Color center creation by dipole stacking in crystals of 2-meth­oxy-5-nitro­aniline

The title compound 2-meth­oxy-5-nitro­aniline forms orange–red crystals and displays shifts in light absorption to longer wavelengths in solution as the concentration increases. Mol­ecular face-to-face stacking with dipoles oriented anti­parallel creates a color center, which is compared to certain inorganic semiconductors.

This work describes the X-ray structure of orange–red crystals of 2-meth­oxy-5-nitro­aniline, C₇H₈N₂O₃. The compound displays concentration-dependent UV-Vis spectra, which is attributed to dipole-induced aggregation, and light absorption arising from an inter­molecular charge-transfer process that decreases in energy as the degree of aggregation increases. The crystals display π-stacking where the dipole moments align anti­parallel. Stacked mol­ecules inter­act with the next stack via hydrogen bonds, which is a state of maximum aggregation. Light absorption by charge transfer can be compared to colored inorganic semiconductors such as orange–red CdS, with a band gap of 2.0–2.5 eV.