Orientation and aggregation of cationic laser dyes in a fluoromica: polarized spectrometry studies

Abstract

Polarized infrared (IR) and polarized ultraviolet–visible light (UV–VIS) absorption spectra were measured for cationic laser dyes, rhodamine 6G (R6G) and oxazine 4 (Ox4), incorporated in an oriented film of synthetic fluoromica (Li taeniolite, LiTN) to determine the orientation of dye molecules in the interlayer space. Dependence of absorbance on the incident angle of polarized light was observed for characteristic vibrational modes of R6G and Ox4 in the infrared region. It indicated qualitatively that dye molecules were tilted with the longest axis of the condensed ring nearly perpendicular to the host layer (vertical orientation). Polarized UV–VIS spectra showed the absorption of monomer, dimer and high-order aggregates of dyes in the interlayer space of TN. The monomer and dimer absorption bands of the dyes did not show any dependence on the polarized incident light. The dichroic ratio dependence on the incident angle of polarized light was observed for H-aggregates, from which the H-aggregates were calculated to be oriented at 71° for Ox4/TN and 73° for R6G/TN against the layer. The obtained molecular orientation in the interlayer space is consistent with the basal spacings derived from powder X-ray diffraction (PXRD) data—22.4 Å for Ox4/TN and 23.7 Å for R6G/TN. Other states of dyes showed only negligible dependence, implying that interlayer distance is determined by the high-order H-aggregate in the interlayer space. Such vertical orientation was ascribed to the high cation exchange capacity (CEC) of LiTN and the dye aggregation in the interlayer space.