Heptametallic, octupolar nonlinear optical chromophores with six ferrocenyl substituents.

New complexes with six ferrocenyl (Fc) groups connected to ZnII or CdII tris(2,2-bipyridyl) cores are described. A thorough characterisation of their BPh4 salts includes two single-crystal X-ray structures, highly unusual for such species with multiple, extended substituents. Intense, visible d(FeII)* metal-to-ligand charge-transfer (MLCT) bands accompany the * intraligand charge-transfer absorptions in the near UV region. Each complex shows a single, fully reversible FeIII/II wave when probed electrochemically. Molecular quadratic nonlinear optical (NLO) responses are determined by using hyper-Rayleigh scattering and Stark spectroscopy. The latter gives static first hyperpolarisabilities 0 reaching as high as approximately 1027esu and generally increasing with -conjugation extension. Z-scan cubic NLO measurements reveal high two-photon absorption cross-sections sigma 2 of up to 5400GM in one case. DFT calculations reproduce the -conjugation dependence of 0, and TD-DFT predicts three transitions close in energy contributing to the MLCT bands. The lowest energy transition has octupolar character, whereas the other two are degenerate and dipolar in nature.