Davy-Louis Versace

Importance of the position of the chromophore group on the dissociation process of light sensitive alkoxyamines.

New photosensitive alkoxyamines have been designed using molecular orbital calculations to improve the selective CO versus NO cleavage. The targeted light-sensitive alkoxyamine is synthesized in one step and its reactivity under UV has been investigated using both ESR and LFP. The ability of this alkoxyamine to control the photopolymerization of n-butyl acrylate is evidenced through a process called nitroxide-mediated photopolymerization NMP(2) . The selected alkoxyamine is finally used to prepare covalently bonded multilayered micropatterns. This original application highlights the high potential of this technique for some specific applications that require spatial control.

Photodissociation rate constants of new light sensitive alkoxyamines.

The photochemical properties of two new photocleavable alkoxyamines bearing a benzophenone-derived chromophore were studied by electron spin resonance (ESR). The CO bond cleavage has been demonstrated and the photolysis rate constants (k(d) ) determined over a large range of light intensity through the monitoring of the nitroxide concentration in aerated conditions. The obtained kinetic data highlight for the first time the linear dependence of k(d) on the light intensity for alkoxyamines: this should be a driving factor for nitroxide mediated radical photopolymerization (NMP(2) ).

π-conjugated sulfonium-based photoacid generators: an integrated molecular approach for efficient one and two-photon polymerization

The cationic photoinitiating abilities of a series of ‘push–pull’ sulfonium-based photoacid generators (PAGs) have been investigated. In this linear π-conjugated series, a 4-N,N-diphenylaminostilbene subunit is associated with different types of sulfonium substituents, which are connected to the stilbene moiety either in the 4′ position or in the 3′ position. This para-to-meta substitution effect leads to a strong increase of the quantum yield for acid generation with a maximum value of ca. 0.5. Such a positioning effect has a strong influence on the efficiency of the S–C bond cleavage. A detailed photolysis mechanism has been proposed. In contrast to commercially available sulfonium salts, these highly reactive π-conjugated PAGs all exhibit large absorption in the visible range as well as large two-photon absorption cross-sections (δmax > 600 GM) in the near-infrared region. As a consequence, efficient one and two-photon polymerization reactions are observed at 405 nm and 800 nm, respectively, using typical monomers such as cyclohexene oxide, n-butyl vinyl ether or SU-8 photoresists. By the fabrication of well resolved two-dimensional microstructures, we finally demonstrate the potential use of these new generation PAGs in the fields of one and two-photon lithography.

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.

Structural effects on the photodissociation of alkoxyamines

The search for photosensitive alkoxyamines represents a huge challenge. The key parameters governing the cleavage process remain unknown. The dissociation process of light sensitive alkoxyamines is studied as a function of their chemical structures. The photochemical properties of 6 selected compounds are investigated by ESR and laser flash photolysis. It is found that (i) the selectivity of the cleavable N-O vs. C-O bond and (ii) the efficiency of the nitroxide formation are strongly related to the alkoxyamine structure. The distance between the chromophore and the aminoxy group is a key parameter for an efficient pathway of the radical generation as displayed by the photopolymerization ability of these alkoxyamines.

Two-photon lithography in visible and NIR ranges using multibranched-based sensitizers for efficient acid generation

We investigated methodically the one- and two-photon absorption properties of a series of multibranched triphenylamine-based chromophores incorporating 4-(methylthio)styryl fragments as external substituents. Some relevant structure–property relationships relative to these highly fluorescent compounds have been derived based on emission anisotropy measurements, quantum chemical calculations and the use of the exciton coupling theory. Even though branching effects lead to a cooperative enhancement of the two-photon absorption (2PA), all compounds exhibit relatively low-to-moderate 2PA cross-sections (δ ≤ 100 GM) in the NIR region. However, the ‘so-called’ one-photon resonance enhancement effect leads to a remarkable increase of δ by more than one order of magnitude in the visible range. This strong 2PA ability has been associated with an efficient photosensitization of iodonium salt to elaborate a new bicomponent photoacid generator, which is readily two-photon activable at 532 nm. In the visible range, the strong enhancement of the efficiency of the two-photon induced polymerization is clearly demonstrated as compared with that observed in the NIR region.

