6-Bromo­indigo dye

6-Bromo­indigo (MBI) [systematic name: 6-bromo-2-(3-oxo-2,3-dihydro-1H-indol-2-yl­idene)-2,3-dihydro-1H-indol-3-one], C₁₆H₉BrN₂O₂, crystallizes with one disordered mol­ecule in the asymmetric unit about a pseudo-inversion center, as shown by the Br-atom disorder of 0.682 (3):0.318 (3). The 18 indigo ring atoms occupy two sites which are displaced by 0.34 Å from each other as a result of this packing disorder. This difference in occupancy factors results in each atom in the reported model used to represent the two disordered sites being 0.08 Å from the higher-occupancy site and 0.26 Å from the lower-occupancy site. Thus, as a result of the disorder, the C—Br bond lengths in the disordered components are 0.08 and 0.26 Å shorter than those found in 6,6′-dibromo­indigo (DBI) [Süsse & Krampe (1979). Naturwissenschaften, 66, 110], although the distances within the indigo ring are similar to those found in DBI. The crystals are also twinned by merohedry. Stacking inter­actions and hydrogen bonds are similar to those found in the structures of indigo and DBI. In MBI, an inter­action of the type C—Br...C replaces the C—Br...Br inter­actions found in DBI. The inter­actions in MBI were calculated quantum mechanically using density functional theory and the quantum theory of atoms in mol­ecules.