Acceptor Substitution Research Articles

Effects of protons and acceptor substitution on the electrical conductivity of La 6WO 12

Transport properties and non-stoichiometry of La 1− x Ca x W 1/6O 2 and La 1− y W 1/6O 2 ( x=0, 0.005, 0.05; y=0.05, 0.1) have been characterized by means of impedance spectroscopy, the EMF-technique, H +/D + isotope exchange, and thermogravimetry in the temperature range 300–1200 °C as a function of oxygen partial pressure and water vapor partial pressure. The materials exhibit mixed ionic and electronic conductivities; n- and p-type electronic conduction predominate at high temperatures under reducing and oxidizing conditions, respectively. Protons are the major ionic charge carrier under wet conditions and predominates the conductivity below ∼750 °C. The maximum in proton conductivity is observed for LaW 1/6O 2 with values reaching 3×10 −3 S/cm at approximately 800 °C. The high proton conductivity for the undoped material is explained by assuming interaction between water vapor and intrinsic (anti-Frenkel) oxygen vacancies.

Nonlinear optical properties of polydiacetylene with donor-acceptor substitution block

The elongation finite-field (elongation-FF) method is applied to donor/acceptor substituted polydiacetylenes (PDAs) for the estimation of substituent effects on nonlinear optical (NLO) properties. The first hyperpolarizability (beta) and the second hyperpolarizability (gamma) of PDA with separated donor and acceptor substitution blocks have much larger values than those of the other substituted PDAs. For the PDAs with donor and acceptor substitution blocks, the relationship between the NLO properties and the block period is examined. It is shown, from the local density of states, that gamma of a system with a quantum well structure has a maximum value at a certain block size. This indicates that by tuning the size of proper block it is possible to achieve the largest gamma value in block polymers. Furthermore, the through-space/bond interaction analysis is performed to examine the pi-conjugation effects on the NLO properties for particular substituted PDA. It is demonstrated by our quantitative analysis that beta is affected by electron transfers in the molecule, and the quantum well structure is critical for gamma improvement.

Donor/Acceptor Effects on the Linear and Nonlinear Optical Properties of Geminal Diethynylethenes (g‐DEEs)

AbstractA series of geminal diethynylethenes (g‐DEEs) with electron‐donating and/or electron‐accepting (D/A) groups were synthesized via a Pd‐catalyzed cross‐coupling sequence. The UV/VIS spectra for donor–acceptor (D–A) functionalized g‐DEEs 5, 8, and 11 show distinctive absorption trends attributable to intramolecular charge‐transfer (ICT). The bond‐length‐alternation (BLA) index for the cross‐conjugated enediyne framework varies slightly with different terminal substituents as determined by density‐functional theory (DFT) calculations and single‐crystal X‐ray analysis. Ultrafast third‐order optical nonlinearities for the g‐DEEs were measured by the differential optical Kerr effect (DOKE) technique and show that terminal donor–acceptor substitution of g‐DEEs enhances molecular second hyperpolarizabilities (γ) in comparison to donor or acceptor g‐DEEs. A small increase in the two‐photon‐absorption cross‐section (σ(2)) is observed in the series 9–11 as a result of increased functionalization. The effects of donor/acceptor substitution on electron delocalization along the cross‐conjugated enediyne structure are evaluated on the basis of natural‐bond‐orbital (NBO) analysis. Solid‐state structures of the four derivatives 3b, 4b, 7 and 8 were characterized by single‐crystal X‐ray structural analysis and show an asymmetric unit cell for one derivative, D–A g‐DEE 8.

Push‐Pull Oligomers with 2,2‐Dicyanovinyl Groups as Electron Acceptors

AbstractThree conjugated oligomer series with terminal donor–acceptor substitution were studied: oligo(1,4‐phenylenevinylene)s DAOPVs 4a–d (n = 1–4), oligo(1,4‐phenyleneethynylene)s DAOPEs 6a–d (n = 1–4) and oligo(2,5‐thienyleneethynylene)s DAOTEs 8a–e (n = 1–5). Dialkylamino or methoxy groups served as electron donors and 2,2‐dicyanovinyl groups as strong electron acceptors. The push‐pull effect polarizes the molecular chains – an effect which is documented by the splitting of the 13C chemical shifts of two carbon atoms of the double or triple bonds in the chain. For higher oligomers (n ≥ 3), the effect is mainly localized at the chain ends. The long‐wavelength absorption results in a charge‐transfer band which loses gradually its CT character since the intramolecular charge transfer (ICT) declines with increasing numbers n of repeat units (increasing chromophore extension, increasing distance D–A). As a consequence, theDAOPVs 4a–d and the DAOPEs 6a–d represent hypsochromic series, for which the λmax values decrease steadily from n = 1 to n = 4. This is also true for the DAOTE series 8a–e, but contrary to 4a–d and 6a–d the decrease is very small in the beginning [λmax(1) – λmax(2) = 1 nm] and grows with increasing n to λmax(4) – λmax(5) = 29 nm. Thus, series 8a–e represents an until now unknown type of conjugated push‐pull oligomers. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Influence of donor and acceptor substitutions on the extrinsic behaviour of PZT piezoceramics

