Luminescent Supramolecular Ionic Frameworks based on Organic Fluorescent Polycations and Polyanionic Phosphorescent Metal Clusters.

Emissive ionic supramolecular frameworks are designed by associating tetraphenylethylene-based tetra-cationic units and di-anionic molybdenum or tetra-anionic rhenium octahedral clusters. Obtained structures were characterized by single-crystal X-ray diffraction. The emission properties of the hybrids were investigated as dry powders or in various solvents by one photon and two photon absorption leading to a O2 concentration dependent luminescence color for the Mo based hybrid.

Game of Crowns: Na+ Is Coming! Red NIR-Emissive Hybrid Liquid Crystals Containing Discotic Crown Ethers and Na2Mo6X8iCl6 (Xi = Cl or Br).

Molecular or supramolecular materials that can self-organize into columns such as discotic liquid crystals are of interest for several applications in the field of optoelectronics. We show in this work that red near-infrared (NIR)-emissive metal cluster compounds of general formula Na2Mo6X8iCl6 (Xi = Cl or Br) can be readily complexed with discotic liquid crystals containing a crown ether. Three cavity sizes have been tested with crown ethers bearing 4, 5, or 6 oxygen atoms. In all cases, 1:1 complexes were formed, thanks to the well-known supramolecular interactions existing between the Na+ cations of the metal cluster salt and the crown ether derivatives. All obtained hybrids are homogeneous, emit in the red NIR region, and show liquid crystalline properties on a wider temperature range than their precursors. Charge transport properties have been investigated by using a space charge limited current device. Obtained results demonstrate that metal cluster compounds can enhance the charge carrier mobility by 5 orders of magnitude compared to the native discotic organic ligands. Considering that the presented organic crown ether derivatives are not the best candidates to design optoelectronic devices because of their inherently low conductivity, but that similar compounds were developed to design proton conductive porous framework, our results open promising perspectives for the use of metal cluster compounds in devices dedicated to such a field.

Asymmetric Ruthenium Catalysis Enables Fluorophores with Point Chirality Displaying CPL Properties.

A novel enantiopure π-allylruthenium(IV) precatalyst allowed the enantioselective and stereospecific allylations of indoles and gave access to indolin-3-ones, containing vicinal stereogenic centers. Facile separation of diastereoisomers exhibiting opposite circularly polarized luminescence (CPL) activities in diverse solvents, including water, demonstrated the potential of these sustainable transformations and of the newly prepared molecules.

Nondestructive All-Optical Readout through Photoswitching of Intramolecular Excimer Emission.

We report the first intramolecular excimer photoswitching induced by molecular motion within a dithienylethene (DTE) molecule without destructive readout. The photochromic compound DTE bears two pyrene chromophores, judiciously positioned to face each other in the DTE's open form. The close proximity of the pyrenes in the open form is confirmed by NMR experiments and geometry optimization. Intense pyrene excimer luminescence is recorded, upon both one- and two-photon excitation (OPE and TPE). The photocyclization reaction of the DTE core induces a molecular motion of one pyrene moiety which thus prevents the possibility of formation of an excimer. Our DTE-based pyrene is stable upon TPE irradiation and shows a high photocyclization quantum yield. Such property specifications allow us to report the original nondestructive readout fluorescence by alternating exposure to OPE and TPE.

Straightforward Access to Multifunctional π-Conjugated P-Heterocycles Featuring an Internal Ylidic Bond.

We report the straightforward one-pot synthesis of novel 5- or 6-membered P-heterocycles featuring an internal ylidic bond: P-containing acenaphthylenes and phenanthrenes. The stability of the compounds tolerates post-functionalization through direct arylation to introduce electron-rich/poor substituents and the synthetic strategy is also compatible with the preparation of more elaborate polyaromatic scaffolds such as acenes and helicenes. Using a joint experimental (X-ray analysis, optical and redox properties) and theoretical approach, we perform a full structure-property relationships study on these new platforms. In particular, we show that molecular engineering allows not only tuning their absorption/emission across the entire visible range but also endowing them with chiroptical or non-linear optical properties, making them valuable dyes for a large panel of photonic or opto-electronic applications.

