Conformation and interactions of 4-(pyridinium-1-yl)-phenolate betaine-dye and its cation in the crystalline state

The crystals of 4-(pyridinium-1-yl)-phenolate dihydrate, 1-(4-hydroxyphenyl)-pyridynium chloride monohydrate, 1-(4-hydroxyphenyl)-pyridynium phosphate dihydrate were obtained to explore the molecular behaviour of the simplest pyridynium betaine-dye in various crystalline environments. 4-(Pyridinium-1-yl)-phenolate (1), as a push–pull chromophore showing the negative solvatochromism of ICT band, can be useful in NLO applications if the polar dipole arrangement is achieved in the solid-state. The attempt to overcome the tendency of betaine dipoles to form anti-parallel arrangements was successful to some extent. X-ray structure analysis of the crystals, obtained from aqueous solutions, revealed space groups Pnna, P212121 and P21/c, respectively. For 4-(pyridinium-1-yl)-phenolate dihydrate and 1-(4-hydroxyphenyl)-pyridynium chloride monohydrate, the anti-parallel arrangements of the betaine-dye molecules/cations force a nonpolar space group. In the centrosymmetrical crystals of 1-(4-hydroxyphenyl)-pyridynium phosphate dihydrate, between the centrosymmetric layers built of phosphate anions and water molecules, there are the polar arrangement of the betaine-dye cations connected to the phosphate-hydrate layers; however, the centrosymmetricity of the layers precludes the polarity of the structure. The alteration of geometrical parameters of the cation, as compared to 4-(pyridinium-1-yl)-phenolate molecule, is observed only in the phenolate moiety due to the protonation of the oxygen atom. A twisted conformation for the betaine-dye molecule observed in the crystalline state, with the dihedral angle of 47.0(1)°, is similar to the results of computational studies for the ground state of the single molecule in an aqueous solution. The influence of crystal environment, and that of the oxygen atom protonation, on the variation of the torsion angle is small.