Ground-state Polarization Research Articles

First Excited State Properties and Static Hyperpolarizability of Ruthenium(II) Ammine Complexes.

First principles calculations were used to study the electronic excitation energies (E), transition dipole moments (μ), and difference of dipole moments between ground and excited states (Δμ) for low-lying singlets of the series of ruthenium(II) ammine complexes. Both cases of the gas phase and the acetonitrile solution were investigated in order to explain the discrepancy between the recent experimental and theoretical results and to develop the optimal way of estimation for the first static hyperpolarizability in the framework of a two-state model introduced by Oudar and Chemla. The present calculations reveal that the effect of solvent on the electronic properties of investigated compounds is not only the change of the excitation energy but also the increasing of ground-state molecular polarization and intensification of metal-to-ligand intramolecular charge transfer for electronic excitations. These effects lead to increasing of the values of Δμ and ground-state dipole moment μg in solution as compared with the gas-phase ones. The proposed theoretical approach gives good agreement with experiment and allows one to apply it for designing a new perspective nonlinear optical active organometallics.

Two-photon absorption in quadrupolar pi-conjugated molecules: influence of the nature of the conjugated bridge and the donor-acceptor separation.

Quadrupolar-type substitution of pi-conjugated chromophores with donor and acceptor groups has been shown to increase their two-photon absorption (TPA) response by up to two orders of magnitude. Here, we apply highly correlated quantum-chemical calculations to evaluate the impact of the nature of conjugated bridge and the charge-transfer distance on that enhancement. We compare chromophores with phenylenevinylene-, thienylenevinylene-, polyene-, and indenofluorene-type backbones substituted by dimethylamino and cyano groups. In all compounds, we find a strongly TPA-active A(g) state (either 2A(g) or 3A(g)) in the low-energy region, as well as a higher lying TPA-active state (mA(g)) at close to twice the energy of the lowest lying one-photon allowed state; the smaller energy detuning in the mA(g) states results in very large TPA cross sections delta. We also investigate the influence of the degree of ground-state polarization on TPA. Independent of the nature of the backbone and the donor-acceptor separation, delta displays the same qualitative evolution with a maximum before the cyanine-like limit; the highest TPA cross sections are calculated for distirylbenzene- and polyene-based systems.

Spin magnetization of a strongly correlated electron gas confined in a two-dimensional finite lattice

The influence of disorder and interaction on the ground state polarization of the two-dimensional (2D) correlated electron gas is studied by numerical investigations of unrestricted Hartree-Fock equations. The ferromagnetic ground state is found to be plausible when the electron number is lowered and the interaction and disorder parameters are suitably chosen. For a finite system at constant electronic density the disorder induced spin polarization is cut off when the electron orbitals become strongly localized to the individual network sites. The fluctuations of the interaction matrix elements are calculated and brought out as favoring the ferromagnetic instability in the extended and weak localization regime. The localization effect of the Hubbard interaction term is discussed.

Ferroelectricity in BaTiO3 nanoscopic structures

The ground-state polarization of BaTiO3 nanosized films and cells is studied using an atomic-level simulation approach based on a shell model with parameters obtained from first-principles calculations. We demonstrate that the critical thickness for ferroelectricity in a free-standing BaTiO3 stress-free film is 3.6 nm, and a decrease in lateral size to nanometric dimensions does not spoil the ferroelectric properties of the film. Nanocells with different lateral faces, BaO or TiO2 planes, present a different domain structure and polarization due to a strong surface effect.

Tuning the two-photon absorption response of quadrupolar organic molecules

We apply correlated quantum-chemical techniques to study the origin of the large two-photon absorption (TPA) cross sections, δ, in stilbene derivatives in which electron-donating and electron-withdrawing substituents provide a quadrupolar charge-transfer arrangement. An additional field created by a set of point charges is used to systematically modify the ground-state polarization to determine its consequences for the TPA response. The effect on the molecular structure can be quantified by the evolution of the π-bond-order alternation (π-BOA) of the conjugated backbone. For moderate ground-state polarizations, a marked increase of the TPA response occurs; for large polarizations, δ peaks and then drops dramatically. Insight into the origin of this evolution is gained by comparing the values of δ obtained via the converged sum-over-states approach to the results of a simple three-state model.

