Sylvie Marguet

Photophysical properties of 5-methylcytidine.

Cytosine methylation, which determines the hot spots for DNA photo-damage, is shown to induce a red-shift of the nucleoside absorption spectrum, making the chromophore more vulnerable to solar radiation, and a tenfold increase of the fluorescence lifetime, making excited statereactions more probable. A femtosecond investigation of the excited state deactivation reveals a quite complex mechanism.

Photophysical Properties of Discotic Dibenzopyrenes

Abstract The photophysical properties of three discogenic dibenzopyrenes substituted by eight pentyloxy (O5DPB), heptyloxy (O7DBPP) or decyloxy (O10DBP) side chains are studied in solution and thin films. It is shown that the absorption and fluorescence spectra of the columnar mesophases are clearly distinguishable from those of the corresponding crystalline phases, allowing the study of phase transitions. Thus, it is found that the shorter the lateral chain length, the slower the crystallisation process. For O5DBP, the supercooled mesophase is stable over a period of at least one year; it crystallises after cooling below the glassy transition. Such a behaviour gives rise to temperature controlled optical bistability based on the position of the lowest energy absorption band and illustrated by a hysterisis loop.

Electronic coupling responsible for energy transfer in columnar liquid crystals

Electronic coupling is the driving force for energy transfer in molecular materials and consists of several components. We determine the strength of dipolarrmultipolar coupling and coupling due to orbital overlap for excitation transport in triphenylene columnar liquid crystals. We use time-resolved fluorescence spectroscopy and computer simulations. The fit of the experimental and simulated fluorescence decays reveals that the transfer process is dominated by short range interactions .multipolar and orbital overlap but the contribution of long range dipolar interactions cannot be neglected. q 1999 Elsevier Science B.V. All rights reserved.

Fano Transparency in Rounded Nanocube Dimers Induced by Gap Plasmon Coupling

Homodimers of noble metal nanocubes form model plasmonic systems where the localized plasmon resonances sustained by each particle not only hybridize but also coexist with excitations of a different nature: surface plasmon polaritons confined within the Fabry-Perot cavity delimited by facing cube surfaces (i.e., gap plasmons). Destructive interference in the strong coupling between one of these highly localized modes and the highly radiating longitudinal dipolar plasmon of the dimer is responsible for the formation of a Fano resonance profile and the opening of a spectral window of anomalous transparency for the exciting light. We report on the clear experimental evidence of this effect in the case of 50 nm silver and 160 nm gold nanocube dimers studied by spatial modulation spectroscopy at the single particle level. A numerical study based on a plasmon mode analysis leads us to unambiguously identify the main cavity mode involved in this process and especially the major role played by its symmetry. The Fano depletion dip is red-shifted when the gap size is decreasing. It is also blue-shifted and all the more pronounced that the cube edge rounding is large. Combining nanopatch antenna and plasmon hybridization descriptions, we quantify the key role of the face-to-face distance and the cube edge morphology on the spectral profile of the transparency dip.

Spectroscopic properties of nematic discoticphenylethynylbenzene derivatives: Symmetry effects

The spectroscopic properties of two phenylethynylbenzene derivatives capable of forming nematic discotic (N D ) liquid crystals are studied in solution and in their N D phase. Their electronic absorption spectra are analysed with the help of fluorescence anisotropy measurements and on the basis of a previous theoretical study. The results presented here illustrate the effects of a symmetry lowering of a radial-symmetric chromophore, arising from the substitution of an ethynylbenzene group by an alkoxy chain, upon the positions and the intensities of the electronic transitions. The absorption and fluorescence spectra of the nematic discotic phases are compared to those obtained in solution. The observed spectral shifts are discussed in terms of both exciton coupling and environment effects on the excitation energy.

Electronic states of multichromophoric phenylethynylbenzene derivatives: exciton theory and CS-INDO-CIPSI calculations

Abstract The singlet electronic states of hexaphenylethynylbenzene derivatives are calculated combining S-CI calculations in the π system and CS-INDO-CIPSI calculations in the σ and π systems (conformations spectra-intermediate neglect of differential overlap-configuration interaction by perturbative selected iterations). S-CI calculations (singly configuration interaction) in the π system using localized π MOs on each chromophore are performed in order to characterize the nature (localized or delocalized) of the excited states of these multichromophoric compounds. It is shown that simple considerations based on exciton theory allow the prediction of these excited states. The effects of symmetry lowering upon the spectroscopic properties of the hexaphenylethylbenzene derivatives arising from orientational disorder of the outer benzene rings are compared to those arising from a methoxy substitution of the central aromatic core. The CS-INDO-CIPSI method, in a version recently modified by us, gives excellent results to interpret the electronic spectra of very large molecules. More generally, this theoretical work provides directions concerning the way to modify the spectroscopic properties of phenylethynylbenzen derivatives.

Pinacyanol chloride as a saturable absorber in Rhodamine B and 6G hybridly mode-locked dye lasers

Abstract Pinacyanol chloride (PC) is shown to be a better saturable absorber than DODCI in hybridly mode-locked rhodamine 6G and rhodamine B dye lasers, due to its higher chemical and photochemical stability. Using PC, stable femtosecond laser pulses can be generated for months without changing the saturable absorber solution. PC also allows a more efficient suppression of satellite pulses.

TICT and triplet states of triarylpyrylium cations

Abstract Triarylpyrylium cations bearing an electron donating group are studied in solution by two pump-probe techniques with picosecond and nanosecond resolution. The transient spectra recorded on the picosecond time-scale as a function of solvent viscosity are in agreement with the formation of a TICT (twisted intramolecular charge transfer) state, predicted by quantum chemistry calculations. The ground state relaxation from the twisted conformation is observed. It is shown that the triplet state is formed only in solid matrices where the non-radiative decay of the excited singlet to the ground state is slowed down.

The fluorescence properties of the phenylated fullerenes C70Ph4, C70Ph6, C70Ph8, and C70Ph10 in room temperature solutions

Abstract The emission and excitation spectra of four phenylated [70] fullerenes, C 70 Ph 4 , C 70 Ph 6 , C 70 Ph 8 , and C 70 Ph 10 in cyclohexane and toluene solutions have been measured. The fluorescence spectra and related excited state properties are found to depend strongly on the number of attached phenyl groups, but with no systematic trends. Quantum yields and fluorescence lifetimes were measured for C 70 Ph 6 , C 70 Ph 8 , and C 70 Ph 10 , allowing the determination of S 1 → S 0 radiative transition rates k R . It is found that k R for C 70 Ph 10 is about six times larger than for the other compounds. This is consistent with measured absorbtivities for these compounds. The particular character of C 70 Ph 10 is also manifested by its higher intersystem crossing rate k ISC .

Two-photon luminescence of single gold nanorods: Influence of the rods volume at a given plasmon resonance

The two-photon luminescence (TPL) of colloidal gold nanorods (GNR) was investigated at the single object level, using correlated topography and polarization resolved intensity and spectral analyses. The origin of the TPL in colloidal GNR is discussed: we show the existence of a specific volume maximizing the TPL intensity.