A study of the substitution influence of Nb 5+ and Fe 3+ impurities on piezoelectric behaviour has been carried out. Different substitution concentrations as well as undoped materials have been performed for ceramics Pb(Zr 1 − x Ti x )O 3 with x = 47 and 40. The different impurity concentrations, and their type, produce variations in the domain wall mobility related to the extrinsic coefficient. This dependence between extrinsic behaviour and impurity concentration is studied for different material structures: the rhombohedral and morphotropic phase boundary (MPB). Two methods are used to characterize the extrinsic and intrinsic behaviours. Elastic and piezoelectric non-linear characterization at resonance has been made in isothermal conditions by impedance and vibration velocity measurements. In order to obtain the intrinsic coefficients, linear characterization at very low temperature has been carried out.

Effect of acceptor and donor dopants on ferroelectric and piezoelectric properties of lead zirconate titanate ceramics

AbstractDielectric, ferroelectric and piezoelectric properties of Pb1−x Lax (Zry Ti1−y )1−(x /4)−(3/4)z Fez O3 ceramics for z = 0 to 6 mol% prepared by a mixed oxide method were investigated. Fe substitution in lead lanthanum zirconate titanate (PLZT) compositions resulted in a phase transformation from coexisting phases (ferroelectric rhombohedral (FERH) and ferroelectric tetragonal (FETET)) to intensified tetragonal phase. Grain size and apparent density increased with increasing acceptor (Fe) content up to 3 mol% in the PLZT system. The optimum dielectric properties (ϵ RT, ϵ Tc and tan δ) were observed in 3 mol% Fe‐doped PLZT composition and then decreased thereafter. Acceptor (Fe) substitution in the PLZT system influenced the ferroelectric remanent polarization (P r) and spontaneous (P s) polarization, reaching saturation at 6 mol% Fe, while the coercive field (E c) increased up to 3 mol% Fe‐doped PLZT composition. The piezoelectric properties are sensitive to acceptor (Fe) modification in the PLZT system which was proved by the decreasing trend of piezoelectric charge coefficient (d 33) up to 3 mol% Fe and then increasing to 6 mol% Fe while the piezoelectric planar coupling coefficient (k p) showed a continuous decreasing trend throughout the series. Dielectric, ferroelectric and piezoelectric properties were characterized as a function of acceptor (Fe) concentration in the PLZT system for possible sensor and actuator applications. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Two-photon absorption in two-dimensional conjugated quadrupolar chromophores

We present ultrafast z-scan measurements of the two-photon absorption (TPA) spectra of a pair of two-dimensionally conjugated quadrupolar donor/acceptor (D/A) chromophores. The all-donor substituted species displays a peak TPA cross section [sigma(2)=520+/-30 GM] that is more than twice that of the D-A species [sigma(2)=240+/-20 GM]. Unlike previous structure-property studies that have evaluated TPA behavior for D/A molecules through the comparison of dipolar and quadrupolar compounds, both molecules investigated herein are quadrupolar, ultimately providing a more consistent evaluation of the effects of donor and/or acceptor substitution on the TPA of conjugated chromophores.

Two-Photon Excitation of Substituted Enediynes

Electronic spectroscopy of nine benzannelated enediynes and a related fulvene was studied under one-photon and two-photon excitation conditions. We utilize measured absorbance and emission spectra and time-resolved fluorescence decays of these molecules to calculate their radiative lifetimes and fluorescence quantum yields. The fluorescence quantum yields for the other compounds were referenced to the fluorescence quantum yield of compound 3 and used to determine relative two-photon absorption cross-sections. Further insight into experimental studies has been achieved using time-dependent density functional (TD-DFT) computations. The probability of two-photon absorption (TPA) increases noticeably for excitation to the higher excited states. The photophysical properties of benzannelated enediynes are sensitive to substitutions at both the core and the periphery of the enediyne chromophore. Considerably enhanced two-photon absorption is observed in an enediyne with donor substitution in the middle and acceptor substitution at the termini. Excited states with B symmetry are not active in TPA spectra. From a practical point of view, this study extends the range of wavelengths applicable for activation of the enediyne moiety from 350 to 600 nm and provides a rational basis for future studies in this field. Our theoretical computations confirmed that lowest energy TPA in benzannelated enediynes involves different orbitals than lowest energy one-photon absorbance and provided further support to the notion that introduction of donor and acceptor substituents at different ends of a molecule increases TPA.

