Jean-Pierre Galaup

Coupling a dendrimer and a fullerene chromophore : a study of excited state properties of C61(poly(aryl)acetylene)2

Abstract A new methanofullerene, C 61 (poly(aryl)acetylene) 2 , where the mono-adduct contains dendrimer units, has been studied with respect to its photophysical properties. The visible absorption and the emission spectra, the triplet molar extinction coefficient as well as the near unity value of the quantum yield of singlet oxygen production upon photoexcitation reveal characteristics in keeping with other methanofullerenes previously studied, where the addend species is not an electron donating group. The results indicate that, even when the methanofullerene is photoexcited, this large addend does not electronically communicate through the methano bridge neither through space. Photophysical properties of methanofullerenes can thus be preserved when they are functionalised with such addends.

Fluorescence and phosphorescence spectroscopy of C70 in toluene at 5 K: site dependent low lying excited states

Abstract We have recorded the dispersed fluorescence and phosphorescence spectra of C70 in toluene matrices at 5 K upon excitation with the green Ar+ laser line (λ=514 nm), along with phosphorescence excitation spectra and also site selected phosphorescence spectra. The dispersed phosphorescence spectrum shows spectral features that originate from at least 14 distinct different sites for C70 within the toluene matrix. Selective excitation of the lowest singlet–singlet transition at 665.8 nm generated a spectrum from a single phosphorescent site which shows prominent Herzberg–Teller induced e1′ and e2′ modes in a 3 A 2 ′ – 1 A 1 ′ transition. The dispersed fluorescence spectrum showed Herzberg–Teller induced e1′ and a1″ modes that were inferred to originate from a 1 A 2 ′ – 1 A 1 ′ transition as well as bands in the origin region that were interpreted to come from 1 E 1 ′ – 1 A 1 ′ and 1 A 2 ″ – 1 A 1 ′ transitions. The selective phosphorescence excitation spectra revealed that the character, the absolute energy and the order of the two lowest excited singlet states 1 A 2 ′ and 1 E 1 ′ are site dependent. An orbitally allowed and thermally activated T2 state lying 240 cm −1 higher in energy from the T1 state is revealed by temperature dependent experiments. Site selective phosphorescence excitation spectra showed evidence of phonon sidebands 12–14 cm−1 from the corresponding zero-phonon line. The energy difference between the S1 and T1 states was determined to be about 2936 cm −1 , however the exact splitting energy is matrix and site dependent.

Amplitude and phase measurements of femtosecond pulses shaped by use of spectral hole burning in free-base naphthalocyanine-doped films

We performed pulse-shaping and time-reversal experiments using spectral holography based on persistent spectral hole burning in free-base naphthalocyanine-doped films. We demonstrate that we can control the pulses diffracted from the hologram by shaping and then by characterizing these pulses in both amplitude and phase. A dephasing time of 29 ps (i.e., a homogeneous linewidth of 69 GHz) was measured from a photon-echo experiment in the chemically accumulated regime.

Influence of the nature of sol–gel matrices on absorption and fine-structure fluorescence spectra of hypericin

Abstract Absorption and fluorescence spectra of hypericin dispersed in an inorganic or a hybrid organo–inorganic have been studied at room or liquid-nitrogen temperature. Changes observed in the absorption spectra and the absence of fluorescence in the inorganic host are assigned to hydrogen bonding between hypericin and hydroxyl groups covering the surface of the pore in the matrix. Analysis of relative intensities and frequency changes in the fine-structure fluorescence and excitation spectra at 4.2 K indicate an increase of the strength of hydrogen bonds under photoexcitation of hypericin. The character of the guest–host interaction in the hybrid xerogel is discussed.

Persistent spectral hole burning in an organic material for temporal pattern recognition

A broad-bandwidth persistent spectral hole-burning organic material is used as a fast optical processor that compares an input temporal-frequency profile with a recorded reference spectral shape. This pattern-recognition procedure relies on a subpicosecond temporal cross-correlation process. The size of the phase-encoding spectral interval exceeds 1 THz. The storage material, the spectral encoder, and the interferometric detector are examined in detail for optimal pattern discrimination. Experimental temporal pattern-recognition results are reported.

