Extended conjugation and donor-acceptor substitution to improve the third-order optical nonlinearity of small molecules

The authors use donor-acceptor substitution to increase the third-order polarizability gamma of small cyanoethynylethene molecules by lowering the energy corresponding to the first optical transition. They identify a weak power-law dependence for gamma depending on the number of conjugated electrons that separate the donor and acceptors (N-1.5), which is the result of the two competing contributions that depend on the conjugation length: the energy separation of the ground and first excited states and the strength of the transition dipole matrix elements. The molecules are highly efficient with respect to their small masses and fall within a factor of 50 of the theoretical limit for centrosymmetric molecules, putting them among the best molecules measured to date. (c) 2007 American Institute of Physics.