Conformation and interactions of 4-(pyridinium-1-yl)-phenolate betaine-dye and its cation in the crystalline state Lukasz Wojtas * , Dariusz Pawlica, Katarzyna Stadnicka Faculty of Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Krako ´w, Poland Received 25 August 2005; received in revised form 26 September 2005; accepted 26 September 2005 Available online 8 November 2005 Abstract 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 P2 1 /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)8, 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. q 2005 Elsevier B.V. All rights reserved. Keywords: Pyridinium betaine-dye; Crystal engineering; Crystal structure; Intermolecular interactions 1. Introduction Designing of a new efficient material for nonlinear optics demands both molecular and crystal structure characterization of the promising compounds. The molecules showing push– pull system were widely investigated in this field [1]. Among them there are pyridinium betaine dyes, often characterized by solvatochromic properties [2]. Batochromic shift of ICT absorption maximum, on going from polar to nonpolar solvents (negative solvatochromism), is attributed to highly polarizable aromatic p-system, which connects donor (O K ) and acceptor (N C ) moieties of the molecule. Relatively high hyperpolarizability of such betaine-dyes creates possibility to utilize the molecules in the field of nonlinear optics [3]. To exploit these properties it is necessary to incorporate the molecule into polar crystalline state to enhance nonlinear response. Zwitterionic character of the molecules favours their anti-parallel arrangement, which forces nonpolar packing in the crystal structure. To overcome this tendency, the co- crystallizations of 4-(2,4,6-triphenylpyridinium-1-yl)-phenol- ate (2, [4]) with various organic counterions were performed, but in all cases only anti-parallel arrangements of the protonated betaine have been achieved resulting in the centrosymmetrical space groups [5]. The aim of this work was to explore the geometry and conformation of the simplest betaine-dye, 4-(1-pyrydinio)- phenolate (1) in a crystalline state and to obtain polar crystals with the protonated molecule connected to the hydrogen bond network formed by selected inorganic anions. The crystallo- graphic results obtained for 1 could be compared with the earlier theoretical calculations, which concerned the geometry and conformation of the molecule, dipole moments in the ground and excited states, solvatochromic shifts and the hyperpolarizability dependence of the dihedral angle [6–8]. The calculations by Ishida and Rossky [6], performed in the gas phase as well as in solutions (acetonitrile or water), Journal of Molecular Structure 785 (2006) 14–20 www.elsevier.com/locate/molstruc 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.09.023 * Corresponding author. Tel.: C48 12 6632059; fax: C48 12 6340515. E-mail address: wojtas@chemia.uj.edu.pl (L. Wojtas).