Joint Venture of Metal Cluster and Amphiphilic Cationic Minidendron Resulting in Near Infrared Emissive Lamellar Ionic Liquid Crystals.

Inorganic red-NIR emissive materials are particularly relevant in many fields like optoelectronic, bioimaging or solar cells. Benefiting from their emission in devices implies their integration in easy-to-handle materials like liquid crystals, whose long-range ordering and self-healing abilities could be exploited and influence emission. Herein, we present red-NIR emissive hybrid materials obtained with phosphorescent octahedral molybdenum cluster anions electrostatically associated with amphiphilic guanidinium minidendrons. Polarized optical microscopy and X-ray analysis show that while the minidendron chloride salts self-organize into columnar phases, their association with the dianionic metal cluster leads to layered phases. Steady-state and time-resolved emission investigations demonstrate the influence of the minidendron alkyl chain length on the phosphorescence of the metal cluster core.

Evidencing ((n-C4H9)4N)2[W6I14] red-NIR emission and singlet oxygen generation by two photon absorption.

We report the first proof of the ability of octahedral tungsten clusters to emit red-NIR light and to produce singlet oxygen upon two-photon absorption, in solution and in the solid-state. Such discoveries open new perspectives for tungsten cluster compounds in several fields like optoelectronics, photodynamic therapy, and bioimaging.

Expanding the Toolbox of Octahedral Molybdenum Clusters and Nanocomposites Made Thereof: Evidence of Two-Photon Absorption Induced NIR Emission and Singlet Oxygen Production.

Bright NIR phosphorescence from octahedral molybdenum clusters has been known since the 1980s. However, their behavior toward NIR excitation has never been investigated. Here we report their abilities to emit NIR light and produce singlet oxygen upon two-photon absorption. This behavior is observed in solution, in the solid state, and when clusters are embedded homogeneously in a poly(dimethylsiloxane) matrix. Such discoveries open new perspectives in several fields like optoelectronics, photodynamic therapy, or bioimaging.

Excited-State Proton Transfer in Oxyluciferin and Its Analogues.

One of the most characterized bioluminescent reactions involves the firefly luciferase that catalyzes the oxidation of the luciferin producing oxyluciferin in its first excited state. While relaxing to the ground state, oxyluciferin emits visible light with an emission maximum that can vary from green to red. Oxyluciferin exists under six different chemical forms resulting from a keto/enol tautomerization and the deprotonation of the phenol or enol moieties. The optical properties of each chemical form have been recently characterized by the investigations of a variety of oxyluciferin derivatives, indicating unresolved excited-state proton transfer (ESPT) reactions. In this work, femtosecond pump-probe spectroscopy and time-resolved fluorescence spectroscopy are used to investigate the picosecond kinetics of the ESPT reactions and demonstrate the excited state keto to enol conversion of oxyluciferin and its derivatives in aqueous buffer as a function of pH. A comprehensive photophysical scheme is provided describing the complex luminescence pathways of oxyluciferin in protic solution.

Chiral Self-Sorting Process with Ditopic Ligands: Alternate or Block Metallopolymer Assembly as a Function of the Metal Ion.

We report an extensive study on the coordination behavior of chiral ditopic bridging ligands, which lead to metallosupramolecular polymers in the presence of Zn(II) and Cu(II) in solution. With the help of UV-vis and circular dichroism spectroscopies, we show that the metallopolymer sequence can be controlled by chirality and by the choice of the metal ion. Although the formation of a block metallopolymer proceeds through the assembly of homoleptic complexes, an alternate metallopolymer may be obtained only when heteroleptic complexes are formed. This demonstrates how the prevalent coordination geometries at metal centers may be used to control the sequences of the metallopolymers.

Simulation and Analysis of the Spectroscopic Properties of Oxyluciferin and Its Analogues in Water.

