Synthesis and X-ray studies of noncentrosymmetric merocyanine dyes—A series of organic nlo crystals

A series of 3-methyl-4-(p′-substituted styryl) pyridine methiodides were readily synthesized under mild condition with excellent yields. The 3-methyl group was expected to direct the molecular packing within the crystalline unit cell. X-ray diffraction analysis of 3-methyl-4-styrylpyridine methiodide crystal clearly showed that this compound was packed in a noncentrosymmetric way about 50 degrees with respect to the crystal polar axis, and proved that the 3-methyl group on pyridine ring did change the molecular packing from centrosymmetric orientation to noncentrosymmetric orientation. However, all other compounds we investigated in this paper were found crystallized centrosymmetrically.