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Theoretical analysis of the effect of stress field gives the variation of nonlinear optical properties of donor-acceptor polyenes with their molecular structure. Changes in the structure lead to an increase in dipole moment due to charge transfer from the donor to the acceptor. The ground states become polarized, with a change in structure from cyanine to zwitterionic. The polarizabilities are the same for different polyenes, confirming the need for a change in bond length to determine the nonlinear optical behavior of conjugated organic molecules.

The geometry and electronic structure of several donor-acceptor diphenylacetylene molecules, that have one-four triple bonds in the conjugated segment, were theoretically examined, and the second-order molecular polarizabilities-beta at the intermediate neglect of differential overlap/configuration interaction (INDO/CI) sum-over-states level were analyzed. The authenticity of the two-state model and the consequence of the sum-over-states methodology was checked. The emergence of many almost isoenergetic excited states, that are the primary causes for the molecular second-order polarizability-beta, is also the cause for the entire breakdown of the two-state approximation when the conjugation length of the molecules exceeds two ripple bonds.

A comprehensive theoretical characterization of the geometric structure, electronic properties and first- and second order polarizibilities of a series of push-pull polyene molecules is performed by ab initio Hartree-Fock approximations. The results indicate that the geometric structure of the polyenic segment is independent of the presence of the donor acceptor groups. It is observed that the average first-order polarizabilities are of the same order of magnitude for the compounds under consideration.

We present the results of the Austin-Model 1 (AM1) geometry optimizations and valence effective Hamiltonian (VEH) band-structure calculations aimed at determining the evolution of the geometric structure and electronic properties (ionization potentials, electron affinities, band gaps) as a function of unit cell content in two sets of regular block copolymers. The first set consists of poly(p-phenylene vinylene)/poly(2,5-dimethoxy-1, 4-phenylene vinylene) copolymers, the second set of poly(p-phenylene)/polyacetylene copolymers. In the latter case, the band gap can be tuned over the whole visible range, depending on the extent of the block copolymer sequences. Special attention is paid to the possible occurrence of localization phenomena. [ABSTRACT FROM AUTHOR]

The expanded self-consistent reaction-field (SCRF) theory is used as an useful theoretical model to analyze the effects of solvent polarity on nonlinear optical properties. This model is able to predict the nature of the solvent which elevates the hyperpolarizability values. The theoretical values for p-nitroaniline molecules in solvents of different polarity are compared with experimental data from hyper-Rayleigh scattering measurements to examine the solvent sensitivity of the nitroaniline.

A new study describes a strong second-order nonlinear optical signal in an ultra-thin film made of centrosymmetric squaraine dyes. The study is considered revolutionary because significant even-order effects are known to occur only in non-centrosymmetric media. It opens up new possibilities for nonlinear optical signals, which are the main components of modern optical communications systems.

The sum-over-states approach to the determination of the relationship between quadratic nonlinear optical properties and molecular structure provides for the identification of the pi-electron excited states that greatly influence the off-resonant quadratic linear response. Three double-degenerate pi-electron excited states, which include states located at 4.64, 5.26 and 6.64 eV above the ground state, are primary contributors.