2-{(E)-[1-(2-Hydroxyethyl)-3,3-dimethyl-3H-indol-1-ium-2-yl]vinyl}-6-hydroxymethyl-4-nitrophenolate dihydrate

The title merocyanine-type molecule, C21H22N2O5·2H2O, crystallizes in a zwitterionic form and has an E configuration at the styryl C=C bond. The styryl part of the molecule 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 molecules. Hydrogen bonds between solvent water molecules, two hydroxyl groups and the phenolate O atom dictate the packing arrangement of molecules in the crystal and join the molecules into a two-dimensional polymeric network which propagates parallel to (001). Four water molecules and four hydroxy groups form a centrosymmetric homodromic cyclic motif of O—H...O hydrogen bonds. Another cyclic centrosymmetric motif is generated by four water molecules and two phenolate O atoms.