Valérie Alain

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).

Elongated push–pull diphenylpolyenes for nonlinear optics: molecular engineering of quadratic and cubic optical nonlinearities via tuning of intramolecular charge transfer

Abstract Push–pull polyenes are of particular interest for nonlinear optics (NLO) as well as model compounds for long-distance intramolecular charge transfer (ICT). In order to tune the ICT phenomenon and control the linear and nonlinear optical properties, we have synthesized and investigated several series of soluble push–pull diphenylpolyenes of increasing length and having various donor (D) and acceptor (A) end groups. Their linear and NLO properties have been studied by performing electro-optical absorption measurements (EOAM) and third-harmonic generation (THG) experiments in solution. Each push–pull molecule exhibits an intense ICT absorption band in the visible characterized by an increase in dipole moment upon excitation (Δ μ ). By lengthening the polyenic chain, linear increases in excited-state dipoles are achieved while the ground-state dipole is maintained constant. The polyenic chain thus allows for long-distance intramolecular charge transfer. In addition, tuning of the magnitude of the ICT phenomenon and of the nonlinear responses is achieved by varying the donor and acceptor end groups: increasing the D/A strength leads to concomitant enhancements of Δ μ , quadratic ( β ) and cubic ( γ ) hyperpolarizabilities values. Giant Δ μ values (up to more than 30 D) and enhanced non-resonant quadratic and cubic NLO molecular responses (i.e. β (0)=500×10 −30 esu and γ (0)=8000×10 −36 esu) have been attained while maintaining suitable solubility, stability and transparency.

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.

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.

Novel photorefractive sol-gel materials

We have developed new photorefractive media based on hybrid organic-inorganic materials containing a charge transporting (CT) molecule either as side-chain or main-chain substituents on the silica backbone. Second order nonlinear optical (NLO) chromophores were introduced either as side chain or as guest units. These materials were prepared by the sol-gel process in the form of thin films of a few μm-thick. NLO and photorefractive properties have been evaluated using electro-optic measurements, two beam coupling experiments and photoconductivity measurements.

Optical and electrochemical properties of soluble donor-π-donor compounds as potential molecular wires and electrochemically-triggered optical switches

With the aim of designing molecular wires operating at easily accessible potentials, a series of soluble bis-donor polyenic compounds (up to 3.2 nm long) has been prepared. Their optical and electrochemical properties have been investigated. These model compounds were shown to exhibit a reversible two-electron oxidation process at low potential (which further decreases with increasing polyenic chain length). The oxidized dicationic species combine convenient stability and solubility. Interestingly, these species also show reduced bond length alternation along the polyenic chain, consistent with a significant red-shift of their absorption band to the NIR region. Such behaviour is expected to lead to a dramatic enhancement of the third-order nonlinear optical responses of the polyenic molecules upon oxidation, opening the way to optical switches. Hence, such molecules represent promising model compounds for molecular wires but also open an innovative route towards voltage-driven optical switches.

Amphiphilic polyenic push–pull chromophores for nonlinear optical applications

Amphiphilic cationic polyenic push–pull chromophores which offer interesting supramolecular possibilities for second harmonic generation have been synthesised and their optical non-linearities studied for the first time by electric field induced second harmonic (EFISH) generation in solution.

Toward stable materials for electro-optic modulation and photorefractive applications: the hybrid organic-inorganic approach

We have developed hybrid organic-inorganic materials based upon the incorporation of nonlinear chromophores in a rigid amorphous inorganic matrix. Functionalized thin films have been prepared using the sol-gel route allowing for mild synthesis conditions. Push-pull chromophores were either incorporated as guests or grafted on a silica based backbone via a spacer. Orientation of the dipolar chromophores within the materials was performed using the Corona technique. By playing on the structure of the push-pull chromophores and on its orientational stability inside the matrix, functionalized materials with large electro-optic coefficients and excellent stability have been designed. By combining both appropriate push-pull chromophores and photoconductors grafted on the matrix, materials showing interesting photorefractive properties can be obtained without requiring the application of an external electric field. Both the presence of a strong internal field in the poled materials and the occurrence of photo-assisted orientational birefringence plays a significant role.

Structure/quadratic hyperpolarizability correlation: solvent effect

In this work, we have investigated theoretically the structure/hyperpolarizability correlations of push-pull molecules sing a two-form two-state model. For this, we have defined a parameter MIX characterizing the mixing between the two-limiting resonance forms and thus ruling the molecular structure and polarization. Also, we have analyzed the solvent effect on the structure and on the polarizabilities of push-pull molecules using the Onsager reaction field theory. We have shown that the dependences of the linear and nonlinear polarizabilities on the solvent dielectric constant look similar to the structure/(hyper)polarizability correlations. Finally, we have investigated experimentally the solvent effect on a series of push-pull polyenes of increasing length. In particular, we have determined the scalar (mu) g(Beta) (0) product (where (mu) g is the ground-state dipole and (Beta) (0) the vector part of static quadratic hyperpolarizability tensor) in different solvents using the electric-field-induced-second-harmonic generation technique. For the different chain lengths, we have obtained a mapping of the positive (mu) g(Beta) (0) peak. This leads for the longest compound of the series to a record high value of (mu) g(Beta) (0) product in chloroform.