Strong Electron-withdrawing Substituents Research Articles

Novel ZnII complexes of 2-(2-hydroxyphenyl)benzothiazoles ligands: electroluminescence and application as host materials for phosphorescent organic light-emitting diodes

A group of novel zinc complexes containing 2-hydroxyphenylbenzothiazole (BTZ) ligands were designed and synthesized, in which different substituents (OCH3, CH3, F, CF3, COOCH2CH3) were attached at the 6-position of the benzothiazole ring in the BTZ ligands. Both photoluminescence (PL) and electroluminescence (EL) behaviors of these zinc complexes were investigated. The emission colors of these zinc complexes were readily tuned from bluish-green to yellow by simply varying the substituent, with strong electron-withdrawing substituents being favorable for longer-wavelength fluorescence. Efficient EL was obtained when these zinc complexes were used as non-doped emitting layers in organic light-emitting diodes (OLEDs). Furthermore, these zinc complexes were proved to be capable of acting as triplet hosts for iridium phosphor in red phosphorescent OLEDs. A high external quantum efficiency of 17.5% was realized for the red phosphorescent OLED with the present zinc complexes as hosts and tris(2-phenylisoquinoline)iridium as doped emitter, which is greatly enhanced compared to that (12.6%) of the device with the traditional 4,4′-bis(N-carbazoly)biphenyl (CBP) as host. The present study successfully exploited novel zinc complexes as electron-transporting host materials for phosphorescent OLEDs.

Application of the dissociative electron transfer theory and its extension to the case of in-cage interactions in the electrochemical reduction of arenesulfonyl chlorides

Important aspects of the electrochemical reduction of a series of substituted arene sulfonyl chlorides are investigated. An interesting autocatalytic mechanism is encountered where the starting material is reduced both at the electrode and through homogeneous electron transfer from the resulting sulfinate anion. This is due to the homogenous electron transfer from the two-electron reduction produced anion (arene sulfinate) to the parent arene sulfonyl chloride. As a result, the reduction process and hence the generated final products depend on both the concentration of the substrate and the scan rate. A change is also observed in the reductive cleavage mechanism as a function of the substituent on the phenyl ring of the arene sulfonyl chloride. With 4-cyano and 4-nitrophenyl sulfonyl chlorides a "sticky" dissociative ET mechanism takes place where a concerted ET mechanism leads to the formation of a radical/anion cluster before decomposition. With other substituents (MeO, Me, H, Cl, and F) a "classical" dissociative ET is followed, where the ET and bond cleavage are simultaneous. The dissociative electron transfer theory, as well as its extension to the case of strong in-cage interactions between the produced fragments, along with gas phase chemical quantum calculations results helped us to rationalize both the observed change in the ET mechanism and the occurrence of the "sticky" dissociative ET mechanism. The radical/anion pair interactions have been determined both in solution as well as in the gas phase. The study also shows that despite the low magnitude of in-cage interactions in acetonitrile compared to the gas phase their existence strongly affects the dynamics of the involved reactions. It also shows that, as expected, these interactions are reinforced by the existence of strong electron-withdrawing substituents. The occurrence of an autocatalytic process and the existence of the radical/anion interaction may explain the differences previously observed in the reduction of these compounds in different media.

Solvent Effects on the Structure-Property Relationship of Some Potentially Pharmacologically Active 3-(4-Substituted Benzyl)-5-Ethyl-5-Phenyl- and 3-(4-Substituted Benzyl)-5,5-Diphenylhydantoins

Two series of 3-(4-substituted benzyl)-5-ethyl-5-phenyl- and 3-(4-substituted benzyl)-5,5-diphenylhydantoins were synthesized and their UV absorption spectra were recorded in the region 200–400 nm in selected solvents of different polarity. The effects of solvent dipolarity/polarizability and solvent/solute hydrogen bonding interactions on the spectral shifts were analyzed by means of the linear solvation energy relationship (LSER) methodology of Kamlet and Taft. The quantitative relationships between hydrogen bonding interactions and the lipophilicity and blood-brain permeation of the studied compounds were discussed. Satisfactory linear dependences were obtained for moderate electron-donating and electron-withdrawing substituents at the benzyl moiety, while the strong electron-withdrawing substituent (NO2) significantly modifies the solvation characteristics of the molecule. The paper clearly demonstrates how the solvatochromic comparison method may be applied to estimate the contributions of various modes of solvation to the pharmaceutically relevant properties of these newly synthetized hydantoin derivatives.

