Solvent effects in the interaction of methyl-β-cyclodextrin with solvatochromic merocyanine dyes

Abstract

The UV-vis spectroscopic behavior of dyes: 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate (1) and 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-2,5-cyclohexadien-1-one (2) was investigated in solutions of methyl-β-cyclodextrin (methyl-β-CyD), using water, methanol, ethanol, propan-2-ol, butan-1-ol, acetone, acetonitrile, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMA), chloroform and dichloromethane as solvents. In aqueous solutions of dye (2) the addition of M-β-CD leads to a bathochromic shift (of the maximum absorption), showing that the probe was transferred to a microenvironment of lower polarity and suggesting the formation of a 1 : 1 dye (2) : CyD inclusion complex, with a binding constant of 128.5 ± 3.5 dm³ mol⁻¹. Data for dye 2 in alcohols showed hypsochromic shifts, which increased in the following order: methanol < ethanol < propan-2-ol < butan-1-ol. These observations appear to reflect dye–solvent interactions through hydrogen bonding. If dye–solvent interactions are strong, the CyD–dye interactions are consequently weak, but the latter increase in importance when the dye–solvent interaction becomes weaker. With hydrogen-bond accepting solvents, data for both dyes showed clearly increasing hypsochromic shifts following the order: DMSO < DMA < DMF < acetone < acetonitrile. This order is exactly the inverse of the increasing order of basicity of the medium. This indicates that the dominant factor for the observed effects in these solvents is the solvent–CyD interaction through hydrogen bonding involving the hydroxyl groups of the CyD and the basic groups of the solvents. These interactions diminish in intensity if the basic character of the medium is reduced, increasing the capability of the dye to interact with the CyD using its phenoxide donor moiety. The largest hypsochromic shifts were obtained in chloroform (66.0 nm) and dichloromethane (67.5 nm) with dye 1 after addition of methyl-β-CyD. In these specific situations, solvents display weak basic and acid properties, that enhanced CyD–dye interactions to such an extent that association complexes formed through hydrogen bonding could be detected (K₁₁ values of 24.8 ± 4.9 dm³ mol⁻¹ in dichloromethane and 66.1 ± 8.0 dm³ mol⁻¹ in chloroform).