Quadratic hyperpolarizability of push—pull polyenes: a two-form analysis of the length effect

We present a two-form two-state description that generalizes to all push—pull molecules the bond length alternation (BLA) approach. All relevant parameters in our model can be derived from electrooptical absorption measurements. Within this framework, we analyse the length effect on the quadratic hyperpolarizability in two series of push—pull polyenes.