Strong Donor Substituents Research Articles

Transformation of fluorinated 1,2-phenylenediamines in polyphosphoric acid medium with or without the benzimidazole 2-carboxylic acid: synthesis of fluorinated 2,2′-bibenzimidazoles and phenazine-2,3-diamines

A series of asymmetrically fluorinated 2,2′-bibenzimidazoles were obtained by reaction of benzimidazole-2-carboxylic acid with fluorinated 1,2-phenylenediamines in polyphosphoric acid at elevated temperature. The tautomers of fluorinated 2,2′-bibenzimidazoles with a predominance of isomers containing F atoms in distant position relative to the NH group of the benzimidazole fragment are detected in DMSO‑d6 using NMR spectroscopy. The ratio of tautomers depends on the number and arrangement of F atoms in the initial fluorinated 1,2-phenylenediamines. Under similar conditions fluorinated 1,2-phenylenediamines in the presence of benzimidazole-2-carboxylic acid, but without its participation as well as without it, give fluorinated phenazine-2,3-diamines. The direction of transformation of fluorinated 1,2-phenylenediamines depends on the number and position of F atoms. Under similar conditions phenazine-2,3-diamine is not formed from non-fluorinated 1,2-phenylenediamine, but it is obtained with the presence of fluorinated 1,2-phenylenediamines. When fluorinated 1,2-phenylenediamines contain F atoms simultaneously at the positions 4 and 5, the hydrodefluorination of initial fluorinated 1,2-phenylenediamines occurs, resulting in less fluorinated 2,2′-bibenzimidazoles and phenazine-2,3-diamines. Hydrodefluorination of fluorinated 1,2-phenylenediamines in a polyphosphoric acid at elevated temperatures is the first example of the hydrodefluorination of fluorinated aromatic compounds containing strong donor substituents, such as amino groups. Polyphosphoric acid is shown to be a suitable medium for redox processes.

Tuning of environment-sensitive 3-hydroxychromone fluorophores based on strong donor substituents in positions 2 or 7

We have synthesized a series of environmentally sensitive fluorophores based on 3-hydroxychromone with systematically varied aromatic groups at positions 2 and 7. The two families of 2,7-diaryl-3-hydroxychromones were synthesized using Suzuki coupling from the corresponding 2-aryl-7-bromo-3-hydroxychromones. These chromophores exhibit a dual fluorescent emission and a strong response to changes in environmental polarity in organic solvents. Both experimental determination and calculations of dipole moment changes show stronger charge transfer in compounds with an electron donor group at position 7 compared to position 2. Their emission is almost totally quenched in the aqueous medium, while the increase in quantum yield upon binding to biological molecules serves as a useful characteristic. Particularly, compounds with strong donors in position 7 are sensitive to diverse lipid environments, whereas those with strong donors in position 2 have different emission profiles for distinct protein interactions. The complex behavior of the probes was evaluated using spectral phasor analysis. Altogether, these novel families of probes may help monitor several biological processes in vitro and in cell cultures.

Self-aggregation of stilbazolium ion pairs in liquid chloroform. A molecular dynamics study

Molecular aggregation plays an important role on optical phenomena, and optimizing the latter requires understanding the aggregation behavior as a function of the molecular structures. The formation of molecular aggregates composed of stilbazolium chromophores and small to medium-size anions is studied by carrying out classical molecular dynamics (MD) simulations, shedding light on the dynamics of spontaneous self-aggregation of anion-cation (AC) pairs and estimating their equilibrium aggregation constants. First, analyses based on atom-atom distances and on contact mapping (Voronoi tessellation) highlight that the interactions occur at the level of the methylpyridinium groups of the cations, which enabled monitoring the clustering dynamics. Then, simulations show that clusters containing up to 5 AC pairs dominate the populations. Among the AC pairs, two of them (where the stilbazolium bears a strong donor substituent and when the anion is SCN- or pTS-) aggregate into small clusters even at high concentration, corroborating related experimental results [ChemPhysChem. 11 (2010) 495–507]. Still, these MD simulations do not reveal strict relationships between the aggregate size and the nature of the anion. Nevertheless, AC pairs bearing iodide anions require longer MD simulation times to reach an asymptotic equilibrium tendency of the average number of clusters. Finally, the aggregates are shown to adopt a preferential Λ-shape with contributions from head-to-head and stacked forms. Complementarily, the analyses of the isoperimetric quotient reinforce the AC pairs are not forming spherical aggregates even for clusters including more than 10 AC pairs. So, when the stilbazolium bears a strong donor substituent and when the anion is pTS-, stacked aggregates are formed, at both low and high concentrations, resulting in excellent agreement with the measured aggregation equilibrium constant.

