Electron-pushing Group Research Articles

Construction of stable heterogeneous catalysts with alkali metals loaded on weakly alkaline carbon-nitrogen materials for transesterification

It is important to enhance the basicity of solid catalysts for the application of heterogeneous catalysts in the synthesis of dimethyl carbonate. Notably, the role of the carrier in the supported solid base catalyst cannot be ignored. Herein, the catalyst (named 1.0Ca/g-C3N4 (M)), prepared by one-step thermal polymerization, achieved a yield of 68.4 % and a selectivity of 96.8 % for DMC and maintained its performance after four consecutive cycles at 130 °C. It could be seen from the characterization that the amino group on the surface of the carrier, as an electron pushing group, endowed the Ca connected at its end with more abundant valence electrons, so that the catalyst exhibited a higher alkalinity. This study provides a new reference for the development of multiphase catalysts in transesterification reactions.

Dual-Ligand Ti-MOFs with Push-Pull Effect for Photocatalytic H2 Production.

Enhancing the photogenerated electrons transfer efficiency is crucial for photocatalytic reactions. Herein, a dual-ligand-induced push-pull effect was manipulated to intensify the transfer of photogenerated electrons between organic ligands and metal clusters using NH2-MIL-125(Ti), a kind of Ti-based metal-organic framework (MOF), as the model system. The dual-ligand MOF, NH2/Cl-MIL-125, was designed and synthesized based on the Hammett constant (σm), in which -NH2 (σm = -0.16) and -Cl (σm = 0.37) were selected as the electron-pushing group and the electron-pulling group, respectively. Meanwhile, -CH3 (σm = -0.07, electron-pushing) and -H (σm = 0, neither electron-pushing nor electron-pulling) were selected as the reference groups to prepare NH2/CH3-MIL-125 and NH2/H-MIL-125, respectively, to validate the electron push-pull effect. NH2/Cl-MIL-125 (5.32 mmol g-1 h-1) exhibits a higher photocatalytic H2 evolution activity than single-ligand NH2-MIL-125 (1.93 mmol g-1 h-1), NH2/CH3-MIL-125 (4.45 mmol g-1 h-1), and NH2/H-MIL-125 (4.73 mmol g-1 h-1) under full-spectrum irradiation. The result can be attributed to the electron push-pull effect between -NH2 and -Cl, which boosts the electron transfer along the ligand-metal-ligand direction. Our dual-ligand-induced push-pull strategy for enhancing the electron transfer may offer some novel insights into the rational design and synthesis of photocatalysts for many other reactions.

Effect of substituents on 1,3,5-triphenylpyrazoline as light-emitting diode-sensitive initiators in photopolymerization

By introducing different substituents at the N1 position, a series of 1,3,5-triphenylpyrazolines were designed and synthesized as photoinitiators (PIs) for cationic and free radical photopolymerization processes. As evaluated through ultraviolet–visible (UV–vis) spectroscopy, fluorescence spectroscopy, theoretical calculations, electrochemistry, and electron spin resonance–spin trapping tests, all PIs show good photoinitiating properties under light-emitting diode (LED) irradiation at 365–425 nm. Their light absorption within the UV–vis range (λmax = 352–390 nm; εmax = 26 000–40 700 M−1 cm−1) is strongly influenced by the substituents on the N1 phenyl ring. Remarkably, the introduction of the carbonyl group redshifts the maximum absorption spectra from 375 nm to 390 nm, which could well match the emission of LEDs in the range of 365–425 nm. The pyrazolines, together with iodonium salt and N-methyldiethanolamine, can generate acid and free radicals under LED irradiation and conduct high-performance LED-induced cationic and free radical photopolymerization processes under 365–425 nm irradiation. Cationic polymerization is enhanced by the introduction of an electron-pushing group, and free radical polymerization is promoted by the introduction of a carbonyl group at the N1 position. These pyrazoline-based photoinitiating systems have promising applications in LED-induced photopolymerization.