Molecular design of mechanofluorochromic dyes and their solid-state fluorescence properties

Heteropolycyclic donor–acceptor π-conjugated (D–π–A) fluorescent dyes with strong electron-withdrawing substituents show mechanofluorochromism (MFC): grinding of as-recrystallized dyes induces a fluorescent color change accompanying an enhanced fluorescence quantum yield, followed by a reversion to the original fluorescent color by heating or exposure to solvent vapor. Interestingly, both absorption and emission peaks are red-shifted by grinding, and the degrees of the red-shift are dependent on the electron-accepting ability of acceptor, steric sizes of the substituents, and D–π–A system. In order to clarify the mechanism of MFC observed with this new class of D–π–A fluorescent dyes, time-resolved fluorescence spectroscopy, X-ray powder diffractometry, differential scanning calorimetry and density measurements were performed before and after grinding of the solids. On the basis of experimental results and semi-empirical molecular orbital calculations (AM1 and INDO/S), we have revealed that the MFC is attributed to a reversible switching between crystalline and amorphous states with changes of dipole–dipole interaction and intermolecular π–π interaction by changes of the densities of the solids before and after grinding. This study demonstrates that the most important point for developing mechanofluorochromic dyes is to design fluorescent dye molecules of strong D–π–A characters with large dipole moments.

Intramolecular Charge Transfer in the Gas Phase: Fragmentation of Protonated Sulfonamides in Mass Spectrometry

The fragmentation of protonated molecules (MH(+)) in mass spectrometry usually results in even-electron product ions, but the MH(+) ions of sulfonamides are different as they often produce dominant radical cations of the constituent amines. For a series of benzenesulfonamides of anilines that bear various substituents, we found that the sulfonamides are preferentially protonated at the nitrogen, which is different from the carboxylic amides. Upon N-protonation, the S-N bond dissociates spontaneously to produce an intermediate [sulfonyl cation/aniline] complex. Within the ion-neutral complex, charge transfer between the two partners occurs in the gas phase to give rise to the ionized anilines. A substantial energy barrier was found to govern the reaction, which is consistent with the outer-sphere electron transfer mechanism. This energy barrier prevents the charge transfer when a strong electron-withdrawing substituent is attached to the aniline moiety. In contrast, when the aniline bears an electron-donating group, charge transfer is still more favorable than the dissociation of the intermediate ion-neutral complex, in spite of the existence of the energy barrier, and therefore dominates. A correlation was observed between the intensities of the ionized anilines and the ionization energies of these anilines.

Hydride transfer reactions via ion–neutral complex: fragmentation of protonated N‐benzylpiperidines and protonated N‐benzylpiperazines in mass spectrometry

An ion-neutral complex (INC)-mediated hydride transfer reaction was observed in the fragmentation of protonated N-benzylpiperidines and protonated N-benzylpiperazines in electrospray ionization mass spectrometry. Upon protonation at the nitrogen atom, these compounds initially dissociated to an INC consisting of [RC(6)H(4)CH(2)](+) (R = substituent) and piperidine or piperazine. Although this INC was unstable, it did exist and was supported by both experiments and density functional theory (DFT) calculations. In the subsequent fragmentation, hydride transfer from the neutral partner to the cation species competed with the direct separation. The distribution of the two corresponding product ions was found to depend on the stabilization energy of this INC, and it was also approved by the study of substituent effects. For monosubstituted N-benzylpiperidines, strong electron-donating substituents favored the formation of [RC(6)H(4)CH(2)](+), whereas strong electron-withdrawing substituents favored the competing hydride transfer reaction leading to a loss of toluene. The logarithmic values of the abundance ratios of the two ions were well correlated with the nature of the substituents, or rather, the stabilization energy of this INC.

