Enhanced Rates of Photoinduced Molecular Orientation in a Series of Molecular Glassy Thin Films.

Photoinduced orientation in a series of molecular glasses made of small push-pull azo derivatives is dynamically investigated for the first time. Birefringence measurements at 632.8 nm are conducted with a temporal resolution of 100 ms to probe the fast rate of the azo orientation induced under polarized light and its temporal stability over several consecutive cycles. To better evaluate the influence of the azo chemical substituents and their electronic properties on the orientation of the whole molecule, a series of push-pull azo derivatives involving a triphenylaminoazo core substituted with distinct electron-withdrawing moieties is studied. All resulting thin films are probed using polarization modulation infrared spectroscopy that yields dynamical linear dichroism measurements during a cycle of orientation followed by relaxation. We show here in particular that the orientation rates of small molecule-based azo materials are systematically increased up to 7-fold compared to those of a reference polymer counterpart. For specific compounds, the percentage of remnant orientation is also higher, which makes these materials of great interest and promising alternatives to azobenzene-containing polymers for a variety of applications requiring a fast response and absolute control over the molecular weight

Small molecule-based photocrosslinkable fluorescent materials toward multilayered and high-resolution emissive patterning

International audiencea Solution-processable green and red-emitting fluorophores possessing photopolymerizable acrylate units have been synthesized. Photocrosslinking was successfully performed in neat thin films at room temperature under low-dose UV irradiation at 365 nm. No further curing step was necessary to achieve insoluble emissive thin films displaying high optical quality. Up to 80% of the green emitting material processed as a non-doped thin film remained after photopolymerization. Despite competitive energy transfer occurring between the excited photoinitiator and the radiative excited state of red-emitting materials, up to 40% of the initial thickness could be achieved after development. The very low RMS roughness of the green and red photocrosslinked thin films after development (RMS o 0.7 nm) allowed us to fabricate multicolored stacks again with high optical quality (RMS roughness o 1.3 nm) after two cycles of irradiation and development involving successively red and green emitters. Resolved patterns as small as 600 nm in width could be obtained upon photolithography performed under an air atmosphere. High adhesion of the photocrosslinked materials on surfaces makes the resulting emissive thin films very promising for realizing complex emissive structures on flat or bend substrates as required in multiple applications such as optical data storage, organic lasers, organic light emitting diodes or counterfeiting

Exploiting Light Interferences to Generate Micrometer-High Superstructures from Monomeric Azo Materials with Extensive Orientational Mobility

Photochromic azo materials have stirred considerable interest for their ability to mechanically respond to polarized light through large photo-induced migration and orientation processes. In order to apprehend the microscopic dynamics behind the extensive mass transport occurring under interferential illumination, two azo compounds differing by their propen-sity to form hydrogen bonds are synthesized and processed as nondoped glassy thin films. Interferential irradiation using polarization and intensity patterns reveals fully distinct responses. Regular nanometer-high surface relief gratings transform into micrometer superstructures with an ampli-tude ten times higher than the initial film thickness when using the latter polarization. Systematic comparisons between the azo materials in terms of thermal properties, photochromism in solution and in the solid state, and photomigration are carried out. The progressive formation of super-structures is ascribed to two successive processes. The first one relates to fast photoinduced migration due to the impinging structured light, and the second one is promoted by slower thermally activated “zig-zag”-like diffu-sion and Z-E thermal relaxation, which in turn requests high orientational mobility of the azo compounds and causes large nanomechanical changes. Such studies should provide novel structural guidelines in terms of material fluidity to rapidly achieve highly structured and rewritable materials at lilogwht irradiance

Tunable double photochromism of a family of bis-DTE bipyridine ligands and their dipolar Zn complexes.

International audienceThe photoinduced ring-closure/ring-opening reactions of a series of bis-dithienylethene derivatives, as free ligands and Zn(II)-complexes, are investigated by resorting to theoretical (time-dependent density functional theory) and kinetic analyses in solution. The originality of the system stems from the tunability of the photoreaction quantum yields and conversion yields as a function of the electronic structure. The latter could be varied by modifying the electron-donating character of the DTE-end substituents L(a-d) (o,o) (a, D = H; b, D = OMe; c, D = NMe(2); d, D = NBu(2)) and/or the Lewis character of the metal ion center L(a-d)ZnX(2) (o,o) (L(a-c), X = OAc; L(d), X = Cl). The orbital description of the doubly-open form (o,o) and half-closed form (o,c) predicts that double closure to the form (c,c) would occur using UV irradiation. Photokinetic studies on the complete series demonstrate that photocyclization proceeds following a sequential ring closure mechanism. They clearly point out distinct quantum yields for the first and second ring closures, the latter being characterized by a significantly lower value. Dramatic decrease in both the quantum yields of the ring-closure and ring-opening processes is demonstrated for the complex L(d)ZnCl(2) exhibiting the strongest charge-transfer character in the series investigated. These studies show that this series of DTE derivatives provides an efficient strategy to tune the photochromic properties through the combination of the electron-donor and electron-acceptor (D-A) groups

