Electron-acceptor Substituents Research Articles

Tuning the antiaromatic character and charge transport of pentalene-based antiaromatic compounds by substitution

Substituting electron-donating and electron-accepting substituents on antiaromatic pentalene cores can significantly tune the optical absorption, energy levels, antiaromaticity, and transistor switch-on behavior.

Synthesis of substituted (N,C) and (N,C,C) Au(III) complexes: the influence of sterics and electronics on cyclometalation reactions.

Cyclometalated Au(III) complexes are of interest due to their catalytic, medicinal, and photophysical properties. Herein, we describe the synthesis of derivatives of the type (N,C)Au(OAcF)2 (OAcF = trifluoroacetate) and (N,C,C)AuOAcF by a cyclometalation route, where (N,C) and (N,C,C) are chelating 2-arylpyridine ligands. The scope of the synthesis is explored by substituting the 2-arylpyridine core with electron donor or acceptor substituents at one or both rings. Notably, a variety of functionalized Au(III) complexes can be obtained in one step from the corresponding ligand and Au(OAc)3, eliminating the need for organomercury intermediates, which is commonly reported for similar syntheses. The influence of substituents in the ligand backbone on the resulting complexes was assessed using DFT calculations, 15N NMR spectroscopy and single-crystal X-ray diffraction analysis. A correlation between the electronic properties of the (N,C) ligands and their ability to undergo cyclometalation was found from experimental studies combined with natural charge analysis, suggesting the cyclometalation at Au(III) to take place via an electrophilic aromatic substitution-type mechanism. The formation of Au(III) pincer complexes from tridentate (N,C,C) ligands was investigated by synthesis and DFT calculations, in order to assess the feasibility of C(sp3)-H bond activation as a synthetic pathway to (N,C,C) cyclometalated Au(III) complexes. It was found that C(sp3)-H bond activation is feasible for ligands containing different alkyl groups (isopropyl and ethyl), although the C-H activation is less energetically favored compared to a ligand containing tert-butyl groups.

Tuning optoelectronic properties of carbazole through substituent effect: TD-DFT investigation

π-conjugated organic materials such as carbazoles have attracted much attention because of their applications in electronic devices such as OLED’s, solar cells and sensors. Due to their optoelectronic properties, high charge carrier mobility, suitable band gaps and orbital energies, carbazoles have received immense attention to serve as a potent photosensitizer for obtaining high performance in DSSC. The present QSPR study of substituted carbazole and carbazole anion with electron donor (NH2) and acceptor (NO2) substituents at various positions is done by the density functional theory (DFT) calculations at B3LYP/6-31G(d,p) level. The longest maximum absorption wavelengths (λmax) in vacuum as well as in polar (acetonitrile) and non-polar (benzene) solvents are studied by using the time-dependent density functional theory (TD-DFT). For the singly substitution of NH2 (at C3) and NO2 (at C4) groups to the carbazole and its anion, the λmax values are observed to be shifted to the longest wavelength from 290.56 nm to 325.73 and 375.77 nm than the other respective positions. Whereas, the respective λmax values for carbazole-anion are found at 384.28 and 590.53 nm respectively. For disubstituted carbazole with NH2 and NO2 at C3 and C4 positions, the longest λmax shifts to 477.15 nm and for carbazole-anion the longest λmax obtained at C4 (NH2) and C5 (NO2) positions with λmax value 694.61 nm. Similar observations are also found in acetonitrile and benzene solvents. Further, it is observed that as the λmax increases, the HOMO-LUMO energy gap (EHL) value decreases accordingly which can be attributed to intramolecular charge transfer from NH2 to NO2 groups. A very good correlation of λmaxwith EHL is observed with correlation coefficient between 0.91 to 0.95 in vacuum as well as in acetonitrile and benzene solvents. Present study may provide valuable guidelines for the choice of suitable substituent to design carbazole moieties as efficient photosensitizer in DSSC. Copyright © 2017 VBRI Press.

