Electron-rich iron/ruthenium arylalkynyl complexes for third-order nonlinear optics: Redox-switching between three states

The new [(n²-dppe)(n⁵-C₅Me ₅)Fe(CξC-1,4-C₆H₄CξC)Ru(n ²-dppe)₂CξC(C₆H₅)] complex (3-H) and its hexanuclear relative [{(n²-dppe)(n⁵-C ₅Me₅)Fe(CξC-1,4-C₆H₄-CξC) Ru(n²-dppe)₂(CξC-1,4-C₆H₄CξC) ₃(1,3,5-C₆H₃)] (4) have been synthesized and characterized. The linear and cubic nonlinear optical properties of these compounds in their various redox states have been studied along with those of the analogous complexes [(n²-dppe)(n⁵-C₅Me ₅)Fe(CξC-1,4-C₆H₄CξC)Ru(n ²-dppe)₂R][PF₆]_n (n=0-2; R=Cl, 2-Cl; R=CξC(4-C₆H₄NO₂), 3-NO₂). We show that molecules exhibiting large third-order nonlinearities can be obtained by assembling such dinuclear Fe/Ru units around a central 1,3,5-substituted C₆H₃ core. These data are discussed with a particular emphasis on the large changes in their nonlinear (third-order) optical properties brought about by oxidation. Experimental and computational (DFT) evidence for the electronic structures of these compounds in their various redox states is presented using 3-Hⁿ⁺ as a prototypical model. Single crystals of this complex in its mono-oxidized state (3-H[PF₆]) provide the first structural data for such carbon-rich Fe^III/Ru ^II heteronuclear mixed-valent (MV) systems. Although experimental evidence for the structure of the dioxidized states was more difficult to obtain, the theoretical study reveals that 3-H²⁺ can be considered to have a biradical structure with two independent spins. The low-lying absorptions that appear in the near-infrared (NIR) range for all these compounds following oxidation correspond to intervalence charge-transfer (IVCT) bands for the mono-oxidized states and to ligand-to-metal charge-transfer (LMCT) transitions for the dioxidized states. These play a crucial role in the strong optical modulation achieved. The possibility of accessing additional states with distinct linear or nonlinear optical properties is also briefly discussed.