A "reverse interrupter": the novel molecular design of a fluorescent photochromic DTE-based bipyridine

International audienceAn original design of a fluorescent dithienylethene (DTE)-based bipyridine, where donor (D) and acceptor (A) groups are located on the same thiophene ring of the DTE unit, is reported; in non-polar solvents, UV or visible excitation triggers a photochromic reaction, disrupting the conjugation and quenching the fluorescence

Highly cohesive dual nanoassemblies for complementary multiscale bioimaging

International audienceInnovative nanostructures made of a high payload of fluorophores and superparamagnetic nanoparticles (NPs) have simply been fabricated upon self-assembling in a two-step process. The resulting hybrid supraparticles displayed a dense shell of iron oxide nanoparticles tightly attached through an appropriate polyelectrolyte to a highly emissive non-doped nanocore made of more than 10 5 small organic molecules. Cooperative magnetic dipole interactions arose due to the closely packed magnetic NPs at the nanoarchitecture surface, causing enhanced NMR transverse relaxivity. Large in vivo MRI T 2 contrast was thus obtained with unusually diluted solutions after intravenous injection in small rodents. Two-photon excited fluorescence imaging could be performed, achieving unprecedented location resolution for agents combining both magnetic nanoparticles and fluorescence properties. Finally, TEM imaging of the sectioned mouse tissue succeeded in isolating the core–shell structures, which represents the first image of intact complex magnetic and fluorescent nanoassemblies upon in vivo injection. Such highly cohesive dual nanoarchitectures should open great horizons toward the assessment with high spatial resolution of the drug or labeled stem cell biodistribution

Matériaux moléculaires amorphes pour la photostructuration de fluorescence

Au cours de ce travail de thèse, des matériaux monomères amorphes bifonctionnels photochromes et fluorescents ont été élaborés et étudiés dans le but de former sous irradiation interférentielle des structures fluorescentes par migration de matière photoinduite . Pour contrer le phénomène d'extinction de fluorescence par l'unité photochrome azoïque, un espaceur saturé de type adduit de Diels-Alder a été introduit entre les deux entités photoactives. De telles molécules ont été obtenues avec succès ainsi que les composés modèles azoïques et fluorescents correspondants.L'introduction de nouveaux fragments comme l'espaceur ne modifiait pas les propriétés de photochromisme que ce fût en solution ou à l'état solide. A l'opposé, des études de spectroscopie de fluorescence stationnaire et résolue en temps ont montré que la présence de cet espaceur limitait efficacement les phénomènes d extinction de fluorescence au sein des composés bifonctionnels. Dans le but de comprendre les mécanismes photophysiques sous-tendant l'extinction de fluorescence observée, des expériences de spectroscopie d'absorption transitoire femtoseconde ont été réalisées sur les composés modèles et bifonctionnels. Elles ont mis en évidence l existence d un transfert d énergie électronique picoseconde de l unité fluorescente vers l unité photochrome.Sous forme de films minces non-dopés, ces nouveaux composés bifonctionnels ont conduit par photostructuration holographique à des reliefs de surface réinscriptibles, stables à température et sous lumière ambiantes et modulés en intensité de fluorescence, les maxima d'émission correspondant aux zones d'accumulation en composés bifonctionnels. Une nouvelle réaction photochimique consistant en la photocoupure de dérivés fluorescents nitrés de la triphénylamine a été mise en évidence. L irradiation sous UV de solutions et de films minces provoquait une photorupture de l état à transfert de charge intramoléculaire, conduisant à la modification de la couleur émise de l orange au bleu. Des structures fluorescentes modulées en énergie ont pu ainsi être obtenues par irradiation au travers d un masque tout en contrôlant la quantité de lumière apportée.This PhD thesis presents the elaboration and the studies of bifunctional amorphous small molecule-based materials combining photochromic and fluorescent units in order to form fluorescent patterns by photoinduced mass transport under interferential irradiation A saturated spacer made of a Diels-Alder adduct was introduced between both photoactive units to inhibit emission quenching of the fluorophore by the azo photochromic moiety. Such compounds as well as their fluorescent and azo model compounds have successfully been synthesised.New fragments such as the spacer brought no modification to the photochromic properties in solution and in the solid state. Conversely, time-resolved and steady-state fluorescence studies showed that the spacer efficiently limited fluorescence quenching within the bifunctional compounds. In order to gain insight into the mechanism responsible for the fluorescence quenching, femtosecond transient absorption spectroscopy measurements have been performed on model and bifunctional compounds. They evidenced picosecond electronic energy transfer from the fluorescent unit towards the azo moiety.As non doped thin films, rewritable surface relief gratings (SRG) stable under ambient light and at room temperature could be written through holographic illumination and displayed fluorescence patterns. Modulation of the emission intensity resulted from the accumulation of fluorophores attached to the azo units undergoing photoinduced mass migration. A novel photochemical reaction leading to the photocleavage of fluorescent nitro-substituted triarylamine derivatives has been proved. UV irradiation in solution and in the solid state caused photodisruption of the intramolecular charge transfer state, yielding neat modification of the emission colour from orange into blue. Fluorescent patterns modulated in energy could have been obtained by using a mask and controlling the amount of photons.CACHAN-ENS (940162301) / SudocSudocFranceF

