Unveiling the relationship between structural and polarization effects on the first hyperpolarizability of a merocyanine dye.

The nonlinear optical response, more specifically the Hyper-Rayleigh Scattering (HRS) response of the Brooker's merocyanine, has been calculated at the time-dependent density functional theory level and rationalized in terms of the structural changes and polarization effects induced by applied external electric fields. The structural change leads to large changes in the HRS response, while only slight variations were observed due to the polarization effects on the fixed quinoid form. Considering both structural and polarization contributions concurrently, the HRS response is dominated by cooperative behavior of those effects for weak and intermediate electric field strengths. At the same time, the competition between both effects was a crucial factor in the region of strong electric fields. The obtained results can lead to an easier understanding for upcoming studies considering more realistic models of solvents where it is not simple to disentangle these contributions.

Unraveling the Electric Field-Induced Second Harmonic Generation Responses of Stilbazolium Ion Pairs Complexes in Solution Using a Multiscale Simulation Method.

The electric field-induced second harmonic generation (EFISHG) response has been largely used to describe the first ß and the second γ hyperpolarizabilities in solution. Although the EFISHG technique cannot be applied to charged compounds (due to the external static electric field), it can be used to describe ion pairs as neutral complexes. A multiscale computational approach is required to generate representative geometrical configurations of such kinds of complexes (using classical force fields), to compute the electronic structure of each configuration (using quantum mechanics methods), and to perform statistical analyses describing the behavior of the nonlinear optical properties. In this work, we target solvated neutral ion pairs complexes, of which the cation is an organic chromophore, and we estimate their EFISHG and hyper-Rayleigh scattering responses. It is shown that the anion-cation relative spatial distribution determines the permanent dipole moment of the complexes, and therefore the relative distance controls the EFISHG response. On the other hand, the ß tensor is independent of the dipole moment and it shows a weak linear correlation with the π-electron conjugation length of the cations. The γ contributions in the global EFISHG response range from 5% to 15%, which is mostly due to the variations of amplitude of the µß∥ contribution, which results from differences in the µ and ß vectors' orientations. The applied multiscale approach provides reasonable results compared with experimental ones, although additional efforts are still required to improve such comparison mainly to consider the possible dissociation effects.

Density-functional theory (hyper)polarizabilities of push-pull pi-conjugated systems: treatment of exact exchange and role of correlation.

The performance of the optimized effective potential procedure for exact exchange in calculating static electric-field response properties of push-pull pi-conjugated systems has been studied, with an emphasis on NO2-(CH=CH)n-NH2 chains. Good agreement with Hartree-Fock dipole moments and (hyper)polarizabilities is obtained; particularly noteworthy is the chain length dependence for beta/n. Thus, the problem that conventional density-functional theory functionals dramatically overestimate these properties is largely solved, although there remains a significant correlation contribution that cannot be accounted for with current correlation functionals.