Theoretical assessment of new molecules for second-order nonlinear optics

The polarizabilities and first hyperpolarizabilities of nine recently synthesized compounds have been calculated ab initio to assess the effects of basis sets, of electron correlation (at the MP2 level), of frequency dispersion, and of the solvent within the polarizable continuum model, as well as to address structure-property relationships. These calculations confirm that moderate-size basis sets like 6-31+G* or 6-311+G* are enough to provide accurate estimates of the polarizabilities and first hyperpolarizabilities for these systems build from the combination of π-conjugated linkers with donor/acceptor moieties. They describe the specific behavior of the hyper-Rayleigh scattering (HRS) first hyperpolarizability and of its depolarization ratio as a function of the incident light frequency. Then, these calculations exemplify the large impact of including electron correlation and solvent effects on the first hyperpolarizabilities, and the crucial issue in view of optimizing the NLO compounds, that these effects are strongly system dependent. Finally, this theoretical investigation draws attention to the interplay between several second-order nonlinear optical molecular responses: the norm of the first hyperpolarizability (β), the projection on the dipole moment (β), which can be obtained from electric field-induced second-harmonic generation measurements, and the HRS response (β).