Quinone Reduction by Organo-Osmium Half-Sandwich Transfer Hydrogenation Catalysts

Organo-osmium(II) 16-electron complexes [OsII(η6-arene)(R-PhDPEN)] (where η6-arene = para-cymene or biphenyl) can catalyze the reduction of prochiral ketones to optically pure alcohols in the presence of a hydride source. Such complexes can achieve the conversion of pyruvate to unnatural d-lactate in cancer cells. To improve the catalytic performance of these osmium complexes, we have introduced electron-donor and electron-acceptor substituents (R) into the para (R1) or meta (R2) positions of the chiral R-phenyl-sulfonyl-diphenylethylenediamine (R-PhDPEN) ligands and explored the reduction of quinones, potential biological substrates, which play a major role in cellular electron transfer chains. We show that the series of [OsII(η6-arene)(R-PhDPEN)] derivatives exhibit high turnover frequencies, enantioselectivities (>92%), and conversions (>93%) for the asymmetric transfer hydrogenation (ATH) of acetophenone-derived substrates and reduce duroquinone and menadione to their di-alcohol derivatives. Modeling of the catalysis using density functional theory (DFT) calculations suggests a mechanism involving formic acid deprotonation assisted by the catalyst amine groups, phenyl-duroquinone stacking, hydride transfer to OsII, possible CO2 coordination, and tilting of the η6-arene ring, followed by hydride transfer to the quinone. These findings not only reveal subtle differences between Ru(II) and Os(II) catalysts, but also introduce potential biological applications.

Synthesis of Fe3O4@L-proline@SO3H as a novel and reusable acidic magnetic nanocatalyst and its application for the synthesis of N-substituted pyrroles at room temperature under ultrasonic irradiation and without solvent

N-Substituted pyrroles have been prepared in high isolated yields (65–90%) by the reaction of hexane-2,5-dione with amines or diamines in the presence of Fe3O4@L-proline@SO3H at ambient temperature under ultrasonic irradiation and without solvent. The experimental procedure involves simple operations, and the products are readily separated by external magnet. The same reaction of hexane-2,5-dione with amines containing electron-acceptor substituents, such as 4-nitroaniline, resulted in fair yields of pyrrole derivatives.

Nonlinear optical properties of a new panchromatic series of water-soluble bicationic push-pull fluorophores

Six water-soluble bications, characterized by a symmetric A+-π-D-π-A+ structure, were synthesized and comprehensively studied for their absorption and fluorescence properties, and their nonlinear optical response. Generally, the photobehavior exhibited by the six systems was found to be dependent on the molecular dimensionality and conjugation, unveiling the role of different electron-donor (dimethoxy-phenylene vs bithiophene) and electron-acceptor (methyl-pyridinium, methyl-quinolinum, and pyrimidin-pyridinium) substituents bound in different positions (ortho/para). Steady-state measurements revealed this series as a panchromatic family of dyes with noticeable absorption and emission abilities, covering the entire visible spectrum - from the near-ultraviolet to the near-infrared spectral region. Femtosecond-resolved spectroscopy disclosed the presence of ultrafast intramolecular charge transfer processes, as also predicted by TD-DFT calculations, from the central electron-rich moiety of the molecule towards the two lateral positively charged units. Further and for the first time, a solvatochromic method was successfully applied to this kind of quadrupolar double-charged compounds in order to estimate their static and dynamic second-order hyperpolarizability coefficients. Thanks to their appreciable fluorescence capability, significant Two-Photon Absorption cross-sections were also measured for all six dyes by means of the Two-Photon Excited Fluorescence technique. These findings, together with their good water solubility, designate these eye-catching fluorophores as potential candidates for bioimaging applications.

