Laurent Legrand

Macroscopic coherence of a single exciton state in an organic quantum wire

Macroscopic quantum coherence has been observed in some many-body systems including superconductors, quantum liquids1 and cold atom condensates2, but never for a single quasi-particle state. In an ideal semiconductor, excitons (electron–hole pairs bound by the Coulomb interaction) can, in principle, exist as delocalized plane waves extending over the entire volume. However, any kind of disorder prevents long-range spatial coherence from emerging. There has been evidence for the formation of macroscopic coherent states only in condensate phases such as in the case of microcavity polaritons condensation3,4 or in a dense quasi-two-dimensional exciton gas5. It is unclear however, whether in this latter case the observations are really related to macroscopic coherence6. Here, we show that a single exciton state in an individual ordered conjugated polymer chain7,8, shows macroscopic quantum spatial coherence reaching tens of micrometres, limited by the chain length. The spatial coherence of the k=0 exciton state is demonstrated by selecting two spatially separated emitting regions of the chain and observing their interference.

Polymorphs and colors of polydiacetylenes: a first principles study.

Polydiacetylenes (PDAs) are exceptional polymeric materials with pi-conjugated backbones. Several of them can undergo chromogenic transitions under a wide range of external stimuli. Herein we investigate the electronic structure and the resulting properties of model and experimental PDAs, by means of first principles condensed matter calculations. It is shown that torsional isomers with a twist of the lateral groups can be formed at small energetic costs. We also show the relationship that exists between these twists and the observed changes in the electronic and physical properties. In particular, the calculated changes in the absorption, Raman and NMR spectra agree with the color and property changes as observed experimentally. Therefore, these isomers are excellent models for the structures involved in the chromogenic transitions.

Strong and specific effects of cations on lysozyme chloride solubility.

The influence of salt nature and concentration on tetragonal lysozyme chloride crystal solubility is presented for a set of mono-, di- and trivalent cations (Cs(+), Rb(+), Mn(2+), Co(2+) and Yb(3+)). The results show that cations have as strong an effect on protein solubility as anions and that they present their own particular effects as co-ions. Indeed, after decreasing at low ionic strength, lysozyme solubility increases with high concentration of polyvalent cations, probably due to co-ion binding and therefore the concomitant increase of the net charge of the protein-salt complex. These new results are discussed in order to progress in the understanding of the crystallisation process at the atomic level.

Reversible quenching of a chromophore luminescence by color transition of a polydiacetylene.

A new reactive diacetylene molecule has been synthesized, incorporating a strongly luminescent chromophore, tetrazine (Tz). It readily polymerizes into the blue polydiacetylene (PDA) form, quenching the Tz luminescence already at concentrations ≤1 %. The blue to red PDA transition is thermally induced in the solid state and the original strong Tz emission is restored. This might lead to a new type of detection for sensors using the PDA color transition.

Coherent control of the optical emission in a single organic quantum wire.

We report on the first coherent control experiments on a purely electronic exciton state in an extended quasi-perfect organic quantum wire, a polydiacetylene chain isolated in the crystalline matrix of its own monomer. The time-integrated luminescence of a single wire is measured as the relative phase between two exciting sub-picosecond laser pulses is varied. From visibility functions the exciton dephasing time is extracted and its temperature dependence studied. Our work points the predominant role of thermalization upon the phase relaxation dynamics. By means of microscopic imaging spectroscopy we also show that despite local excitation coherent control is achieved on states delocalized over the chain at the micrometric scale.

Exciton-phonon coupling in a CsPbBr3 single nanocrystal

We have performed micro-photoluminescence measurements on a single CsPbBr3 nanocrystal (NC) with a size comparable to the Bohr diameter (7 nm). When the NC has an orthorhombic crystal symmetry, we observe an exciton fine structure composed of three peaks linearly polarized. We took advantage of the polarization properties of micro-photoluminescence to monitor in situ both the energy and linewidth of individual peaks when increasing temperature. We reveal that two regimes exist, at low and high temperature, which are dominated by acoustic or longitudinal optical phonon (Frohlich term) couplings, respectively. The acoustic contribution does not change when the energy of the excitonic transition varies in the range of 2.46–2.62 eV, i.e., with NC sizes corresponding to this range. We find that line broadening is mainly ruled by the Frohlich term, which is consistent with the polar nature of CsPbBr3.

Weak protein-cationic co-ion interactions addressed by X-ray crystallography and mass spectrometry.

The adsorption of Rb(+), Cs(+), Mn(2+), Co(2+) and Yb(3+) onto the positively charged hen egg-white lysozyme (HEWL) has been investigated by solving 13 X-ray structures of HEWL crystallized with their chlorides and by applying electrospray ionization mass spectrometry (ESI-MS) first to dissolved protein crystals and then to the protein in buffered salt solutions. The number of bound cations follows the order Cs(+) < Mn(2+) ≃ Co(2+) < Yb(3+) at 293 K. HEWL binds less Rb(+) (qtot = 0.7) than Cs(+) (qtot = 3.9) at 100 K. Crystal flash-cooling drastically increases the binding of Cs(+), but poorly affects that of Yb(3+), suggesting different interactions. The addition of glycerol increases the number of bound Yb(3+) cations, but only slightly increases that of Rb(+). HEWL titrations with the same chlorides, followed by ESI-MS analysis, show that only about 10% of HEWL binds Cs(+) and about 40% binds 1-2 Yb(3+) cations, while the highest binding reaches 60-70% for protein binding 1-3 Mn(2+) or Co(2+) cations. The binding sites identified by X-ray crystallography show that the monovalent Rb(+) and Cs(+) preferentially bind to carbonyl groups, whereas the multivalent Mn(2+), Co(2+) and Yb(3+) interact with carboxylic groups. This work elucidates the basis of the effect of the Hofmeister cation series on protein solubility.

Coherent Exciton State In An Organic Quantum Wire

For the first time, we evidence the delocalization of a single quasiparticule state, an exciton, over the entire length of the organic quantum wire on which it is photocreated. This quantum wire, a polydiacetylene chain, has the properties of a perfect quasi‐1D semiconducting system where, moreover, a strong exciton‐photon coupling regime can be reached without the need of a cavity for the photons. The macroscopic coherence of the exciton state on the chain is demonstrated using an interference experiment.