Color center creation by dipole stacking in crystals of 2-meth-oxy-5-nitro-aniline.

This work describes the X-ray structure of orange-red crystals of 2-meth-oxy-5-nitro-aniline, CHNO. The compound displays concentration-dependent UV-Vis spectra, which is attributed to dipole-induced aggregation, and light absorption arising from an inter-molecular charge-transfer process that decreases in energy as the degree of aggregation increases. The crystals display π-stacking where the dipole moments align anti-parallel. Stacked mol-ecules inter-act with the next stack 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.