TOWARD LIGAND-DRIVEN LIGHT-INDUCED SPIN CHANGE - INFLUENCE OF THE CONFIGURATION OF 4-STYRYLPYRIDINE (STPY) ON THE MAGNETIC-PROPERTIES OF FEII(STPY)4(NCS)2 COMPLEXES - CRYSTAL-STRUCTURES OF THE SPIN-CROSSOVER SPECIES FE(TRANS-STPY)4(NCS)2 AND OF THE HIGH-SPIN SPECIES FE(CIS-STPY)4(NCS)2

A method to obtain ligand-driven light-induced spin changes (LD-LISC) in transition-metal molecular compounds is described. It is based on the utilization of ligands capable of being photochemically modified, such as cis-trans photoisomerizable ligands. A prerequisite for the spin conversion to be observed upon ligand photoisomerization is that one at least of the two complexes formed with ligand isomers presents a thermally-induced spin crossover. Two complexes of the type FeL4(NCS)2 with L = trans-stpy (complex 1) and L = cis-stpy (complex 2), stpy standing for 4-styrylpyridine, are presented. Their syntheses are described. Their X-ray diffraction structures were determined. 1 crystallizes in the orthorhombic space group Pna2(1), with Z = 4, a = 32.004(26) angstrom, b = 15.394(13) angstrom, and c = 9.842(10) angstrom. 2 crystallizes in the monoclinic space group P2(1)/n, with Z = 2, a = 13.115(2) angstrom, b = 12.569(3) angstrom, c = 14.574(3) angstrom, and beta = 91.90(1)-degrees. In both complexes, the coordination core has the same geometry, with the NCS- groups in the trans position. Variable-temperature magnetic susceptibility measurements show that 1 presents a S = 2 <--> S = 0 crossover, centered around 108 K, whereas 2 retains the high-spin state down to 4.2 K. So, this pair of compounds do fulfill the spin-state conditions for the LD-LISC effect to be observed. However, the highest temperature at which the experiment might be carried out is about 90 K. For ligand photoisomerization to occur with suitable quantum yields, this temperature has now to be increased by modifying the complexes chemically.