Hélène Chaumeil

Two-photon absorption and polymerization ability of intramolecular energy transfer based photoinitiating systems.

We design a new photoinitiating system where the two-photon absorption of a 2,7 bisaminofluorene moiety leads to the photoactivation of a camphorquinone subunit through a Förster-type intramolecular energy transfer: the application to a two-photon polymerization reaction is demonstrated.

One-pot structural conversion of magadiite into MFI zeolite nanosheets using mononitrogen surfactants as structure and shape-directing agents

MFI-type materials with a lamellar morphology were successfully synthesized by using mononitrogen surfactants specifically designed by molecular modelling. The mononitrogen surfactants directed the recrystallization of a crystalline layered polysilicate formed in situ, the magadiite, into a zeolite ZSM-5. Moreover, the surfactants allow the preservation of the lamellar shape of the magadiite and inhibit a further growth into one dimension, leading to the formation of zeolite ZSM-5 nanosheets with a thickness comprised between 2 and 3 nm and a Si/Al ratio of 24. This simple approach paves a new way for obtaining zeolite materials of controlled size and shape for specific catalytic applications.

Abnormal enhancement of the photoisomerization process in a trans-nitroalkoxystilbene dimer sequestered in β-cyclodextrin cavities.

We report on the synthesis and the photophysical properties of a trans-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Φ(f)), the Stokes shift, and the quantum yield for the trans-to-cis photoisomerization (Φ(t→c)) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Φ(f) increases together with the decrease of Φ(t→c). This solvent-induced reverse behavior mainly stems from the progressive stabilization of a highly polar twisted internal charge transfer state (TICT) at excited singlet level which opens a competing channel to photoisomerization. In the presence of hydroxylic substrates (i.e., alcohols or water), fluorescence of DPNS is strongly quenched due to a hydrogen bonding interaction at excited state. The efficiency of the process is clearly correlated to the H-bond donor ability of the quencher. In aqueous solution, the major formation of a 2:1 host-guest complex with β-cyclodextrins (β-CD) prevents the quenching by H(2)O and leads to a 50-fold increase of the fluorescence signal together with a strong band blue-shift with respect to that of the free chromophore. This latter effect was rationalized in terms of a severe reduction of the solvent-induced stabilization of the TICT state. As a consequence, the trans-to-cis photoisomerization reaction is reactivated and leads to a paradoxical 14-fold increase of Φ(t→c) even though DPNS is sequestered in β-CD cavities.

Photoinduced intramolecular electron transfer in a 2,7-diaminofluorene chromophore decorated with two benzophenone subunits

An extensive photophysical analysis of a 2,7-bis-(N-4-methoxyphenyl-N-phenylamino)fluorene derivative covalently linked with two benzophenone moieties is presented. A systematic comparison with a model chromophore without benzophenone was performed. For both chromophores, the electronic properties of the ground states are completely equivalent indicating that benzophenone subunits do not exhibit any electronic interaction with the diaminofluorene core. However, at the singlet excited state, the presence of benzophenones induces the occurrence of additional non-radiative de-excitation pathways. Even the intersystem crossing rate is significantly increased with respect to that of the model one. A photoinduced intramolecular electron transfer (PIET) from diaminofluorene to benzophenone subunits is proposed as the most efficient quenching process. At low polar solvent, the emission of an exciplex confirms the PIET process and the occurrence of a partial charge separation between donor and acceptor parts.

