Nonlinear optical properties of linear chains and electron-correlation effects

The large third-order nonlinear optical properties ${\gamma }_{\mathrm{ijkl}}(-{\omega }_4;{\omega }_1,{\omega }_2,{\omega }_3)$ observed for conjugated organic linear chains, such as polyenes, are explained by strong electron-correlation behavior in virtual, two-photon ${}_{}{}^1{}_{}{}^{}A_g^{}{}_{}{}^{}$ states. The dominant chain axis component ${\gamma }_{\mathrm{xxxx}}$ is found to possess a power-law dependence on the chain length with an exponent of 4.6 ± 0.2, independent of chain conformation. Good agreement in sign and magnitude is obtained between calculated and experimental gas-phase values of ${\gamma }_{\mathrm{ijkl}}(-2\mathrm{}\omega ;\omega ,\omega ,0)$.