Copper photoredox catalysts for polymerization upon near UV or visible light: structure/reactivity/efficiency relationships and use in LED projector 3D printing resins

Copper complexes (CuCs) bearing pyridine–pyrazole ligands are synthesized and evaluated as new photoredox catalysts/photoinitiators in combination with an iodonium salt (Iod) for the free radical polymerization of (meth)acrylates and the cationic polymerization of epoxides upon visible light exposure using a Light Emitting Diode (LED)@405 nm. The structure/reactivity/efficiency relationships for the copper complexes are studied as well as the chemical mechanisms involved. The different substituents on the pyrazole moiety of the ligand allow tuning of the oxidation potential and the visible light absorbance of the complexes and to optimize the performance of the polymerization photoredox catalysts. The use of a novel additive (CARET) in a three-component system (CuC/Iod/CARET) highly improves the performance. Finally, the high performances of the Cu(I) complexes for the development of new 3D printing resins using an LED projector are demonstrated. Currently, LED projector printing is really advantageous in 3D printing i.e. this technology projects the profile of an entire layer at one time.

Photoinduced development of antibacterial materials derived from isosorbide moiety.

A straightforward method for immobilizing in situ generated silver nanoparticles on the surface of a photoactivable isosorbide-derived monomer is developed with the objective to design a functional material having antibacterial properties. The photoinduced thiol-ene mechanism involved in these syntheses is described by the electron spin resonance/spin trapping technique. The resulting materials with or without silver nanoparticles (Ag NPs) were used as films or as coatings on glass substrate. The surface of the synthesized materials was characterized by X-ray photoelectron spectroscopy and scanning electron microscopy, and their thermal and mechanical properties were evaluated by dynamic-mechanical thermal tests, differential scanning calorimetry, thermogravimetric analyses, along with pencil hardness, nanoindentation, and scratch resistance tests. The photoinduced formation of Ag NPs is also confirmed by UV spectrophotometry. Finally, a primary investigation demonstrates the antibacterial properties of the isosorbide-derived material against Staphylococcus aureus and Escherichia coli, as well as its cytocompatibility toward NIH 3T3 fibroblastic cells.

Photoinitiating systems of polymerization and in situ incorporation of metal nanoparticles into polymer matrices upon exposure to visible light: push–pull malonate and malononitrile based dyes

Novel push–pull dyes containing a (substituted) hydrocarbon moiety and a malonate (or a malononitrile) moiety are proposed as photoinitiators for the ring opening polymerization of epoxides as well as the synthesis of interpenetrated polymer networks (IPNs) upon exposure to visible light (laser diode, halogen lamp, etc.). Excellent polymerization profiles are obtained. The role of the acceptor and donor moieties in these dyes towards their light absorption properties, the associated photochemical processes and their photoinitiating ability is investigated. A very efficient dye has been selected for the reduction of Ag+ and the in situ formation of Ag(0) nanoparticles in the synthesized IPNs.

Natural biopolymer surface of poly(3-hydroxybutyrate-co-3-hydroxyvalerate)-photoinduced modification with triarylsulfonium salts

The graft copolymerization of 2-hydroxyethylmethacrylate (HEMA) and methacrylic acid (MAA) onto a biodegradable and biocompatible polymer, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV), has been investigated. The one-step process was conducted in aqueous media using for the first time a cationic photoinitiator, triarylsulfonium hexafluoroantimonate salts, upon UV irradiation. Under appropriate conditions, the remaining radicals abstract hydrogen from the PHBHV backbone which initiate the UV-mediated grafting of polymers from the PHBHV surface. This process provides the opportunity to adapt the functionality of the film, thus permitting the control of its wettability as a function of the time of irradiation. In the last demonstration, we demonstrated that UV-mediated graft polymerization is an effective method to micropattern coatings onto PHBHV surface to develop microdevices for biological applications.