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

Abstract

Two N-alkyl derivatives of 2-meth­oxy-5-nitro­aniline, namely, 2-meth­oxy-N-(4-methyl­benz­yl)-5-nitro­aniline (C₁₅H₁₆N₂O₃, orange crystals) and 2-meth­oxy-5-nitro-N-(tri­phenyl­meth­yl)aniline ethanol quatersolvate (C₂₆H₂₂N₂O₃·0.25C₂H₅OH, yellow crystals), have been prepared and analyzed by single-crystal X-ray diffraction to determine if these compounds display the same face-to-face π-stacking arrangement as observed in the un-alkyl­ated parent compound. The tri­phenyl­methyl derivative crystallizes with 0.25 ethanol mol­ecules, the contribution of which has been modeled with a solvent mask due to a high degree of disorder. The UV-Vis spectra show that the benzyl derivative displays concentration-dependent shifts in the visible portion of the spectrum, shifting to longer wavelengths with higher concentration, which is attributed to aggregation in solution. This behavior is similar to that observed for the parent compound. In contrast, the tri­phenyl­methyl derivative shows an attenuated concentration dependence on visible absorption, which does not display as great a red shift, and therefore represents a lesser tendency for aggregation in solution. The crystal packing for these compounds is distinguishing, with the benzyl derivative π-stacking in a face-to-face manner with dipole moments oriented anti-parallel, while the sterically demanding tri­phenyl­methyl group prevents direct π-stacking for this compound. The packing observed for these compounds supports the hypothesis that an inter­molecular charge-transfer process is responsible for the orange color of crystals of the benzyl derivative. It is concluded that sterically undemanding groups attached at the amino group could deliver materials which mimic the color center formation hypothesized for the parent compound. These types of structural changes may have useful applications in the field of organic semiconductors.