Hydrogen bonds and π-π inter-actions in two new crystalline phases of methyl-ene blue

Two unprecedented solid phases involving the 3,7-bis-(di-methyl-amino)-pheno-thia-zin-5-ium cation, i.e. methyl-ene blue (MB ), have been obtained and structurally characterized. In the crystals of 3,7-bis-(di-methyl-amino)-pheno-thia-zin-5-ium chloride dihydrate, C₁₆H₁₈N₃S⁺·Cl^-·2H₂O (I) and 3,7-bis-(di-methyl-amino)-pheno-thia-zinium bis-ulfite, C₁₆H₁₈N₃S⁺·HSO₄^- (II), the cationic dye mol-ecules are planar and disposed in an anti-parallel mode, showing π-π stacking inter-actions, with mean inter-planar 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 mol-ecules allows a network of hydrogen bonds involving MB as both a donor and an acceptor, whereas in compound (II), the homo-inter-action of the anions causes an effective absence of classical hydrogen-bond donors. This substantial difference has important consequences for the stacking geometry and supra-molecular inter-actions of the MB cations, which are analysed by Hirshfeld fingerprint plots and subsequently discussed.