C70Ph8 and C70Ph10: a computational and solid solution spectroscopic study

Mapping of the potential energy surface of the torsions of the phenyl groups of C70Ph8 and C70Ph10 by molecular mechanics reveals the existence of two conformers for each molecule. Energetically, both pairs are quasi- degenerate and therefore they could be experimentally detectable despite the low activation barriers for the inter-conversion (2.4 kcal mol−1 for C70Ph8 and 3.9 kcal mol−1 for C70Ph10, respectively). In an effort to pin down the existence of the conformers, the laser induced luminescence spectra of C70Ph8 and C70Ph10 were recorded in several solid solutions. The spectral features for C70Ph8 are rather independent of the nature of the environment and closely resemble those observed before in liquid solution with two main bands separated by more than 2000 cm−1. For C70Ph10, two new low-energy bands appear which were previously undetected in the liquid phase studies and which depend on the matrix. It is proposed that the doublet bands are fingerprints of the presence of the conformers.

Fluorescence spectroscopy of C60 in toluene solutions at 5 K

We have recorded the dispersed fluorescence and the fluorescence excitation spectra of C60 in toluene matrices at 5 K. Upon excitation with the green Ar a laser line (ka 514 nm) we obtained for the first time in this matrix well resolved visible fluorescence spectra which we have compared with those observed in other low temperature matrices. Our spectra were interpreted and assigned using theoretical assessments of vibronic activities of transitions between the three lowest excited electronic states 1 T1g, 1 T2g, 1 Gg and the totally symmetric ground state, and on the basis of a single 0 0 level which has pseudo-Jahn‐Teller (JT) components of the three near-degenerate excited states. The fluorescence spectra exhibit prominent JT induced hg(1) progressions, Herzberg‐Teller-induced hu and other ungerade mode vibrations, including a very active t1u(4) mode. Excitation wavelength independent bands are assigned to the fluorescence of C60 molecules in toluene microcrystals embedded in the toluene glass whereas excitation wavelength dependent features are interpreted as originating from C60 molecules isolated in the toluene glass itself. These interpretations are supported by the results of spectrally selective detected fluorescence excitation spectra. ” 2001 Elsevier Science B.V. All rights reserved.

Vibrational dynamics of CO stretching in W(CO)6-doped hybrid xerogels from 5 K to room temperature with the CLIO-FEL

One color pump-probe and photon-echo experiments have been performed on the CO stretching mode of W(CO) 6 at about 1982 cm -1 in organo-mineral xerogels. Over the whole temperature range studied, from room temperature to 5 K, the population lifetime T 1 remains constant when the vibrational dephasing time T 2 lengthens slightly. The temperature evolution of the pure dephasing time T * 2 reveals two activation energies corresponding to molecular motions of the guest in the xerogel pore.

Optical information storage in condensed matter with stochastic excitation

We have performed a temporal structure analysis of subpicosecond signals retrieved in the time domain from a spectral modulation engraved in a photochemical hole-burning material. Two situations are examined experimentally. The first one corresponds to a spectrally programmed stimulated-photon-echo configuration, and the second one corresponds to simple optical data storage. The results are obtained in a sample of octaethylporphine molecules embedded in a polystyrene matrix. Broadband stochastic excitations are used both to write the modulation inside the inhomogeneous width of the ground level of the zero-phonon line and then to read the spectral information. An original field cross-correlation technique is used to sample the retrieved signals.

Vibrational dynamics of deuterium chloride in solid nitrogen probed by linear and nonlinear spectroscopy

The vibrational dynamics of DCl isolated in nitrogen matrices is investigated by means of time-resolved one-color degenerate four-wave mixing experiments with the help of infrared absorption spectroscopy. Coherence times are found in the tens to hundreds of picoseconds, showing a strong coupling between the DCl stretch and the low-energy local phonons of the lattice. Spectral diffusion due to vibrational energy transfers between the guest molecules is also examined. Fast energy transfers between isotopic species explained the specific shape of the time-resolved signals. The efficiency of such processes is discussed, suggesting an enhancement of the DCl vibrational transition dipole moment induced by the nitrogen environment.