BODIPY-DERIVATIVES AS PHOTOSENSITIZERS IN PHOTODYNAMIC THERAPY: THE RELATIONSHIP BETWEEN ELECTRONIC PROPERTIES AND LIGHT ABSORPTION EFFICIENCY

In this study, the structural and electronic properties of BODIPY and its twelve derivates, including meso-amino, meso-alkoxy, meso-phenyl, meso-alkyl and other compounds, were investigated using an accurate and broadly parametrized self-consistent tight-binding quantum chemical method named GFN2-xTB. The influence of the substituent groups on the geometrical parameters, the HOMOLUMO gap, and electronic properties of BODIPYs was analyzed. The obtained results have shown that the presence of substituents at the R2 position did not significantly change the structural parameters of BODIPY. The electron-acceptor or electron-donor substituents in the meso-position have different effects on the electronic parameters of the molecules. A correlation between the global electrophilicity index and the HOMO-LUMO gap was found. Among the studied systems, the BODIPY-derivative with a -NO2 group at the meso-position was evaluated as the most promising candidate for photosensitizers in PDT.

Substituent Effect on the Photophysics and ESIPT Mechanism of N,N'-Bis(salicylidene)-p-phenylenediamine: A DFT/TD-DFT Analysis.

Excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) processes are widely exploited in the designing of organic materials for multifarious applications. This work explores the aftereffects of combining both ESIPT and ICT events in a single molecule, namely, N,N'-bis(salicylidene)-p-phenylenediamine (BSP) exploiting DFT and TD-DFT formalisms. The PBE0 functional employed in the present study is found to yield results with better accuracy for excited-state calculations. The results reveal that introduction of electron donor (-NH2) and electron acceptor (-NO2) substituents on BSP produces a strikingly red-shifted emission with respect to the corresponding emission from the unsubstituted analogue in polar solvents. This red-shifted emission originated due to the coupled effect of ESIPT and planar-ICT (PICT) processes from the coplanar geometry adopted by the substituted molecule (s-BSP). Based on the computed potential energy curves, the ground-state intramolecular proton transfer (GSIPT) was found to take place more favorably in s-BSP than in BSP under all solvent conditions. In the case of ESIPT, the barrier and relative energies of the phototautomers of s-BSP were slightly higher than BSP, which shows that simultaneous substitution of -NH2 and -NO2 groups causes slight perturbation to the ESIPT process. Overall, the computed results show that simultaneous substitution of suitable electron donor and acceptor substituents provides profitable changes in the photophysical properties of ESIPT molecules like BSP. These molecular-level insights will pave way for designing better materials for diverse applications.

Optical properties improvement for the [14]annulene using different donor-acceptor substituents

In this research, the annulene molecule was selected as pi electron skeleton and the electron donor and acceptor groups were attached to it and their energetic, electronic properties, and optical properties were calculated. It was shown that these molecules have exothermic formation reaction which is the evidence of their stability. It was demonstrated that these substituents lowers the Eg of annulene and significantly increased its dipole moment especially for CN-AN-OLi and CF3-AN-OLi molecules. The UV-visible absorption of annulene drastically increased by the existence of these donor and acceptor groups. Improved visible absorption achieved for CN-AN-NHLi which the annulene molecule sandwiched between two -NHLi as highly electron donor and other –CN electron acceptor substituents.

Selective Extraction of Gold with Polymeric Inclusion Membranes Based on Salen Ligands with Electron- Accepting Substituents

Salen-type ligands with electron-accepting substituents on the aromatic ring were synthesized. These ligands were used as extractant agents in polymeric inclusion membranes for the extraction of metal ions from aqueous solutions. Measurements of the membrane potential indicated selectivity of the membrane, which was confirmed by the quantification of the extracted metal. Ten types of metal ions were studied to establish the selectivity of each membrane. Salen-type ligands had higher selectivity to gold than to other metals and allowed for the extraction of acceptable percentages of this metal. The results showed that the percentage of extracted gold depends on the type of substituent present in the ligand`s aromatic ring, the pH of the working solution and the area of the membrane. There was no effect of membrane thickness on the metal extracted. The time required to reach a certain extraction percentage decreased considerably as the membrane area increased.

