Journal of Molecular Liquids 109 (2004) 149–154 0167-7322/04/$ - see front matter  2003 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2003.06.004 Absorption anisotropy and molecular association of some ionic dyes in liquid crystalline solution A. Ghanadzadeh*, M.S. Zakerhamidi Department of Chemistry, Faculty of Science, Gilan University, Rasht, Iran Received 5 February 2002; accepted 6 June 2003 Abstract Dichroic ratios R and order parameters S of solutions of methylene blue, rhodamine 6G and rhodamine B in the nematic liquid crystalline solvent were investigated by the measuring of the intensity of the absorption bands in the visible region of parallel aligned samples. The molecular association of the dyes in a nematic solvent and also in an amphiphilic solution was studied using optical spectroscopy and in a guest–host system. The nature of the interacting pairs in these dyes was discussed using the exciton theory.  2003 Elsevier B.V. All rights reserved. Keywords: Methylene blue; Rhodamines; Ionic dyes; Aggregation; Anisotropic solvent; Guest–host interaction 1. Introduction It is generally believed that ionic dye molecules in solution can interact electrostatically to form aggregates such as dimers, and even higher-order aggregates, which are in turn influenced by several factors including the concentration, pH, temperature and solvent nature. As the dimer absorption spectrum of the ionic dyes is different to that of the monomeric form, molecular association in the dye molecules can be detected from visible absorption spectrum investigations as a reliable technique in the dimer or aggregate formation studies. The aggregative properties of rhodamines, and meth- ylene blue (MB) and their analogous structures have been the subject of renewed interest in recent years w1– 5x. Such revival in the interest of the solution chemistry of these dyes stems from their unique photophysical behavior, which until now has remained ambiguous. Although the aggregate formation of the ionic dyes in the isotropic solvents (ordinary solvents) been studied by a number of researchers w1–5x, there are a lot of works carried out in ordered liquid crystal solvents w6– 9x. The use of nematic liquid crystals as anisotropic solvents provides a quick and simple method for meas- *Corresponding author. Fax: q98-131-30912. E-mail address: aggilani@guilan.ac.ir (A. Ghanadzadeh). uring of the anisotropic physical molecular properties of solutes w10–16x. The optical polarization measurements made using nematic solvents give important information on the orientation of the solute molecules, the direction of transition moments and the polarizations of electronic absorption bands. The linear dichroism of a sample is defined as the difference in absorbance between two linearly polarized beams of light possessing electric vectors perpendicular to each other. If a system has uniaxial symmetry, such as a nematic, the absorbances are conveniently measured parallel and perpendicular to the long molecular axis (director). In this case, the dichroic ratio R is defined as w10,11x RsA yA (1) ≤ H where A and A are the absorbances for light polarized ≤ H parallel and perpendicular to the director, respectively. The dichroic ratio R of a uniaxial nematic liquid crystal can be related to the order parameter Ss N3 cos uy 1 2 2 1M, where u is the angle between the long molecular axis and the director of the liquid crystal. Then (Ry1) 1 Ss (2) 2 (Rq2) 1y2 3 cos by1 Ž .