High-performance LED induces cationic photopolymerization using novel 1,3,5-triaryl-2-pyrazoline as photosensitizer

A series of 1,3,5-triaryl-2-pyrazolines was designed and synthesized as photosensitizers for cationic photopolymerization (CP). The light absorption within the ultraviolet-visible (UV–vis) range (λmax = 344–376 nm; εmax = 17,300–20,000 M−1 cm−1), which was strongly influenced by the substituents on the C3 atom of pyrazoline ring, matched the emission of light-emitting diodes (LEDs) within 365–425 nm. The pyrazolines, together with iodonium salt (Iod) or sulfonium salts, can generate acid upon the irradiation with LEDs, and the quantum yields of the systems were high enough (0.11–0.17 for Iod and 0.04–0.12 for sulfonium salts) for high-performance LED-induced CP at 365–425 nm irradiation. UV–vis spectra, theoretical calculation, electrochemistry, electron spin resonance–spin trapping, and fluorescence quenching were investigated to determine the photochemical mechanism. The introduction of electron-pushing group and heterocycle rings can efficiently enhance the sensitization capabilities of pyrazolines to onium salts. These pyrazoline-based photoinitiating systems have promising applications in LED-induced photopolymerization.

One/two-photon sensitive sulfonium salt photoinitiators based on 1,3,5-triphenyl-2-pyrazoline

Three 1,3,5-triphenyl-2-pyrazoline-based sulfonium salt photoinitiators (PIs) were synthesized, and the relationship between their molecular structures and photophysical and photochemical properties under one/two-photon irradiation were presented. The sulfonium salts had absorption peaks from 343 nm to 372 nm that overlapped the emission of 365–425 nm light-emitting diode (LED) light sources. The energy levels of molecular orbitals were closely related to the quantum yield of photoacid generation (ΦH+) as determined by the electrochemical and absorption properties of the PIs, and the introduction of an electron-pushing group could increase ΦH+ due to the enlarged energy gap between the singlet excited states S1 and S2. Interestingly, the wavelength dependence of ΦH+ was due to the nature of the transition. The π–π* transition showed high ΦH+ values (0.56–0.66) under excitation by UV LEDs, while the π–σ* transition showed low efficiency under visible LED irradiation. Intramolecular charge transfer from the pyrazoline ring to sulfonium salts induced efficient photolysis and high ΦH+. The sulfonium salts could efficiently initiate cationic polymerization and free radical polymerization. In addition, their high two-photon absorption cross-sections (δ780 nm = 75–105 GM) reduced the threshold of two-photon microfabrication. These PIs should have potential in LED photocuring and two-photon 3D microfabrication.

Modulation of fluorescence sensing properties of quinoline-based chemosensor for Zn2+: Application in cell imaging studies

Abstract An electron pushing group has been introduced to a Schiff-base compound, 2-((2-(quinolin-2-yl)hydrazono)methyl)phenol (HQP) to get another Schiff-base molecule, namely, 4-methyl-2-((2-(quinolin-2-yl)hydrazono)methyl)phenol (HMQP), to modulate the Zn2+ sensing properties of the probe. HQP and HMQP have been synthesized by Schiff-base condensation between 2-hydrazinylquinoline and the respective aldehyde in 1:1 ratio and characterized by standard methods. Both the compounds show their Zn2+ sensing ability but to different extents. The fluorescence intensity of HQP at ~490 nm (λex = 410 nm) increases by about 4.5 folds in the presence of one equivalent of Zn2+ ion. However, the fluorescence intensity is enhanced by around 30 folds in the presence of one equivalent of Zn2+ in case of HMQP. The fluorescence peak of HMQP at 502 nm (λex = 420 nm) is shifted to 515 nm in the presence of Zn2+ whereas same type of shifting for HQP is found to be negligible. Some theoretical calculations have been performed to look into the spectral transitions of HMQP. Fluorescence intensity enhancement is explained by PET mechanism. Limit of detection (LOD) value for HMQP has been determined to be 220.6 nM, showing its good sensitivity towards zinc. The color change of HMQP under visible light is not conclusive in the presence of different metal ions, but only Zn2+ can induce green coloration to HMQP solution under UV light, thus indicating the possibility of naked eye detection by the analyte. HMQP with one equivalent of Zn2+ displays high fluorescence intensity in the pH range of 7.0–9.0 indicating its suitability towards biological applications. The probe has been utilized to live cell imaging studies using C6 cells.