Equilibrium molecular structure of orotic acid from gas-phase electron diffraction data and quantum-chemical calculations

The accurate equilibrium structures of planar anti and syn orotic acid conformers with different positions of the carboxyl group with respect to the C=C bond were determined from gas-phase electron diffraction data using the dynamic model and taking into account anharmonic vibrational corrections calculated with ab initio force field. The high accuracy of coupled-cluster computations was exploited in the analysis of structural effects due to electron-acceptor substituents.

Directing excited state dynamics via chemical substitution: A systematic study of π-donors and π-acceptors at a carbon-carbon double bond.

Functional group substituents are a ubiquitous tool in ground-state organic chemistry often employed to fine-tune chemical properties and obtain desired chemical reaction outcomes. Their effect on photoexcited electronic states, however, remains poorly understood. To help build an intuition for these effects, we have studied ethylene, substituted with electron acceptor (cyano) and/or electron donor (methoxy) substituents, both theoretically and experimentally: using ab initio quantum molecular dynamics and time-resolved photoelectron spectroscopy. Our results show the consistent trend that photo-induced ethylenic dynamics is primarily localized to the carbon with the greater electron density. For doubly substituted ethylenes, the trend is additive when both substituents are located on opposite carbons, whereas the methoxy group (in concert with steric effects) dominates when both substituents are located on a single carbon atom. These results point to the development of rules for structure-dynamics correlations; in this case, a novel mechanistic ultrafast photochemistry for conjugated carbon chains employing long-established chemical concepts.

Nitrofluorene derivatives trapped within MWCNTs for electrocatalysis of NADH: Substituent effects on π-π stacking interaction strength

We describe nanostructured electrode platforms composed of multiwalled carbon nanotube/glassy carbon electrodes (MWCNT/GCEs) with entrapped nitroaromatic compounds such as 2-nitrofluorene (2-NF), 2-nitro-9-fluorenone (2-NFN), 2,7-dinitrofluorene (2,7-NF) and 2,7-dinitro-9-fluorenone (2,7-NFN). All of the above nitroaromatic compounds were tested as precursors for the production of redox mediator couples useful for NADH electrocatalysis.In this communication, we reveal the effect of the substituents on both the π-π stacking interaction strength and the electrocatalysis of NADH.The results show that the nitro group plays a triple role: it acts as a precursor of the mediator redox couple for NADH electrocatalysis; it increases the π-π stacking interaction with the electrode; and it acts as an electron acceptor substituent that promotes the electrocatalysis of NADH.

Study of glycoluril and its derivatives by 1H and 13C NMR spectroscopy

Bicyclic bisureas, especially 2,4,6,8-tetraazabicyclo[3.3.0.]octan-3,7-dione (glycoluril), have a special place in chemistry of heterocyclic compounds. The carbamide fragment in glycoluril structure determines the properties of the molecule, which are due to the presence of two reaction centers (NH-groups and C=O-groups) in the molecule. In this work, we analyzed the proton and carbon chemical shifts of glycoluril and its derivatives (86 compounds) in the NMR spectra to reveal the effect of the donor-acceptor substituents on the changes in the electron density in the bicyclic framework from the position of symmetry and asymmetry. A general analysis of the 1H and 13C NMR spectra of glycolurils makes it possible to accurately determine the spatial configurations of molecular symmetry, in the presence of which (σ1 and / or σ2) the enantiotopic hydrogen and carbon atoms of the bicyclic framework are manifested by equivalent signals. Also, according to the 1H and 13C chemical shifts in the NMR spectra, glycolurils with electron-acceptor substituents can be clearly distinguished by the shielding of carbon atoms of C=O-groups, and with electron-donating substituents by the deshielding of CH-CH-carbons, due to the rearrangement of electron density and the occurrence of local paramagnetic contributions owing to anisotropy.

