Enhancing second-order nonlinear optical properties by controlling the wave function in one-dimensional conjugated molecules

We have developed a way to control the wave function to improve the second-order optical nonlinearity in organic materials. We simulated second-order hyperpolarizability β of four types of molecules with a polydiacetylene structure with donor and acceptor substitutions with the Austin Model 1 method of Dewar et al. [J. Am. Chem. Soc. 107, 3902 (1985)]. To improve β by wave-function control, the transition dipole moment $r_{\mathrm{gn}}$ and dipole-moment difference Δr between the ground and excited states must be optimized simultaneously by controlling the charge separation of the wave function by adjusting conjugated lengths and donor- and acceptor-substitution sites. We expect a maximum second-order hyperpolarizability per unit molecular length of about 3000×${10}^{\mathrm{-}30}$ esu/(10 Å) at a detuning energy of 0.2 eV, which corresponds to an electro-optic coefficient >100$r_{33}$(${\mathrm{LiNbO}}_3$). The sharpening absorption band should induce an exceptionally large electro-optic coefficient >10 000$r_{33}$(${\mathrm{LiNbO}}_3$).