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

This work describes the X-ray structure of orange–red crystals of 2-methoxy-5-nitroaniline, C 7 H 8 N 2 O 3 . 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.