Involvement of Glycine 141 in Substrate Activation by Enoyl-CoA Hydratase

Raman spectroscopy has been used to investigate the structure of a substrate analogue, hexadienoyl-CoA (HD-CoA), bound to wild-type enoyl-CoA hydratase and G141P, a mutant in which a hydrogen bond to the substrate carbonyl has been removed. Raman spectra of isotopically labeled HD-CoAs, together with normal mode calculations, confirm the selective ground-state polarization of the enone fragment previously suggested to occur on binding to the wild-type enzyme [Tonge, P. J., Anderson, V. E., Fausto, R., Kim, M., Pusztai-Carey, M., and Carey, P. R. (1995) Biospectroscopy 1, 387-394]. In addition, Raman spectra of HD-CoA bound to the G141P mutant enzyme demonstrate that the hydrogen bond between the G141 amide NH group and the substrate carbonyl is critical for polarization and activity. Replacement of G141 with proline results in an approximately 10(6)-fold decrease in k(cat) and eliminates the ability of the enzyme to polarize the substrate analogue. As G141 is part of a consensus sequence in the enoyl-CoA hydratase superfamily, the results presented here provide direct evidence for the importance of the oxyanion hole in the reactions catalyzed by other family members.

Theoretical investigation of the origin of the large non-linear optical response in acceptor-substituted carotenoids

Correlated semiempirical quantum-chemical calculations are performed to rationalize the origin of the large first- ( α), second- ( β), and third- ( γ) order molecular polarizabilities found in carotenoid-like molecules capped at a single end by an acceptor group. The ground state polarization in the apocarotenal H(-CHCH-) 10CHC(CN) 2, is made to vary via the application of an external homogeneous electric field. The important structural characteristics found for this long polymethine are (i) the relative instability of the cyanine-like (equal-bond length) structures and (ii) the double-step evolution of the dipole moment caused by the transfer of a first and then a second electron towards the acceptor end of the molecule as the electric field increases in the range 10 7–10 8 V/cm. The abruptness of these steps results in very large values of the molecular polarizabilities. The longitudinal components of the molecular polarizabilities are analyzed as a function of the bond-order alternation (BOA) parameter.

Nonlinear optical response in acceptor-substituted carotenoids: A theoretical study

Correlated semi-empirical quantum-chemical calculations are performed to rationalize the origin of the large first- (α), second- (β), and third- (γ) order molecular polarizabilities in carotenoidlike molecules capped at a single end by an acceptor group. The degree of ground-state polarization of the chromophores is made to vary via the application of an external homogeneous electric field in the range 107–108 V/cm. The most interesting feature is a stepwise evolution of the longitudinal component of the dipole moment with field, caused by charge transfer toward the acceptor end of the molecule. Since the steps are abrupt, this results in very large values of the molecular polarizabilities, in accordance with the derivative relationships among the dipole moment and α, β, and γ. The longitudinal components of the molecular polarizabilities are analyzed as a function of both the external field and the bond-order alternation (BOA) parameter. The ability of simple models to describe the evolution of the molecular polarizabilities is assessed.

