Pulsed electric linear dichroism of triphenylmethane dyes adsorbed on montmorillonite K10 in aqueous medial

Electric linear dichroism (ELD) spectra of two cationic triphenylmethane dy es, crystal violet (CV) and malachite green (MG), bound to sodium montmoril lonite K10 (MK-10) were studied at 20 degrees C in aqueous media at two mix ing ratios, D/S, of 0.10 and 0.24 in the 700- to 400-nm wavelength region a nd in the applied electric field strength range between 0 and 3 kV/cm, The specific parallel and perpendicular dichroism (Delta A(parallel to)/A and D elta A(perpendicular to)/A) spectra of dye-adsorbed MK-10 suspension were m easured at a fixed field strength with an apparatus equipped with a 512-cha nnel photodiode array detector. By changing the field strength over a wide range, a series of the reduced dichroism values of the bound dyes were meas ured at a fixed wavelength. By fitting these dichroism values to theoretica l orientation functions, the intrinsic reduced dichroism (Delta A/A)int spe ctra at the limiting high fields (ELD spectrum) were determined for CV and MG bound to MK-10. No appreciable difference was observed at the two DIS va lues. The ELD spectra of these bound dyes are undulatory but never constant , throughout their absorption region; thus, the dye plane does not lie flat ly either on the surface or between layers of MK-10 particle. The isotropic absorption spectra, A, of bound CV and MG were each deconvoluted to eight partial absorption bands, which were grouped into three differently polariz ed transitions, i.e., one out-of-plane and two mutually perpendicular in-pl ane. The optical transition dipole moment direction of each group was found to make a considerable angle with respect to the symmetry axis of the disk like MK-10 particle, whose plane (or surface) tends to orient toward the ap plied electric field at the limiting high fields. By simulating the observe d ELD spectra of bound CV and MG with those deconvoluted bands, the roll, t ilt, and inclination angles of both dyes were evaluated quantitatively with a new analytical method. The average angles (+/-theta(R), +/-theta(T),\the ta(N)\) are -(34-47)degrees, 34 degrees, and 51 degrees for bound CV and -4 4 degrees, 32 degrees, and 53 degrees for bound MG at two D/S values; thus, the triangular plane of each dye is rolled as well as tilted with a large inclination angle. (C) 2000 Academic Press.