Photophysics and Reactivity of Photoinitiators Two-photon activated : Application in Multiphoton Microfabrication

L’avènement des lasers impulsionnels nanosecondes à femtosecondes a permis un développement rapide de techniques permettant de sonder et/ou de transformer les matériaux à l’échelle locale par des processus d’absorption non linéaire. Ce saut technologique a vu l’émergence de nombreuses applications associées au phénomène de confinement spatial. La stéréolithographie 3D par photopolymérisation biphotonique constitue un exemple typique d’application à forte valeur ajoutée qui offre de prometteuses perspectives en terme d’écriture à l’échelle nanométrique. Un enjeu fondamental constitue alors l’élaboration de nouveaux photoamorceurs très réactifs et activables à deux photons. Dans ce contexte, ce manuscrit présente une étude photophysique et photochimique de deux séries de photoamorceurs biphotoniques ‘Donneur/Accepteur’ intégrants des stilbènes comme relais électroniques avec pré-organisation dans des structures bichromophores. Les processus primaires photoinduits, les mécanismes de photoamorçage, la photoréactivité à l’echelle locale sont décrits et étudiés méthodiquement. Enfin, le potentiel appliqué de cette nouvelle génération de photoamorceurs est mis en évidence en microfabrication multiphotonique à travers l’élaboration de structure 3D à l’échelle µm.The advent of pulsed laser technologies has promoted the rapid growth of new emerging research domains which aim at probing and/or transforming materials at local scale using non linear absorption processes. A large range of applications takes benefit of the inherent spatial containment observed in non linear absorption processes so as to control photoreactions at nm-scale. The field of multiphoton fabrication (or stereolithography) addresses this fundamental issue and has developed rapidly so that it is no longer a rapid prototyping technology but a real manufacturing technique that is commercially available. The development of multiphoton stereolitography also requires highly reactive two-photon activable (2PA) initiators whose design and elaboration are the subject of considerable molecular engineering research. In this context, the present manuscript describes the photophysical and photochemical properties of two series of 2PA initiators. Such novel D--A structures have be designed by associating distinctive Donor and Acceptor groups into stilbene arms used as ‘electron relay’ and organized into a (multi)branched architecture. The photoinduced primary processes, the global photoinitiating mechanisms as well as the photoreactivity are described methodically. We finally demonstrate the applied potential of this new type of two-photon initiators in multiphoton stereolitography

Roles of Wettability and Supercooling in the Spreading of Cyclopentane Hydrate over a Substrate

We use transmission optical microscopy to observe cyclopentane hydrate growth in sub-mm, open glass capillaries, mimicking cylindrical pores. The capillary is initially loaded with water and the guest fluid (cyclopentane) and thus possesses three menisci, that between water and cyclopentane (CP) in the middle and two menisci with the vapors at the ends. At temperatures T below the equilibrium temperature Teq ≈ 7 °C, the hydrate nucleates on the water–CP meniscus, rapidly coating it with an immobile, polycrystalline crust. Continued movement of the other two menisci provides insights into hydrate growth mechanisms, via the consumption and displacement of the fluids. On water-wet glass, the subsequent growth consists of a hydrate “halo” creeping with an underlying water layer on the glass on the CP side of the meniscus. Symmetrically, on CP-wet glass (silane-treated), a halo and a CP layer grow on the water side of the interface. No halo is observed on intermediate wet glass. The halo consists of an array of large monocrystals, over a thick water layer at low supercooling (ΔT = Teq – T below 5 K), and a finer, polycrystalline texture over a thinner water layer at higher ΔT. Furthermore, the velocity varies as ΔTα, with α ≈ 2.7, making the early stages of growth very similar to gas hydrate crusts growing over water–guest interfaces. Beyond a length in the millimeter range, the halo and its water layer abruptly decelerate and thin down to submicron thickness. The halo passes through the meniscus with the vapor without slowing down or change of texture. A model of the mass balance of the fluids helps rationalize all of these observations

Help from a Hindrance: Using Astigmatism in Round Capillaries To Study Contact Angles and Wetting Layers

International audienceRound glass capillaries are a basic tool in soft-matter science, but often are shunned due to the astigmatism they introduce in micrographs. Here, we show how refraction in a capillary can be a help instead of a hindrance to obtain precise and sensitive information on two important interfacial properties: the contact angle of two immiscible fluids and the presence of thin films on the capillary wall. Understanding optical cusps due to refraction allows direct mesurement of the inner diameter of a capillary at the meniscus, which, with the height of the meniscus cap, determines the contact angle. The meniscus can thus be measured without intrusive additives to enhance visibility, such as dyes or calibrated particles, in uniform, curved, or even tapered capillaries or under demanding conditions not accessible by conventional methods, such as small volumes (μL), high temperatures, or high pressures. We further elicit the conditions for strong internal reflection on the inner capillary wall, involving the wall and fluid refractive indices and the wall thickness, and show how to choose the capillary section to detect thin (submicron) layers on the wall, by the contribution of total internal reflection to the cusps. As examples, we report the following: (i) CO2-water or -brine contact angles at glass interfaces, measured at temperatures and pressures up to 200 °C and 600 bar, revealing an effect apparently so far unreported-the decrease in the water-wet character of glass, due to dissolved salts in brine, is strongly reduced at high temperatures, where contact angles converge toward the values in pure water; (ii) A tenuous gas hydrate layer growing from the water-guest contact line on glass, invisible in transmission microscopy but prominent in the cusps due to total internal reflection.[on SciFinder (R)

