Differently coloured polymorphs of 4-[(2-nitrophenyl)azo]phenol

The crystal structures of the brown-yellow and orange polymorphs of the tit le compound, 4-[(2-nitrophenyl)diazenyl]phenol, C12H9N3O3, have been determ ined and their visible reflection spectra recorded. Both structures adopt a stacking arrangement with interstack hydrogen bonds. Ab initio and semi-em pirical (AM1 and INDO-CISD) calculations were performed in order to rationa lize the difference in colour. It can be attributed neither to the subtle d istinctions in molecular geometry nor to the effect of intermolecular elect rostatic interactions. The most probable origin of this difference is the m ixing of intramolecular n --> pi* and intermolecular charge-transfer excita tions.