Recognition of molecular planarity of cationic dyes by anionic, crystalline bilayer aggregates. Evidence using metachromatic and solvatochromic properties

Abstract

Steric selectivity of metachromatic, cationic dyes related to molecular planarity has been investigated using a number of representative anionic bilayer aggregates described in the preceding paper. The anionic bilayer membranes specifically incorporated planar cationic dyes such as methylene blue (MB) and acridine orange (AO) as monomers in the hydrophobic region of crystalline bilayer aggregates, whereas bulky dyes such as crystal violet (CV), cyanine dye (NK-737) and rhodamine 6G (R6G) could not be incorporated and existed mostly as aggregates in the hydrophilic region. The incorporated, planar dyes underwent monomer-to-aggregate transitions during the gel-to-liquid crystalline phase transition of the bilayer host, whereas the unincorporated, bulky dyes underwent aggregate-to-monomer transitions under the same conditions. With these properties in mind, the conditions for planarity recognition of cationic dyes were investigated in relation to steric hindrance among intramolecular aromatic rings, bulkiness of substituted groups attached to the aromatic rings, and hydrophobicity. Solvatochromic dyes which ordinarily showed no aggregation were also investigated for the direct observation of the microenvironment in which the dyes exist and for the generalization of planarity recognition. It was revealed that distortion of aromatic rings caused by the steric hindrance significantly interfered with the incorporation of the dye. In a series of planar dyes with the same aromatic rings, an increase in molecular distortion by introducing bulky substituents made incorporation more difficult in spite of there being an increase in the hydrophobicity. These results indicate that complete molecular planarity, resulting from minimal distortion of aromatic rings and the absence of bulky substituents, is more important than hydrophobicity for preferential incorporation. Variations in the molecular structure of the L-glutamate amphiphiles also makes less clear the classification of dyes according to the extent of molecular packing among L-glutamate residues. An optimum molar ratio of amphiphile to dye also exists in which the selectivity between planar and hindered dyes is maximized.