F. Dupuy

Multiplet structure of the emission bands of coronene and perylene in n-heptane single crystal

Abstract Highly resolved emission spectra of coronene and perylene molecules included in single crystals of n -heptane have been observed at 4.2 K. Comparison with spectra obtained in polycrystallized solutions indicates that solute molecules are trapped in similar substitutional sites in both matrices. Polarization study of the multiplet components and ESR measurements reveal that multiplets are due to molecules differing only slightly in orientation.

Multiple luminescence from ‘push-pull’ diphenyl polyenes revealed by picosecond spectroscopy. Evidence for TICT and bicimer states

Abstract Picosecond time-resolved spectroscopy has been used to study the emission behavior of 1-( p -N,N-dimethylaminophenyl)-4-( p -cyanophenyl)-1,3-butadiene (DCB) and 1-( p -N,N-dimethylaminophenyl)-6-( p -cyanophenyl)-1,3,5-hexatriene (DCH) in different solvents and experimental conditions (temperature, concentration, excitation intensity). At low concentrations ( c −4 M) in polar solvents, we clearly resolved two emission bands: the first one is attributed to the planar conformer and is the direct precursor of the second one attributed to the emission of a TICT state. At higher concentration ( c > 10 −4 M) and higher excitation intensity a third emission appears, assigned to a ‘bicimer’ species, as observed previously in stilbene derivatives. This βicimer’ species is formed by the association of at least two excited molecules. Under certain experimental conditions, laser action can be observed from the bicimer emission.

Photochimie à l'État Solide: Réactivité, Structure Cristalline et Fluorescence des Dichloro-1,8 Méthyl-9 et Méthyl-10 Anthracènes

Abstract Photoreactivity of the two isomers has been compared in solution as well as in the solid state (microcristalline powder, KBr matrices) and sites of nucleation of the dimers have been investigated by microscopic observation on monocrystals. In solution, 1,8-dichloro 10-methyl anthracene (6) photodimerises faster than its 9-methylated isomer (5). The reverse order of reactivity is observed in the solid state (microcrystals and microcrystalline powder). Crystalline and molecular structure of 6 have been established and compared to that, already known, of 5 in view to interpretate fluorescence spectra (excimer type) and the difference of reactivity. As for several monosubstituted derivatives, the photodimerisation of 5 and 6 is not topochemical but photodimers have been shown to appear in areas of crystalline defects, especially along the c axis in the compound 6. Compacity of crystalline lattices is considered to play a major role in the difference of reactivity.

Auto-association of tetracene in solution

Abstract A systematic study of the absorption and fluorescence spectra of solution of tetracene led us to the discovery of two auto-associated species: microcrystals, improperly called a dimer by several authors and another species, which could be the true dimer.

Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal. High resolution excitation of the two Davydov components of the N-d8 host crystal

Abstract Direct excitation spectra of the triplet exciton band of deuterated naphthalene are presented. The high resolution spectra (0.1 cm −1 ) of the two Davydov components show a substructure that we attribute to the presence in the crystal of natural traps and local strains. A model based on the calculation of the eigenstates of a finite plane of molecules is provided for lineshape simulations. In this work the artificial edge effects introduced by the finite size of the model crystal are eliminated by appropriate boundary conditions. Our method allows us to introduce a random distribution of traps, with variable energies, located inside and outside the exciton band, thus featuring a real crystal. Comparison between calculations and experimental results accounts for the salient effects of the traps on the spectroscopic properties of a mixed crystal.

Calculation of phosphorescence excitation spectra of clusters in isotopically mixed crystals of deuteronaphthalene

Abstract Using a model of generating random numbers for 100 × 100 sites of a two-dimensional lattice and the approximation of deep traps, we calculate phosphorescence excitation spectra for N h 8 /N- d 8 mixed crystals. We show that observed spectra are consistent with couplings between states AB + and AB − , and AA + and AA − which give strong intensity to the weak peak AB − and makes observable the electronically forbidden state AA − . A strong variation in the local concentration of Clusters is discussed.

Spin—spin and spin—orbit interactions in the triplet states of naphthalene clusters

Abstract The phosphorescence lineshape of triplet clusters is analysed using the direct spin—orbit coupling mechanisms. Triplet spin-quantisation and spin—orbit routes are calculated for any configurations of cluster. This and the consideration of molecules with one substituted 13 C allow us to understand better the napthalene phosphorescence lineshape.

Resonant secondary emission and spectral diffusion in isotopically mixed naphthalene crystals

Abstract Resonant secondary radiation (zero-phonon lines) flourescence spectra of isotopic 3D aggregates of traps (NH 8 ) in a host crystal (ND 8 ), as well as transfer-induced fluorescence at trap concentrations between 0.01 and 10% and at temperatures between 1.6 and 10 K, are presented. Laser-excited (0–1) resonant secondary (zero-phonon) emission is used to resolve close-lying electronic resonances, corresponding to monomers, aggregates and to their host and trap satellite lines, in a quasi-continuum of 3D trap states. Such states are also calculated by numerically diagonalizing a mixed-crystal 3D hamiltonian. Measured and calculated delocalization indices are given for all trap species. In addition, low-temperature spectral diffusion of photoselected trap species is measured by plotting the attenuation of the zero-phonon lines. We give a simple discussion on the modulation of the resonant secondary radiation spectra by concentration, excitation wavelength and temperature. We show that low-temperature exciton diffusion, assited by acoustical phonon creation, occurs above a very low concentration threshold ( C > 0.05% for monomers at T = 1.6 K). Thermally activated transfer of the photoselected excitons is also plotted for temperatures up to 10 K above which the zero-phonon lines are quenched.

Random walks on an ultra-metric space: a model for transport by tunnel effect in a mixed crystal

Hopping transport by tunnel effect in a disordered solid often implies a wide spread of the hopping times. This is described here by an ultra-metric space in which the distance between two centres is defined as the longest hopping time on the shortest path between them. The authors derive numerically and analytically the mean and the second moment of the number of distinct sites visited as functions of time. These results are applied to the calculation of the decay of luminescence due to hopping and trapping in the much studied triplet state of isotopically mixed crystals of naphthalene. The numerical simulations are quite well described by the scaling theory of percolation.

Direct and delayed emissions from a naphthalene crystal doped with pyrene molecules

Abstract Direct and delayed emissions from a naphthalene crystal doped with pyrene molecules are investigated at temperatures between -196 and +70°C. Data are obtained for different optical excitations and for different concentrations of the host molecules. It is shown that these observed emissions are due to the deformation of the crystalline cell by the presence of the guest molecules.