1,2-Diborolanes with strong donor substituents: Synthesis and high antimicrobial activity

1,2-diborolanes with strong and without strong donor substituents have been described, and are also referred to as 1,2-diboracyclopentane. The 1,2-diaryl/alkyl-amino-1,2-diboracyclopentanes 2, 3, and 4 were obtained in good yield after the reaction of 1,2-dichloro-1,2-diboracyclopentane 1 with ArNHLi and Me3Si-NR2. The structures of these new derivatives were characterized by nuclear magnetic resonance spectroscopy. The molecular structures of 2b, 2c, 2e, 4, and 5f were also determined by single-crystal X-ray diffraction. The newly synthesized 1,2-borolanes are stable in air and showed particularly high activity against some Gram-positive bacteria.

Rigidified Push-Pull Dyes: Using Chromophore Size, Donor, and Acceptor Units to Tune the Ground State between Neutral and the Cyanine Limit.

A series of rigidified dyes with either dodecyloxy or piperidyl donors in combination with either malononitrile or barbiturate acceptors were synthesized. To analyze the influence of varying the donor (piperidyl vs. dodecyloxy), chromophore (mono-, bis-, or tricyclic system), and acceptor moiety (methyl barbiturate vs. malononitrile) on the ground-state polarity, a combination of several methods was employed: UV/Vis absorption and emission, quantum chemical computations, 1 H as well as 13 C NMR spectroscopy, and cyclic voltammetry measurements. Depending on the acceptor and donor moieties, the ground state was shifted from a neutral form to the cyanine limit and further to a zwitterionic structure. When the dye had strong piperidyl donor and barbiturate acceptor substituents, the ground state was close to the cyanine limit, which resulted in strong cyanine-like absorption properties with a dominant 0-0 transition. When the dye combined a weak dodecyloxy donor with a malononitrile or barbiturate acceptor, the neutral resonance form significantly contributed to the ground state, leading to weaker but broader absorption spectra featuring transitions to higher vibronic states.

Relationships between geometrical and electronic structures and optical properties of 1,8-naphthosultam substituents and derivatives: TDDFT study

Geometrical and electronic structure and optical properties of several substituents and derivatives of 1,8-naphthosultam are studied by quantum-chemical DFT and TDDFT. It is found that the substituents –NO2, –CF3, and –N(CH3)2 affect the geometrical and electronic structure the most. It is shown that the Stokes shift is greatest (≈260 nm) for compounds with the strong donor substituent –N(CH3)2, while strong acceptor substituents provide a quite small Stokes shift. The dependence of the Stokes shift on the difference in energies of the frontier orbitals of the ground and excited states of molecules is found. Of the 1,8-naphthosultam substituents considered, compounds with –N(CH3)2 substituent, which emit in the biological window region, can be advised for use in optical bioimaging. The results can be used as a basis for the development and creation of new functional materials and biologically active compounds.

Hole Transfer Processes in meta- and para-Conjugated Mixed Valence Compounds: Unforeseen Effects of Bridge Substituents and Solvent Dynamics.

To address the question whether donor substituents can be utilized to accelerate the hole transfer (HT) between redox sites attached in para- or in meta-positions to a central benzene bridge, we investigated three series of mixed valence compounds based on triarylamine redox centers that are connected to a benzene bridge via alkyne spacers at para- and meta-positions. The electron density at the bridge was tuned by substituents with different electron donating or accepting character. By analyzing optical spectra and by DFT computations we show that the HT properties are independent of bridge substituents for one of the meta-series, while donor substituents can strongly decrease the intrinsic barrier in the case of the para-series. In stark contrast, temperature-dependent ESR measurements demonstrate a dramatic increase of both the apparent barrier and the rate of HT for strong donor substituents in the para-cases. This is caused by an unprecedented substituent-dependent change of the HT mechanism from that described by transition state theory to a regime controlled by solvent dynamics. For solvents with slow longitudinal relaxation (PhNO2, oDCB), this adds an additional contribution to the intrinsic barrier via the dielectric relaxation process. Attaching the donor substituents to the bridge at positions where the molecular orbital coefficients are large accelerates the HT rate for meta-conjugated compounds just as for the para-series. This effect demonstrates that the para-meta paradigm no longer holds if appropriate substituents and substitution patterns are chosen, thereby considerably broadening the applicability of meta-topologies for optoelectronic applications.

Optical Response of Terpyridine Ligands to Zinc Binding: A Close Look at the Substitution Effect by Spectroscopic Studies at Low Temperature.

