Stephan Guy

Efficient Dibenzo[c]acridine Helicene-like Synthesis and Resolution: Scaleup, Structural Control, and High Chiroptical Properties

Herein, we describe our recent expeditious synthesis of dibenzo[c]acridine helicene-like compounds on a large scale in pure enantiomeric form. This flexible synthesis allows for variation at several positions on the skeleton. Geometrical parameters related to these compounds have been obtained from monocrystal X-ray structure resolution. Additionally, chiroptical parameters have been recorded, highlighting the versatility of this family showing for example optical rotation at 589 nm varying between 135 and 150 deg g(-1)cm(2).

Room temperature photon avalanche in Ho3+ doped YAG, YAP, YLF and ZBLAN

Abstract Upconverted, green emission from the 5 S 2 level of Ho 3+ ion in two oxides, YAG and YAP, and two fluorides, YLF and ZBLAN, was observed at room temperature under cw 580–590 nm excitation. Under these excitation wavelengths, which are off resonance with any ground state absorption, characteristic features of photon avalanche process are observed. A time-dependent rate equation model was employed to describe this behavior using experimentally determined parameters.

Fluorescence imaging of submicrometric lattices of colour centres in LiF by an apertureless scanning near-field optical microscope

We report fluorescence imaging of colour centres in Lithium Fluoride (LiF) using an apertureless Scanning Near Field Optical Microscope (SNOM). The sample consists of periodically spaced submicrometric coloured areas F2 laser-active colour centres produced by low-energy electron beam lithography on the surface of a LiF thin film. A silicon Atomic Force Microscope (AFM) tip is used as an apertureless optical probe. AFM images show a uniform surface roughness with a RMS of 7.2 nm. The SNOM images of the red fluorescence of colour centres excited at lambda = 458 nm with an argon ion laser show that the local photon emission is unambiguously related to the coloured areas and that topographic artefacts can be excluded.

Efficient 4f3(4F3/2)→4f25d excited-state absorption in Nd3+ doped fluoride crystals

Abstract Excited state absorption (ESA) from the 4 F 3/2 metastable level of Nd 3+ to the 4f 2 5d excited configuration has been investigated in YLiF 4 , LiLuF 4 and BaY 2 F 8 fluoride crystals doped with Nd 3+ ions. Efficient ESA cross-sections are obtained around 213 nm and suggest that an up-conversion pumping scheme could be used to reach 4f 2 5d for UV tunable source. ESA from the 4 D 3/2 level to the 4f 2 5d configuration completes the study. A simple interpretation of the results on observed interconfigurational transitions is given using electric dipole selection rules.

Pure chiral organic thin films with high isotropic optical activity synthesized by UV pulsed laser deposition

We show that the pulsed laser deposition (PLD) technique allows us to elaborate pure molecular solid state thin films based on binaphthyl molecules featuring high isotropic optical rotation (HIOR). The transfer of chirality from the ablated target to the substrate was probed by polarimetry and circular dichroism (CD) measurements. A rotary power of 12° mm at 546 nm has been obtained. CD spectroscopy confirms both the chiral and the isotropical nature of the deposited thin films and put into evidence the disorder between the naphthyl chromophores inside the molecular material. Finally, we highlight three regimes for the deposition mechanism depending on the laser fluence: desorption without racemisation, desorption with racemisation, and ablation with degradation. These results demonstrate that, by properly adjusting the growth conditions, isotropic purely organic chiral films can be grown by PLD, thereby opening the prospect for the synthesis of high value organic materials for applications such as waveguiding.

Au10(SG)10: A Chiral Gold Catenane Nanocluster with Zero Confined Electrons. Optical Properties and First-Principles Theoretical Analysis

We report facile synthesis of the Au10(SG)10 nanoclusters, where SG stands for glutathione, found to be promising as a new class of radiosensitizers for cancer radiotherapy. The homoleptic catenane structure with two Au5SG5 interconnected rings, among different isomer structures, gives the best agreement between theoretical and experimental optical spectra and XRD patterns. This catenane structure exhibits a centrosymmetry-broken structure, resulting in enhanced second harmonic response and new characteristic circular dichroism signals in the spectral region of 250-400 nm. This is the first determination of the nonlinear optical properties of a ligated cluster with an equal Au-to-ligand ratio, thus without a metallic core and therefore zero confined electrons. Insight into the nonlinear and chiroptical efficiencies arising from interplay between structural and electronic properties is provided by the TD-DFT approach.

Large-Scale Synthesis of Helicene-Like Molecules for the Design of Enantiopure Thin Films with Strong Chiroptical Activity

Helicenes are fascinating molecules owing to their unusual properties and applications in many fields from catalysis to organic electronics. Herein, we report a straightforward pathway for the synthesis of helicene-like molecules on a gram scale in an enantiopure form. Thin-film materials with good propagating optical properties and very high chiroptical responses have been grown by using pulsed laser ablation without altering the structure or the enantiopurity of the molecules. Moreover, electronic and vibrational circular dichroism spectroscopies coupled with theoretical calculations enabled some dependences of the chiroptical properties with the structure to be highlighted, for example, effects of rigidification, aromatization, or the state of matter (liquid versus solid).

Reversibility of the supramolecular chirality of bridged binaphtol derivatives at the air-water interface

Second Harmonic Generation (SHG) was used to observe the reversible formation of chiral molecular aggregates at the air-water interface. The aggregates were formed by the chiral bridged binaphthol derivative 1( + ) under lateral compression in a Langmuir trough. The S polarized output SHG intensity was measured as a function of the input polarization angle of the fundamental beam during several compression cycles of the molecular film. The chirality of the supramolecular aggregates was established from the non vanishing SHG intensities collected for the p and s input polarization angles. It is demonstrated that the formation of the chiral domains with compression is a reversible process.

Optical properties of Tm3+ ions in GeGaSSb glasses

Tm3+ are new candidates for optical amplifiers in the 1.5micrometers region, the S band. For the moment, Tm3+-doped Fibers Amplifiers (TDFA) h ave been studied in fluoride matrices because of the lowest phonon energy of this glasses compared to the silica ones. We proposed to evaluate the spectroscopic potential of this ion in sulfide GeGaSSb glasses where the phonon energy is around 350cm-1. Absorption and emission cross sections were measured. Lifetime measurements were carried out. Results were compared with the Judd-Ofelt calculations.

Photon avalanche upconversion in YAIO3: Ho3+ crystals

Abstract We report room temperature upconverted emission in Ho3+-doped YAIO3 single crystals. Intense green fluorescene was observed under yellow, orange or red cw excitation. Under 580–590 nm excitation, where the characteristic features of a photon avalanche process are observed, the excitation power threshold is twice lower in the 5 at.% Ho3+-doped sample than in the 1 at.% Ho3+-doped one.