Sensitive magnetometry based on nonlinear magneto-optical rotation

Application of nonlinear magneto-optical (Faraday) rotation to magnetometry is investigated. Our experimental setup consists of a modulation polarimeter that measures rotation of the polarization plane of a laser beam resonant with transitions in Rb. Rb vapor is contained in an evacuated cell with antirelaxation coating that enables atomic ground-state polarization to survive many thousand wall collisions. This leads to ultranarrow features $(\ensuremath{\sim}{10}^{\ensuremath{-}6} \mathrm{G})$ in the magnetic-field dependence of optical rotation. The potential sensitivity of this scheme to sub-$\ensuremath{\mu}\mathrm{G}$ magnetic fields as a function of atomic density, light intensity, and light frequency is investigated near the $D1$ and $D2$ lines of ${}^{85}\mathrm{Rb}.$ It is shown that through an appropriate choice of parameters the shot-noise-limited sensitivity to small magnetic fields can reach $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}12} \mathrm{G}/\sqrt{\mathrm{Hz}}.$

Microscopic model of ferroelectricity in stress-free PbTiO3 ultrathin films

The ground-state polarization of PbTiO3 thin films is studied using a microscopic effective Hamiltonian with parameters obtained from first-principles calculations. Under short-circuit electrical boundary conditions, (001) films with thickness as low as three unit cells are found to have a perpendicularly polarized ferroelectric ground state with significant enhancement of the polarization at the surface.

Bond-length alternation in rigidized merocyanines

The two merocyanines 7-(4-dimethylaminophenyl)-2,3,4,4a,5,6,10,10a-octahydroanthracen-2-one [C 22 H 25 NO, (2)] and [2,3,4,4a,5,6,10,10a-octahydro-7-(5-piperidino-2-thienyl)-2-anthrylidene]methanedicarbonitrile [C 26 H 27 N 3 S, (4)] show somewhat shortened single bonds [1.434 (4)-1.452 (4) A in (2) and 1.421 (4)-1.425 (4) A in (4)] and elongated double bonds [1.345 (4)-1.358 (4) A in (2) and 1.355 (4)-1.365 (4) A in (4)] compared to standard bond lengths in polyenes. A comparison of the merocyanines reveals that (4) exhibits a slightly smaller bond length alternation within the rigidized hexatriene moiety than (2), probably because of an increased ground-state polarization.

Highly Dipolar, Optically Nonlinear Adducts of Tetracyano-p-quinodimethane: Synthesis, Physical Characterization, and Theoretical Aspects

A new series of nonlinear optical molecules are described where the ground state polarization is predominantly zwitterionic when the molecules are dissolved in solution. The molecules, which are derived in general from facile reactions between tertiary amines and tetracyano-p-quinodimethane (TCNQ), are of a type where the stabilization of the charge-separated ground state is favored by an increase in aromaticity over the neutral, quinoidal forms of the molecules. The measured second-order optical nonlinearity of one in the series has been measured by hyper-Rayleigh scattering and a figure of merit value, μβ(0), being the product of the dipole moment and static first hyperpolarizability, is found to be 9500 × 10^(-48) esu. This value, which is higher than most other reported values, is taken from studies in chlorinated solvents of relatively low polarity, but the discussion emphasizes the evolution of μβ(0) with solvent polarity, showing that even higher values could be expected with only modest increases in the polarity of the surrounding medium. The analysis of experimental data taken during dipole moment studies is thoroughly examined, and it is concluded that full account must be taken of the molecular shape to correlate the results with theoretical calculations. An ellipsoidal reaction field model is preferred for these highly one-dimensional molecules having strongly anisotropic polarizabilities.

Experimental Demonstration of the Dependence of the First Hyperpolarizability of Donor-Acceptor-Substituted Polyenes on the Ground-State Polarization and Bond Length Alternation

