Crystal structure of (E)-2-cyano-3-(12-methyl-12H-benzo[b]pheno­thia­zin-11-yl)acrylic acid

In the title compound, C(21)H(14)N(2)O(2)S, a donor–acceptor type of benzo[b]pheno­thia­zine (bpz) derivative, the thia­zine ring adopts a boat conformation and the bond-angle sum at the N atom is 360.0°. The dihedral angle between the benzene ring and the naphthelene ring system fused to the thia­zine ring is 32.76 (5)°. In the crystal, carb­oxy­lic-acid inversion dimers linked by pairs of O—H⋯O hydrogen bonds generate R (2) (2)(8) loops. Aromatic π–π stacking [shortest centroid–centroid separaton = 3.5242 (13)Å] consolidates the structure and very weak C—H⋯O and C—H⋯N inter­actions also occur.