Ligand-Driven Light-Induced Spin Change in Transition-Metal Complexes: Selection of an Appropriate System and First Evidence of the Effect, in FeII(4-styrylpyridine)4(NCBPh3)2

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Abstract
The first observation of a ligand-driven light-induced spin-state change (LD-LISC effect) in a transition-metal complex is reported. The compounds under investigation are of the type FeIIL4X2, where L is a cis/trans photoisomerizable ligand. For an iron(II) spin-state change to result from ligand cis ↔ trans conversion, the FeII(trans-L)4X2 species had to exhibit a thermally-induced high-spin state ↔ low-spin state crossover. This property was checked by variable-temperature magnetic susceptibility measurements, for compounds with X- = NCBPh3- or NCBH3- and L = 1-phenyl-2-(4-pyridyl)ethene (or 4-styrylpyridine, abbreviated as Stpy), 1-(4-R-phenyl)-2-(4-pyridyl)ethene (R = CH3, COOCH3), or 1-(1-naphthyl)-2-(4-pyridyl)ethene. The results are comparatively discussed. The best candidate for the LD-LISC effect to be observed is found to be Fe(Stpy)4(NCBPh3)2: the complex (Ct) formed with trans-Stpy undergoes a thermally-induced spin crossover centered around 190 K; the one (Cc) formed with cis-Stpy retains the high-spin state at any temperature. Photoisomerization of the Stpy ligand, at 140 K, in the complex embedded within a cellulose acetate matrix, is effectively shown, on the basis of UV−vis absorption measurements, to trigger the spin-state change of the iron(II) ions.

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