Systematic Investigation of CD Spectra of Aryl Benzyl Sulfoxides Interpreted by Means of TDDFT Calculations

The CD spectra of 13 crystalline aryl benzyl sulfoxides 1a-m with various substituents on the two aromatic rings were recorded in solution and in the solid state. Solution CD spectra were very homogeneous along the series, consisting in most cases of a couplet-like feature in the 200-300 nm region. The red-shifted component of the couplet, corresponding to the sulfoxide-centered n-pi* transition, is always positive for (R) absolute configuration in accordance with Mislow's rule. The presence of a strong electron-withdrawing substituent on the phenyl ring (nitro or ester group) alters the shape of the CD spectrum. CD calculations using the TDDFT method were run for eight representative compounds using DFT-optimized geometries. In all cases, calculated spectra were in very good agreement with experimental ones and allowed for rationalization of the diverse spectral behaviors. It is demonstrated that TDDFT//DFT calculations represent a reliable option for assigning the absolute configuration of this class of compounds whenever crystals suitable for X-ray are not available. Solid-state CD spectra recorded with the KCl pellet technique were in some cases in agreement with those in solution. However, in other cases new and strong CD signals appeared which were interpreted as being due to intermolecular couplings in the crystals.

Electrochromic performance and ion sensitivity of a terthienyl based fluorescent polymer

A novel terthienyl based fluorescent polymer bearing strong electron-withdrawing substituents directly attached to the 3,4-positions of the central thiophene ring was synthesized by electrochemical polymerization of diethyl 2,5-di(3,4-ethylenedioxythiophen-2-yl)thiophene-3,4-dicarboxylate. The corresponding polymer was characterized by cyclic voltammetry, FT-IR and UV–vis spectroscopy. The polymer has a well-defined redox process ( E p ,1/2 = 0.74 V) and demonstrates a reversible electrochromic behavior; lilac in the neutral state and transparent sky blue in the oxidized state. Also, the polymer had low band gap ( E g = 1.82 eV) and high redox stability (retaining 94.0% of its electro-activity after 500th switch). Moreover, the sensitivity of both the monomer and its polymer towards metal cations was investigated by monitoring the change in the fluorescence intensity. Among various common ions both the monomer and its polymer were found to be selective towards Cu 2+ and Cu + ions by quenching the fluorescence efficiency with a Stern–Volmer constant ( K sv) of (1.4–1.6 × 10 3 M −1) and (1.5–1.8 × 10 2 M −1) for monomer and polymer solutions, respectively.

Halogenation of N-substituted p-quinone monoimines and p-quinone monooxime esters: X. Halogenation of N-aroyl-2,5(2,3)-dialkyl-1,4-benzoquinone monoimines and their reduction products

Introduction of a strong electron-withdrawing substituent to the nitrogen atom of 2,5(2,3)-dialkyl-1,4-benzoquinone imines makes their halogenation products, the corresponding cyclohexene derivatives, very unstable and favors halogenation of methyl groups in the quinoid ring. Bromination of 4-amino-N-aroyl-2,5-dialkyl-6-bromophenols gave 2,5-dialkyl-6-benzoyloxy-3,5-dibromocyclohex-2-ene-1,4-diones.

A short route to 3-alkynyl-4-bromo(chloro)cinnolines by Richter-type cyclization of ortho-(dodeca-1,3-diynyl)aryltriaz-1-enes

Cleavage of ortho-(dodeca-1,3-diynyl)triazenes in HCl or HBr medium and subsequent cyclization of the resulting diazonium salts is investigated. In the absence of a strong electron-withdrawing substituent, the reaction affords 3-alkynyl-4-bromo(chloro)cinnolines as the only product. A methoxycarbonyl group promotes hydrolysis of 4-halocinnolines which results in the formation of by-products: furo[3,2- c]cinnoline and cinnolinone. Substitution of bromine in 3-(alk-1-ynyl)-4-bromocinnolines is achieved with methylamine, Na 2S and ethynylbenzene affording pyrrolo[3,2- c]cinnoline, thieno[3,2- c]cinnoline and 3,4-diethynylcinnoline, respectively.

