1980 Volume 53 Issue 8 Pages 2146-2151
The long-wavelength bands of the absorption spectra of 10-methylacridinium (MeAcr), Acridine Orange (AO), and Acridine Yellow (AY) in the poly(vinyl alcohol) film were investigated by the linear dichroism method. The parallel- and perpendicular-dichroic spectra of each dye were resolved into the long-axis and short-axis polarized absorption components. From these component bands the isotropic spectrum was reconstituted and the reduced dichroism spectrum was simulated by computer with a new technique which utilizes a general expression for the reduced dichroism of planar molecules with the multiple band structure. A weak absorption band (or bands) hidden in a strong one could be revealed by reproducing the observed isotropic and linear-dichroic spectra. The vibrational sub-bands of MeAcr were shown to have the average intervals of ca. 1.5×103 cm−1 for the short-axis polarized 1La band and of ca. 0.7×103 cm−1 for the long-axis polarized 1Lb band. In the visible region of the isotropic spectrum, the weak 1La band of AO was clearly shown to consist of the fine structure with a regular energy separation, although it is overlapped with the intense 1Lb band which is also composed of a vibrational structure. The same spectral feature was observed for AY. The dichroic results are discussed under considerations of (1) the overlap of the mutually orthogonal transition moments, whose angles were determined relative to the long axis of the C2v-symmetric dye molecule, (2) the angle between the orientation axis and the long axis of the molecular plane, and (3) the average degree of orientation of dye molecules in the film.
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