Hyperpolarizabilities βzzz and βzxx of π-extended molecules tetra-substituted by ortho-positioned donor–acceptor pairs as analyzed by homologous substitution model

Hyperpolarizability β of fused-ring molecules tetra-substituted by two ortho-positioned donor (D)–acceptor (A) were studied for three types of C2v isomers, using simplified sum-over-states (SOS) calculation, missing-orbital analysis (MOA), and homologous substitution model. The model is comprised of two molecules, one di-substituted by two Dʹs, the other by two Aʹs at the sites homologous as a whole to tetra-substitution. Ortho1-type with two D–A pairs at molecular x-edges showed larger βzxx than that of the model, exhibiting charge-transfer (CT), nevertheless all isomer types could show CT enhancement according to resonant structures. The substitution model successfully described qualitatively magnitudes and signs for β for all isomers.