High catalytic activities in the norbornene polymerization with neutral palladium complexes containing N4-type tetradentate chelating ligands

Norbornene polymerization catalyzed by new Pd(II) complexes bearing N4-type tetradentate ligands obtained from the reaction between a 6-methyl-2-picolinic acid or picolinic acid and appropriate diamines has been studied. A class of new palladium complexes, [Pd(X 1X 2bpb)] and [Pd(X 1X 2-6-Me 2bpb)] (X 1 = Me, X 2 = Me ( 1 and 4); X 1 = H, X 2 = H ( 2 and 5); X 1 = H, X 2 = NO 2 ( 3 and 6); bpb = N,N′-( o-phenylene)bis(pyridine-2-carboxamidate); 6-Me 2bpb = N,N′-( o-phenylene)bis(6-methylpyridine-2-carbox-amidate)) were synthesized and characterized. The molecular structure of Pd complex 5 was determined by X-ray crystallography, showing distorted square planar configurations. Using modified methylaluminoxanes (MMAO) as an activator, the palladium complexes exhibited high catalytic activities for the polymerization of norbornene. The catalytic activities up to 4.0 × 10 6 g of PNBEs/mol Pd·h and M w up to 8.34 × 10 5 g/mol with PDI < 2.53 were observed. Amorphous polynorbornenes (PNBEs) were obtained with good solubility in halogenated aromatic solvents. Interestingly, the structural modification with the methyl groups of pyridyl rings and the strong electron-withdrawing substituents induced improvement in solubility, thermal stability and catalytic activity. FT-IR, 1H, and 13C NMR analyses of the polymers suggest that the catalytic polymerization occurs via vinyl addition mechanism.

Facile 1,3-diaza-Claisen Rearrangements of Tertiary Allylic Amines Bearing an Electron-Deficient Alkene

Tertiary allylic amines with an electron-deficient alkene react with isocyanates and isothiocyanates to give highly substituted ureas and thioureas arising from formal 1,3-diaza-Claisen rearrangements. Isocyanates and isothiocyanates with strong electron-withdrawing groups are more reactive. Similarly, the data suggest that a stronger electron-withdrawing substituent on the alkene favors a faster reaction, but this may be offset by sterics in the cyclic transition state.

Low Bandgap Polymers by Copolymerization of Thiophene with Benzothiadiazole

Three low bandgap copolymers of thiophene and benzothiadiazole with electron-donating and electron-withdrawing substituents, P1, P2, and P3, have been synthesized by Pd-catalyzed Stille-coupling. Electronic energy levels of the polymers are estimated by cyclic voltammetry. The polymer films show a broad absorption band in the wavelength range from 300 to 750 nm. Among the polymers, the polymer that contains the 5,6-dinitrobenzothiadiazole unit, P3, possesses the smallest bandgap of 1.55 eV calculated from its absorption band-edge at ≈800 nm. With the increase of the electron-withdrawing ability of the substituents on the benzothiadiazole unit, the energy bandgap of the polymers decreased in the order P1 > P2 > P3. The results indicate that stronger electron-withdrawing substituents on the acceptor unit can effectively decrease the bandgap of the polymers.

Theoretical studies on charge transport character and optional properties of Alq3 and its difluorinated derivatives

Electronic structures and spectroscopic properties of tris(8-hydroxyquinoline) aluminum (Alq3) and its difluorinated derivatives are studied by using ab initio and DFT methods. The B3LYP and CIS methods have been used to optimize the ground- and excited-state geometries, respectively. Absorption and emission spectra are also calculated by using the time-dependent density functional theory (TD-DFT) based on the ground- and excited-state geometries. We have found that for difluorinated Alq3 the substituents cause energy lowering in the energy levels of the lowest unoccupied molecular orbitals (LUMO) and the electron affinity ( EA) increase, which facilitates injection of the electron carrier from the metal electrode. By comparing with the non-substituted Alq3, 5,6-difluoro-substituted ( 2) and 5,7-difluoro-substituted Alq3 ( 3) undergo slight red-shift in the spectra, while the 6,7-difluoro-substituted Alq3 ( 4) shows blue-shift. In view of the electronic reorganization energy ( λ e), the complexes 2 and 4 are potential materials for electron injection and transport. In addition, the results reveal that important blue luminescence materials can be obtained by applying strong electron-withdrawing substituents at the 6,7-positions of phenoxide.

