant rubber blends.

Preparation and characterization of Schiff base metal complexes for high performance supercapattery

With the urgent need for portable electrochemical energy storage (EES) devices, it is significant to develop energy storage devices with superior energy-power densities, long-term cycling durability and low cost. Supercapattery combines the benefits of supercapacitors and batteries with splendid specific power and good cycling stability. In this work, we synthesize Schiff base of O-tolidine and terephthalaldehyde (OTTP) and its metal complex (M-OTTPs, M= Co2+, Ni2+, Zn2+) via a feasible one-step strategy as electrode materials. The as-obtained M-OTTPs possess remarkable electrochemical property on account of the coordination effect on metal ions and N atoms, and the volume hindrance caused from metal ions entering affects the π-π accumulation in the skeleton of the Schiff base. With metal ions entering into the Schiff base molecular structure, the electrochemical properties of M-OTTPs improve apparently in comparison with OTTP. Ni-OTTP electrode exhibits the excellent specific capacitance of 561.8 C g − 1 at the current density of 0.5 A g − 1 with extraordinary rate capability in three electrode system. An alkaline (6 M KOH electrolyte) supercapattery hybrid device (Ni-OTTP//AC) is manufactured from Ni-OTTP as the positive electrode and activated carbon (AC) as the negative electrode, which possesses the high energy density of 34.58 Wh kg−1 at the power density of 613.24 W kg−1. The M-OTTPs possess remarkable electrochemical property due to the coordination effect on metal ions and N atoms, and the volume hindrance caused from metal ions entering affects the π-π accumulation in the skeleton of the Schiff base. This study provides an innovative strategy to prepare electrode materials with prominent electrochemical performances by conjugated Schiff bases synthesis and metal ion coordination.

Antimicrobial activity of silver sulfide quantum dots functionalized with highly conjugated Schiff bases in a one-step synthesis.

In the present paper, low-dimensional Ag2S QDs were fabricated for the first time, with four different dithiocarbazate derivative Schiff bases (SB) as capping agents in a one-pot synthesis. These SB-capped Ag2S QDs were almost spherical with an average size range of 4.0 to 5.6 nm, which is slightly smaller than conventional thioglycolic acid (TGA)-capped Ag2S QDs. We demonstrate that the growth of Gram-positive bacteria (Bacillus subtillus and Staphylococcus aureus), Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa), and a prevalent fungal pathogen (Candida albicans) are inhibited more when the bacterial and fungal cells were nurtured with the synthesized SB-Ag2S QDs, compared with TGA-Ag2S QDs or free unbound Schiff bases. The minimum inhibitory concentration (MIC) results confirmed that even low concentrations of SB-Ag2S QDs were able to inhibit bacterial (MIC 5–75 μg mL−1) and fungal growth (MIC 80–310 μg mL−1), and in some cases they performed better than streptomycin (8–25 μg mL−1). Lethality bioassay results confirmed that SB-Ag2S QDs were not toxic to brine shrimp (Artemia salina). The results show that capping agents are essential in the design of functional Ag2S QDs, and highlight that Schiff bases provide an excellent opportunity to optimize the biological activities of silver based QDs.

Phenolphthalein Conjugated Schiff Base as a Dual Emissive Fluorogenic Probe for the Recognition Aluminum (III) and Zinc (II) Ions.

In this study, a new phenolphthalein derivative, FFIZNA, has been planned and successfully prepared in an uncomplicated way. The probe FFIZNA could selectively monitor Al3+ and Zn2+ among other relevant cations with diverse colors through a turn-on emission response in EtOH:HEPES (9/1;v/v) media owing to the chelation enhanced fluorescence (CHEF), prevention of ESIPT, -C=N- isomerization and PET of the probe FFIZNA. The interactions of Al3+ and Zn2+ with the probe FFIZNA were confirmed by emission spectroscopy, Job's plot and 1H-NMR titration substantiated 1:2 reaction stoichiometry between FFIZNA and Al3+ and Zn2+. The time-response study displayed that the emission of FFIZNA with Al3+ and Zn2+, rapidly boosted and reached the stable value in less than 3.0 and 4.0min, respectively. Therefore, the FFIZNA has successfully been utilized to the dual recognition of Al3+ and Zn2+ in solutions. Phenolphthalein conjugated schiff base as a dual emissivefluorogenic probe for the detection aluminum (III) and zinc (II) ions.

Hydroquinone-based conjugated Schiff base polymer as anode material for lithium ion batteries

Redox-active organic compounds have emerged as promising candidates for lithium-ion batteries because of their potentially high specific capacities, cost-effective, safe and environmental-friendly. Herein, a hydroquinone-based conjugated Schiff base polymer (HQ-CSBP) was synthesized by the condensation of 2,5-diamino-1,4-benzenediol dihydrochloride (DABD) and 2,5-dihydroxyl-terephthalaldehyde (DHTA). At a current density of 100 mA g−1, The material provides a remarkable specific capacity of 576 mA h g−1 which corresponds to 96.5% the theoretical value (596 mA h g−1). The capacity retention rate is as high as 94% after 100 charge-discharge cycles. These results demonstrate the great potential of HQ-CSBP as a novel polymer anode material for high-performance lithium ion batteries.