Highly efficient third-order optical nonlinearities in donor-substituted cyanoethynylethene molecules

We investigated the third-order nonlinear optical properties of several donor-substituted cyanoethynylethene molecules in the zero-frequency limit. We observed nonlinearities that are extraordinarily large relative to the small molecular mass of these molecules and that are within a factor of 50 from the fundamental limit. At a wavelength of 1.5 microm, the rotational average of the third-order molecular polarizability is 53 +/- 13 x 10(-48) m5 V(-2) (3.8 x 10(-33) esu) for the best molecule, which corresponds to 1.7 x 10(-48) m5 V(-2) per delocalized electron. The high nonlinear efficiency of these molecules is due to the compact two-dimensional conjugated system and the effective donor-acceptor substitution patterns.

Magnesium/nitrogen and beryllium/nitrogen coimplantation into GaN

The nitrogen coimplantation characteristics in the Mg- and Be-implanted GaN with different dopant concentration ratios have been systematically investigated. The Hall-effect measurements show that the p-type characteristics are produced in the Mg- and Be-implanted GaN by the coimplantation of N atoms and subsequent annealing, which is essentially related to the column II/V dopant concentration ratio and annealing condition. This behavior may be attributed to the reduction of self-compensation induced by N vacancies and the enhanced acceptor substitution, which is in reasonable agreement with the surface stoichiometric switching determined by x-ray photoelectron spectroscopy measurements. From photoluminescence data, the activation energy of the Be acceptor level is evaluated to be about 145meV, which is shallower than that of the Mg acceptor. These experimental results indicate that the selective-area N coimplantation with Mg and Be atoms into GaN is an effective method to enhance the p-type conductivity and to improve the p-type Ohmic contact resistance.

Conjugated Oligomers with Terminal Donor—Acceptor Substitution

AbstractFor Abstract see ChemInform Abstract in Full Text.

Two-Photon Absorption and First Nonlinear Optical Properties of Ionic Octupolar Molecules: Structure−Function Relationships and Solvent Effects

We present a quantum-chemical analysis of the two-photon absorption properties and first hyperpolarizabilities of a series of ionic octupolar molecules and a comparison of their characteristics with corresponding neutral molecules. The molecular geometries are obtained via BL3YP/6-31G (d,p) level optimization including the SCRF/PCM approach, while the dynamic NLO and two-photon absorption properties are calculated with the ZINDO/CV method including solvent effects. The effects of donor or acceptor substitution and elongation of the conjugation path length are established to demonstrate the engineering guidelines for enhancing two-photon absorption cross section and molecular optical nonlinearities. It is found that the chain length dependence of the two-photon absorption and the first nonlinearity follow the same trend, displaying a saturation limit at n = 5. The solvent induced effect on the two-photon absorption and NLO properties are studied using the ZINDO/CV/SCRF method. It has been observed that two-photon absorption and the first nonlinearity peaks at epsilon approximately = 20 and then decreases slightly, approaching saturation. We also compare our theoretical findings with the experimental results wherever available in the literature.

Effect of Donor−Acceptor Substitution on the Nonlinear Optical Properties of Oligo(1,4-phenyleneethynylene)s Studied by Third Harmonic Generation Spectroscopy

Third-harmonic generation (THG) spectroscopy was performed for oligo(1,4-phenyleneethynylene)s (OPEs) with terminal donor-acceptor (DA) substitution and compared to the results of merely donor substituted OPEs and regular OPE chains with 2,5-dipropoxy benzene rings. Both, extension of the conjugation and push-pull effect enhance the molecular hyperpolarizability gamma, even for the DAOPEs, which exhibit a hypsochromic shift of the long-wavelength absorption for increasing length L of the conjugated chain.

Conjugated Oligomers with Terminal Donor–Acceptor Substitution

Conjugated oligomers represent a prominent class of compounds from a viewpoint of their theory, synthesis, and applications in materials science. Push-pull substitution with an electron donor D at one end of the conjugation and an electron acceptor A at the other end results in them having outstanding optical and electronical properties. This Review highlights fundamental synthetic strategies for the preparation of such oligomers with n repeat units (n=1, 2, 3, 4, ..) and the rules that govern their linear and nonlinear optical properties (absorption, frequency doubling and tripling). The unification of chemical, physical, and theoretical aspects with an interdisciplinary image of this class of compounds is attempted herein.

Effect of Acceptor and Donor Dopants on the Dielectric and Tunable Properties of Barium Strontium Titanate

The effects of different concentrations of Mn2+, Mg2+, Al3+, Fe3+, La3+, and Nb5+ on the dielectric and tunable properties of Ba0.6Sr0.4TiO3 ceramics were investigated. It was found that doping in small amounts with acceptor ions such as Mg2+, Fe3+, and Al3+ could meliorate the dielectric properties clearly. Decrease of dielectric loss was attributed to the formation of compensating defects originating from acceptor substitution. It was concluded that the tunability was linked to both the dielectric constant and the grain size. A higher figure of merit was obtained by doping the ceramics with smaller ions of Al and Fe, compared to Ti.

