Structure and NLO properties of layered bimetallic oxalato-bridged ferromagnetic networks containing stilbazolium-shaped chromophores

A series of 35 layered compounds A[(MCrIII)-Cr-II(C2O4)(3)].n solvent has been synthesized with five different metals (M = Mn, Fe, Co, Ni, Cu) and seven hyperpolarizable stilbazolium-shaped A chromophores: (4-[4-(dimethylamino)-alpha-styryl]-N-alkylpyridinium), alkyl = methyl, ethyl, isopropyl (DAMS, DAES, DAPS respectively); (4-[4-methoxy-alpha-styryl-N-alkylpyridinium), alkyl = isopentyl, heptyl (MIPS, MHS); DAZOP and CINDAMS are DAMS analogues, where the central [C=C] core has replaced by azo or butadiene cores, respectively]. These compounds have been designed as possible multiproperty materials associating ferromagnetism and second-order optical nonlinearity. Two-third of these compounds exhibit second harmonic generation (with efficiency up to 100 times that of urea at 1.9 mu m), the others being inactive. All of them order ferromagnetically below Curie temperatures that range from 6 to 13 K. The structures of DAPS[Mn-II- Cr-III(C2O4)(3)].CH3CN and MIPS[(MnCrIII)-Cr-II(C2O4)(3)] have been resolved. Both of them belong to the centrosymmetric P2(1)/c space group, which accounts for their NLO inactivity. The long axes of the chromophores in a given layer are parallel, but the dipolar moments are antiparallel. Two successive chromophore layers along the stacking direction have approximately orthogonal orientations, giving rise to a doubling of the c parameter. X-ray powder diffraction shows that the high crystalline compounds of the series possess the same monoclinic unit cell as A[(MnCrIII)-Cr-II(C2O4)(3)] (A = DAPS, MIPS). A search for structure-property correlation emphasizes the relationship between a short interlayer distance and the alignment of the chromophore dipoles.