5-(1-Acetoxyvinyl)-cycloSaligenyl-2′,3′-dideoxy-2′,3′- didehydrothymidine Monophosphates, a Second Type of New, Enzymatically Activated cycloSaligenyl Pronucleotides

In our attempt to further develop the cycloSal pronucleotide concept, we report on 5-(1-acetoxyvinyl)-cycloSal-d4TMPs as a new type of enzyme-activated pronucleotides. These compounds were converted into 5-acetyl-cycloSal-d4TMPs by (carboxy)esterase cleavage inside the cells. The enzymatic reaction led to the formation of a strong electron-withdrawing substituent that strongly accelerates the chemical hydrolysis of the cycloSal nucleotide to give d4TMP. For some cycloSal-d4TMPs a separation into the diastereomers was achieved. The absolute configuration was assigned by correlation of circular dichroism spectra with similar compounds. Most of the compounds showed complete retention of antiviral activity in TK-deficient CEM/TK(-) cells, which proves the TK-bypass potential of this approach. Interestingly, (S(P))-isomers of cycloSal phosphate triesters showed better antiviral activity in HIV-2-infected thymidine-kinase deficient CEM/TK(-) cells than their (R(P))-counterparts.

Substituent effects on the tautomerism of monochalcogenocarboxylic acids XC( [dbnd]O)YH (X = H, F, NH 2, OH, CN, and CH 3; Y = S, Se, and Te): A theoretical study

Substituent effects on the conformational and energetic properties of keto–enol tautomerism of monochalcogenocarboxylic acids XC( O)YH (X = H, F, NH 2, OH, CN, and CH 3; Y = S, Se, and Te) were investigated systematically by theoretical calculations. The relative stabilities of the tautomers, associated barrier heights and transition state geometries were reported. Results from different levels of calculations show that the keto forms are thermodynamically more stable than the enol forms by over 3 kcal/mol. Generally, compared with the H-substituted systems, the relative energies between the keto and enol isomers and activation energies of the keto–enol isomerizations are not affected much by the substituents NH 2, CN, and CH 3, which could form conjugations with the C O group of the keto forms or the C Y moieties of the transition states and enol forms. However, both the relative energies and activation energies would be increased by the strong electron-withdrawing substituents OH and F. The factors that influence the kinetic and thermodynamic properties of the tautomerizations were analyzed. Analysis of the transition state geometries show all these 1,3-H shift transition states are more similar to the products than the reactants, and the F- and OH-substituted ones are later than others.

Synthesis and Characterization of Dicyclopalladated Complexes of Azobenzene Derivatives by Experimental and Computational Methods

A series of doubly cyclopalladated complexes of azobenzene and its unsymmetrical substituted derivatives, namely, {LPdCl(mu-AZB)LPdCl}, where AZB is azobenzene, 4-methylazobenzene, 4-aminoazobenzene, or 4-(dimethylamino)-4'-nitroazobenzene, while L is N,N-dimethylformamide, dimethylsulfoxide, or pyridine, have been prepared. Their structural and spectroscopic properties were determined by X-ray diffraction analysis as well as by (1)H NMR, IR, UV-vis, and fluorimetric studies. Experimental results were rationalized by quantum chemical calculations. Crystal structures of several complexes have been resolved, and for the first time, it was demonstrated that the cyclopalladation may take place at the azobenzene aromatic ring having the strong electron-withdrawing substituent at the para position. In all cases, the metalated carbon and N,N-dimethylformamide or dimethylsulfoxide ligands are mutually trans, whereas the pyridine ligands are in the cis arrangement. cis/trans isomerism in the isolated compounds is explained by comparing the calculated energies of isomeric structures. All of the complexes absorb strongly in the visible region, and according to time-dependent density functional theory calculations, most of the absorptions can be attributed to intraligand pi --> pi* or metal-to-ligand charge-transfer transitions. The fluorescence emission was observed for the complexes with 4-aminoazobenzene or 4-(dimethylamino)-4'-nitroazobenzene. The aromaticity of palladacycles is evaluated by several aromaticity indices and related to relevant experimental findings.

