Correlated quantum-chemical techniques are applied to the description of electronic excitations in interacting conjugated chains. The focus is on the magnitude and conjugation-length dependence of the splitting of the lowest optically allowed excitonic state, which is induced by interchain interactions. We first examine cofacial dimers formed by linear polyene chains of various lengths and use two strategies to compute the exciton coupling energy. One is based on molecular exciton theory, which assumes that the excited-state wave functions of the isolated chains remain unperturbed by the intermolecular forces; in the other, the supermolecular approach, the wave functions are obtained from molecular orbital calculations performed for the whole system and are therefore not constrained to a single chain. We find that the two techniques lead to consistent results, provided an appropriate form for the interchain Coulomb interactions is adopted in the excitonic model. In particular, both formalisms indicate a peak behavior for the evolution of the exciton splitting energy with the length of the interacting conjugated chains. As an illustration, the chain-length dependence of the Davydov splitting is evaluated in the case of oligothiophenes on the basis of the experimental x-ray crystal structures; the results are compared to recent polarized absorption data.

The present paper deals with the photophysical properties of columnar liquid crystals formed by hexakis-(alky1oxy)triphenylenes. Absorption and fluorescence spectra of solutions are analyzed on the basis of quantum chemical calculations performed by the CS-INDO-CI (conformations spectra-intermediate neglect of differential overlap-configuration interaction) m e t h d . the absorption maximum is due to the SO -Sq transition while fluorescence originates from the weak SO -S1 transition. In columnar aggregates, the former transition corresponds to delocalized excited states while the latter corresponds to localized ones; calculation of intermolecular interactions shows that, at the temperature domain of the mesophases, all the molecules have the same excitation energy and, therefore, no spectral diffusion of the fluorescence is expected, in agreement with the time-resolved emission spectra. Excitation transfer is investigated by studying the fluorescence decays of mesophases doped with energy traps. Their analysis is made by means of Monte Carlo simulations considering both intracolumnar and intercolumnar jumps and using four different models for the distance dependence of the hopping probability. The best description is obtained with a model based on the extended dipole approximation and taking into account molecular orientation.

The core–valence correlation is introduced into ab initio relativistic pseudopotential calculations by modifying the existing core polarization potential. The salient feature of the method presented here is the use of an l-dependent cutoff parameter (which is related to spherical harmonic functions) for solving the multicenter integrals over the 1/r4 - and r/r3 -type operators. The method is tested on the Rb2 and Cs2 molecules considered as two valence-electron problems. Reliable results for the molecular spectroscopic constants (Re, Te, De, and ωe ) are obtained for the ground state and the lowest excited states. Deviation from the experimental values ranges from 0.05 to 0.1 Å for Re, seldom exceeds 2 cm−1 for ωe, and is of the order of 100 cm−1 for De for most of the excited states.

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The present communication examines how the dynamics of the double helix affects the Frenkel excitons that correspond to the low-energy absorption band of DNA. Two types of oligomers, (dA) n .(dT) n and (dAdT) n /2.(dAdT) n /2, are studied theoretically, in the framework of the exciton theory in combination with quantum chemical calculations. The properties of the exciton states (energy, oscillator strength, degree of delocalization, “anisotropy”, etc.) found for canonical B-DNA geometries are compared to those obtained for conformations extracted from molecular dynamics simulations. It is shown that, although structural fluctuations reduce both the mixing between different monomer transitions and the spatial extent of the eigenstates, excitations still remain delocalized over several bases. The presence of alternating base sequences makes the eigenstates of the double-stranded oligomers more sensitive to disorder. All these effects result from a variation of the coupling terms, with the diagonal energy being only slightly altered by the structural fluctuations. The experimental absorption spectra presented here corroborate the theoretical results according to which the absorption of (dA) n .(dT) n is centered at higher energies than that of (dAdT) n /2.(dAdT) n /2. Finally, it is shown that, in contrast to what is commonly admitted, the formation of collective excited states in double-stranded oligomers is not expected to induce large spectral shifts, with respect to a homogeneous mixture of monomers.

Theoretical calculations for the ground state and for 83 excited states of the Na2 molecule are presented in the framework of two independent approaches. The electron–core interaction is represented either by a pseudopotential or by a model potential, and a core polarization potential is introduced in both cases. The basis set contains either Gaussian orbitals or two-center generalized Slater orbitals. The two methods appear to give similar results, one being more accurate for the ground and first excited states, the other being better adapted to the intermediate Rydberg states. A very good agreement is obtained with the experimental spectroscopic constants determined for 26 states, the mean deviation being ΔRe=0.05a0, Δωe=0.86 cm−1, and ΔDe=57 cm−1.

The present study combines both experiment and molecular dynamics simulations in order to document the ionization behavior of the C 6 H 6 -H 2 O and C 6 H 6 -D 2 O complexes close to the ionization threshold, in particular its nonadiabatic character. Using the two-color two-photon resonant ionization laser technique, the ionization thresholds of these species have been measured together with the threshold for dissociative ionization. A binding energy has been deduced for the neutral species: D 0 (C 6 H 6 -H 2 O) ) 106 ( 4 meV and D 0 (C 6 H 6 -D 2 O) ) 116 ( 5 meV, which significantly increases the precision compared to literature. Using a semiempirical potential model, the minimum energy structures of the neutral and ionic species have been determined, and the potential energy surfaces have been analyzed using a two-dimensional approach. As a result, the formation of a stable C 6 H 6 + -H 2 O complex close to the threshold is found to be controlled by a pure quantum effect and is ascribed to the classically forbidden region of the neutral ground state wave function for the intermolecular vibrational motion. Using classical molecular dynamics simulations in order to sample this region, it has been shown that the neutral conformations involved in the production of stable ions at the ionization threshold exhibit a strong geometry change compared to the neutral equilibrium conformation; i.e., the water molecule is strongly shifted off the benzene C 6 axis and is also flipped over backward the benzene ring. The difference in the ionization energy of the C 6 H 6 -H 2 O and C 6 H 6 -D 2 O complexes, which cannot be explained by the difference in the neutral binding energies alone, supports this result.