Hydrogen bonds and π–π inter­actions in two new crystalline phases of methyl­ene blue

Two unprecedented solid phases involving the 3,7-bis­(di­methyl­amino)­pheno­thia­zin-5-ium cation, i.e. methyl­ene blue (MB(+)), have been obtained and structurally characterized. In the crystals of 3,7-bis­(di­methyl­amino)­pheno­thia­zin-5-ium chloride dihydrate, C(16)H(18)N(3)S(+)·Cl(−)·2H(2)O (I) and 3,7-bis­(di­methyl­amino)­pheno­thia­zinium bis­ulfite, C(16)H(18)N(3)S(+)·HSO(4) (−) (II), the cationic dye mol­ecules are planar and disposed in an anti­parallel mode, showing π–π stacking inter­actions, with mean inter­planar distances of 3.326 (4) and 3.550 (3) Å in (I) and (II), respectively. In compound (I), whose phase was found affected by merohedral twinning [BASF = 0.185 (3)], the presence of water mol­ecules allows a network of hydrogen bonds involving MB(+) as both a donor and an acceptor, whereas in compound (II), the homo-inter­action of the anions causes an effective absence of classical hydrogen-bond donors. This substantial difference has important consequences for the stacking geometry and supra­molecular inter­actions of the MB(+) cations, which are analysed by Hirshfeld fingerprint plots and subsequently discussed.