Abstract
The crystal structures of the brown-yellow and orange polymorphs of the title compound, 4-[(2-nitrophenyl)diazenyl]phenol, C(12)H(9)N(3)O(3), have been determined and their visible reflection spectra recorded. Both structures adopt a stacking arrangement with interstack hydrogen bonds. Ab initio and semi-empirical (AM1 and INDO-CISD) calculations were performed in order to rationalize the difference in colour. It can be attributed neither to the subtle distinctions in molecular geometry nor to the effect of intermolecular electrostatic interactions. The most probable origin of this difference is the mixing of intramolecular n --> pi* and intermolecular charge-transfer excitations.
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