Arnaud Spangenberg

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.

Photoswitchable interactions between photochromic organic diarylethene and surface plasmon resonance of gold nanoparticles in hybrid thin films

Hybrid materials combining gold nanoparticles (GNP) of variable diameter and an organic thin layer of photochromic diarylethenes were achieved. Solid-state photoswitching based on ring-closure/ring-opening reaction was carried out under alternate UV and visible irradiations. In addition to the spectral changes due to the photochromism itself, the surface plasmon resonance related to the GNP is significantly modified, influenced by a photoinduced change in the refractive index of its environment. These two contributions were sorted out, showing the possibility of probing a photochromic switch by following the plasmon band. The shape change of the plasmon band was consistently compared to calculations based on the Mie theory. Additionally, with one given diarylethene compound, both UV-visible spectroscopy and surface enhanced Raman scattering (SERS) spectroscopy showed an acceleration of the ring-opening photochromic reaction in the presence of GNP.

Room-temperature preparation of metal-oxide nanostructures by DUV lithography from metal-oxo clusters

A versatile, fast and easy route towards preparation of metal-oxide nanostructures by a full-optical method at room temperature is presented here. The concept relies on the preparation of photosensitive metal-oxo clusters (MOCs) that can be crosslinked and mineralized in a single step process, by Deep-UV (DUV) irradiation (ArF laser emission band at 193 nm). The oxo-clusters are prepared by complexation between metal alkoxides and methacrylic acids followed by a partial hydrolysis. These molecular building blocks are designed to absorb DUV light and they can react from an excited state to give rise to crosslinking reactions. Photocrosslinking of Ti, Zr and Hf oxo-clusters was investigated by means of in situ FTIR and spectroscopic ellipsometry. In the case of Ti-oxo clusters, we demonstrate that the material can be fully mineralized into TiO2 by DUV irradiation. A photocatalytic process involving TiO2 nanoparticles is proposed to explain the DUV mineralization process. Finally, we used DUV interferometric lithography to illustrate nanopatterning based on these photoresists. These inorganic photoresists open new doors towards room temperature preparation of high-resolution inorganic nanostructures with strong interest for practical applications in electronics, optics, photonics or biology.

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.

Two-photon absorption in a conformationally twisted D–π–A oligomer: a synergic photosensitizing approach for multiphoton lithography

A comparative study of the linear and nonlinear optical properties of a novel triphenylamine–pyrimidine alternated oligomer and its corresponding V-shaped quadrupolar monomer is presented. Both chromophores strikingly exhibit the same spectral shape when considering their respective one- and two-photon absorption spectra. This effect was attributed to a weak interchromophore coupling within the oligomer which exhibits a highly distorted geometry resulting in a strong reduction of the effective conjugation length. The recursive implementation of nine monomers into a three-dimensional architecture leads however to a cooperative enhancement of the two-photon absorption (2PA) cross-section with a δMAX of 5093 GM at 800 nm. This very high 2PA ability has been oriented to improve the two-photon induced polymerization efficiency of a bicomponent photoinitiator system implying a hexaarylbiimidazole used as a H-abstractor and an aliphatic amine used as a H-donor. The photosensitizing mechanism is investigated and we clearly show that the intrinsic photoinitiation efficiency of the oligomer is increased by a factor 3 as compared to its corresponding monomer. We therefore demonstrate that such a two-photon sensitizing strategy leads to a synergy effect combining a higher photoinitiation reactivity and a very large two-photon absorption cross-section.

Direct nanopatterning of 100 nm metal oxide periodic structures by Deep-UV immersion lithography

Deep-UV lithography using high-efficiency phase mask has been developed to print 100 nm period grating on sol-gel based thin layer. High efficiency phase mask has been designed to produce a high-contrast interferogram (periodic fringes) under water immersion conditions for 244 nm laser. The demonstration has been applied to a new developed immersion-compatible sol-gel layer. A sol-gel photoresist prepared from zirconium alkoxides caped with methacrylic acids was developed to achieve 50 nm resolution in a single step exposure. The nanostructures can be thermally annealed into ZrO(2). Such route considerably simplifies the process for elaborating nanopatterned surfaces of transition metal oxides, and opens new routes for integrating materials of interest for applications in the field of photocatalysis, photovoltaic, optics, photonics or microelectronics.