Amino acid and cinnamaldehyde conjugated Schiff bases as proficient corrosion inhibitors for mild steel in 1 M HCl at higher temperature and prolonged exposure: Detailed electrochemical, adsorption and theoretical study

Three Schiff bases composing of cinnamaldehyde and three different amino acids, namely glycine (CGSB), histidine (CHSB) and tryptophan (CTSB) have been synthesized and investigated for their anti-corrosion propensity on mild steel in 1 M HCl. Both gravimetric and electrochemical experimentation establish the CTSB as superior inhibitor, while the CGSB as the least effective among the three. Effectiveness of the Schiff bases is tested upto 60 °C for an exposure of 6 h in HCl. Effect of various exposure times, maximum being 96 h, is also investigated at a fixed temperature of 30 °C. All the three inhibitors impart appreciable extent of corrosion inhibition efficiency under these extreme conditions. Thermodynamic adsorption and kinetic parameters ascertain chemisorption of these mixed-type corrosion inhibitors on mild steel. Mode and extent of interaction between Schiff bases and mild steel are evaluated from quantum mechanical calculation and molecular dynamics simulation.

Synthesis of biochar conjugated Schiff base composites and their enhanced antimicrobial activity against five pathogenic organisms

The search for antimicrobial drug of high bio-efficacy not prone to multiple microbial resistance has been on the rise in recent time. This study focused on the preparation, characterization and antimicrobial evaluation of Schiff functionalized HNO3 activated plantain peel and rice husk biochar against five pathogenic bacteria. The activated rice husk and plantain peel biochar were characterized using Braunauer-Emmett-Teller, X-ray diffraction and Fourier transform infra-red spectroscopy. Based on the result, the activated rice husk and plantain peel biochar were amorphous and crystalline respectively with pore surface area and pore size for activated rice husk and plantain peel biochar as 9.369 and 27.32 (m2.g-1) and 8.790 and 16.65 (cc.g-1), respectively. The Fourier transform infra-red spectroscopy result indicated bonds, such as –OH, C=O and –N-H, where actual chelation and electrostatic attraction mechanism are prevalent responsible for antimicrobial potency. The cell bioactivity was hampered due to permeation of the biocidal agents, cell membrane disruption and generation of reactive oxygen species enhanced by conjugation of Schiff base based biochar to the microbial cell wall leading to death of microbes. The ease of Schiff base release and reusability of the nanocomposite favors the product as an efficient, low cost, effective and promising nanocomposite for decontamination of bacteria infested media.

Template‐Assisted Regioselective Identification of Metal Ions on Coumarin‐Furan Conjugated Chemosensors: AIEE Effect and Photo‐Switching pH Indicator by ICT

A coumarin‐furan conjugated Schiff base chemosensor (R1) of straightforward design showed an excellent aggregation‐induced emission enhancement effect in ACN‐H2O mixtures of different ratios, which was confirmed by field emission scanning electron microscopy (FESEM) and supported by Dynamic Light Scattering (DLS). R1 identified three metal ions (Pb2+, Hg2+ and Cu2+) in colorimetric and fluorometric techniques with an intramolecular charge transfer (ICT) off‐on mechanism. The space specificity of the metal ions (Hg2+ and Cu2+) resulted in hydrolysis of ‐CH=N‐ functionality of the sensor. The latter also exhibited excellent sensitivity in acidic conditions by blocking ICT and remained stable in neutral to basic pH. Cell viability and microscopic fluorescence experiments revealed that R1 is cytocompatible and may be used for the detection of Pb2+, Hg2+, and Cu2+ ions in living cells.

Non-destructive determination of chemical and microbial spoilage indicators of beef for freshness evaluation using front-face synchronous fluorescence spectroscopy

We investigated the potential of front-face synchronous fluorescence spectroscopy (△λ = 75 nm) to non-destructively evaluate beef freshness and quality decline during chilled storage. The total volatile basic nitrogen (TVB-N), thiobarbituric acid reactive substances (TBARS) and total viable count (TVC) values were used as standard freshness indicators. The fluorescent substances, including amino acids, collagen and conjugated Schiff bases, were highly correlated with the chemical and microbial deterioration of the beef. Quantitative models for simultaneously predicting the three freshness indicators were built combined with partial least squares (PLS) algorithm and showed good reliability. For TVB-N and TBARS values, Rc2 and Rp2 were both above 0.900, and for TVC values Rc2 and Rp2 were 0.912 and 0.871, respectively. The qualitative model established by partial least squares discriminant analysis (PLS-DA) algorithm could accurately classify beef samples as fresh, acceptable or spoiled. The accuracy of the calibration and validation sets were 92.54% and 86.96%, respectively.

