Lionel Ventelon

Optimization of quadrupolar chromophores for molecular two-photon absorption

Abstract Multiphoton absorption is attracting considerable attention owing to applications in various fields including optical power limitation, and high resolution imaging of biological tissues. Within this framework, our aim has been the design of optimized NLO-phores with very high two-photon absorption (TPA) cross-sections in the visible and/or NIR region and high fluorescence quantum yield. Our strategy was based on the push–push or pull–pull functionalization of rigid conjugated cores built from a rigidified biphenyl or bithiophene moiety. A series of push–push and pull–pull elongated chromophores have been prepared and their nonlinear absorption properties investigated by determining their two-photon excited fluorescence (TPEF) characteristics. The TPA spectra of theses derivatives can be tuned and their TPA responses boosted by playing on the nature of the rigid core and electroactive end groups. As a result, quadrupolar molecules showing very large TPA cross-sections (up to 1.3×10 −47 cm 4 s photon −1 ) in the visible red and/or NIR region have been obtained. Such derivatives are promising for optical limitation applications as well as for TPEF imaging of cellular membranes.

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.

A convenient synthesis of push-pull polyenes designed for the elaboration of efficient nonlinear optical materials

Abstract We present a convenient and versatile synthetic approach towards push-pull phenylpolyenes and diphenylpolyenes bearing a hydroxyl functional group. These chromophores are of particular interest for the design of efficient nonlinear optics materials due to their large optical nonlinearities, their transparency in the near IR and their capacity to be incorporated into polymeric materials.

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.

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 probes for nonlinear optical imaging of biological membranes

Second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) are nonlinear optical (NLO) phenomena that scale with excitation intensity squared, and hence give rise to an intrinsic 3-dimensional resolution when used in microscopic imaging. TPEF microscopy has gained widespread popularity in the biology community whereas SHG microscopy promises to be a powerful tool because of its sensitivity to local asymmetry. We have implemented an approach toward the design of NLO-probes specifically adapted for SHG and/or TPEF imaging of biological membranes. Our strategy is based on the design of nanoscale amphiphilic NLO-phores. We have prepared symmetrical bolaamphiphilic fluorophores combining very high two-photon absorption (TPA) cross-sections in the visible red region and affinity for cellular membranes. Their incorporation and orientation in lipid membranes can be monitored via TPEF anisotropy. We have also prepared amphiphilic push-pull chromophores exhibiting both large TPA cross-sections and very large first hyperpolarizabilities in the near-IR region. These NLO-probes have proved to be particularly useful for imaging of biological membranes by simultaneous SHG and TPEF microscopy and offer attractive prospects for real-time imaging of fundamental biological processes such as adhesion, fusion or reporting of membrane potentials.

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.

Molecular probes for two-photon excited fluorescence and second harmonic generation imaging of biological membranes

Novel microscopies based on nonlinear optical (NLO) phenomena are attracting increasing interest in the biology community owing to their potentialities in the area of real-time, non-damaging imaging of biological systems. In particular, second-harmonic generation (SHG) and two-photon excited fluorescence (TPEF) are NLO phenomena that scale with excitation intensity squared, and thus give rise to an intrinsic 3-dimensional resolution when used in microscopic imaging. In this perspective, we have implemented a molecular engineering approach toward NLO-probes specifically designed for SHG and/or TPEF imaging of cellular membranes. We have designed nanoscale rod-like fluorophores showing very large TPEF cross-sections in the visible red, outperforming standard fluorophores such as fluorescein by up to two orders of magnitude. Bolaamphiphilic derivatives combining high TPEF cross-sections and affinity for cellular membranes were prepared. Their incorporation into model or cell membranes can be monitored by TPEF microscopy. Amphiphilic push-pull chromophores showing both high TPA and SHG cross-sections in the near-IR region were designed as NLO-probes for imaging of biological membranes by simultaneous SHG and TPEF microscopy. These NLO-phores offer intriguing potentialities for imaging of fundamental biological processes such as adhesion, fusion or for reporting of membrane electrical potentials.

Towards Stable Materials for Electro-Optic Modulation and Photorefractive Applications

A large effort have been devoted to the preparation of organic polymeric materials for electro-optic modulation [1, 2, 3]. These materials contain push-pull chromophores (i.e. combining electron-releasing and electron-withdrawing groups interacting through a conjugated linker) either incorporated as guest in the polymeric matrix (doped polymers) or grafted onto (or into) the polymeric matrix (functionalized polymers) [45]. By heating the polymer above its glass transition temperature (T g ), orientation of the dipolar chromophores can be achieved via application of a strong external electric field. After cooling to room temperature, noncentrosymmetrical poled-polymeric materials are obtained. Such materials are interesting in view of both processability and chemical flexibility that allows for “engineering” of the nonlinear responses. When optimized chromophores are incorporated and poled in polymers, materials with large electrooptical coefficient can in principle be obtained. However, besides the optimization of chromophores aiming at obtaining large molecular figure of merit (EOM), a number of additional issues are to be considered if materials with both large and permanent nonlinear responses are to be achieved. For instance, an interesting way to improve the orientational stability consists in using polymers with very high T, (such as polyimides for instance) [6]. However, this demands organic chromophores with high thermal stability. Also, increasing the chromophore concentration in order to have larger χ (2) (and electro-optic coefficient) values can be detrimental since dipolar interactions between push-pull chromophores can favor antiparallel association thus impeding the poling process, as pointed out by Dalton and coworkers [7].