Diimine-dithiolene complexes, mixed-ligand

With rich luminescent properties, long-lived CT excited states and a variety of bimolecular photochemical reaction pathways, the mixed-ligand square-planar diimine dithiolene complexes of d8 metal ions show great promise for solar-energy conversion, as luminescent probes or in photocatalytic applications. Complexes of this type have also received attention for the nonlinear optical properties, such as second harmonic generation, related to the MMLL CT excited state (14, 129). 

Mixed-ligand complexes such as nickel dithiolene diimine can be prepared via ligand substitution reactions (Eq. 6) (52). 

These complexes are redox active. The two one-electron reductions resemble more closely the reduction of the corresponding bis(diimine) complex than those of the corresponding bis(dithiolene) complex. The redox potentials are more sensitive to diimine ligand variation than to dithiolene variation. One-electron oxidation is relatively insensitive to diimine ligand variation. However, the dependence of one-electron oxidation on dithiolene variation has not been assessed directly due to the limited amount of data available on the oxidation of the corresponding dithiolene complexes. It has been proposed that the LUMO of the neutral mixed-dithiolene diimine complexes possesses more diimine than dithiolene character and that the HOMO is mainly metal d orbital in nature (52). 

C. Square-Planar Mixed-Ligand Dithiolene-Diimine and Related Complexes / 339... 

Mixed-ligand diimine dithiolate complexes of Zn(II) were among the first compounds classified as having a LLCT excited state (131). The complexes Zn(phen)(tdt) (41), Zn(bpy)(tdt) (42), and Zn(biq)(tdt) (43), where biq = 2,2 -biquinoline, were reported to have absorption bands at wavelengths of 475 nm (80 M-1 cm-1), 465 nm (65 A/-1 cm-1), and 590 nm (40 M-1 cm-1), respectively. The LLCT transition is attributed to a HOMO that is localized on the dithiolene and a LUMO that is localized on the diimine, with a clear relationship to the MMLL CT transition described for the square-planar M(di-imine)(dithiolate) complexes (M = Ni, Pd, Pt) described in Section II.C. However, the relative orientations of the two planar ligands in the <78 square planar and the dw pseudotetrahedral complexes are quite different, and have a profound influence on the absorption and emission properties. 

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