Crystal structures of 2-meth­oxy-5-nitro­aniline N-alkyl derivatives support color-center creation by a dipole-stacking mechanism

A striking color change from orange to yellow is observed in crystals of two N-alkyl derivatives of 2-meth­oxy-5-nitro­aniline. The orange compound has a sterically unencumbered 4-methyl­benzyl group and the yellow compound the more sterically demanding tri­phenyl­methyl group. The crystal structures are used to rationalize a charge transfer color center in the former compound, which is less favorable in the latter.

Synthesis and structure of 9-(di­methyl­amino)-1,10-phenanthrolin-1-ium nitrate

In the title salt, the cation is almost planar, implying significant conjugation of the lone pair of the secondary amino group N atom with the aromatic ring system. In the extended structure, N—H⋯O and C—H⋯O hydrogen bonds link the components into infinite chains of alternating cations and anions propagating along the b-axis direction.