J.P. Lemaistre

Electronic excitations in organized molecular systems. A model for columnar aggregates of ionic compounds

Abstract A model for the description of electronic excitations in organized molecular systems of finite size in which intermolecular distances are comparable to molecular dimensions is presented. A methodology based on the excitonic theory coupled to quantum chemistry is developed and applied to columnar aggregates of triarylpyrylium tetrafluoroborates. Excitation energy and interactions among transition moments (diagonal and off-diagonal terms of the matrix Hamiltonian) are calculated taking into account the precise geometry of the aggregate. Its energetic topography shows that diagonal energy is sensitive to edge and orientational effects. Diagonalization of the electronic Hamiltonian in the strong exciton interaction limit, provides the eigenstates from which localization indexes, radiative lifetimes, absorption and fluorescence spectra are calculated. It is found that aggregate growth induces a blue-shift in absorption and an increase in radiative lifetime. Orientational disorder causes a red-shift in fluorescence. Previously published experimental data are discussed in the frame of the present model.

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.

Degeneracy, orientational disorder and chromophore size effects on Frenkel excitons in columnar mesophases

The properties of Frenkel excitons in columnar mesophases are analysed using numerical calculations with a particular emphasis on the off-diagonal disorder induced by rotation of the molecular disks around the column axis. The influence of degeneracy of the molecular electronic transitions and the effect of the chromophore size are illustrated by means of the extended dipole model. The localisation/delocalisation behaviour of the exciton states is characterised by calculation of the participation ratio. It is shown that the existence of two orthogonal dipolar transitions per chromophore makes the optical spectra less sensitive to chromophore rotation and maintains the eigenstates bearing the oscillator strength quite delocalised, even in the presence of complete orientational disorder.

Localization length and intraband scattering of excitons in linear aggregates

Abstract A theoretical model to describe the intraband scattering of excitons in linear aggregates of finite size which exhibit strong intermolecular interactions is presented. From the calculation of the aggregate eigenstates, the localization length of excitons is evaluated for various configurations featuring physical situations like trapping, edge effects, inclusion of diagonal and/or orientational disorders. The intraband scattering is studied by considering the exciton–phonon stochastic coupling induced by the thermal bath. This coupling creates local dynamical fluctuations in the site energies which are characterized by their amplitude ( Δ ) and their correlation time ( τ c ). Expressions of scattering rates are provided and used in a Pauli master equation to calculate the time dependence of the eigenstates populations after initial excitation of the quasi exciton-band. It is shown that the time evolution of the lowest state population as well as the Stokes shift strongly depend on τ c . Comparison of the theoretical results to time-resolved experiments performed on triaryl pyrylium salts allows us to interpret the observed Stokes shift and to derive an average value of the exciton–phonon correlation time.

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.

Multi-channel transfer model for triplet excitons in aromatic compounds☆

Abstract We have investigated the ESR line-shape of molecular triplet excitons in thermal equilibrium. Our formula is applied to the triplet ESR line-shapes of aromatic compounds and elucidates the mechanism of their temperature dependence.

Energy relaxation of exciton states in molecular aggregates

Abstract Energy relaxation of exciton states was recently investigated in columnar molecular aggregates of ionic compounds. Quantum chemistry calculations revealed very strong interactions (1000 cm−1 or more) among the chromophores. Such strong interactions are expected to induce large cooperation effects, even at room temperature. A model to describe the intraband scattering among the eigenmodes of the aggregate, in the strong coupling limit, is presented. Energy relaxation is analysed as an incoherent energy transfer process induced by the stochastic coupling of the exciton states to a thermal bath characterized by the amplitude and the correlation time of the site-energy fluctuations. Transfer rates are then calculated and used in a Pauli master equation to describe the time evolution of the eigenstates populations and the optical line shapes during the scattering process. The theoretical results are discussed for various configurations of linear aggregates as a function of the exciton-bath correlation time and are compared to time-resolved fluorescence experimental data.

Disorder-induced exciton scattering in molecular aggregates

Abstract Static and dynamic disorders are investigated to analyze the exciton scattering in molecular aggregates of finite size like columnar aggregates or J-aggregates which exhibit strong intermolecular dipolar interactions ( V ). The static disorder ( σ ), of Anderson type, stems from the inhomogeneities in the site energies while the dynamic disorder originates from the exciton-phonon stochastic coupling induced by the thermal bath. The dynamic coupling induces the exciton diffusion at rates depending on the amplitude ( Δ ) and on the correlation time ( τ ) of the fluctuations. A theoretical model, based on the numerical calculation of the exciton eigenstates and their participation ratios is used to simulate the role of both disorders on the optical responses. It is shown that an increase of the static disorder diminishes the coherence length, i.e. the number of coherently coupled molecules and that the dynamic disorder induces a temperature-dependent scattering among the quasi-exciton band states. Simulation of the effects of static and dynamic disorders are presented and discussed as a function of the fundamental parameters.

Exciton states in organized molecular systems: A model for columnar aggregates

Abstract A model to describe the collective electronic excited states in organized molecular systems of finite size featuring a columnar aggregate of triaryl pyrylium salt is presented. Calculations are used to evaluate the diagonal and off-diagonal matrix elements of the aggregate Hamiltonian. Diagonalization of the electronic Hamiltonian provides the eigenstates from which the localization properties are calculated. Delocalized states may exist, even at room temperature, due to the very strong coupling among chromophores. Furthermore, the scattering among eigenmodes of the aggregate is studied as an incoherent energy transfer process induced by stochastic coupling to the thermal bath. Time evolution of populations of the eigenstates is numerically calculated and checked against experimental data.

The ESR Line Shape of Triplet Excitons in Disordered Systems : The Anderson Theory Approach

The ESR triplet line-shape of randomly oriented molecular aggregates in the solid phase reflects static and dynamic properties of the excited triplet state. By static properties we refer to the spin hamiltonian parameters describing the fine and hyperfine structure, while by dynamic property we refer to the lifetime for the triplet exciton transfer caused by resonance exchange forces between the units of the aggregate.