A large βzzz of π-extended molecules tetra-substituted by meta-positioned donor–acceptor pairs

We computationally studied hyperpolarizability β of C2v fused-ring molecules substituted by two pairs of meta- or para-positioned donor (D)–acceptor (A). In two types of meta-substituted isomers, the meta1-type showed small βzzz, being plausible since the meta-positioned D–A pairs indicated the resonance structures not preferable for the large charge transfer (CT). Nevertheless, the meta2-type showed a large βzzz resulting from a large forward CT associated with the HOMO–LUMO transition. This βzzz increased with the number of fused-rings (the x-extension) and decreased slightly with small structural changes by the geometry optimization, but these changes did not vary the dominant CT structure. The x-extension was ineffective to the large βzzz for the para-substituted isomer.