Recognition of Al3+ through the off-on mechanism as a proficient driving force for the hydrolysis of BODIPY conjugated Schiff base and its application in bio-imaging

Two new BODIPY azine bearing quinoline and pyrazine attached Schiff base chemosensors (R1 and R2) have been synthesized and applied for the detection of Al3+ in CH3CN/H2O medium. Intramolecular hydrogen bonding makes both the sensors rigid and helps to encapsulate Al3+ in the cavity. The pink colour of R1 and R2 has been changed to green fluorescent upon excess addition of Al3+ which is only because of the hydrolysis of imine bond to regenerate compound 8. Another nitrogen atom present in quinoline and in pyrazine moiety made R1 and R2 more efficient in sensing of Al3+ ion compare to R1D, R2B and R2D. Cell viability and fluorescence microscopic experiments showed that the chemosensors are cytocompatible and can be used as an effective fluorescent probe for detecting Al3+ ion in the living cell.

Zn2+ conjugated Schiff base organic nanoparticles for selective quantification and degradation of diethyl chlorophosphate in aqueous media: Application to green vegetables

In the present work, zinc conjugated Schiff base organic nanoparticles have been fabricated and explored for the quantification and degradation of diethyl chlorophosphate (a common pesticide for green leafy vegetables). Primarily, a Schiff base (SB) was derived by the condensation of 2-hydroxy-4-methoxybenzophenone and 2-aminoethanol and then, it was transformed into organic nanoparticles by reprecipitation method. Further, the Zn2+ ions were added to dispersion of organic nanoparticles (SB-ONPs) to form zinc conjugated Schiff base organic nanoparticles (ZnSB-ONPs). Their conjugation turns ‘ON’ the spectrofluorimetric response and displayed an elevated emission band centered at 435 nm. Further, the emission signal of ZnSB-ONPs was quenched in the presence of diethyl chlorophosphate with visual appearance of HCl gas which indicated breakage/formation of bonds. Therefore, a detailed investigation of recognition and degradation of diethyl chlorophosphate was accomplished by spectrofluorimetric, spectroscopic (1H NMR and 31P NMR) and mass spectrometric experiments. The use of Zn2+ ions in conjugation with Schiff base organic nanoparticles (in ZnSB-ONPs) offered some advantages like selective spectrofluorimetric recognition and degradation of diethyl chlorophosphate, quantification of diethyl chlorophosphate up to nano-molar levels (LOQ = 55.2 nM), and application of developed protocols in aqueous media to green leafy vegetables and tea samples.

Benzimidazole and benzothiazole conjugated Schiff base as fluorescent sensors for Al3+ and Zn2+

Two benzimidazole/benzothiazole based azomethines, (E)-2-(1H-benzo[d]imidazol-2-yl)-4-(4-(diethylamino)-2-hydroxybenzylideneamino)phenol (HBZA) and (E)–2-(benzo[d]thiazol–2-yl)–4-(4-(diethylamino)–2-hydroxybenzylideneamino)phenol (HBTA) were designed and synthesised. Investigations of solvatochromic behaviour of these fluorophores in solvents of varying polarities showed large Stokes shift of 134–210 nm. Time resolved Laser induced fluorescence measurements revealed the excited state dynamics of the fluorophores. Molecules were also found to be emissive in aggregated state as seen from the aggregation induced emission studies. Appreciable absorption and emission spectral changes upon co-ordination of HBZA with Al3+/Zn2+ and HBTA with Al3+, as well as good sensitivity and selectivity, indicated their capability of detecting the two analytes. The binding stoichiometry was determined using electrospray ionization mass spectrometry (ESI-MS) and the binding mechanism was studied using density functional theory.

Rapid evaluation of quality deterioration and freshness of beef during low temperature storage using three-dimensional fluorescence spectroscopy

We investigated three-dimensional (3-D) fluorescence spectroscopy for its potential to evaluate beef quality deteriorative changes and freshness. The fluorescence characteristics of heme, conjugated Schiff base and amino acids, could be indicators of internal biochemical reactions associated with beef deterioration, including color changes, lipid oxidation, and protein degradation, as well as a measure of freshness decline. To classify beef quality in terms of color (sensory index) and pH (chemical index), cluster analysis method (CA) was used. Three classes were identified: “fresh”, “acceptable”, “spoiled”. We then developed a qualitative model to classify stored beef into these three classes using 3-D front-face excitation-emission matrices (EEMs) of fat tissue, combined with a parallel factor analysis (PARAFAC) algorithm. The resulting model had calibration and validation accuracies of 95.56% and 93.33%, respectively. These results demonstrate the potential of fluorescence spectroscopy to accurately and non-destructively monitor beef quality decline.

Ultrafast Dynamics of a "Super" Photobase.

Molecular reactivity can change dramatically with the absorption of a photon due to the difference of the electronic configurations between the excited and ground states. Here we report on the discovery of a modular system (Schiff base formed from an aldehyde and an amine) that upon photoexcitation yields a more basic imine capable of intermolecular proton transfer from protic solvents. Ultrafast dynamics of the excited state conjugated Schiff base reveals the pathway for proton transfer, culminating in a 14-unit increase in pKa to give the excited state pKa * >20 in ethanol.