Relation between Crystal Packing and Optical Anisotropy for Schiff Base-Nickel Complexes that Form Various Ladder-like Hydrogen-Bonding Networks

A series of nickel(II) complexes of a 2,6-dihydroxynaphtho-1-aldehyde Schiff base was synthesized. By means of in situ formation of metal-containing tectons, some of those complexes afforded sub-millimeter-scale single crystals directly from the reaction mixture. The complexes consisted in part of various ladder-like hydrogen-bonded networks, which forced the chromophore to arrange in anisotropic fashion, with the effect dependent on the length of the alkyl chains. The crystals exhibited a marked linear dichroism, and their solid-state polarization absorption spectra were recorded with an optical microscope. Using data from quantum chemical calculations and the crystal structure, we were able to elucidate the relationship between crystal packing and optical anisotropy. Analyses based on theoretical equations allowed us to explain the observed absorbance and to interpret the mechanism of the dichroism in detail.