Postpartum Depression and Anxiety among Lebanese Women: Correlates and Scales Psychometric Properties

Background: We found that it was important to fill a gap in the literature and check the psychometric properties of the Edinburgh Postnatal Depression Scale (EPDS) and Perinatal Anxiety Screening Scale (PASS) in the Arabic language and delineate factors associated with postnatal depression (PPD) and anxiety (PPA) among Lebanese women 4–6 weeks after delivery. Methods: This cross-sectional study carried out between July 2018 and March 2019 enrolled 295 participants who came for a postnatal checkup at four clinics. Results: The EPDS and PASS scales’ items converged over two- and four-factor solutions, explaining 62.51% and 53.33% of the variance, respectively (KMO EPDS = 0.816, αCronbach EPDS = 0.826; KMO PASS = 0.878, αCronbach PASS = 0.920; Bartlett’s test of sphericity p Conclusion: Depression and anxiety prevalence rates in the Lebanese population were higher compared with other countries, which may be due in part to the differences in regional, social and environmental culture

Enhancement of Two-Photon Initiating Efficiency of a 4,4'-Diaminostyryl-2,2'-bipyridine Derivative Promoted by Complexation with Silver Ions

International audienceWe report on the two-photon-induced polymerization (TPIP) ability of a new class of free radical two-photon initiator based on a cationic silver(I) complex incorporating 4,4′-diaminostyryl-2,2′-bipyridine (DES) derivatives as ligands. Coordination with Ag+ induces a strong increase of the charge transfer character at excited state, which enhances the two-photon absorption properties of the complex with respect to that of the free ligand. A comparative analysis of the photophysical properties of DES and DES2Ag+ shows that the presence of silver cation increases the efficiency for the generation of radical cations (DES*+), which can be used as hydrogen abstractor in free radical photopolymerization. We show that the addition of an aliphatic amine used as hydrogen donor also opens a parallel route for the regeneration of DES. The improvement of the two-photon polymerization efficiency is then evidenced by the fabrication of microstructures. We finally demonstrate that the use of DES2Ag+ as two-photon initiator offers new opportunities for the fabrication of functional nanostructures composed of metal-polymer nanocomposites

Versatile Photochemical Surface Modification of Biopolyester Microfibrous Scaffolds with Photogenerated Silver Nanoparticles for Antibacterial Activity

International audienc

Near-Infrared Laser Direct Writing of Conductive Patterns on the Surface of Carbon Nanotube Polymer Nanocomposites

International audienceNear-infrared (NIR) laser annealing is used to write conductive patterns at the surface of Polypropylene/Multi-walled carbon nanotube nanocomposite (PP/MWCNT) plates. Before irradiation, the surface of the nancomposite is not conductive due to the partial alignment of the MWCNT which occurs during injection molding. We observe a significant increase of the surface sheet resistance using NIR laser irradiation, which we explain by a randomization of the orientation of MWCNTs in the PP matrix melt by NIR laser irradiation. After only 5s of irradiation, the sheet resistance of PP/MWCNT, annealed with a laser at a power density of 7 W/cm 2 , decreases by more than 4 decades from ~ 100 MΩ/sq to ~ 1 kΩ/sq. Polarized Raman, TEM, and SEM are used to investigate the changes in the sheet resistance and confirm the physico-chemical processes involved. This allows directwriting of conductive patterns using NIR laser at the surface of nanocomposite polymer substrates, with the advantages of a fast, easy, and low-energy consumption process

Two-Photon Initiating Efficiency of a Ditopic Alkoxynitrostilbene Reacting through a Self-Regenerative Mechanism

International audienceThe photophysical properties and the photoinitiating reactivity of a ditopic alkoxynitrostilbene were compared to those of its single branch chromophore used as a reference. Whereas a trivial additive effect is observed when considering the one-and two-photon absorption properties, a clear and very significant amplification has been highlighted for the photoreactivity of this free radical photoinitiator which was used as a hydrogen abstractor in presence of an aliphatic amine co-reactant. We indeed demonstrate that the proximity of two nitroaromatics moieties within the same molecular architecture gives rise to an original cycling mechanism based on a stepwise photo triggering of each photoredox center followed by a subsequent regenerative process. The combination of a high two-photon absorption cross-section (delta 780nm ≈ 330 GM) with a strong enhancement in photoreactivity makes this nitrostilbene bichromophore a very suitable candidate for two-photon polymerization applications

Abnormal Enhancement of the Photoisomerization Process in a <i>trans</i>-Nitroalkoxystilbene Dimer Sequestered in β‑Cyclodextrin Cavities

We report on the synthesis and the photophysical properties of a <i>trans</i>-nitroalkoxystilbene dimer (DPNS). The fluorescence quantum yield (Φ_f), the Stokes shift, and the quantum yield for the <i>trans</i>-to-<i>cis</i> photoisomerization (Φ_<i>t→c</i>) are strongly dependent on the nature of the solvent. Upon increasing solvent polarity, Φ_f increases together with the decrease of Φ_<i>t→c</i>. 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 H₂O 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 <i>trans</i>-to-<i>cis</i> photoisomerization reaction is reactivated and leads to a paradoxical 14-fold increase of Φ_<i>t→c</i> even though DPNS is sequestered in β-CD cavities