Pyrido triazin-nucleus synthesis and theoretical studies: 2,3,6-trioxo-8-aryl-1,3,4,6-tetrahydro-]2H[pyrido]1,2-b] [1,2,4[triazin-7,9-dicarbonitryl derivatives

An efficient method for the synthesis of potentially biologically [2H]-pyrido [1,2-b]1,2,4-triazines derivatives is described via reaction of 1,6-diamino-2-oxo-4-phenyl-1,2-dihydropyridine-3,5-dicarbonitrile derivatives with oxalyl chloride in DMF in the presence of pyridine as a base. The intramolecular hydrogen bonding (IMHB) interactions have been investigated at M06-2X/6-311++G(d,p) level of theory. The eight compounds (5a-h) characterized by geometries and energies. The Natural bond orbital (NBO) and quantum theory of atoms in molecules (QTAIM) were performed to explore the nature of the hydrogen-bonding interactions in these compounds. The IMHB formed between the CO and NH groups. The theoretical calculations showed that the IMHB strength increases in the presence of electron-donor substituents. An opposite behavior was observed for electron-acceptor substituents.

Enhancement of Terbium(III)-Centered Luminescence by Tuning the Triplet Energy Level of Substituted Pyridylamino-4-R-Phenoxo Tripodal Ligands.

A series of luminescent phenoxo-bridged dinuclear TbIII complexes with tripodal ligands, 2,2'-[[(2-pyridinylmethyl)imino]di(methylene)]-bis(4-R-phenol), where R = CH3 (LCH3) (I), Cl (LCl) (II), CH3O (LCH3O) (III), COOCH3 (LCOOCH3) (IV), were prepared to probe the effect of para-substitution on the phenol ring of the ligand on the TbIII luminescence. For these TbIII complexes a complete suppression of the ligand-centered fluorescence is observed, which demonstrates an efficient ligand-to-metal energy transfer. Complex IV was found to be the one that shows the greater intensity of the emission at room temperature. The obtained quantum yields follow the trend IV > II ≫ I > III. The quantum yield for II and IV is approximately five times greater than those obtained for I and III, indicating that the LCl and LCOOCH3 are better sensitizers of the TbIII ions. These results were rationalized in terms of the variation of the energy gap between the triplet level (T1) of the ligand and the emissive 5D4 level of TbIII, due to the electron-acceptor or electron-donor properties of the substituents. The τav values are in the millisecond range for all the studied complexes and resulted independent of temperature. The Commission International d'Eclairage coordinates (CIE) for all complexes are in the green color region, being insensitive to the variation of temperature. Moreover, the color purity (CP) is ca. 90% for all complexes, being ca. 100% for IV. Thus, the introduction of electron-acceptor substituents on the ligand permitted us to improve the luminescent properties of the TbIII complexes.

Novel Heteroleptic Ruthenium(II) Complexes with 2,2'- Bipyridines Containing a Series of Electron-Donor and Electron-Acceptor Substituents in 4,4'-Positions: Syntheses, Characterization, and Application as Sensitizers for ZnO Nanowire-Based Solar Cells.

A novel series of complexes of the formula [Ru(4,4′-X2-bpy)2(Mebpy-CN)](PF6)2 (X = −CH3, −OCH3, −N(CH3)2; Mebpy-CN = 4-methyl-2,2′-bipyridine-4′-carbonitrile) have been synthesized and characterized by spectroscopic, electrochemical, and photophysical techniques. Inclusion of the electron-withdrawing substituent −CN at one bpy ligand and different electron-donor groups −X at the 4,4′-positions of the other two bpy ligands produce a fine tuning of physicochemical properties. Redox potentials, electronic absorption maxima, and emission maxima correlate well with Hammett’s σp parameters of X. Quantum mechanical calculations are consistent with experimental data. All the complexes can be anchored through the nitrile moiety of Mebpy-CN over ZnO nanowires in dye-sensitized solar cells that exhibit an improvement of light to electrical energy conversion efficiency as the electronic asymmetry increases in the series.