Terpyridine (tpy) ligands are popular building blocks to bind metal ions. Several tpy ligands with different substituents were synthesized and examined for their binding with zinc cation. The study revealed a large substituent effect on the zinc binding-induced fluorescence quenching. With the aid of a liquid nitrogen Dewar, the tpy molecules were frozen to their ground-state conformation, preventing (or minimizing) molecular reorganization in the photoinduced excited state. This allowed us to detect the fluorescence spectra from the locally excited state (having a minimum of charge transfer interaction) and the temperature-dependent fluorescence. The fluorescence response to low temperature provided useful information about the intramolecular charge transfer (ICT) interaction between the donor and acceptor groups. Furthermore, a strong donor substituent (such as Me2N) played an essential role in observed fluorescence quenching. The study also provides a useful example to elucidate the ICT mechanism by using low-temperature fluorescence spectroscopy.

Structural features of substituted triazole-linked chalcone derivatives as antimalarial activities against D10 strains of Plasmodium falciparum: A QSAR approach

A quantitative structure–activity relationship (QSAR) was performed to analyze antimalarial activities against the D10 strains of Plasmodium falciparum of triazole-linked chalcone and dienone hybrid derivatives using partial least squares regression coupled with stepwise forward–backward variable selection method. QSAR analyses were performed on the available IC50 D10 strains of Plasmodium falciparum data based on theoretical molecular descriptors. The QSAR model developed gave good predictive correlation coefficient (r2) of 0.8994, significant cross validated correlation coefficient (q2) of 0.7689, r2 for external test set (rpred2) of 0.8256, coefficient of correlation of predicted data set (rpred,se2) rpred,se of 0.3276. The model shows that antimalarial activity is greatly affected by donor and electron-withdrawing substituents. The study implicates that chalcone and dienone rings should have strong donor and electron-withdrawing substituents as they increase the activity of chalcone. Results show that the predictive ability of the model is satisfactory, and it can be used for designing similar group of antimalarial compounds. The findings derived from this analysis along with other molecular modeling studies will be helpful in designing of the new potent antimalarial activity of clinical utility.

Influence of Strong Electron-Donor Monomers in Sequence-Controlled Polymerizations.

A series of styrenic monomers containing strong donor substituents (i.e., 4-methylstyrene, 4-acetoxystyrene, and 4-tert-butoxystyrene) were tested in sequence-controlled radical copolymerizations in the presence of acceptor comonomers (i.e., N-substituted maleimide). The copolymers were synthesized by nitroxide-mediated polymerization using the commercially available alkoxyamine BlocBuilder MA. This technique allowed synthesis of copolymers with controlled molecular weights and molecular weight distributions for all monomers. Moreover, a beneficial effect of the donor substituents on the sequence-controlled copolymerization was noted. Indeed, all studied styrenic derivatives led to a very precise incorporation of functional N-substituted maleimides in the copolymer chains. The most favorable donor/acceptor copolymerization was observed with 4-tert-butoxystyrene. This particular monomer was therefore tested in the presence of various functional N-substituted maleimides (i.e., N-benzyl maleimide, 4-(N-maleimido) azobenzene), N-(1-pyrenyl)maleimide, and N-(2-(amino-Boc)ethylene)maleimide). In all cases, sequence-controlled microstructures were obtained and characterized. Moreover, the formed copolymers were hydrolyzed into poly(4-hydroxystyrene-co-N-substituted maleimide) derivatives. In all cases, it was verified that the sequence-controlled microstructure of the copolymers is preserved after hydrolysis.

Unexpected cyclization route for o-ethynylbenzoic acids hydrazides in the presence of base

The reaction of o-ethynylbenzoic acids hydrazides with base has been studied. In the presence of a strong donor substituent (1,5-dimethylpyrazol-4-yl) it has been found that an unusual cyclization route occurs to give the corresponding benzopyridazinone instead of the expected isoindolinone.

In silico optimization of merocyanine-spiropyran compounds as second-order nonlinear optical molecular switches

Time-dependent Hartree-Fock and Møller-Plesset second-order calculations have been used to unravel the relationships between structure and first hyperpolarizability in spiropyran/merocyanine couples and therefore to design efficient second-order nonlinear optical switching compounds. Large first hyperpolarizabilities for the merocyanine form as well as large contrasts of first hyperpolarizability have been obtained when, on the same species, (i) substituents at R(1) and R(2) positions on the phenolate ring of the merocyanine form are strong acceptor and donor substituents, respectively, (ii) the ethylenic bridge is substituted by donor groups, (iii) the other aromatic part of the system is benzimidazolo rather than indolino or benzothiazolo, and (iv) strong donor substituents are placed on the benzimidazolo moiety.

