Aromatic Schiff Bases Research Articles

Protocatechualdehyde-based epoxy vitrimer with low dielectric, excellent flame retardancy, and recyclability

Conventional bisphenol A epoxy resins cannot meet the requirements of the new generation of electronic/electrical industries in terms of dielectric properties and flame retardancy and are challenging to reprocess after molding, resulting in waste of resources and environmental hazards. Therefore, a tetrafunctional active ester curing agent (TIE) was successfully synthesized by a one-pot method using biomass protocatechualdehyde as raw materials, followed by curing DGEBA to prepare an epoxy vitrimer (TIE/DGEBA). MHHPA/DGEBA was chosen for comparison since the –OH was also absent after curing. The results showed that TIE/DGEBA had good combinatorial performance, thanks to the unique structure of TIE, including active ester units, siloxane chains, and aromatic Schiff base groups. TIE/DGEBA possessed a lower dielectric constant (2.9 vs. 4.0) compared to MHHPA/DGEBA, which was attributed to the synergistic effect of the less-polar siloxane chain segments, bulky benzene structure, and lower cross-linking density (1971 vs. 733 mol/cm3) of TIE/DGEBA. The aromatic Schiff base not only imparted good thermal stability (T30% = 418.0 vs. 422.2 °C) to TIE/DGEBA but ensured that it allowed for recycling and reprocessing. After solvent recycling, the resin can be remolded without significant change in tensile strength (53.0 vs. 46.3 MPa). TIE/DGEBA displayed better flame retardancy with higher residual weight (28.2 % vs. 5.5 %), lower total heat release (29.6 vs. 43.5 kJ/g), and good resistance to thermal oxidization. The structural and performance characteristics of TIE/DGEBA offer a strategy for designing multifunctional epoxy-based materials in electrical/electronic applications.

Comparison of conventional and green approaches to the synthesis of aromatic Schiff bases

Abstract The chemical industry is one of the key elements in improving the quality of human life. At the same time, it generates pollution influencing the ecosystem and our health. To limit or remove some of the reasons of pollution for two decades less harmful approaches to the synthesis of various organic compounds were developed. Among organic compounds, azomethines, also known as Schiff bases, are of particular interest in biochemistry, medicine, and pharmacy. For over twenty years many novel approaches to the synthesis of these compounds have been investigated. Toxic solvents (benzene, toluene) have been replaced with water, fruit juice, or white egg, and in many cases, temperature and time of process were significantly reduced. In the review, conventional and green approaches to the synthesis of Schiff bases are presented with a focus on the effectiveness of these methods, including advantages and disadvantages.

Visible light-induced self-healing fluorinated polyurethane-acrylate containing aromatic Schiff base

Light-induced self-healing materials are of great practical value due to their precise local modulation. Aromatic Schiff bases with imine bonds have self-healing property due to their fast dynamic bond exchange properties under visible light. In this paper, we prepared a fluorinated polyurethane-acrylate containing aromatic Schiff base with visible light-induced self-healing ability by Pickering emulsion polymerization. The effect of the aromatic Schiff base content on the particle size of the fluorinated polyurethane-acrylate emulsion, the hydrophobic and oleophobic, mechanical and self-healing properties of the latex films was systematically investigated. The maximum water contact angle and tensile strength of fluorinated polyurethane-acrylate latex film could reach 113.25° and 8.64 MPa, respectively. The tensile strength could be restored to 88.64 % of the original one under visible light irradiation after damage. This work is the first report on visible light-induced self-healing fluorinated polyurethane-acrylate containing aromatic Schiff base, and provides a new design idea for the development of sustainable waterborne polyurethane-acrylate.

Dispersion of Single-Walled Carbon Nanotubes by Aromatic Cyclic Schiff Bases via Non-Covalent Interactions.