Tuning the Intramolecular Charge Transfer of Alkynylpyrenes: Effect on Photophysical Properties and Its Application in Design of OFF−ON Fluorescent Thiol Probes

Green and yellow-emitting 1,6- and 1,8-bis(phenylethynyl) pyrenes (dyes 7, 8, 9, and 10) with different intramolecular charge transfer (ICT) feature were synthesized and the effect of ICT on the photophysical properties of these derivatives were studied by UV-vis absorption spectra, fluorescence emission spectra, and DFT/TDDFT calculations. For the dyes with electron-pushing group (e.g., -dimethylamino, dye 8 and dye 10), structureless and solvent polarity-sensitive fluorescence emission spectra were observed. Conversely, dye with electron-withdrawing group (e.g., -CN, dye 7) shows structured and solvent polarity-independent emission spectra. OFF-ON fluorescent thiol probes 11 and 12 with 2,4-dinitrobenzenesulfonyl protected ethynylpyrene fluorophore were designed based on DFT/TDDFT calculations, which predicts dark state (S(1)) for these thiol probes (e.g., oscillator strength f = 0.0086 for S(1)<--S(0) transition of the probe 11). This dark state is induced by the ICT effect with ethynylated pyrene fluorophore as electron donor and 2,4-dinitrobenzenesulfonyl unit as electron acceptor. Cleavage of the 2,4-dinitrobenzenesulfonyl unit by thiol releases the free fluorophore, for which the lowest-lying excited state S(1) is no longer a dark state, but an emissive state (f = 0.9776 for S(1)<--S(0) transition). These theoretical predictions on the photophysical properties of the molecular probes were fully proved by experimental results. Our results demonstrated that the fluorescence OFF-ON switching of this kind of thiol probe is due to the termination of the ICT effect (which quenches the emission, by a dark S(1) state) by cleavage of the 2,4-dinitrobenzenesulfonyl unit (as acceptor of ICT effect) with thiols, not the re-establishment of the D-pi-A feature of the fluorophore. These investigation on the pyrene derived green-emitting fluorophores and the DFT/TDDFT calculation aided probe design suggest that future application of these results may prove useful toward the rational design of fluorophores or fluorescent probes with predetermined photophysical properties.

Relationship between optical nonlinearities of H-shaped chromophores and electronegativities of their substituent groups

The novel H-shaped chromophores containing two D–π–A units locked parallel to each other are very effective chromophores, because they exhibit much higher first hyperpolarizability ( β) values than the corresponding mono-D–π–A unit reference compounds and good optical transparency at 532 nm. The first molecular hyperpolarizabilities ( β) of these H-shaped chromophores were determined via HRS method. In investigating relation between NLO properties and substituent groups on benzene rings, it is found that the β values of H-shaped chromophores vary directly with the difference between the Hammett constants ( σ) of the electron withdrawing and electron pushing groups in the D–π–A units.

A density functional study of substituted pyrazole derivatives

Computations by density functional theory (DFT) method were performed on a series of pyrazole derivatives. The heats of formation (HOFs) were predicted using B3LYP and B3P86 functionals with aug-cc-pVDZ and 6-311++G ∗∗ basis sets via designed isodesmic reactions. In the isodesmic reactions the pyrazole was chosen as a reference compound. The general trend is that the HOFs increase with increasing number of –NH 2 and –N 3 groups. And the HOFs initially decrease then increase as the number –NO 2 and –NF 2 groups increasing. The HOF of 3,5-didifluoroaminopyrazole is the smallest (134.4 kJ/mol), and the HOF of 3,4,5-triazidopyrazole is the largest (1240.6 kJ/mol) at the B3LYP/aug-cc-pVDZ level. The position of the group also affects the HOFs. Judged by the HOF values, the 4- and 3-monosubstituted isomers are the most stable monosubstituted isomers when the electron pulling and pushing groups are attached to the pyrazole ring, respectively. The 3,5-isomers are the most stable di-substituted isomers. The values of HOFs decrease as the –NO 2 groups being replaced by the –NH 2 and –NF 2, but increase dramatically as the –NO 2 groups being replaced by –N 3 with the average increment of 332.0 kJ/mol. The relative stability of the title compounds was evaluated based on the calculated HOFs and the energy gaps between the frontier orbits. Thermal stabilities were evaluated via bond dissociation energies (BDE) at the UB3LYP/aug-cc-pVDZ level. The value of C–N 3 bond dissociation energy, 524.5 kJ/mol in average, is relatively larger than other out-of-ring C–N bonds. The BDE ZPE value of C–NO 2 bond is smaller than all the other C–N bonds, which is 270.7 kJ/mol in average. These results provide basic information for the molecular design of novel high energetic density materials.

