Eléna Ishow

Synthesis and photochromism of two new 1,2-bis(thiazolyl)perfluorocyclopentenes with chelating sites

Two new dithiazolylethenes, 1a {1,2-bis[5′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene} and 2a {1,2-bis[4′-methyl-2′-(2′′-pyridyl)thiazolyl]perfluorocyclopentene}, have been synthesized. Their photochromic behavior has been fully investigated in solution as well as in the crystalline phase. They display both fatigue-resistant and thermally irreversible photochromic reactions, with more than 80% of the closed-ring forms (1b and 2b, respectively) in the photo-stationary state. Upon UV irradiation, a colorless solution of 1a turns purple while the yellow color of a 2a solution becomes more intense. Such marked differences in their electronic absorption spectra are reproduced by DFT calculations. Finally, although structurally closely related, only 1a shows photochromism in the crystalline phase whereas 2a displays reversible photo-modulation of fluorescence in solution.

Molecular Structure – Optical Property Relationships for a Series of Non-Centrosymmetric Two-photon Absorbing Push-Pull Triarylamine Molecules

This article reports on a comprehensive study of the two-photon absorption (2PA) properties of six novel push-pull octupolar triarylamine compounds as a function of the nature of the electron-withdrawing groups. These compounds present an octupolar structure consisting of a triarylamine core bearing two 3,3′-bis(trifluoromethyl)phenyl arms and a third group with varying electron-withdrawing strength (H < CN < CHO < NO2 < Cyet < Vin). The 2PA cross-sections, measured by using the femtosecond open-aperture Z-scan technique, showed significant enhancement from 45 up to 125 GM for the lowest energy band and from 95 up to 270 GM for the highest energy band. The results were elucidated based on the large changes in the transition and permanent dipole moments and in terms of (i) EWG strength, (ii) degree of donor-acceptor charge transfer and (iii) electronic coupling between the arms. The 2PA results were eventually supported and confronted with theoretical DFT calculations of the two-photon transition oscillator strengths.

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

Stimulated 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 nondoped 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 distributed Bragg resonator configuration, as well as under the form of random lasing at high pump intensities.

Solvatochromic dissociation of non-covalent fluorescent organic nanoparticles upon cell internalization

Amorphous red-emitting materials involving solvatochromic small molecules have been processed by the reprecipitation method as non-doped nanospheres characterized by a remarkably low polydispersity. Their mean diameter could simply be tuned by the concentration of the organic solution giving rise to time-stable dispersion of 85-200 nm-sized nanoparticles. Time-resolved measurements performed on solid nanoparticles showed significant size-dependence effects of the emission lifetime and maxima evidencing populations with distinct molecular conformations. Nanoparticle internalization has proved successful in NIH-3T3 murine fibroblasts with normal toxicity effects after 48 h. Fluorescence confocal microscopy under one- and two-photon excitations revealed dual emission enabling localization of the organic material within the plasma membrane and the cytoplasm. Model experiments resorting to suspended artificial lipid bilayers allowed us to conclude on the dissolution of nanoparticles by the phospholipid membrane during the internalization process. They let us to assume that uptake of hydrophobic nanoparticles by living cells implies an endocytosis mechanism operating through the formation of plasmic vesicles.

All-Solution-Processed Organic Light-Emitting Diodes Based on Photostable Photo-cross-linkable Fluorescent Small Molecules

We demonstrate herein the fabrication of small molecule-based OLEDs where four organic layers from the hole- to the electron-transporting layers have successively been deposited by using an all-solution process. The key feature of the device relies on a novel photopolymerizable red-emitting material, made of small fluorophores substituted with two acrylate units, and displaying high-quality film-forming properties as well as high emission quantum yield as nondoped thin films. Insoluble emissive layers were obtained upon UV irradiation using low illumination doses, with no further need of postcuring. Very low photodegradation was noticed, giving rise to bright layers with a remarkable surface quality, characterized by a mean RMS roughness as low as 0.7 nm after development. Comparative experiments between solution-processed OLEDs and vacuum-processed OLEDs made of fluorophores with close architectures show external quantum efficiencies in the same range while displaying distinct behaviors in terms of current and power efficiencies. They validate the proof of concept of nondoped solution-processable emissive layers exclusively made of photopolymerized fluorophores, thereby reducing the amount of components and opening the way toward cost-effective fabrication of solution-processed OLED multilayer architectures.

Nanoparticle Organization through Photoinduced Bulk Mass Transfer

A series of dipolar triphenylaminoazo derivatives, with largely distinct charge transfer and glass transition temperatures, has been synthesized. Their photomigration capability in the solid state to form surface relief gratings (SRGs) under interferential illumination has been investigated with respect to their photochromic properties and showed a prevailing influence of the bulkiness of the azo substituent. The azo mass transfer was utilized to efficiently photoalign 200 nm polystyrene nanoparticles along the SRG crests, which were initially deposited on nonirradiated azo surfaces. In contrast, nanoparticles spin cast on prestructured surface relief gratings were localized in the troughs of the periodic structures. These distinct locations point out the ability of isotropic and amorphous photochromic thin films to collectively move and organize nano-objects in an ordered fashion through the use of polarized illumination. This versatile approach opens the path to optically aligned ensembles of individual nano-objects over large areas, which can be further combined with metallic conductive or magnetic coating to create novel functional nanostructures.