One of the challenging issues that hinders the application of single-walled carbon nanotubes (SWCNTs) is the poor solubility and the inevitable formation of bundles. Efforts still need to be made towards solving the problem. Herein, we report a non-covalent strategy to disperse aggregated SWCNTs by aromatic cyclic Schiff bases assisted by ultrasonic techniques. The aromatic cyclic Schiff base (OMM) was synthesized via Schiff base reactions, and the molecular structure was determined by ATR-FT-IR, solid-state 13C-NMR, and HRMS. Although the yielded product showed poor solubility in aqueous solution and organic solvents, it could interact with and disperse the aggregated SWCNTs in dimethyl formamide (DMF) under the condition of ultrasound. UV-vis-NIR, FL, Raman spectra, AFM, and TEM, along with computer simulations, provide evidence for the interactions between OMM molecules and SWCNTs and the dispersion thereof. The semiconductive (7,5), (8,6), (12,1), and (9,7)-SWCNTs expressed a preference for dissolution. The capability of dispersion is contributed by π-π, C-H·π, and lone pair (lp)·π interactions between OMM and SWCNTs based on the simulated results. The present non-covalent strategy could provide inspiration for preparing organic cyclic compounds as dispersants for SWCNTs and then facilitate their further utilization.

Ultrafast excited state intramolecular proton transfer and isomerization of long-chain linked Schiff bases.

The ultrafast proton transfer and the following dynamics for aromatic Schiff bases N,N'-bis(salicylidene)ethylenediamine (salen) and N,N'-bis(salicylidene)-1,4-butylenediamine (salbn) were investigated with experimental and theoretical methods. A dual emission property with a large Stokes shift in salen and salbn indicates that excited state intramolecular proton transfer occurs with photoexcitation. An efficient single proton transfer was confirmed within 200fs for both molecules. Subsequently, a fast twisted motion of the keto moiety carries cis-keto to a relaxed stable geometry in the S1 state. Following the twisted motion, the phenol ring at keto moiety further rotates to a conical intersection with the ground state and a cis-trans isomerization occurs. The isomerization rate is high, which dominates the competition with the radiative transition, resulting in weak emission intensity. It is confirmed that the length of alkyl chain affects the direction of phenol ring twisting and rotation during the whole subsequent relaxation of excited cis-keto tautomer. Compared with polar solvent acetonitrile, the barrier of isomerization is higher and the hydrogen bond on keto moiety is stronger in nonpolar solvent toluene. It makes fluorescence radiation channels competing with isomerism more likely to occur, contributing to the observed difference of enol/keto emission ratios of salen and salbn in toluene and acetonitrile.

Tailoring cross-linking temperature via hydrogen bonding and steric hindrance towards high-performance fire-Safe Polymer

Constructing thermally cross-linkable structures into thermoplastic polymers has emerged as a compelling strategy to enhance flame retardancy. However, the cross-linking reactions are hard to control, leading unexpected deterioration of processability and mechanical properties. Here, we report a novel strategy to modulate cross-linking reactions utilizing intramolecular hydrogen bonding and steric hindrance effects. Taking aromatic Schiff base (SA) as an example, novel monomers SAP and SAPN were designed and synthesized by incorporating phenolic and naphthalene units. As expected, the resultant copolymer PET-co-SAPNn exhibited a 120 °C higher cross-linking temperature compared with that of PET-co-SAn, powering them with great processability. Robust mechanical properties were obtained owing to the enhanced polymerization stability and intramolecular hydrogen bonding of SAPN, as evidenced by the tensile strength of 86.0 MPa for PET-co-SAPN15, 1.6 times to that of pure PET. Additionally, PET-co-SAPNn exhibits the highest fire safety, whose peak heat release rate (pHRR) and maximum smoke density (Dm) decreased by 49% and 53%, attributing to enhanced char-forming ability from the cross-linking and the phenolic and naphthalene units in SAPN. Considering these adequate improvements, this study provides a novel strategy for designing “self-cross-linking” flame-retardant polymers with improved processability and fire-safety.

Di-imine Schiff base inhibitor for carbon steel corrosion in 1 M HCl: Electrochemical, surface and theoretical investigations