DFT studies on the molecular orbitals and related properties of [Ru(phen) 2(9,9′-2R-dpq)] 2+(R=NH 2,OH,H and F)

Theoretical studies on complexes [Ru(phen) 2 (9,9′-2R-dpq)] 2+ (phen=1,10-phenanthroline; dpq=dipyrido[3,2-d:2′,3′-f]quinoxaline; R=NH 2,OH,H and F) were carried out using the density functional theory (DFT) method. The results show that the 9,9′-site-substituents in intercalative ligand(dpq) have very important effects on the frontier molecular orbitals and related properties of the complexes. The HOMO, NHOMO, LUMO and NLUMO energies are gerenally decreased with increase of π electron-withdrawing ability of R except the LUMO energy of complex II (R–OH) being the lowest due to the rather strong intramolecular hydrogen-bonds in complex II. Both the π electron-pushing groups (NH 2,OH) and the electron-withdrawing (F) can make red shifts in the max ( λ) electronic absorption bands of the substitution derivates, in particular, OH group can lead to this effect more. Moreover, both electron-withdrawing group(F) and the electron-pushing groups (NH 2,OH) can all make the LUMOs of the corresponding derivatives predominantly populate on π-orbitals of their main ligands, whereas the LUMO of the parent complex [Ru(phen) 2(dpq)] 2+ is almost equally populated on the π-orbitals of three ligands. In addition, the coordination stabilities of the four complexes are reduced with increase of π electron-withdrawing ability of R. The above theoretical results can be applied to reasonably analyze the trend in DNA-binding of the complexes, and thus they will be useful references for designs of new DNA-probes and photoreagents.

Studies on 4,7-di-substitution effects of one ligand in [Ru(phen)3]2+ with DFT method.

Studies on the complex [Ru(phen)3]2+ (phen = 1,10-phenanthroline) and its derivatives with 4,7-di-substitution on one ligand(phen) were carried out using the DFT method at the B3LYP/LanL2DZ level of theory. The trends in the substituent effects caused by the electron-pushing group (OH) and the electron-withdrawing group (F), on the electronic structures and the related properties, for example, the energies and the components of some frontier molecular orbitals, the spectroscopy properties, and the net charge populations of some main atoms of the complexes, etc., have been investigated. The computational results show that the substituents have some interesting effects on the electronic structures and the related properties of the complexes. First, according to the analysis of components of LUMO of the complexes, the electron-withdrawing group (F) can activate the main ligand (the substituted ligand, i.e., 2R-phen) and passivate the coligands, on the contrary, the electron-pushing group (OH) can activate the coligands and passivate the main ligand in the first electronic excited states of complexes. Second, both the electron-pushing group (OH) and the electron-withdrawing group (F) can cause a red shift in the electronic ground bands. Third, the characteristics of the atomic net charge populations on the main ligand can also be analyzed in detail by means of a schematic map expressed by several series of arrowheads based on the law of polarity alternation and the idea of polarity interference. The most negative charges are populated on N1, the next most net negative charges are populated on C3 among the skeleton atoms for the three complexes, etc. The computational results can be better used to explain some experimental phenomena and trends.

Studies on electronic structures and related properties of [Ru(bpy)2(dpq)]2+ and its 9,9′-substituted derivates with DFT method

Studies on electronic structures and related properties of [Ru(bpy)2(dpq)]2+ and its 9,9′-substituted derivates are carried out using DFT method at B3LYP/LanL2DZ level. The substituent effects caused by the electron-pushing group (OH) and the electron-withdrawing groups (F) on the electronic structures and its related properties, e.g. the energies and components of some frontier molecular orbitals, the spectral properties, and the net charge populations of some main atoms of the complexes, etc. have been investigated. The computational results show that: first, the substituents have some important effects on the first excited state properties. Both electron-withdrawing group (F) and the electron-pushing group (OH) can all activate the main ligand and passivate the co-ligands in the first excited states of [Ru(bpy)2(9,9′-2R-dpq)]2+, and both can lead to a little red shift in the electronic ground bands of the substitutive derivates, respectively, in particular, the electron-pushing group (OH) can lead to that more. Secondly, the most negative charges are populated on N1 or N5, and the next most negative charges are populated on C8 among all atoms of aromatic ring skeleton. In addition, the substituents have slight effects on coordination-bond lengths of complexes. The above theoretical results should be important to further inquire into the interaction mechanism between the complexes and DNA active units from the interactions between molecular orbitals, or from the interactions between atomic charges.