Firefly bioluminescence is a quite efficient process largely used for numerous applications. However, some fundamental photochemical properties of the light emitter are still to be analyzed. Indeed, the light emitter, oxyluciferin, can be in six different forms due to interexchange reactions. In this work, we present the simulation of the absorption and emission spectra of the possible natural oxyluciferin forms in water and some of their analogues considering both the solvent/oxyluciferin interactions and the dynamical effects by using MD simulations and QM/MM methods. On the one hand, the absorption band shapes have been rationalized by analyzing the electronic nature of the transitions involved. On the other hand, the simulated and experimental emission spectra have been compared. In this case, an ultrafast excited state proton transfer (ESPT) occurs in oxyluciferin and its analogues, which impairs the detection of the emission from the protonated state by steady-state fluorescence spectroscopy. Transient absorption spectroscopy was used to evidence this ultrafast ESPT and rationalize the comparison between simulated and experimental steady-state emission spectra. Finally, this work shows the suitability of the studied oxyluciferin analogues to mimic the corresponding natural forms in water solution, as an elegant way to block the desired interexchange reactions allowing the study of each oxyluciferin form separately.

Non-Ising and chiral ferroelectric domain walls revealed by nonlinear optical microscopy.

The properties of ferroelectric domain walls can significantly differ from those of their parent material. Elucidating their internal structure is essential for the design of advanced devices exploiting nanoscale ferroicity and such localized functional properties. Here, we probe the internal structure of 180° ferroelectric domain walls in lead zirconate titanate (PZT) thin films and lithium tantalate bulk crystals by means of second-harmonic generation microscopy. In both systems, we detect a pronounced second-harmonic signal at the walls. Local polarimetry analysis of this signal combined with numerical modelling reveals the existence of a planar polarization within the walls, with Néel and Bloch-like configurations in PZT and lithium tantalate, respectively. Moreover, we find domain wall chirality reversal at line defects crossing lithium tantalate crystals. Our results demonstrate a clear deviation from the ideal Ising configuration that is traditionally expected in uniaxial ferroelectrics, corroborating recent theoretical predictions of a more complex, often chiral structure.

Polarization state studies in second harmonic generation signals to trace atherosclerosis lesions.

We have performed multi-photon image reconstructions as well as polarization state analyses inside an artery wall affected by atherosclerosis to investigate the changes in collagen structure. Mice, either healthy or affected by spontaneous atherosclerosis, have been used for this purpose. A two-photon imaging system has been used to investigate atherosclerotic lesions in the ascending aorta of mice. Second harmonic imaging has been performed alternatively on healthy samples and on affected region. The reconstructed images show that the spatial distribution of the collagen network seems disorganized by the disease. The polarization state studies reveal however that the apparent disorganization of the collagen is related to its spatially diffuse distribution and that the internal structure of the collagen fibers is not affected by the disease. In addition, a theoretical simulation of the second harmonic polarization states shows that they are consistent with the known 3D structure of the collagen network.

Broadband transient dichroism spectroscopy in chiral molecules.

We perform time-resolved broadband transmission ellipsometry using the ultrafast pump-probe technique in an original setup. Our setup allows us to measure, over a wide spectral range, the optical rotary dispersion of the sample or, when needed, its circular dichroism by adding a broadband quarter-wave plate. While our experiment has been designed to study the transient states in chiral molecules, it performs sufficiently well to also characterize the ground state.

External stimulus driven variable-step grating in a nematic elastomer.

We report on the creation of micro-patterns in an oriented nematic elastomer (an artificial muscle material) by photopolymerization of surface aligned nematic liquid crystal monomers. We demonstrate that microscopic techniques are able to create accurate patterns in rubber-like liquid crystal materials. Two approaches, based on one and two-photon excitations respectively, are implemented using a microscope-based setup. Due to its high spatial selectivity, the two-photon excitation mode yields finer patterns. Benefitting from the intrinsic, thermally-induced, contractile properties of the material, gratings with variable steps in response to temperature changes were fabricated.