Synthesis, spectral and redox switchable cubic NLO properties of chiral dinuclear iron cyanide/isocyanide-bridged complexes

Abstract

Two chiral dinuclear cyanide/isocyanide-bridged complexes (R)-[Cp(dppe)Fe–CN–Fe(dppp)Cp]PF₆ (1[PF₆]) and (R)-[Cp(dppe)Fe–NC–Fe(dppp)Cp]PF₆ (2[PF₆]), and their mono-oxidation products (R)-[Cp(dppe)Fe^II–CN–Fe^III(dppp)Cp] [PF₆]₂ (1[PF₆]₂) and (R)-[Cp(dppe)Fe^III–NC–Fe^II(dppp)Cp][PF₆]₂ (2[PF₆]₂) were synthesized and fully characterized. The electronic spectra of both the mixed-valence complexes 1[PF₆]₂ and 2[PF₆]₂ exhibit a strong and broad absorption band with two discernable peaks in the NIR region, which are attributed to Fe(II)–Fe(III) IVCT transitions. The attributions are supported by the DFT calculations. Under irradiation with a nanosecond laser at 1064 nm, the measured third-order NLO results of all four cyanide-bridged complexes showed that complexes 1⁺ and 2⁺ do not exhibit an NLO response, but their one-electron oxidation complexes 1²⁺ and 2²⁺ exhibit a strong NLO response due to a resonance enhanced effect. In addition, both complexes 1²⁺ and 2²⁺ display RSA and self-defocusing effects and show good optical limiting behavior in a broadband range.