Mireille Blanchard-Desce

Very large quadratic nonlinearities in solution of two push-pull polyene series: Effect of the conjugation length and of the end groups

Abstract By use of the electric-field-induced-second-harmonic generation (EFISH) technique in solution the quadratic hyperpolarisability (β) of two series of push-pull polyenes of increasing length is measured. It is shown that the sharp increase of β with the number of double bonds n is not exponential, the dependence being in n 2.4 for one of the series. For the longest ever measured compounds ( n = 8) the static μβ(0) values are exceptionally large, about 50 times that of 4-nitroaniline, while the effect still does not level-off. The screening effect of the triple bond, as compared to the double bond, is also evidenced. The benzodithia group is shown to be less effective as an electron donor than the dimethylanilino one, however, this difference appears to fall-off as n increases,

Chain-length dependence of the quadratic hyperpolarizability of push-pull polyenes and carotenoids. Effect of end groups and conjugation path

Abstract Numerous push-pull polyenes and carotenoids of increasing length and bearing various donor and acceptor end groups have been synthesized in order to investigate the chain-length dependence of their quadratic hyperpolarizability β. The variation of β was studied using the electric-field-induced second-harmonic (EFISH) generation technique. In each series of homologous compounds, the lengthening of the polyenic conjugation path results in a sharp increase in β with the number n of double bonds of the polyenic chain. This behaviour leads to very large values of β for the longest molecules whose length reaches 25 A. A steeper increase of β was observed when the end groups are weaker electron donors and electron acceptors. Moreover, β-kna relationships have been established, the a exponent depending on the end groups. Finally, the effect of the introduction of a triple bond in the middle of the polyenic linker has been studied. The replacement of a double bond by a triple bond in the conjugation path leads to smaller β and to a blue shift of the charge transfer absorption band. However, this phenomenon is modulated by the nature of the end groups. Therefore, in some cases, the presence of the triple bond may advantageously result in a gain of transparency with almost no decrease in β.

Broadband optical limiting optimization by combination of carbon nanotubes and two-photon absorbing chromophores in liquids

Nowadays, it seems evident that a unique nonlinear optical (NLO)material cannot offer simultaneously linear transparency,colour neutrality and broadband optical limiting efficiency at the performance levels required for sensor and eye protection against all laser threats.Several combinations of NLO materials were investigated last few years, including multicell or multilayer geometries. The approach presented here combines multiphoton absorption with nonlinear scattering. For that purpose, singlewall carbon nanotubes are suspended in various solutions of multiphoton absorbing chromophores. Such combinations allow us to obtain optical limiters of high linear transmittance and excellent colour neutrality. Broadband optical limiting is expected from the association of these two broadband materials,and enhanced optical limiting efficiency is expected from cumulative effects in the nanosecond regime. We report here on the optical limiting studies performed with nanosecond laser pulses on several families of multiphoton absorbers in chloroform,with carbon nanotubes suspended in the solutions. The performances of these samples are compared with those of simple multiphoton absorber solutions and carbon nanotube suspensions, and the differences observed are interpreted in terms of cumulative NLO effects and adverse aggregation phenomenon. Ways to optimise stability of the suspensions are also experimented and discussed.

A novel amphiphilic ferrocene derivative containing a barbituric acid unit: synthesis and quadratic optical non-linearity

Abstract A novel amphiphilic ferrocene derivative ( P ) containing barbituric acid was designed and synthesized. It has a π-donor-switch-acceptor structure, in which the N , N -dioctadecylaniline unit serves as the electron donor, the ferrocene moiety as the switch, and the barbituric acid unit as the electron acceptor. The non-linear optical experiments show the lipid P displays efficient optical second harmonic generation (SHG) with a molecular hyperpolarizability (β) as high as 1.8 × 10 −28 e.s.u. The lipid P alone is unable to form a high quality monolayer at the gas-water interface. The molecular recognition of the lipid P and its complementary substrate-melamine derivative ( M ), at the gas-water interface, may significantly improve the monolayer formation of the lipid P . The improvement of the monolayer has been found to exhibit a temperature-dependent effect: at about 24.5 °C it reaches the optimum.

Second harmonic generation in mixed carotenoid-fatty acid and carotenoid-cyclodextrin Langmuir-Blodgett films

In this paper we report on the second harmonic generation properties of non-centrosymmetrical Langmuir-Blodgett films built from mixtures of push-pull carotenoids with ω-tricosenoic acid or amphiphilic cyclodextrin. The effects of the carotenoid as well as of the diluent on the orientational order have been studied both in monolayers and alternate active-passive multilayers.

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.

