is highly likely that only the small changes in the geometry and mutual arrangement of
dye ions determine the difference between spectra of aggregated dye.
Composition of the IA can be, in principle, obtained if we divide the molar
absorptivity of the IA and molar absorptivity of aggregated dye taking into account the
charges of dye and anion. For example, such ratio for IA (AP)
5
GaW
12
O
40
should equal
to 5 : 1. As can be seen from the Table 1, this rule is overall accomplished in the studied
system but there are also significant differences.
It is also important that molar absorptivity of the dye would be appropriately small
at analytical wavelength. It is seen from the results presented in the table 3 that molar
absorptivity of Chrompyrazol-1 is equal to 9250 mol
-1
l cm
-1
, and for Astra Phloxine –
3359 mol
-1
l cm
-1
.
Theoretical Study of UV-Vis Absorption Spectrum of Astra Phloxine.
Polymethine dyes are extensively used as spectral sensitizers in photographic
materials, as probe for biological systems, as laser materials etc. In chemical analysis,
their use as a countercations in ion association complexes with anionic species is well
known. One of the most often used representatives of such dyes is Astra Phloxine. Over
the last few decades, quantum chemical calculations of molecular spectra have become
a fruitful field of research.
The primary task in quantum chemical calculations used for the estimation of
spectral characteristics of the dyes in specific IAs was to find the calculation method,
which would lead to the most accurate reproduction of the absorption spectra in the
visible and ultraviolet region. First, the optimization of the geometry of Astra Phloxine
polymethine dye was carried out by means of conformational analysis of the molecule
using the method of molecular mechanics (Fig. 5). The optimal conformation of Astra
Phloxine has been obtained. It was found out that correct quantum-chemical simulation
of Astra Phloxine spectrum requires the calculation of the vibrationally-resolved
spectrum using the Franck-Condon principle. The comparison of DFT functionals for
the calculation of vibrationally-resolved spectrums of Astra Phloxine has shown that
for the calculation of an electronic spectrum the pbe1pbe functional is the most suitable
among the applied ones: m06, CAM-B3LYP, WB97XD, M11 (Fig. 6).
In this work, the experimentally obtained UV/Vis absorption spectrum of Astra
Phloxine was the subject of a theoretical study including density functional theory
calculations. It was shown that the absorption spectrum of Astra Phloxine is best
simulated by PBE1PBE density functional. In addition, the time spent on calculations
was moderate being compromise in relation to the accuracy. To consider the solvent
effect, the calculation of harmonic vibration frequencies was performed in water with
basis set 6-31 G(d) by Franck-Condon principle. Calculation of harmonic vibrational
frequencies was done using the same level of theory and allowed to introduce the
thermochemical corrections to ΔG at 298.15 K. It was also found that by using DFT
- 1426 -