Many strategies for an efficient inhibitor preparing were proposed for metals protection against corrosion in last decades. Herein, the adsorption and inhibitory effect of aromatic di-imine Schiff base inhibitor (Ar-Sb) namely (N1-(2-(((E)− 4-(dimethylamino)benzylidene)amino)ethyl)-N2-(2-((2-((2(Z)− 4-dimethylamino)ben- zylidene)amino) ethyl) amino) ethyl) amino) ethyl)ethane-1,2-diamine) on carbon steel (CS) in 1 M HCl solution was investigated experimentally and theoretically. The electrochemical measurements (EMs) such as electrochemical impedance spectroscopy (EIS), Potentiodynamic polarization (PDP) and cyclic voltammetry (CV) techniques exhibited the anticorrosion performance of Ar-Sb. Also, the adsorption of Ar-Sb on CS surface at different temperatures and immersion times was studied using PDP and EIS which indicated formation of a protective film layer of Ar-Sb responsible for CS corrosion mitigation. Ar-Sb inhibitor provided 95.79% inhibition efficiency at 5 × 10−4 M which reflected blocking of CS active sites through the adsorption process which was studied using various adsorption isotherms. CS surface morphology was studied using scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and atomic force microscope (AFM) reflecting the inhibition role of Ar-Sb in CS protection. The data obtained from PDP suggested that Ar-Sb inhibitor acted as mixed-type inhibitor. Also, quantum calculations such as density functional theory (DFT) and Monte Carlo (MCs) verified the inhibitive performance of Ar-Sb for CS surface with the experimental results.

Anti-cancer Schiff bases as photostabilizer for poly(vinyl chloride)

To prevent or reduce the drawback effect and prolong the life of PVC and preserve its composition and efficiency, compounds are incorporated with PVC. In this work, three highly aromatic anti-cancer Schiff base compounds in symbol (C2, C3 and C5) were applied to photostabilize of PVC against photodegradation, by added it to the polymer in the form of Schiff bases, to increase the stability of polymer when irradiated with ultraviolet (UV) light, (λmax = 365 nm) at 30 ºC, with the intensity of the light wavelength at 1920 lux for a long duration. The PVC was blended with Schiff's base in concentration (0.5 %wt.), with thickness films (40 μm) were made from the blended materials. Several methods were used, including infrared spectroscopy, surface examination of irradiated films to know the effect of additives on PVC photostabilization and conductivity of films were measured by LCR meter. The results showed that the additives significantly reduced the photodegradation of PVC. C2 compound as additive, stabilize PVC more than other additives, it can absorb harmful radiation, deactivate hydrogen chloride, and scavenge high-energy species such as peroxides. Several mechanisms of photostabilization for C2 compound have been suggested. Therefore it is acting as stabilizers for PVC.
 KEY WORDS: PVC, Photostabilizer, UV radiation, AFM, FESEM, LCR-meter
 Bull. Chem. Soc. Ethiop. 2024, 38(1), 135-146. DOI: https://dx.doi.org/10.4314/bcse.v38i1.11

Facile strategy of preparing excellently thermal-insulating and fire-safe building materials based on an aromatic Schiff base dicarboxylic acid without additional flame retardants

Developing excellently fire-safe and thermal-insulating building materials is a critical subject in the building sector. In this work, a novel aromatic Schiff base dicarboxylic acid (FN) was synthesized through a facile strategy to prepare high-performance rigid polyurethane foam (RPUF) composites with excellent thermal insulation and fire safety performance. The uniform dispersion of FN in polyether polyol improved the cellular structure of RPUF composites, which was conducive to increasing the thermal insulation of RPUF composites. Therefore, incorporating only 10 wt% FN into RPUF composite (RPUF/10FN) possessed the lowest wall temperature (51.9 °C) and room temperature (38.8 °C) with the lowest temperature growth rate under the continuous thermal radiation of the heat source among all samples in the small chamber test. Moreover, FN remarkably reduced the peak heat release rate, peak smoke production rate, and peak CO production rate of RPUF/10FN by 41.9%, 58.0%, and 59.6% without additional flame retardants, respectively, compared with those of pure RPUF. In addition, FN endowed RPUF/FN composites with better fire resistance and structural integrity compared with pure RPUF in the building facade fire test and fire protection test. The increased fire safety of RPUF/FN composites is due to the barrier effect of the formation of a dense and continuous char layer and the dilution effect of nonflammable gas generated by the thermal degradation of FN. Furthermore, FN enhanced the water contact angle of RPUF/FN composites from 112.2° for pure RPUF to 120.8° for RPUF/10FN, indicating that FN also enhanced the hydrophobicity of RPUF/FN composites. Therefore, the as-designed RPUF composites containing FN possessed a broad application prospect in the building field.