Photoinduced intramolecular charge-transfer dynamics of a red-emitting dicyanovinyl-based triarylamine dye in solution

The photophysics of a red-emitting push-pull triarylamine compound (fvin) comprising a triphenylamino electron donor core and a dicyanovinylene electron acceptor group is investigated by steady-state absorption and emission and femtosecond time-resolved absorption spectroscopy in room-temperature n-hexane, toluene, ethanol, and acetonitrile solvents. Fvin is strongly fluorescent in apolar solvents upon excitation of the S0 → S1 intramolecular charge transfer (ICT) transition, but hardly emissive in polar solvents. Time-resolved spectra reveal a strong dependence of the excited-state dynamics on both the solvent polarity and viscosity. Unlike n-hexane solutions where the fluorescent ICT excited state remains weakly solvated and keeps a structure close to that of the Franck-Condon level, significant stabilization of the ICT state operates in toluene, leading to a relaxed fluorescent ICT′ form with stronger charge transfer character. This was featured by a notable dynamic Stokes shift of the stimulated emission (SE) band occurring in 15 ps. However the similarly high fluorescence quantum yield and lifetime in n-hexane and toluene excludes any large structural distortion on going from ICT to ICT′. In acetonitrile and ethanol, the SE shift dynamics is enhanced and clearly biphasic. The first step can be ascribed to an ICT → ICT′ conformational relaxation as in toluene. The second step, accompanied by a drastic drop of the SE intensity and a strong evolution of the excited-state absorption spectrum (time-constant 1.5 ps in acetonitrile, 9 ps in ethanol), corresponds to the appearance of a new ICT″ state with enhanced charge localization and separation, most probably associated with a rotation of the phenyldicyanovinylene group. This large-amplitude distortion is followed by fast internal conversion to the ground state (9 ps in acetonitrile, 21 ps in ethanol), accounting for the almost nonemissive character of fvin in polar solvents

Laser operation in non-doped thin films made of a small-molecule organic red-emitter

International audienceStimulated emission in small-molecule organic films at a high dye concentration is generally hindered by fluorescence quenching, especially in the red region of the spectrum. Here we demonstrate the achievement of high net gains (up to 50 cm-1) around 640 nm in thermally evaporated non-doped films of 4-di(4'-tert-butylbiphenyl-4-yl)amino-4'-dicyanovinylbenzene, which makes this material suitable for green-light pumped single-mode organic lasers with low threshold and superior stability. Lasing effect is demonstrated in a DBR resonator configuration, as well as under the form of random lasing at high pump intensities

Photochemical Mapping of the Multimodal Plasmonic Response of 2D Gold Crystals

cited By 0International audienceThe properties of hybrid plasmonic structures comprising a molecular ultrathin film of photoactive azobenzene derivatives and individual mesoscopic crystalline gold platelets are studied upon monochromatic irradiation. The spatial variation of the optical near-field produced by the excitation of high-order plasmon modes induces a net modification of the film corrugation with subwavelength pattern features. Unlike previous reports exploiting azo dye-grafted polymers, the photomigration of neat films composed of glass-forming push–pull azo compounds yields a highly resolved negative imprint of the local electric field intensity variations able to discriminate the different plasmon resonances sustained in these multimodal triangular prisms. Our experiments are in full agreement with green dyadic method simulations of near-field intensity maps and constitute both an efficient imaging technique and a way to self-align patterns of an optically active molecular medium in registry with the optical hotspots, which could be a major asset for the integration of filtering or gain media near plasmon-enhanced devices