Langmuir-blodgett films on the basis of an amphiphilic nitronylnitroxyl radical

The 2-(4-hexadecylmethylamino)phenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy-3-oxide radical, which contains a long alkyl chain and a strong donor substituent conjugated with a nitronyl nitroxide group (an acceptor), was synthesized. The behavior of monolayers formed by this radical on a water surface and the conditions of their transfer onto solid substrates by the Langmuir-Blodgett method were examined. UV-VIS and IR spectroscopies, atomic force microscopy, and small-angle X-ray scattering were used to study the optical and structural characteristics of films prepared from the pure radical and its mixtures with behenic acid. It was revealed that radical aggregates are formed both in solutions and Langmuir-Blodgett films.

Kinetics and mechanism of the thermal decomposition of 1-bromo-4-nitroxymethylcubane

The thermal decomposition kinetics of 1-bromo-4-nitroxymethylcubane in the liquid phase is typical of C-ONO2 bond heterolysis, which occurs if the nitro ester has a strong donor substituent. A comparison between 1-bromo-4-nitroxymethylcubane and tert-butyl nitrate shows that bromocubyl is close to the tert-butyl group in induction properties and cubyl itself is a stronger donor than this group.

Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis.

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Donor‐Substituted 1,1,4,4‐Tetracyanobutadienes (TCBDs): New Chromophores with Efficient Intramolecular Charge‐Transfer Interactions by Atom‐Economic Synthesis

A wide variety of monomeric and oligomeric, donor-substituted 1,1,4,4-tetracyanobutadienes (TCBDs) have been synthesized by [2+2] cycloaddition between tetracyanoethylene (TNCE) and donor-substituted alkynes, followed by electrocyclic ring opening of the initially formed cyclobutenes. Reaction yields are often nearly quantitative but can be affected by the electron-donating power and steric demands of the alkyne substituents. The intramolecular charge-transfer (CT) interactions between the donor and TCBD acceptor moieties were comprehensively investigated by X-ray crystallography, electrochemistry, UV-visible spectroscopy, and theoretical calculations. Despite the nonplanarity of the new chromophores, which have a substantial twist between the two dicyanovinyl planes, efficient intramolecular CT interactions are observed, and the crystal structures demonstrate a high quinoid character in strong donor substituents, such as N,N-dimethylanilino (DMA) rings. The maxima of the CT bands shift bathochromically upon reduction of the amount of conjugative coupling between strong donor and acceptor moieties. Each TCBD moiety undergoes two reversible, one-electron reduction steps. Thus, a tri-TCBD derivative with a 1,3,5-trisubstituted benzene core shows six reversible reduction steps within an exceptionally narrow potential range of 1.0 V. The first reduction potential E(red,1) is strongly influenced by the donor substitution: introduction of more donor moieties causes an increasingly twisted TCBD structure, a fact that results in the elevation of the LUMO level and, consequently, a more difficult first reduction. The potentials are also strongly influenced by the nature of the donor residues and the extent of donor-acceptor coupling. A careful comparison of electrochemical data and the correlation with UV-visible spectra made it possible to estimate unknown physical parameters such as the E(red,1) of unsubstituted TCBD (-0.31 V vs Fc+/Fc) as well as the maxima of highly broadened CT bands. Donor-substituted TCBDs are stable molecules and can be sublimed without decomposition. With their high third-order optical nonlinearities, as revealed in preliminary measurements, they should become interesting chromophores for ultra-thin film formation by vapor deposition techniques and have applications in opto-electronic devices.

Effect of Substitution on the Change of Refractive Index in Dithienylethenes: An Ellipsometric Study

AbstractAn ellipsometric study was carried out in order to investigate the photoinduced change of the refractive index (Δn) of a series of diarylethenes across the entire spectral range 260–1700 nm. This technique allowed examination of the influence of the resonance conditions on the refractive index. Changing the chemical design of diarylethenes by introducing electron‐donating or electron‐withdrawing substituents in 5,5′‐positions of thiophyene allowed derivation of some correlations between chemical structure and Δn. A strong donor substituent directly linked to the 5,5′‐position of the diarylethene moiety gave rise to an increase in the change of the refractive index. 1,2‐bis(2‐methyl‐5‐formyl‐3‐thienyl)perfluorocyclopentene phenyl hydrazone showed a large and stable Δn of 0.005 at 1.5 μm and appeared to be a good candidate for the development of telecommunication devices.