Studies of substituent effects on the electronic structure and related properties of [Ru(phen)3]2+ with DFT method

Studies on the complex [Ru(phen)3]2+ (phen=1,10-phenanthroline) and its 5,6-di-substituted derivates are carried out using DFT method and B3LYP/LanL2DZ level of theory. The trends in the substituent modification effects caused by the electron-pushing group (OH) and the electron-withdrawing group (F), on the electronic structure and related properties, e.g. the components and the energies of some frontier molecular orbitals, the spectroscopy characteristics, and the net charge populations of some main atoms of the complexes, etc, have been investigated. The computational results show: First, the substituents have some interesting effects on the excited state characteristics of complexes. For the excited states of the complexes, the electron-withdrawing group (F) can activate the main ligand and passivate the co-ligands, whereas the electron-pushing group (OH) can activate different sites of the main ligand (relative to the ground state). Second, both the electron-pushing group (OH) and the electron-withdrawing group (F) can cause a red shift in the electronic ground band. Third, the following characteristics of the atomic charge populations can been seen: Along path (a) R→C5→C12→C11→N1, the atomic negative charges fully alternate with the atomic positive charges for I (ROH) and III (RF), and the extent of alternation is greater for III than for I. However, such a atomic charge alternation does not obviously exist for II (RH). Along path (b) R→C5→C12→C4→C3→C2→N1, the atomic charge populations incompletely show alternation for I and III. Changing substituent R can greatly influence charge populations on its neighboring atoms, but it only slightly influences those on farther atoms. Such complicated characteristics of the charge populations in the complexes can be explained by the law of polarity alternation and the idea of polarity interference.

Studies on 6,6‘-Disubstitution Effects of the dpq in [Ru(bpy)2(dpq)]2+with DFT Method

Theoretical studies on 6,6‘-disubstitution effects of the dpq in [Ru(bpy)2(dpq)]2+ are carried out by using DFT method at the B3LYP/LanL2DZ level. The substituent effects caused by the electron-pushing group (OH) and the electron-withdrawing group (F) on the electronic structures and the related properties, e.g., the energies and the components of some frontier molecular orbitals, the spectral properties, and the net charge populations of some main atoms of the complexes, etc., have been investigated. The computational results show that the substituents have some interesting effects on the electronic structures and related properties of the complexes. First, on the basis of the analysis of the frontier molecular orbitals, the substituents influence the first excited-state properties of the substitutive derivates. The electron-withdrawing group (F) can activate the main ligand and passivate the co-ligands in the first excited state of [Ru(bpy)2(2F-dpq)]2+, whereas the electron-pushing group (OH) does not h...

Conjugatged Oligomeric Coordinated Silver Phenylacetylide Derivatives With Ultrafast Optical Kerr Effect

ABSTRACTSilver phenylacetylide prepared by one-step process was characterized as a linear coordinated polymeric compound by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy. Its prominent third-order optical nonlinearity was detected by heterodyned ultrafast optical Kerr effect (OHD-OKE) measurement. The second-order hyperpolarizability was enhanced by 2 orders of magnitude by means of incorporation of proper electron-pushing groups. The structure status was further investigated by quantum chemical calculation and molecular modeling.

A model exact calculation of SHG coefficients in polysubstituted push—pull polyenes

We report the model exact second-harmonic generation coefficients (β exact) in polysubstituted all-trans push—pull polyenes up to ten carbon atoms. The Pariser—Parr—Pople (PPP) model with standard molecular parameters has been used in the calculations. The two-state charge-transfer model in these systems is inadequate and an exact calculation is necessary to obtain reliable SHG coefficients. We find that for a fixed geometry, β is enhanced if all the electron withdrawing substituents (pull) are successively placed from one end of the polyene chain and all the electron pushing groups (push) from the other. A twisted conformation of the polyene backbone leads to large SHG coefficients at the YAG laser frequency since the system approaches two-photon resonance. For a given chain length, the SHG coefficients increase with increase in the number of the push and pull groups.