Studying the photodecomposition rate constant and morphology properties of modified poly(vinyl chloride) with novel Schiff's bases

AbstractThe photodegradation of a poly(vinyl chloride) (PVC) film filled with Schiff bases in its structure was investigated and studied by comparing it with a blank PVC film. A PVC film containing novel Schiff's bases was synthesized and used as a photostabilizer for PVC. PVC was exposed to ultraviolet light and underwent harmful changes, where the photostabilization effect differed before and after filling aromatic Schiff bases within the structure. After 300 h of UV irradiation, the photodecomposition rate constants were calculated to identify the impact of Schiff bases on PVC films. In order to evaluate the impact of adding Schiff bases as photo‐stabilizers, the evolution of different functional groups during irradiation was monitored using FTIR spectra. Also, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were exploited to examine the surface morphology of produced polymers. These investigations showed that filling novel Schiff bases inside the polymers performed much better photostabilization than plain PVC. Hence, the photodecomposition rate constant, on the other hand, was calculated alongside the irradiation time.

A phosphorus- and nitrogen-containing aromatic Schiff base derivative in waterborne polyurethane backbone for excellent flame retardant, UV-shielding and mechanical properties

Waterborne polyurethane (WPU) is widely used in the fields of coatings, adhesives and treatment agents. However, the WPU is highly flammable and has a high demand for UV-shielding property in practical application. To date, it remains a challenge to effectively improve the flame retardant and UV-shielding properties without compromising the inherent mechanical property. Here, we report a novel flame retardant WPU (FRWPU) with 9, 10-dihydro-9-oxo-10-phosphophenanthrene-10 oxide (DOPO)-based aromatic Schiff base derivative containing phenolic hydroxyl (DOPO-HAMB) in the backbone. Compared to the pristine WPU, the FRWPU reaches the limiting oxygen index (LOI) value of 27.9 %, and achieves a V-0 rating in the UL-94 test with only 3 % of DOPO-HAMB. Meanwhile, in the cone calorimetry test, the peak heat release rate (pHRR) and total heat release (THR) of FRWPU are reduced by 37.7 and 29.3 %, respectively. In addition, the FRWPU can suppress melt dripping and smoke release during combustion effectively. These enhancements are attributed to the cooperation between DOPO and aromatic Schiff base derivatives with both gas-phase and condensed-phase flame retardant mechanisms. More interestingly, the FRWPU exhibits excellent UV shielding property while maintaining the inherent mechanical property of polyurethane, which provides a useful reference for the design and fabrication of flame retardant waterborne polyurethane.

Rational design of an organic–inorganic hybrid with Schiff base cations for an efficient quadratic nonlinear optical switch

By introducing conjugated aromatic Schiff base cations, we successfully obtained a novel organic–inorganic solid-state quadratic nonlinear optical switching material with a high operating temperature.

Comprehensive Empirical Model of Substitution-Influence on Hydrogen Bonding in Aromatic Schiff Bases.

In this work, over 500 structures of tri-ring aromatic Schiff bases with different substitution patterns were investigated to develop a unified description of the substituent effect on the intramolecular hydrogen bridge. Both proximal and distal effects were examined using Density Functional Theory (DFT) in the gas phase and with solvent reaction field (Polarizable Continuum Model (PCM) and water as the solvent). In order to investigate and characterize the non-covalent interactions, a topological analysis was performed using the Quantum Theory of Atoms In Molecules (QTAIM) theory and Non-Covalent Interactions (NCI) index. The obtained results were summarized as the generalized, empirical model of the composite substituent effect, assessed using an additional group of simple ring-based Schiff bases. The composite substituent effect has been divided into separate increments describing the different interactions of the hydrogen bridge and the substituent: the classical substituent effect, involving resonance and induction mediated through the ring, steric increment based on substituent proximity to the bridge elements, and distal increment, derived from substitution on the distal ring.