Absorption, Luminescence, and Sizing of Organic Dye Nanoparticles and of Patterns Formed Upon Dewetting

Organic nanoparticles made of a push-pull triarylamine dye with an average diameter of 60 nm, were prepared by reprecipitation. We study their photophysical properties by a combination of photothermal and fluorescence microscopy. Photothermal contrast provides a quantitative measure of the number of absorbers. The size of nanoparticles estimated from the absorption measurements was compared with sizes measured by AFM. Fluorescence and absorption microscopy provide quantum yield on the single-particle level as a function of excitation intensity. The quantum yield strongly decreases at high intensities because of singlet-singlet or singlet-triplet annihilation. We also report the formation of molecular thin layers and of labyrinth-shaped structures on glass substrates, presumably induced by dewetting.

Experimental and theoretical investigation of the first-order hyperpolarizability of a class of triarylamine derivatives.

This paper reports on the static and dynamic first-order hyperpolarizabilities of a class of push-pull octupolar triarylamine derivatives dissolved in toluene. We have combined hyper-Rayleigh scattering experiment and the coupled perturbed Hartree-Fock method implemented at the Density Functional Theory (DFT) level of theory to determine the static and dynamic (at 1064 nm) first-order hyperpolarizability (βHRS) of nine triarylamine derivatives with distinct electron-withdrawing groups. In four of these derivatives, an azoaromatic unit is inserted and a pronounceable increase of the first-order hyperpolarizability is reported. Based on the theoretical results, the dipolar/octupolar character of the derivatives is determined. By using a polarizable continuum model in combination with the DFT calculations, it was found that although solvated in an aprotic and low dielectric constant solvent, due to solvent-induced polarization and the frequency dispersion effect, the environment substantially affects the first-order hyperpolarizability of all derivatives investigated. This statement is supported due to the solvent effects to be essential for the better agreement between theoretical results and experimental data concerning the dynamic first-order hyperpolarizability of the derivatives. The first-order hyperpolarizability of the derivatives was also modeled using the two- and three-level models, where the relationship between static and dynamic first hyperpolarizabilities is given by a frequency dispersion model. Using this approach, it was verified that the dynamic first hyperpolarizability of the derivatives is satisfactorily reproduced by the two-level model and that, in the case of the derivatives with an azoaromatic unit, the use of a damped few-level model is essential for, considering also the molecular size of such derivatives, a good quantitative agreement between theoretical results and experimental data to be observed.

Are Fluorescent Organic Nanoparticles Relevant Tools for Tracking Cancer Cells or Macrophages

Strongly solvatochromic fluorophores are devised, containing alkyl chains and enable to self-assemble as very bright fluorescent organic nanoparticles (FONs) in water (Φf = 0.28). The alkyl chains impart each fluorophore with strongly hydrophobic surroundings, causing distinct emission colors between FONs where the fluorophores are associated, and their disassembled state. Such color change is harnessed to assess the long-term fate of FONs in both cancer cells and monocytes/macrophages. Disintegration of the orange-emitting FONs by monocytes/macrophages is evidenced through the formation of micrometer green-yellowish emitting vesicles. By contrast, cancer cells retain longer the integrity of organic nanoparticles. In both cases, no significant toxicity is detected, making FONs as valuable bioimaging agents for cell tracking with weak risks of deleterious accumulation and low degradation rate.

Photochromic Organic Nanoparticles as Innovative Platforms for Plasmonic Nanoassemblies

The fabrication of hybrid core-shell nanoassemblies involving a nondoped azo photochromic core coated with a dense shell of gold nanoparticles is reported to investigate the influence of localized plasmons onto the azo core photoisomerization. Photochromic organic nanoparticles, regarded as a novel class of high-density photoswitchable nanomaterials, are first elaborated upon precipitation in water of push-pull azo molecules, containing sulfur-terminated units to chelate gold nanoparticles. Photoisomerization studies of the azo nanoparticles reveal significantly higher E → Z photoconversion yields and Z → E thermal back relaxation rate constants compared to those of dyes processed as thin films and in solution, respectively. These unexpected results are ascribed to the large surface-to-volume ratio and cooperative effects encountered in nanoparticles that deform without disassembling under polarized illumination as a result of the weak change in the azo dipole moment. UV-vis spectroscopy and Raman microscopy of the hybrid nanoassemblies show strong optical coupling between both photoactive constituents, confirming that gold nanoparticles are tightly positioned on the azo core surface. Such coupling causes partial quenching of the azo photoisomerization but does not impact the thermal back relaxation. Longer sulfur-terminated chains provide reduced quenching of the photoreaction by the localized plasmons, thereby opening perspectives toward plasmon-mediated deformation of nano-objects for light-controlled nanomechanics.