Metal-induced dimensionality tuning in a series of bipyrimidine-based ligands: a tool to enhance two-photon absorption

We report the synthesis, the photophysical and the two-photon absorption (2PA) properties of a series of octupolar bipyrimidine-based ligands incorporating N-substituted amines as terminal donor groups. The effect of replacing phenylvinylene π-conjugated linkers by fluorenylvinylene ones was also investigated. The linear absorption spectrum of these compounds is dominated by an intensive charge transfer band which is sensitive to N-substitution and the π-bridge nature. The excitation anisotropy spectrum indicates that this band encompasses multiple S0 → Sn transitions, whose occurrence is well rationalized on the basis of the Frenkel exciton model. The 2PA spectrum also corroborates the presence of several electronic transitions. In apolar or moderately polar medium, excited ligands mainly deactivate through a highly emissive intramolecular charge transfer (ICT) state localized within a single branch of the chromophore. In highly polar medium, the solvent-induced stabilization of the low emissive twisted intramolecular charge transfer (TICT) state leads to a severe quenching of the fluorescence. The same mechanism is observed upon complexation with Zn2+. According to single-crystal X-ray analyses, metal-induced planarization of the bipyrimidine chelating site was observed for the short length ligands. Such a dimensional change from D2d to D2h symmetry leads to a decrease of 2PA cross-sections with respect to the free ligands. A divergent effect is observed for the complex with the long length ligand since the three-dimensional structure is maintained which induces a sizeable increase of the 2PA cross-section with a maximum value of up to 2000 GM.

Rapid Prototyping of Chemical Microsensors Based on Molecularly Imprinted Polymers Synthesized by Two-Photon Stereolithography

Two-photon stereolithography is used for rapid prototyping of submicrometre molecularly imprinted polymer-based 3D structures. The structures are evaluated as chemical sensing elements and their specific recognition properties for target molecules are confirmed. The 3D design capability is exploited and highlighted through the fabrication of an all-organic molecularly imprinted polymeric microelectromechanical sensor.

Probing photochromic properties by correlation of UV-visible and infra-red absorption spectroscopy: a case study with cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene

Quantification of the relative composition of the isomers in a photochromic system at any irradiation time interval is a critical issue in determining absolute quantum yields. For this purpose, we have developed a simple and convenient protocol involving combination of UV-visible and infra-red absorption spectroscopy. Photochromic cyclization reaction of cis-l,2-dicyano-l,2-bis(2,4,5-trimethyl-3-thieny1)ethene (CMTE) is analyzed to demonstrate the efficiency of the proposed methodology. This approach is based on the fact that the two isomers show distinctive infra-red bands. Detailed investigations of the UV-visible and infra-red spectra of the mixture obtained at different irradiation times in CCl(4) supported by quantum chemical computations lead to the unambiguous estimation of molar absorption coefficients of the closed isomer (epsilon(CF) = 4650 L mol(-1) cm(-1) at 512 nm). It facilitates the first determination of absolute quantum yields of this reversible photochromic reaction in CCl(4) by fitting the UV-visible spectral data (Phi(OF-->CF) = 0.41 +/- 0.05 and Phi(CF-->OF) = 0.12 +/- 0.02 at 405 nm and 546 nm, respectively).

Rapid Prototyping of Polymeric Nanopillars by 3D Direct Laser Writing for Controlling Cell Behavior

Mammalian cells have been widely shown to respond to nano- and microtopography that mimics the extracellular matrix. Synthetic nano- and micron-sized structures are therefore of great interest in the field of tissue engineering, where polymers are particularly attractive due to excellent biocompatibility and versatile fabrication methods. Ordered arrays of polymeric pillars provide a controlled topographical environment to study and manipulate cells, but processing methods are typically either optimized for the nano- or microscale. Here, we demonstrate polymeric nanopillar (NP) fabrication using 3D direct laser writing (3D DLW), which offers a rapid prototyping across both size regimes. The NPs are interfaced with NIH3T3 cells and the effect of tuning geometrical parameters of the NP array is investigated. Cells are found to adhere on a wide range of geometries, but the interface depends on NP density and length. The Cell Interface with Nanostructure Arrays (CINA) model is successfully extended to predict the type of interface formed on different NP geometries, which is found to correlate with the efficiency of cell alignment along the NPs. The combination of the CINA model with the highly versatile 3D DLW fabrication thus holds the promise of improved design of polymeric NP arrays for controlling cell growth.