Marguerite Barzoukas

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).

Nonlinear optical chromophores containing dithienothiophene as a new type of electron relay

Nonlinear optical (NLO) donor-acceptor (D-A) chromophores containing a fused terthiophene, namely dithienothiophene (DTT) as electron relay, and various strength acceptors were synthesized to evaluate the relays role in the solvatochromism as well as in the molecular nonlinearity, µβ(0). Their solvatochromic behavior, which is dependent on acceptor strength, is correlated to their µβ(0). The static quadratic hyperpolarizability, β(0), of DTT-containing chromophores was determined by EFISH measurements. From the comparison of µβ(0) values of D-A chromophores based on oligothiophenes, it was assessed that DTT as relay is significantly more efficient compared to bithiophene (the same number of conjugated double bonds) and terthiophene. An additional merit of DTT is that chromophores containing DTT have a noticeably high thermal stability.

Boron derivatives containing a bithiophene bridge as new materials for non-linear optics

New molecules with dimesitylboron as the acceptor group and bithiophene as the unsaturated chain have been synthesized and their non-linear optical properties investigated. The theoretical first hyperpolarizabilities determined by calculations are compared with experimental results.

The linear and non-linear optical properties of some conjugated ferrocene compounds with potent heterocyclic acceptors

Abstract We have synthesized donor-acceptor chromophores containing ferrocene as the donor and several different heterocyclic acceptors. In each case the chromophores have an intense band at low energy which is ascribed to a metal to ligand charge-transfer band. Neutral compounds containing the 1,3-diethyl-2-thiobarbituric acid and dicyanomethylene-2,3-dihydrobenzothiophene-1,1-dioxide acceptors have large first molecular hyperpolarizabilities (β) in comparison to a previously examined compound 1-ferrocenyl-2- E -(4nitrophenyl)ethylene, as measured by electric field induced second harmonic generation (EFISH), in chloroform, with 1.907,μm fundamental radiation. Incorporation of 1-ferrocenyl-4- E,E (4-nitrophenyl)butadiene into polymethylmethacrylate at 2% molar loading, yielded after poling at 100 V μm −1 , a polymer film with an r 33 = 0.8 pm V −1 measured at 820 nm.

Synthesis and stability studies of conformationally locked 4-(diarylamino)aryl- and 4-(dialkylamino)phenyl-substituted second-order nonlinear optical polyene chromophores

A series of chromophores with high second-order nonlinearities has been synthesized; the chromphores consist of triarylamine or dialkylarylamine donors linked by a conformationally locked polyene bridge to a dicyanomethylidene acceptor. The use of bridges of this type, combined with the replacement of dialkylarylamine with triarylamine donors, leads to high thermal stability without adverse affects on the nonlinear optical properties of the chromophores.

Intramolecular charge transfer and enhanced quadratic optical non-linearities in push-pull polyenes

Abstract Push-pull polyenes, which have an electron-donating group (D) and an electron-withdrawing group (A) grafted on opposite ends of a conjugated polyenic chain, are of particular interest as model compounds for long-distance intramolecular charge transfer (ICT), as well as potent non-linear optical chromophores. Several series of push-pull polyenes of increasing length, combining aromatic donor moieties and various acceptor groups, have been prepared and studied. Their linear and non-linear optical properties have been investigated by performing electro-optical absorption measurements (FOAM) and electric-field-induced second-harmonic generation (EFISH) experiments in solution. Each molecule shows a broad and intense ICT absorption band in the visible associated with an increase in the dipole moment (Δμ). Lengthening the polyenic chain linking the D and A groups results in a bathochromic shift of the ICT absorption band and induces a linear increase in the excited state dipole. In contrast, the ground state dipole remains roughly constant. As a result, the longest molecules exhibit huge Δμ values (up to 42 D) as well as markedly enhanced quadratic hyperpolarizabilities (β). In addition, the nature of the end groups has been found to influence strongly both the ICT and optical non-linearities: larger β and Δ μ values, as well as steeper length dependences, are obtained with push-pull phenylpolyenes bearing strong acceptors.

Large optical nonlinearities with conjugated ferrocene and ruthenocene derivatives

Abstract Donor—acceptor polyenes of increasing length combining the ferrocene or ruthenocene electron-donating moieties and different electron-withdrawing end groups have been synthesized in order to achieve enhanced quadratic optical nonlinearities. These compounds exhibit two absorption bands, the less intense red-shifted one being attributable to a metal-to-ligand charge transfer (MLCT) transition, and the sharper and more intense blue-shifted one to a π → π* charge transfer transition. Both bands exhibit positive salvatochromic behavior and red-shifted with increasing polyenic chain length and stronger acceptors. In addition, the lowest energy band gains in relative intensity, suggesting increased coupling between the metal center and the acceptor. The varition of their quadratic hyperpolarizability β in solution has been investigated using the electric-field-induced second-harmonic (EFISH) generation technique, at 1.907 μm in chloroform. In each series of homologous compounds, lengthening the conjugation path results in a marked increase in μβ values. This phenomenon leads to very large μβ values. The potent 3-(dicyanomethylidene)-2,3-dihydrobenzothiophene-1,1-dioxide acceptor moiety leads to even higher quadratic nonlinearities than the effective electron-withdrawing 1,3-diethyl-2-thiobarbituric acid end group.

Crystal structures and non-linear optical properties of borane derivatives

[4′-(Dimethylamino)biphenyl-4-yl]dimesitylborane (BNB) and 4-[4′-(Dimethylamino)pnenylazo]phenyldimesityl-borane (BNA) have been investigated by X-ray diffraction; crystallographic data were used to calculate dipole moments in the ground state and the hyperpolarizability coefficient βo was measured by the EFISH technique for each derivative. The results show that the trivalent boron group is an efficient electron acceptor compared to other known acceptors such as the nitro group. Organoboron derivatives are promising candidates for nonlinear optical materials.