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Optimizing specific third-order polarizabilities and approaching the fundamental limit in donor substituted cyanoethynylethene (CEE) molecules

We review the molecular third-order polarizability of a series of compact two-dimensional cyanoethynylethene derivatives and show that the best molecules have a nonlinearity that is only by a factor of 50 lower than the fundamental limit derived by Kuzyk, which is revealed to be a quite useful concept in discussing the observed trends in molecular nonlinearities. We also show that the nonlinearity of this family of molecules contains a large two-photon contribution, and that the best molecules have a very high specific third-order polarizability, meaning that they pack a large nonlinearity in a relatively small size.