Nonlinear optical (NLO) properties of two series of octupolar molecules are investigated to establish the structure-NLO-property relationships by carrying out the ab initio (HF/6-31G) calculations of the molecular polarizability (R), the first hyperpolarizability (β), and the second hyperpolarizability (γ). It is found that, as the charge-transfer character of the ground state increases by introducing a stronger donor, (i) the absolute BLA value decreases and (ii) R, β, and γ increase. Thus, the design strategy to maximize the NLO properties of the octupolar molecule is established. Also, the solvation-induced effects on the NLO properties are studied by using the self-consistent reaction field (SCRF) method. As the solvent polarity increases, R, β, and γ increase monotonically. These observations are in good agreement with the predictions based on the valencebond three-charge-transfer model.

Extended conjugation and donor-acceptor substitution to improve the third-order optical nonlinearity of small molecules Appl. Phys. Lett. 90, 251106 (2007); 10.1063/1.2750396 Role of spatial distortions on the quadratic nonlinear optical properties of octupolar organic and metallo-organic molecules Theoretical descriptions of the molecular nonlinear optical properties of tetrahedral donor-acceptor molecules are presented by using a valence-bond and four charge-transfer state model. Based on this five-state model, as the extent of the charge transfer from the peripheral donors ͑acceptors͒ to the central acceptor ͑donor͒ increases, the first hyperpolarizability monotonically increases. The theoretical predictions are confirmed by carrying out ab initio calculations of the first hyperpolarizabilities of three different series of tetrahedral molecules. The -electron delocalization effect on the nonlinear optical property is elucidated by making a comparison of the first hyperpolarizability of the tetrahedral molecule with that of fictitious tetrahedrally assembled linear polyynes.