“…Thus, a typical push-pull stilbene, 4-amino-4 -nitrostilbene, has been recently calculated to have [23] a molecular quadratic hyperpolarizability, β, as high as 200 × 10 −30 esu. Push-pull chromophores also serve as solvation probes [24,25] and as model systems for electron transfer [26][27][28].…”