Claude Runser

Chromophores with Strong Heterocyclic Acceptors: A Poled Polymer with a Large Electro-Optic Coefficient

Syntheses of a series of conjugated donor-acceptor chromophores, based on a strongly electron-withdrawing heterocyclic acceptor, have led to compounds with large second-order optical nonlinearities. Incorporation of one of these chromophores into polycarbonate at 20 percent weight loading yielded, after poling at 150 volts per micrometer, a polymer film with an electro-optic coefficient, r33, of 55 picometers per volt at 1.313 micrometers. This value is roughly twice that of lithium niobate. A variant of one of these chromophores exhibited improved thermal stability as needed for use in polymers with higher glass transition temperatures. The chromophore was soluble in common organic solvents, had a scalar product of the dipole moment, μ, and the molecular first hyperpolarizability, β (corrected for dispersion), of roughly 5000 × 10−48 electrostatic units, and showed less than 10 percent decomposition after heating for 20 minutes in air and at 200°C in an inert organic solvent.

A two-state description of (hyper) polarizabilities of push-pull molecules based on a two-form model

Abstract We have established analytical structure/(hyper)polarizability relationships for donor-acceptor conjugated molecules, within the framework of the two-state approximation. In this two-form model, a molecule is described as a mixture between neutral and charge-separated resonance forms. We defined a parameter MIX, characterizing the mixing between the limiting-resonance forms, that is measurable in solution. MIX is proportional to the change in dipole between the ground and excited states. We studied analytically the variation of (hyper)polarizabilities with MIX. Finally, we showed how this model can account for external perturbation, such as the solvent electric reaction field.

LARGE QUADRATIC HYPERPOLARIZABILITIES WITH DONOR-ACCEPTOR POLYENES FUNCTIONALIZED WITH STRONG DONORS. COMPARISON WITH DONOR-ACCEPTOR DIPHENYLPOLYENES

Abstract Donor-acceptor polyenes and carotenoids of defined and increasing length, bearing electron-donating ferrocene or julolidine moieties, and formyl or dicyanovinyl electron-withdrawing end groups have been prepared in order to achieve enhanced quadratic optical nonlinearities. The variation of their quadratic hyperpolarizability β in solution was investigated using the electric field induced second harmonic (EFISH) generation technique, and was compared to the behavior of two series of soluble push-pull diphenylpolyenes. The chainlength behavior for each series of homologous compounds can be modeled by μs(0) = kna relationships with respect to the number n of double bonds in the polyenic chain, the exponent value a depending markedly on the end groups. Steeper increases were observed with the ferrocene donor moiety as compared to the strong julolidine donor group, and with the stronger dicyanovinyl acceptor (a = 2.4 and 1.45), as compared to the weak formyl acceptor (a = 1.6 and 1.3). In contrast, the series of push-pull diphenylpolyenes display the weakest dependencies, and show very little influence of the terminal acceptor substituents on the length behavior (a ≈ 1).

Solvent effect on the intramolecular charge transfer of zwitterions. Structures and quadratic hyperpolarizabilities

Abstract Solvent effect on intramolecular charge transfer of zwitterions was investigated by combining nuclear magnetic resonance and non-linear optical measurements. 1 H nuclear magnetic resonance spectra of two phenoxide-pyridinium derivatives evidenced large chemical shifts on changing solvent polarity. In order to check if the ground state geometry is changed, spin-lattice relaxation rates for one compound were measured and the angle θ between the two aromatic rings calculated. The value of θ appears to be large and solvent independent within estimated accuracy. These results indicate that structural changes are less important than expected, and chemical shifts on rising solvent polarity result mainly from increasing donor to acceptor charge transfer in the ground state. Consequently, the benzenoid resonance form seems to dominate the ground state structure. Furthermore, the weight of this contribution increases with solvent polarity. The sign of the electric field induced second harmonic mean hyperpolarizability is negative for all three compounds. This also indicates that the benzenoid contribution dominates the ground state geometry in polar solvents. Moreover, the absolute value of the dipole quadratic hyperpolarizability product increases sharply on rising solvent polarity. This behaviour implies an enhancement of donor to acceptor charge transfer taking place in the ground state, and also of acceptor to donor charge transfer resulting from the transition ground state to first excited state.