Synthesized and Designed New Modified Poly(vinyl chloride) Structures to Enhance Their Photo-Resistance Characteristics

We synthesized and designed four new modified poly(vinyl chloride) structures to develop various polymer derivatives to enhance their photo-stability. Commercially available poly(vinyl chloride) was synthetically modified into a polymer containing aromatic Schiff base moieties. First, the PVC polymer was reacted with ethylene diamine from one side by using an equivalent of extra ethylene diamine to increase the probability of preventing cross-linking versus a two-sided reaction. In the second step, the corresponding aldehydes were reacted with an amino group to create the Schiff base new molecules. Different approaches were applied to confirm the structures of synthesized modified polymers such as FTIR, 1H NMR, and EDX. Using these methods, the structures and the percentage of modification for the studied materials were demonstrated. The percentage of modification exhibited between 30–35% using integration in the 1H NMR spectra and EDX analytical composition percentage of elements. In order to lessen the photo-degradation of the PVC polymeric chains, synthetic compounds have been employed as photo-stabilizers of the PVC films. In order to assess the effectiveness of modified PVC as a photo-stabilizer, a variety of techniques were used, including the weight loss method and monitoring the development of different functional groups while being exposed to radiation using FTIR spectra. Utilizing atomic force microscopy (AFM), scanning electron microscopy (SEM), and microscope pictures, the surface morphology of produced polymers was also investigated. These experiments demonstrated that novel modified Schiff base polymers perform as excellent photo-stabilizers in comparison to pure PVC. As a result, after being exposed to UV light for 300 h, PVC films exhibit less photo-degradation than blank PVC, so that light in the UV range may be absorbed by the connected units’ high conjugation mechanism.

Design, Synthesis, Drug-Likeness and In Silico Prediction of Polycyclic Aromatic Schiff base Tethered Organosilatranes

The present article includes the generation of aromatic Schiff base tethered organosilatranes via two step synthetic pathways starting from anthracene-10-carbaldehyde and 1H-indole-3-carbaldehyde. The synthesis was achieved by employing condensation reaction followed by transesterification methodology. The prepared organosilatranes containing aromatic schiff based ligands were structurally authenticated using spectroscopic techniques such as FT-IR, NMR and mass analysis. The prepared compounds were also scrutinized for in silico physiochemical properties via Molininspiration and ADMET software and the results revealed their good biological potential for drug discovery.

Excited state intramolecular proton transfer and topography of conical intersection of aromatic hydrazone Schiff bases

We investigated the energy landscapes of several aromatic hydrazone Schiff base molecules using the SA-CASSCF and MS-CASPT2 theories. We located the energy-minimized conical intersection (MECI) of the S1/S0 surfaces. Furthermore, we evaluated the dot products of the S1 energy gradient in the Franck-Condon geometry with the gradient difference and the interstate coupling vectors. Additionally, we obtained the topographical parameters of the MECIs. We examined the excited state intramolecular proton transfer (ESIPT) using TD-DFT, which reveal that ESIPT on the S1 surfaces involve very small barriers. Some of the investigated Schiff base compounds are potential photoswitches.

Aromatic Schiff bases confer inhibitory efficacy against New Delhi metallo-β-lactamase-1 (NDM-1)

The irregular use of antibiotics has created a natural selection pressure for bacteria to adapt resistance. Bacterial resistance caused by metallo-β-lactamases (MβLs) has been the most prevalent in terms of posing a threat to human health. The New Delhi metallo-β-lactamase-1 (NDM-1) has been shown to be capable of hydrolyzing almost all β-lactams. In this work, eight aromatic Schiff bases 1–8 were prepared and identified by enzyme kinetic assays to be the potent inhibitors of NDM-1 (except 4). These molecules exhibited a more than 95 % inhibition, and an IC50 value in the range of 0.13–19 μM on the target enzyme, and 3 was found to be the most effective inhibitor (IC50 = 130 nM). Analysis of structure–activity relationship revealed that the o-hydroxy phenyl improved the inhibitory activity of Schiff bases on NDM-1. The inhibition mode assays including isothermal titration calorimetry (ITC) disclosed that both compounds 3 and 5 exhibited a reversibly mixed inhibition on NDM-1, with a Ki value of 1.9 and 10.8 μM, respectively. Antibacterial activity tests indicated that a dose of 64 μg·mL−1 Schiff bases resulted in 2–128-fold reduction in MICs of cefazolin on E. coli producing NDM-1 (except 4). Cytotoxicity assays showed that both Schiff bases 3 and 5 have low cytotoxicity on the mouse fibroblast (L929) cells at a concentration of up to 400 μM. Docking studies suggested that the hydroxyl group interacts with Gln123 and Glu152 of NDM-1, and the amino groups interact with the backbone amide groups of Glu152 and Asp223. This study provided a novel scaffold for the development of NDM-1 inhibitors.

