Maximizing Third Order Optical Polarizability of Small Organic Molecules Through Donor-Acceptor Substitution
We show that a donor-acceptor or push-pull system is an important design strategy for maximizing the off-resonant third-order optical nonlinearity (gamma) in small organic molecules, cyanoethynylethenes. Our work details the two competing contributions to gamma that depend on the conjugation length: the energy corresponding to the first optical transition, and the strength of the transition dipole matrix elements. This competition leads to the weak power-law dependence of gamma that we measured, and depends on the number of conjugated electrons that separate the donor and acceptor (N^1.5). Our molecules have record high nonlinearity relative to their molecular mass and fall within a factor of 50 of their theoretical limits, making them highly attractive candidates for all-optical device integration.