Synthesis, characterisation and some reactions of alkynyl derivatives [Ni(η5-C5H5)(PPh3)CCX

Oxalic acid catalyses the hydrolysis of the Ni(II) acetylide [Ni(η5-C5H5)(PPh3)CCCH(OEt)2] 1, to the alkynylaldehyde [Ni(η5-C5H5)(PPh3)CCCHO] 2, in high yield. Condensation reactions of 2 with phenylhydrazine and dinitrophenylhydrazine in the presence of acetic acid, and with malononitrile and 3-phenyl-5-isoxazolone (C9H7NO2) in the presence of triethylamine yield [Ni(η5-C5H5)(PPh3)CCX] derivatives where X = CHNNHC6H53, CHNNHC6H3(NO2)2-2,4 4, CHC(CN)25, and CHC9H5NO26. The reactivity of [Ni(η5-C5H5)(PPh3)CCX] complexes towards [Co2(CO)8] is a function of X. Thus 1 and 2, where X = CH(OEt)2 or CHO, react readily to give the bridging alkyne derivatives [{μ-η1:η1-Ni(η5-C5H5)(PPh3)CCCH(OEt)2}{Co2(CO)6}] 7, and [{μ-η1:η1-Ni(η5-C5H5)(PPh3)CCCHO}{Co2(CO)6}] 8, but 5, where X is the strongly electron-withdrawing CHC(CN)2 group, does not react even after 24 h at room temperature. Furthermore, coordination of the alkyne to a Co2(CO)6 fragment appears to inhibit the normal reactions of the group X in 7 and 8. Thus the acetal grouping in 7 does not undergo oxalic acid-catalysed hydrolysis to an aldehyde in 8, and the aldehyde function in 8 does not undergo a Knoevenagel condensation with CH2(CN)2. The IR spectra of 1, and 3–6 show a single ν(CC) band the frequency of which decreases along the series X = CH(OEt)2 > CHNNHC6H5 > CHNNHC6H3(NO2)2-2,4 > CHC(CN)2 ≈ CHC9H5NO2; that of 2 is anomalous in that it can show two ν(CC) bands. The UV-visible spectra of 1–6 show a strong charge transfer absorption band which increases in wavelength 1 < 3 < 2 < 4 < 5 < 6. These spectroscopic data and the 13C chemical shifts suggest that the (η5-C5H5)(Ph3P)Ni moiety is a donor and, when X is an acceptor, charge separated cumulenic mesomers such as Ni+CCX− contribute to the description of the bonding in 1–6. This is not reflected in the molecular dimensions of 1, 2 and 5 as determined by X-ray diffraction. However, the crystal structure of [{μ-η1:η1-Ni(η5-C5H5)(PPh3)CCCHO}{Co2(CO)6}], 8, shows that the C2Co2 cluster core is severely distorted because of the strong donor (Ni) and acceptor (CHO) substituents on the acetylenic carbon atoms.

Photophysics of arylsubstituted tropylium ions

Phenyl tropylium ion (TRPH) bearing different donor substituents were investigated both experimentally (absorption and fluorescence) and theoretically (semiempirical and ab initio calculations for ground and excited state structures and transitions). Generally, an increased donor strength of the substituent leads to a favoring of the planar geometry in the ground state. In the S1-state, however, energetic minimum develops for orthogonal structures, which are characterized by strong charge shift (CSh) (electron transfer from the substituted aryl to the tropylium fragment), forbidden transition moments and near-zero singlet–triplet energy gaps. These results can serve as a basis to interpret the completely non-emissive behavior of the tropylium derivatives with strong donor substituents.

Organometallic nickel(II) complexes with substituted benzonitrile ligands. Synthesis, electrochemical studies and non-linear optical properties. The X-ray crystal structure of [Ni( η5-C 5H 5){P(C 6H 5) 3}(NCC 6H 4NH 2)][PF 6

A new family of organometallic Ni(II) benzonitrile derivatives bearing strong donor substituents of general formula [Ni( η 5-C 5H 5){P(C 6H 5) 3}NCC 6H 4R][PF 6] (R=NH 2, N(CH 3) 2, C 6H 5, OCH 3, H, F) has been synthesized. A structural study by X-ray diffraction of the compound with R=NH 2 showed crystallization on the centrosymmetric monoclinic space group, P2 1/ n, with a quasi planar structure of the coordinated nitrile. First and second hyperpolarizabilities evaluated by hyper-Rayleigh scattering and Maker fringe techniques showed essentially the same values for the complexes and the uncoordinated chromophores. Theoretical studies by the extended Hückel method found a small amount of π delocalization due to the cancelling effects on the interaction of the metal fragment orbitals with π and π * nitrile orbitals.