Direct Synthesis of Aromatic Imine Schiff Bases from β-Phenol Hydroxy Ketone

Abstract: A facile methodology has been developed to build carbon nitrogen double bond from ketones promoted by the hydroxyl groups in β-phenol hydroxy ketone. It is noteworthy that the halogenated β-phenol hydroxy ketone can chemoselectively react with the amine to afford halogenated phenol imine. It is suitable for certain natural products and also suitable for water-based heteroamines. The method possesses low toxicity and is widely applicable. This strategy is usually used to obtain moderate to good yields of aromatic amine Schiff base.

Benzaldehyde decorated octadecylamine for tailor-made molecular firefighting and efficient thermal energy management

• Benzaldehyde decorated octadecylamine for tailor-made thermal management. • Self-crosslinking of aromatic Schiff base for molecular firefighting. • Extensively reversible thermal transformation in a wide range of 30–100 °C. • Open a window for designing multifunctional phase change materials. More than 40% of the energy generated globally is consumed by residential and commercial buildings, so energy efficient materials for buildings are urgently needed at present. On-demand thermal energy storage/release using phase change materials (PCMs) can improve thermal energy utilization efficiency, however, they are facing low adaptability to temperature and easy flammability in the case of fulfilling high thermal management capacity. Herein, we report octadecylamine-based PCMs with tailor-made molecular firefighting and thermal energy management capability. These synthesized octadecylamine-based Schiff base-containing PCMs (SB-PCMs) may self-extinguish while encountering short-time fire and possess variable phase transition temperatures of 33, 42, 92, and 101 °C. Meanwhile, these SB-PCMs own high phase transition enthalpies of 161.4–183.4 J g −1 and excellent cyclic stability. When SB-PCMs as thermal energy management materials in wood plastic-based building materials, they show high adaptability to ambient temperature and excellent thermal management capability, achieving temperature regulation of 9.0 °C. Meanwhile, fire safety of the composite wood plastic materials is significantly improved. Detailed analysis confirms that the incorporated benzaldehyde with various positional substituted hydroxyl may induce noncovalent interactions (mainly hydrogen bond and π-π stacking) of SB-PCMs and further control their self-assembling processes, thus achieving multiple thermal responses of SB-PCMs; self-crosslinking charring of benzaldehyde-based Schiff base structure may retard fire spreading, therefore presenting molecular firefighting. Additional multistage thermochromic behaviors of SB-PCMs may extend their application in temperature-sensitive paint, intelligent fabric, etc. This work firstly represents a method for manipulating the multistage thermal and fire responses of PCM in the case of fulfilling high thermal energy management capacity and confirms its molecular-firefighting and energy efficient application in building materials.

Aromatic Schiff Base-Based polymeric phase change materials for Safe, Leak-Free, and efficient thermal energy management

Polymeric phase change materials (PCMs) have garnered enduring interest owing to no leakage and considerable latent heat in thermal management. However, the flammability, poor recyclability, and low energy storage density of typical polymeric PCMs have long-term constraint on their applications in sustainable energy. Herein, we fabricated a novel linear polymeric solid–solid PCM (HPPEG-PU) with facile recyclability, fire safety, and simultaneous high heat transition capacity via introducing aromatic Schiff base structure into the hard segment of HPPEG-PU and meanwhile using high-molecular-weight polyethylene glycol as energy-storage soft segment. HPPEG8000-PU achieved the latent heat of 131.1 J g−1, higher than the reported polymeric solid–solid PCMs, and realized thermal reprocessing at 80 ℃ and fire extinguishment once being ignited. Mechanisms for safe, leak-free, and highly efficient thermal energy storage/release were discussed deeply, in which aromatic Schiff base structure induced network confinement, crystallization, and self-crosslinking charring are highly responsible for shape stability, high thermal storage capacity, and fire safety, respectively. Based on the phase change characteristic of HPPEG-PU, thermosensitive imaging and convenient thermal management applications were developed in this work. Cunningly targeting aromatic Schiff base structure to polymeric PCMs may provide a promising route for safe, leak-free, and efficient thermal energy management.