Ligand-driven light-induced spin change in transition-metal complexes: Selection of an appropriate system and first evidence of the effect, in Fe-II(4-styrylpyridine)(4)(NCBPh(3))(2)

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 Fe(II)L(4)X(2), where L is a cis/trans photoisomerizable ligand. For an iron(II) spin-state change to result from ligand cis (<->) trans conversion, the Fe-II(trans-L)(4)X(2) 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(-) = NCBPh(3)(-) 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)-(NCBPh(3))(2): the complex (C-t) formed with trans-Stpy undergoes a thermally-induced spin crossover centered around 190 K; the one (C-c) 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 W-vis absorption measurements, to trigger the spin-state change of the iron(II) ions.