First hyperpolarizabilities of ionic octupolar molecules: structure–function relationships and solvent effects

We present a quantum-chemical analysis of the first hyperpolarizabilities of a series of ionic octupolar molecules and comparison of their characteristics with corresponding neutral molecules. The molecular geometries are obtained via BL3YP/6-31G (d,p) optimization including SCRF/PCM approach, while the dynamic NLO properties are calculated with the ZINDO/CV method including solvent effects. The effect of donor or acceptor substitution and elongation of the conjugation path length are established to demonstrate the engineering guidelines for enhancing molecular optical non-linearities. The solvent induced effect on the NLO properties are studied using ZINDO/CV/SCRF method. It was found that solvents play a significant role on the first hyperpolarizabilities of ionic octupolar molecules.

Quadrupolar donor–acceptor substituted oligo(phenylenevinylene)s—synthesis and solvatochromism of the fluorescence

AbstractThe electronic spectra of oligo(phenylenevinylene)s with five benzene rings and an acceptor–donor–acceptor substitution pattern were investigated. Although the absorption spectra are mostly unaffected by an increasing solvent polarity, both bathochromic shifts and reduced efficiencies of the fluorescence arise with the quadrupolar electronic structure. The chromophores with the lowest oxidation or reduction potentials are the most sensitive towards the environment. The synthesis of the centrosymmetrical oligomers was performed in a sequence of two two‐fold Wittig–Horner olefinations. Copyright © 2004 John Wiley & Sons, Ltd.

Red‐ and Blue‐Shifts in Oligo(1,4‐phenyleneethynylene)s Having Terminal Donor−Acceptor Substitutions

AbstractFour series of oligo(1,4‐phenyleneethynylene)s (OPEs), 1−4 (a−d), each having a terminal dialkylamino group as their electron donor, were prepared by applying Sonogashira−Hagihara reactions and a protecting group strategy. To study the influence that the push−pull effect has on the long‐wavelength absorption, three of the four series of OPEs contain terminal acceptor groups (CN, CHO, NO2). Extending the conjugation (increasing the number of repeat units, n) lowers the energy E(n) of the electron transition in the purely donor‐substituted series 1a−4a (bathochromic shift). This effect is superimposed in the push−pull series 1−4 (b−d) by the effect of the intramolecular charge transfer (ICT), which decreases with increasing the distance between the donor and acceptor groups. In the case of the relatively weak acceptor CN, both effects annihilate one another, whereas the reduction of the ICT predominates in the CHO and NO2 series. Therefore, E(n) is virtually independent of n in the CN series 1b−4b and shows a hypsochromic effect in the CHO and NO2 series (1c−4c and 1d−4d, respectively). We rationalize these results based on AM1−INDO/S calculations. The results of the commonly used intuitive VB model having electroneutral and zwitterionic resonance structures are compared with those of a more appropriate MO model having dipole segments at both chain ends. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Effects of dipole alignment and channel interference on two-photon absorption cross sections of two-dimensional charge-transfer systems

The relationship between two-photon absorption cross sections and the architecture of donor–acceptor substitutions in two-dimensional charge-transfer cumulene-containing aromatic molecules is analyzed. It is shown that a molecular design strategy proposed for one-dimensional charge-transfer molecules must be considerably altered for two-dimensional systems. It is found that the maximal two-photon cross section of such two-dimensional systems can often be well described by a four state model. The importance of the alignment of the transition dipole moments and of channel interference on the two-photon absorption cross sections is emphasized.

One- and Two-Photon Spectroscopy of Donor−Acceptor−Donor Distyrylbenzene Derivatives: Effect of Cyano Substitution and Distortion from Planarity

The one- and two-photon spectroscopic properties of four symmetrically substituted donor−acceptor−donor distyrylbenzenes with either di-n-butyl- or diphenylamino donor groups and cyano acceptor groups are reported. It has been found that the position of the substitution of the electron-withdrawing cyano groups on the central phenylene ring as compared to the vinylene bond strongly affects the observed properties. In particular, the molecules with cyano substitution on the α-carbon of the vinylene linkage are characterized by weak fluorescence, short fluorescence lifetimes, and two-photon cross sections (δ) that are comparable to analogous molecules with no acceptor groups. In contrast, the molecules with acceptor substitution on the central phenylene ring are strongly fluorescent and have δ values roughly twice those of the vinyl-substituted molecules. These results are discussed in terms of the larger deviation of the conjugated backbone from planarity and the smaller distance between the donors and acce...