2-{(E)-[1-(2-Hydroxy-ethyl)-3,3-dimethyl-3H-indol-1-ium-2-yl]vin-yl}-6-hydroxy-meth-yl-4-nitro-phenolate dihydrate.

The title merocyanine-type mol-ecule, C(21)H(22)N(2)O(5)·2H(2)O, crystallizes in a zwitterionic form and has an E configuration at the styryl C=C bond. The styryl part of the mol-ecule and the indolium ring are slightly twisted and form a dihedral angle of 13.4 (1)°. The 1.274 (3) Å C-O bond length in the phenolate fragment is the longest among similar mol-ecules. Hydrogen bonds between solvent water mol-ecules, two hydroxyl groups and the phenolate O atom dictate the packing arrangement of mol-ecules in the crystal and join the mol-ecules into a two-dimensional polymeric network which propagates parallel to (001). Four water mol-ecules and four hydr-oxy groups form a centrosymmetric homodromic cyclic motif of O-H⋯O hydrogen bonds. Another cyclic centrosymmetric motif is generated by four water mol-ecules and two phenolate O atoms.