Electronic responses of donor acceptor substituted twisted biphenyls

Donor-acceptor biphenyl derivatives are particularly interesting model compounds presenting a charge transfer absorption band because the extent of conjugation, and therefore the amount of charge transfer between the substituents, may be varied by a controllable structural feature, namely the torsion angle between the two phenyl rings. This feature has become even more interesting since chromophores of strong zwitterionic character, synthesized with a twist of nearly 90°, have been shown to exhibit unprecedented quadratic responses [1], over an order of magnitude above that of the best conventional push-pull chromophores. In this context, we have investigated both experimentally and theoretically, two biphenyl based systems with varying inter-aryl angles: a nitro-piperidinyl series [2] of conventional push-pull character and a pyridinium-phenoxide series [3] of zwitterionic character. The results agree qualitatively with semiempirical simulations based on the AM1 Hamiltonian [4] used with the COSMO solvation model [5]. For the first series, the decrease in quadratic response with increasing dihedral angle indicates that oscillator strength loss is the dominant factor. In the second series, the corresponding increase in quadratic response points to the change in dipole moment upon excitation as the leading effect. Here, we will analyze to what extent the more ab initio electronic structure calculations based on the density functional theory may provide more quantitative results in spite of the problems they face in the description of charge transfer systems.

Recent advances in two-photon 3D laser lithography with self-Q- switched Nd:YAG microchip lasers

We review our recent results towards the development of a turnkey 3D laser printer, based on self-Q-switched microchip Nd:YAG lasers, with reproducible sub-100nm resolution, and with large-scale (cm) and fast-speed (cm/sec) capability at micron resolution. First of all, we report on line fabrication with 70nm lateral, and 150nm longitudinal resolutions without significant shrinking. This is due to the tight focusing with green visible wavelength, large numerical aperture, and excellent resin properties. Secondly, we report on two-photon sensitive photoacid generators that lead to efficient 3D microfabrication with epoxy SU-8 resin. Thirdly, we demonstrate high-speed microfabrication of large scale, millimeter size, scaffolds and cemtimeter height needle with high repetition rate (130Khz), and high average power (1W) amplified microchip laser. Finally we demonstrate the two-photon induced cross-linking of antibodies to determine the type of red blood cells in microfluidic channels.

Environmental and steric effects on electronic and vibrational properties of pyridinium phenoxide derivatives

Our present understanding of the dependence of non linear optical (NLO) properties of charge transfer compounds on their dielectric environment is generally summarized as a set of universal response functions versus the ground state ionicity. Experimentally, these behaviors are recovered by piece-wise assembling measurements performed in solvents of increasing polarities on series of chromophores with varying acceptor and donor groups and/or conjugation paths. In this work, we will take advantage of the recent success in synthesizing pyridinium phenoxides with or without tert-butyl oxygen protection groups as well as with or without steric methyl groups to modify the twisting of the central diaryl bond. Thus, we are in a position to sweep over a wide range of zwitterionic character with essentially the same chromophore. This opens up a great opportunity to investigate the relationships between geometrical structure and NLO properties, to examine the validity of current formalisms, and to test numerical simulations at the semi empirical and density functional levels. In this work, we have carried out an experimental and theoretical study combining UV visible, IR, Raman, and Hyper-Rayleigh spectroscopies to extract information concerning the geometry, the electronic structure, and the NLO response of our compounds. In particular, we show that the steric effect is sufficient to push the chromophore to the full zwitterionic limit. More generally, the approach we followed here shows great potential in probing chromophore-environment interactions.

Synthesis of a New Diaazacrown Ether Compound Interconnected with an Azacrown Ether and Decorated with a Long Lipophilic Chain

Abstract The synthesis of an original compound consisting of an azacrown ether interconnected with a diazacrown ether bearing an alkyl chain is described herein. This derivative is promising for numerous applications. GRAPHICAL ABSTRACT

Sterically hindered pyridinium phenoxides as chromophores for quadratic optics

Polymers doped with non-linear optical (NLO) molecules are key materials in the elaboration of organic NLO devices. In this field, there is an ongoing need for chromophores with large dipole moments and optical non linearities. Here, we consider pyridinium phenoxides, a type of zwitterionic biphenyl-like molecule. A combination of mathematical modelling and some preliminary experimental measurements indicate that the NLO properties of these molecules depend on the twist angle existing between the two aromatic rings. In order to corroborate this structure/activity relationship, different sterically hindered pyridinium phenoxides were synthesized using the Suzuki coupling reaction involving a boronic ester and an aryl halide. We analyze the solvatochromism of the substituted zwitterions in details, determine the chemical equilibrium of protonation and perfom nonlinear optical measurements which are interpreted with the help of semi-empirical calculations.