Effect of conjugated length on the calculated nonlinear second‐order optical susceptibilities of some organic molecules

Calculations of the nonlinear second‐order optical susceptibilities (βijk) for disubstituted conjugated organic molecules are reported. The calculations are performed using the semiempirical INDO/CI method combined with a sum‐over‐states expression for βijk. The calculated results show that the second‐order static susceptibility (β0) varies as the 2.6 power of the length of the conjugated π‐electron system. Furthermore, it is found that the amount of substituent charge transfer from ground state to first excited state decreases with inccreasing chain length. A limiting value of β0 with the length of the conjugated system is discussed.