Fluorescence improvement of pyridylacrylonitrile by dimethylaminophenyl-substitutions: The effect of packing modes of conjugated compounds

Four (dimethylamino)arylacrylonitrile derivatives in the solid state are presented. The compounds were characterized by single crystal X-ray diffraction to investigate the effects of substituents on the resulting crystals lattices and to examine the effects of the factors controlling their solid state on photochemical behavior. The molecules included 2-(phenyl)-3-(4-dimethylaminophenyl)acrylonitrile (I) 2-(2′-pyridyl)-3-(4-dimethylaminophenyl)-acrylonitrile (II), 2-(3′-pyridyl)-3-(4-dimethyl-aminophenyl)acrylonitrile (III) and 2-(3′-pyridyl)-3-(4-dimethylaminophenyl)acrylonitrile (IV). The dimethyl-, CN groups and the position of nitrogen atom in the pyridyl groups affected the nature of the molecular packing and consequently their fluorescence properties. The lattice of each compound is compared with acrylonitriles previously reported and the effect of substitution on the crystal properties with the strongest emission in solid state is examined. Based on analyses of molecular packing in the single crystals, the differences in the fluorescence could be attributed to aggregates that showed non-classical herringbone packing with π–π overlap between neighbor molecules. Also the crystal structure studies showed that the N atom position is located anti to the CN group, giving a more planar geometry. This N-dimethyl substituent for I–IV appears to be general for more planar ground-state geometry about the nitrogen atom and consistent with an “amino conjugation effect” to obtain sufficient quinoid structures.

► X-ray characterization gave evidence about nitrogen position in structure affecting the fluorescence.
► Molecular structure and CH⋯π interactions determined the packing modes in conjugated compounds.
► Highly dipole–dipole of A–π–D structures exhibit different fluorescence in solution and solid state.