Electric linear dichroism. A powerful method for the ionic chromophore-colloid system as exemplified by dye and montmorillonite suspensions

The electric linear dichroism (ELD) measures the anisotropic absorption of a chromophore in solution under externally applied electric pulse field, wh ich orients the solute molecule (particle) rotationally to the field direct ion. The ELD method is used to study at 20 degrees C the binding mode and e lectrooptic properties of a nonlinear optical molecule (DAES), 4-[2-[4-(dim ethylamino)phenyl]ethenyl]-1-ethylpyridinium, in the presence of montmorill onite K10 (MK-10) in the absorption region at mixing ratios (D/S) between 0 .1 and 0.97. where D is the concentration of DAES and S is the cation excha nge capacity of MK-IO. Descriptions are given on a new home-built ELD appar atus. The measured quantities are the wavelength (600-300 nm) and field-str ength (0-3 kV cm(-1)) dependence of the reduced dichroism, Delta A/A. The o ptical transition moment directions of bound DAES is determined from the ob served ELD spectra relative to the symmetry axis of the field-oriented disk like MK-10 particle. Discussion is given both on the field-strength depende nce of Delta A/A and on the ELD spectral analysis in terms of the tilt, rol l, and inclination angles of the molecular plane of DAES bound to the surfa ce and/or interlayer of MK-IO particle. The DAES molecule is not bound flat ly on the surf;ice or between the layers, but inclined at 35-39 degrees, ti lted by +/-21 degrees, and rolled by +/- (27-33)degrees relative to the cla y plane. The DAES-MK-10 system is no more associative than the clay itself on the basis of the electric-dichroism-average relaxation time, evaluated f rom the field-off decay signal, of DAES bound to MK-10 particle in solution . (C) 2000 Elsevier Science B.V. All rights reserved.