It has been suggested that optimizing the first hyperpolarizability, β, of donor-acceptor compounds requires a specific donor-acceptor strength for a given conjugated bridge. For donor-acceptor polyenes, β can be maximized when an optimal degree of mixing between neutral and charge-separated canonical resonance forms This degree of mixing is related to the donor-acceptor strength and a molecular parameter, bond length alternation (BLA), defined as the difference between the average carbon-carbon single and double bond lengths in the polymethine backbone. The degree of BLA arises from the linear combination, or mixing, of the two-limiting charge-transfer resonance forms of the molecule (Figure 1).4 For unsubstituted polyenes or chromophores with weak donors-acceptors, the neutral canonical form is the dominant contributor to the ground state, resulting in large positive BLA. As the donor-acceptor strength increases, the charge-separated resonance structure contributes more to the ground state, resulting in smaller BLA, until both resonance forms contribute equally and the ground-state structure possesses essentially zero BLA, analogous to a symmetrical cyanine. Increasing the ground-state polarization further results in the charge-separated canonical form dominating the ground-state structure, leading to negative BLA.

The dependence of the molecular first hyperpolarizabilities of merocyanines on ground-state polarization and length

We report here the dipole moment (µ) and first hyperpolarizability (β) determined by electric field-induced second harmonic generation, for several merocyanine dyes containing an 1,3,3-trimethylindoline heterocycle as a ‘donor’ in which the ‘acceptor’ end of the molecule and the polyene bridge length was systematically varied; dyes with hexamethine bridges gave positive β, while that with a dimethine bridge gave a negative β value.

Lines of interacting quantum-dot cells: A binary wire

The behavior of linear arrays of cells composed of quantum dots is examined. Each cell holds two electrons and interacts Coulombically with neighboring cells. The electrons in the cell tend to align along one of two axes resulting in a cell ‘‘polarization’’ which can be used to encode binary information. The ground-state polarization of a cell is a highly nonlinear function of the polarization of its neighbors. The resulting bistable saturation can be used to transmit binary information along the line of cells, thus forming a binary wire.

An N.M.R. Investigation of Polarization of Pyridine by +R Substituents and a Proposed New Method for Determination of the Substituent Resonance Parameter (σR)

The previously established 1H n.m.r. method of probing bond order1 has been used to show that electron-donating (+R) substituents at C2 of pyridine cause significant ground-state polarization of the heteroaromatic system. This has led to a new and independent method of estimating the substituent resonance parameter (σR ).

The Relationship Between Second-Order Nonlinear Optical Properties And Ground-State Polarization

ABSTRACTA review is presented describing our recent work to correlate the first hyperpolarizability, β, of organic materials with the molecular parameter bond length alternation (BLA). Donor-acceptor polyenes displaying a wide BLA range were synthesized. For a particular chromophore, BLA was fine-tuned by varying solvent polarity. The degree of BLA was analyzed by X-ray diffraction, 1H-NMR and electronic absorption spectroscopy. Non-resonant, solvent-dependent, electric field induced second harmonic generation (EFISH) Measurements were performed to probe the variation in the second-order nonlinearity as a function of ground-state polarization. The resulting trend, which is fully consistent with theoretical predictions, identified chromophores possessing optimized positive and negative hyperpolarizabilities. An optimized chromophore was incorporated in a polymer matrix and poled. The resulting electro-optic coefficient was found to be significantly enhanced relative to the longer chromophore Disperse Red 1.

ChemInform Abstract: NMR Investigation of Ground‐State Polarization of Some Substituted Aromatic Systems

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis and properties of [4,5]dihomotropone (bicyclo[5.1.1]Nona-2,5-dien-4-one)

The potentially through-bond “dihomoaromatic” title compound has been elaborated by means of a tandem acyloin-ring expansion sequence and shown by spectroscopic means not to possess significant ground-state polarization.

An N.M.R. Investigation of Ground-State Polarization of Some Substituted Aromatic Systems

The previously established n.m.r. method for estimating mobile bond orders was applied to the investigation of ground-state polarization of benzene derivatives with ortho or para pairs of +R/-R substituents, naphthalene and five heteroaromatic systems (furan, thiophen, pyrrole, quinoline and pyrazole). Evidence for significant ground-state polarization which is solvent-independent was found in a number of these systems, especially benzene, pyrrole and pyrazole.