Biphenyl derivatives with enhanced nonlinear absorptivities for optical limiting applications

Novel conjugated chromophores were designed and investigated for optical power limitation based on multiphoton absorption processes. Their design is based on the push-push functionalization of a semi-rigid elongated system derived from the extension of biphenyl cores. Biphenyl moieties with tunable twist angle were examined. Phenylene-vinylene rods were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency. These derivatives combine wide linear transparency and enhanced nonlinear absorptivities in the visible range. Pump-probe Kerr ellipsometry indicates large excited-state absorption cross-sections (with typical σe values of 5 10-16 cm2) while nanosecond nonlinear transmission measurements and optical limitation experiments reveal very strong nonlinear absorption that can be fitted by a three-photon absorption process (leading to α3 values up to 18000 cm3 GW-2). Such behavior results from a sequential multiphoton process involving excited-state absorption subsequent to two-photon excitation (with typical σ2 values of 5 10-20 cm4 GW-1). Both the linear transparency, the photostability and the nonlinear absorption spectral characteristics of these derivatives can be tuned by playing on the biphenyl twist angle. As a result, chromophores combining good linear transparency and enhanced nonlinear absorptivities in the visible range have been obtained.

Molecular engineering of nanoscale quadrupolar chromophores for two-photon absorption

Our aim has been the design of optimized NLO-phores with very high two-photon absorption (TPA) cross-sections (s2) in the red-NIR region, while maintaining high linear transparency and high fluorescence quantum yield. Our molecular engineering strategy is based on the push-push or pull-pull functionalization of semi-rigid nanoscale conjugated systems. The central building blocks were selected as rigid units that may assist quadrupolar intramolecular charge transfer by acting either as a (weak) donor or acceptor core. Quadrupolar molecules derived either from a phenyl unit, a rigidified biphenyl moiety or a fused bithiophene unit have been considered. Conjugated oligomers made of phenylene-vinylene and/or phenylene-ethynylene units were selected as connecting spacers between the core and the electroactive end groups to ensure effective electronic conjugation while maintaining suitable transparency/fluorescence. The TPA cross-sections were determined by investigating the two-photon-excited fluorescence properties using a Ti:sapphire laser delivering fs pulses. Both the nature of the end groups and of the core moiety play an important role in determining the TPA spectra. In addition, by adjusting the length and nature of the conjugated extensor, both amplification and spectral tuning of TPA cross-sections can be achieved. As a result, push-push fluorophores which demonstrate giant TPA cross-sections (up to 3000 GM) in the visible red, high fluorescence quantum yields and good transparency in the visible range have been obtained.

Nanoscale boomerang-shaped octupolar molecules derived from triphenylbenzene

The octupolar framework provides a promising route towards molecular compounds combining enhanced NLO responses and improved nonlinearity-transparency trade-off. In this perspective, we have designed original three-branched boomerang-shaped nanoscale molecules. Their molecular design is based on the grafting of three conjugated blades bearing either an electron-withdrawing or an electron-releasing end group on a triphenylbenzene core which can act as a (weak) donor or acceptor counterpart. We selected oligomeric phenylene-vinylene conjugated rods to allow for efficient charge transfer between the center and the periphery of the molecule while preserving transparency. Based on this strategy, we have prepared homologous nanoscale molecules with size varying between 2 and 5 nm. These molecules exhibit a definite solvatochromic behavior, in consistence with a multidimensional intramolecular charge transfer (MDICT) taking place between the core and the peripheral groups. Large first-order hyperpolarizabilities could be achieved by taking advantage and boosting of the MDICT phenomenon while maintaining wide transparency in the visible region (up to ||β|| = 800 10-30 e.s.u., with λmax = 377 nm). The superlinear dependence of β on size and their concave shape make elongated analogues attractive candidates for future developments.

Hyperpolar multichromophoric nanoassembly for molecular nonlinear optics

A multichromophoric nanoassembly was designed by gathering seven push-pull chromophores on a β-cyclodextrin assembling unit via covalent linkers. Such supermolecule provides a valuable model for the investigation of confinement effects on the linear and nonlinear optical properties of push-pull chromophores in the condensed phase. Push-pull chromophores display a significant ground-state dipole, thus promoting dipolar interactions that are expected to influence both the conformation and the optical properties of the multichromophoric assembly. In this perspective, the photophysical and nonlinear optical properties of the mutichromophoric bundle were investigated and compared to those of the monomeric chromophore. The absorption, circular dichroism and fluorescence investigations provide evidence that the push-pull chromophores do not behave as isolated independent chromophores within the multichromophoric assembly. The nanoscale supermolecule is hypsochromically and significantly hypochromically shifted with respect to its monomeric analogue. In addition, the close proximity promotes excitonic coupling, as well as excimer formation phenomena. The nanoscopic assembly also shows a very large dipolar moment (μ = 38 D), and a significant molecular first-order hyperpolarisability, which reveal a spontaneous sheaf-type self-arrangement of the dipolar chromophores within the supermolecule. Such chiral hyperpolar nanoassemblies are promising candidates as model systems for nanophotonics.