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 b een synthesized with five different metals (M = Mn, Fe, Co, Ni, Cu) and sev en hyperpolarizable stilbazolium-shaped A chromophores: (4-[4-(dimethylamin o)-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, resp ectively]. These compounds have been designed as possible multiproperty mat erials associating ferromagnetism and second-order optical nonlinearity. Tw o-third of these compounds exhibit second harmonic generation (with efficie ncy up to 100 times that of urea at 1.9 mu m), the others being inactive. A ll 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 cen trosymmetric 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 di polar 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 c ell as A[(MnCrIII)-Cr-II(C2O4)(3)] (A = DAPS, MIPS). A search for structure -property correlation emphasizes the relationship between a short interlaye r distance and the alignment of the chromophore dipoles.