Nonlinear optical properties of push–pull polyenes for electro-optics

Improved nonlinear organic chromophores of varying conjugation length with either thiobarbituric acid or 3-dicyanomethylene-2,3-dihydrobenzothiophene-1,1-dioxide (FORON® Blue) acceptors have been synthesized and investigated for their nonlinear optical properties. Very large quadratic hyperpolarizabilities β(−2ω; ω, ω) have been found, up to 25,700×10^(−48) esu at λ=1.91 μm. In a guest–host polymer very high electro-optic (EO) coefficients, of up to 55 pm/V, have been determined at λ=1.31 μm with 20-wt % chromophore loading. We find good agreement between molecular parameters evaluated by electric-field-induced second-harmonic generation (EFISH) and the measurements of guest–host solid–solid solutions. The latter method is well suited to the determination of the product of dipole moment μ and hyperpolarizability β quickly and reliably at the wavelength of interest for EO applications without the complications associated with EFISH measurements.

Intramolecular dipolar coupling enhancement of the first-order molecular hyperpolarizability in a polar solvent

Abstract The effect of intramolecular coupling on the first-order molecular hyperpolarizability in a bis-chromophoric system was theoretically and experimentally investigated. According to finite-field calculations using AM 1 parameters, the effective hyperpolarizability of a bis-chromophore is essentially a vectorial addition of that of the corresponding monomers. However, the EFISH-derived effective hyperpolarizabilities of the bis-chromophores do not follow the additive model. Interestingly, the effective hyperpolarizabilities of the bis-chromophores are significantly enhanced as the solvent polarity increases, which is not observed in those of the corresponding monomeric counterparts.

Phosphine oxides as novel acceptor groups in molecules presenting non-linear optical properties

In search for molecules possessing high hyperpolarizabilities, β, the diphenylphosphine oxide group is used as a novel acceptor. Three phosphorus model compounds were synthesized and their non-linear optical (NLO) properties investigated. The results obtained from EFISH measurements are complemented by semi-empirical calculations performed with the AM1 hamiltonian method taking account of the angle, θ, between the dipole moment, µ, and the vector part of the tensor, β. Their high non-linear efficiencies [βx(O)=38–63 × 10–30 esu] make these compounds interesting for their potential use as NLO materials.

Push-pull polyenes and carotenoids with enhanced quadratic nonlinear optical susceptibilities

A wide choice of push-pull polyenes and carotenoids of increasing length (up to 30 angstrom) and bearing various donor and acceptor end groups has been synthesized in order to investigate the chain-length dependence of their quadratic hyperpolarizability (beta) . (beta) measurements have been performed using the electric field induced second harmonic generation (EFISH) technique. In each series of homologous compounds, the lengthening of the conjugation path results in a pronounced increase in (mu) (beta) values. This behavior leads to very large static (mu) (beta) (0) values for the longest molecules and can be modeled by (mu) (beta) (0) equals kna relationships with respect to the number n of double bonds in the polyenic chain. The exponent value (a) was found to depend on the end groups. The replacement of a double bond by a triple bond in the middle of the polyenic chain results in a blue shift of the charge transfer absorption band and leads to smaller (mu) (beta) (0) values. However, this phenomenon is modulated by the end groups.

Solvent-dependent quadratic hyperpolarizabilities of two zwitterions

We have investigated the effect of solvent polarity on the conformations of two zwitterions. A Nuclear Magnetic Resonance (NMR) study shows that changing the solvent results in large intramolecular charge transfer. However NMR measurements indicated that a benzenoid ring structure predominates the ground state structure for both zwitterions in polar and non polar solvents. In spite of their very different solvatochromic behaviors, both compounds present a sharp enhancement of the quadratic hyperpolarizability on increasing the solvent polarity.