Donor–acceptor units modulate the electronic and photoluminescence characteristics of thiophene oligomers

Organic small-molecule luminescent materials have high fluorescence quantum efficiency, are easy to purify, and show advantages related to brightness. However, the low mobility of hole carriers hinders their practical applications. To obtain luminescent materials with high fluorescence quantum efficiency, we investigated organic small-molecule thiophene oligomers with electron-donating or electron-accepting substituents at the terminal groups using the density functional theory. Compared with the electron-donating groups (–NH2, –CH3, –C2H5, and –C3H7), the electron-withdrawing groups (–NO2 and –COOH) resulted in unusual electronic and photoluminescent properties. The ultraviolet spectra of the thiophene oligomer derivatives could be modulated in the range of 200–500 nm, making them suitable for application in organic light-emitting devices. In particular, the group with the strongest electron-withdrawing effect (–NO2) resulted in blue light emission in the wavelength range of 400–480 nm, suitable for application in blue chips. Finally, the calculation results show that the donor–acceptor-modified oligomers are worthy of synthesis due to their relatively high electron mobilities.

Long-range photoinduced charge separation in tröger bases D/A dyads

Tröger´s base (and its derivatives) are compounds comprised of two aromatic (or polyaromatic) rings bridged by a diazocine aliphatic cycle. We report herein the photophysical properties of two series of novel Tröger´s bases (TB) asymmetrically substituted by electron donor (D) and electron acceptor (A) substituents. In TB series 3, a carbonitrile group (ACN) lies at the position 2 of the heterocycle, while position 8 is occupied by a series of D with increasing reductant capacity: H (3a), CH3 (3b), OCH3 (3c) or N(CH3)2 (3d). A novel TB series (5a-5d) which comprise the same D, but a 2,2-dicyanovinyl group (A = CHC(CN)2) as electron acceptor, was synthesized and fully characterized. TB absorption (νAmax) and emission energies (νFmax), fluorescence quantum yields (ΦF) and emission lifetimes (τF) were determined in a series of aprotic solvents covering a wide range of medium polarity (ε∼2-38). νFmax, ΦF and τF largely depend on the polarity of the medium and the nature of D/A pair. From the solvatochromic study on νFmax, it is concluded that upon excitation TB´s develop large degrees of charge separation (CT). Photophysically, 3a-3c resembles 4-(N,N-dimethylamino)benzonitrile derivatives showing internal CT state dipole moments (μ1*) of ∼ 15–17 D. For 3d and the entire series 5, CT occurs throughout the diazocine ring giving rise to giant μ1* (> 25 D). This is indeed an unusual result, because it strongly suggests that the aliphatic diazocine ring can couple the D/A redox centers as a π bridge would do.

In Ukrainian

Search for the “structure-reactivity” correlations of the thermally stimulated reactions of cinnamic acids on the surface of catalysts is important for the development of pyrolytic conversion methods of lignocellulosic biomass components into products with high added value, in particular, into styrene. Therefore, in this work, the kinetics of pyrolysis of the reaction series of para-substituted derivatives of trans-cinnamic acid (Н, -СН 3 , -С(СН 3 ) 3 , -ОСН 3 , -F) on the surface of nanosized silica by the method of thermoprogrammed desorption mass spectrometry (TPD MS) was investigated. The products of pyrolytic reactions on the surface are identified ‒ the corresponding, para-substituted vinylbenzenes, phenylacetylenes, and phenylketenes. The kinetic parameters of the decarboxylation, ketenization, and decarbonylation reactions were calculated. The correlations "structure-reactivity" between the kinetic parameters (activation energy) and thermodynamic parameters (Hammett substituents constants) have been obtained, which indicate that electron-donor substituents reduce the activation energy of these three reactions while the electron-acceptor substituents increase it. That is, in the transitional state of the rate-limiting step, a decrease in the electron density is observed at the reaction center. The calculated values of the reaction constants ρ show that the studied reactions dependent on the sensitivity to the effect of the substituents are placed in a sequence: decarbonylation> decarboxylation> ketenization. The formation of phenylacetylenes is the most sensitive to structural changes in the molecule and proceeds through the most polar transition state.