Palladium(0)-Catalyzed Cis−Trans Alkene Isomerizations

The cis−trans isomerization of some olefins bearing strong electron-withdrawing substituents (dmma and E-tse) is catalyzed by their mere coordination to a suitable Pd(0) fragment without any external reagents. The choice of spectator ligand is fundamental to the efficiency of the process, and the phosphanyl-quinolines dppq and dppq-Me have proven to be particularly effective. The mechanism of the rearrangement is discussed, and the presence of electron-withdrawing and conjugated substituents on the olefins appears to be an essential prerequisite for the isomerization.

Preparation of cobalt-containing bulky monodentate phosphines with electron-withdrawing/donating substituents on bridged arylethynyl and their applications in Suzuki coupling reactions

Several cobalt-containing bulky monodentate phosphines (μ-PPh 2CH 2PPh 2)Co 2(CO) 4(μ,η-( t Bu) 2PC C(C 6H 4R)) ( 4a: R=H; 4b: R= p-F; 4cp: R= p-CF 3; 4cm: R= m-CF 3; 4d: R= p-OMe) were prepared from the reactions of ( t Bu) 2PC C(R–C 6H 4) ( 2a: R=H; 2b: R= p-F; 2cp: R= p-CF 3; 2cm: R= m-CF 3; 2d: R= p-OMe) with Co 2(CO) 6(μ-PPh 2CH 2PPh 2) 3. Further reactions of 4a, 4b, 4cp, 4cm, and 4d with Pd(OAc) 2 yielded unique palladium complexes (μ-PPh 2CH 2PPh 2)Co 2(CO) 4(μ,η-( t Bu) 2PC C(C 6H 3R)-κC 1)Pd(μ-OAc) ( 5a: R=H; 5b: R= p-F; 5cp: R= p-CF 3; 5cm: R= m-CF 3; 5d: R= p-OMe, respectively). The strong electron-withdrawing substituents, –F and –CF 3, assist the ortho-metalation process during the formation of 5b, 5cp, and 5cm. The more positively charged palladium center in 5b (or 5cp, 5cm) enhances the probability for PhB(OH) 3 − to attack the metal center and the rate of reduction thereafter. DFT studies on the charges of these palladium centers support this assumption. The enhancement of the reaction rates of the Suzuki–Miyaura cross-coupling reactions using 5b, 5cp, and 5cm is thereby attributed to this effect.

Reduction of radiation-induced conductivity in poly(ethylene terephthalate): Effect of dopant structure

This paper focuses on developing radiation tolerant polymeric films by incorporating small molecule dopants into the material formulation. The radiation tolerance is imparted to the polymer by dopants capturing the mobile electrons generated upon radiation exposure. A wide range of conjugated small molecules was utilized for the reduction of radiation-induced conductivity (RIC) in semi-crystalline poly(ethylene terephthalate). Using these results, the process of electron and hole trapping by π-conjugated molecules may be generalized to many organic systems. All conjugated ring cores examined, when substituted with an appropriate electron-withdrawing species, led to excellent RIC reduction. The nitro group is the strongest electron-withdrawing substituent, and therefore leads to the lowest RIC, but other substituents also reduce RIC, and the interactions may be quantified by the Hammett parameter. It was also found that the dopant must be present in a certain range of concentrations, between 10 and 200mol/m3 (6×1018 and 1×1020molecules/cm3), for proper trapping of the photo-induced electron-hole pairs. This understanding of the interactions between conjugated dopants, photons, and electrons will benefit other applications, such as controlled conductivity in coatings and organic electronics.