Ph. Kottis

Picosecond solvation of electronically excited solutes in alcoholic solvents: non-debye behaviour of the time-dependent fluorescence shift related to h

Time-dependent fluorescence shifts (TDFS) of polar compounds in a series of alcoholic solvents have been studied on a pico-second time scale. A non-exponential TDFS kinetic behaviour is observed, particularly at short times (0<t< 50 ps). It cannot be represented by a sum of decreasing exponentials. The effects of solute-solvent interactions on the kinetics of TDFS have been analysed. It is shown that the non-standard kinetics is due to hydrogen bonding in the solvent, particularly to the presence of alcoholic hydrogen-bonded aggregates. Following electronic excitation of the polar solute reorganization of solvent aggregates controls the TDFS time development. Our results show that a simple Debye model is inadequate to explain such a process.

Picosecond hydrogen-bond formation in excited states: The case of an intramolecular charge-transfer (ICT) state

Abstract Time-resolved emission spectra of 7-amino-3-methyl-1,4-benzoxazine-2-one (AMBO) are plotted in the time range 0

Effect of ground and excited singlet-state geometry on the level ordering of 1,4-diphenylbutadiene: a theoretical study using MNDO

Abstract Using a MNDO formalism, we have calculated the more stable geometries of the ground and excited singlet states of 1,4-diphenylbutadiene (DPB). We show that the angle between the phenyl plane and the polyene plane is important. In the ground state, the phenyl angle corresponding to equilibrium is near 75°. In the excited B u + state, the corresponding value is near 30°. Our results show that significant conformational change occurs in the excited singlet states. Such a change determines the level ordering of the singlet excited states, and particularly the B u + and A g − excited states.

Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal

(1978). Spectra of direct excitation of phosphorescence in an isotopic mixed naphthalene crystal. Molecular Physics: Vol. 35, No. 2, pp. 595-599.

Pure electronic, vibronic, and two‐particle surface excitons on the anthracene crystal. I. General theoretical basis, experimental study, and analysis of the (0,0) region

These two papers (papers I and II) present a complete study—theoretical and experimental—of the first singlet–singlet electronic and vibronic transitions of the anthracene crystal. We interpret the new observed structures in terms of pure electronic, vibronic, and two‐particle site shift surface exciton states. The first paper (I) begins by a model description of the bulk and surface excitons in a rigid lattice. We then describe the experimental study of the anthracene crystal reflectivity and surface emission excitation at low temperatures with and without nitrogen coating of the crystal surface. For the first time, we clearly established the following results: (1) The existence and the accurate position of the upper a polarized Davydov component for the pure electronic surface exciton; (2) The equality within our experimental accuracy (≊2 cm−1), of the surface and bulk excitonic Davydov splitting (Δ=223 cm−1) and the translational equivalence (δ=206 cm−1) of the surface structures with their bulk coun...

Triplet exciton transfer and fusion in isotopically mixed molecular crystals

We present experimental results on triplet energy transfer from traps to super‐traps and on triplet exciton fusion in substitutionally disordered naphthalene crystals, monitored by the phosphorescence and by the delayed fluorescence of the traps and of the super‐traps. The concentration dependence of these emissions at fixed temperatures has been discussed abundantly by other authors. In this work, we present new results on their dependence on temperature and on the source of excitation, which have, respectively, been little or not at all reported on before. Comparison of experiments done with selective and with nonselective excitation of the traps shows that the results depend on the method of preparation of the trap excitons. The existence of threshold temperatures for these emissions is interpreted as evidence of phonon assisted processes, for which mechanisms are proposed. A significant drop in transport efficiency at high temperatures is observed for trap concentrations from 4% to 8%. This drop with ...

Characterisation of species of triplet miniclusters in the system N-h8/N-d8. Cluster—cluster interactions and model inhomogeneous distributions in a two-dimensional lattice

Abstract A full spectroscopic characterisation of triplet miniclusters of N-h8 in a crystai of N-d8 and their interactions via the lattice states is presented in a two-dimensional lattice. Triplet mini-exciton energy levels, transition moments and polarisations are calculated. A randon trial model is used to calculate the effective (spectroscopic) concentration of the different clusters and their topology (spatial distribution and orientation in a two-dimensional 100 × 100 lattice, Spectra, and cluster—cluster couplings leading to transfer or exciton fusion are calculated for local concentrations. It is shown that the average concentration is a very poor parameter for predicting cluster-cluster interactions or cluster spectra lineshapes. The knowledge of inhomogeneity of traps is also important when direct excitation of triplet traps is performed.

Pure electronic, vibronic, and two‐particle surface excitons on the anthracene crystal. II. Experimental study and analysis of the (0,1) region

In this second part, we present the experimental and theoretical study concerning vibronic surface transitions for the vibration modes 390 and 1400 cm−1. We begin by a theoretical discussion of the different limiting cases of exciton‐vibration coupling. In the experimental part, evidence of the surface character for the observed structures in surface emission excitation spectra is provided by their shifts upon surface coating. A general smilarity of the surface states with the previously described bulk states, including a translational equivalence (δ=206 cm−1), is established. The observed structures are assigned to interacting single particle (vibron) resonances and two‐particle bands. However, some slight discrepancies between surface and bulk band structures lead us to question the complete validity of their translational equivalence. Finally, our observations show that the surface two‐particle excitation profile (peak shaped) is quite different from the surface two‐particle absorption profile (stepwise threshold profile). We propose a model mechanism of the surface excitations relaxation which accounts qualitatively for this specific surface excitation profile.

Picosecond spectroscopic investigation of the internal conversion rate of excited perylene in solution

Abstract The dynamics of relation from upper vibrational levels of the S 1 * state and from the upper vibronic states S 3 * and S 4 * of perylene dissolved in organic solvents of various viscosities and polarities have been investigated. Unexpected slow decay times have been measured, of the order of 30 ps for vibrational relaxation in the S 1 * manifold and of the order of 50–60 ps for internal conversion from states S 3 * and S 4 *. Significant solvent effects have not been observed on changing the polarity and the viscosity of the solvent.

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.