PHOTOPHYSICAL AND OPTICAL SPECTRA PROPERTIES OF POLYCHLORINATED DIBENZO-P-DIOXIN DERIVATIVES

Klimenko V.G.
Central New Mexico Community College 525 Buena Vista Dr SE, Albuquerque.


PHOTOPHYSICAL AND OPTICAL SPECTRA PROPERTIES OF POLYCHLORINATED DIBENZO-P-DIOXIN DERIVATIVES

Compounds of the dibenzo-p-dioxin series, also called dioxins, became widely known and fell into disrepute after high toxicity of some polychlorinated dibenzo-p-dioxins (PCDD) had been established. Due to their high stability, dioxins formed in industrial processes can be retained in soil and water for long periods, enter living organisms and be accumulated in them. Due to the extreme hazard of dioxin ecotoxicants, investigation of their physicochemical properties and development of highly sensitive methods for their detection and quantification have become urgent.
Molecular structure
Dibenzo-p-dioxin (DD) is an aromatic cyclic ether consisting of two benzene rings connected through oxygen bridges.
X-Ray diffraction data for DD and for five of its chloroderivatives [mono-, di-, tetra-, hexa-, and octachlorodioxins (OCDD) have been described in the literature[1-3]. In the crystals, the dioxin molecules are nearly planar, although some atoms including Cl atoms deflect from the plane. Thus in di- and hexachlorodioxin molecules, the angle between the C-Cl bond and the plane amounts to 3-40, while in mono-, octa- and tetrachlorodioxins, this angle does not exceed 2o. The C-Cl bond length in mono-, di- and hexachlorodioxin molecules varies from 1.69 to 1.75_A. The geometric parameters of the central fragment of the molecules differ insignificantly: the C7O bond length is equal to 1.36-1.38 A and the angle between the C-O bonds is 116o. Thus, the size of the COC angle in dioxins is intermediate between those in anisole (110o) and diphenyl oxide (120o).
According to X-ray diffraction data, the central fragment of dioxin molecules can be considered to be planar. In the 2,3,7,8-tetrachlorodibenzo-p-dioxin (b-TCDD),{ the oxygen atoms deflect slightly from the plane through the four carbon atoms in different directions, thus forming a `chair' configuration of the central fragment. However, the angle of deflection is only 0.8o.
In the DD molecule, all six atoms of the central fragment lie in one plane but this plane as a whole is somewhat (by 0.5o) rotated around axis 1. The C-O bond length in dibenzo-p-dioxin is 1.383A, the internal and external CCO angles are 122o and 118o, and the central COC angle is 116o. Note that the geometric parameters determined from X-ray diffraction data refer to molecules forming the crystal lattice. No experimental data for these parameters in free dioxin molecules have been reported in the literature.
Electronic structure of molecules
The ground electronic state (S0) of the planar DD molecule is characterised by the following charges (re) on the atoms of the central fragment, oxygen and four carbons: re(O)=0.37 and re(C)=+0.22 (in the electron charge units) on each of equivalent atoms. The calculation was carried out by the INDO/S semiempirical approximation. The charges on the C-H group atoms are small in magnitude (0.05-0.09 units) and have opposite signs (for carbon re<0).
Vibrational states
The molecules of dioxins contain 22 atoms and have 60 normal modes. In planar molecules, 41 modes are in-plane vibrations and 19 modes are out-of-plane vibrations. In the infrared, Raman, fluorescence and phosphorescence spectra, which display the vibrations of molecules in the ground electronic state, only vibrations with particular types of symmetry can be observed, depending on the symmetry of the molecule, in accordance with the selection rules.
The attention was focused on the IR spectra of the most toxic b-TCDD. The IR spectrum of this compound in a KBr pellet was measured in a wider frequency range (4000-375 cm-1) than the spectra of other tetrachloro-substituted DD. Figure1 shows a fragment of this spectrum.
If the molecular configuration of any dioxin has an inversion centre, only about a half of the fundamental modes are displayed in the IR spectra, while the other frequencies are active in the Raman spectra. The most comprehensive IR and Raman data have been reported for DD and OCDD (Raman spectra of OCDD are shown in Fig. 2). For other PCDD, to the best of our knowledge, no data on the Raman spectra are available.
Singlet electronically excited states
Dioxins, like most benzene derivatives, are colourless, i.e. they do not absorb light in the visible region. The UV absorption spectra of dioxins [Sm(ππ*)S0] have been studied in solution and in the vapour phase. The absorption maxima and the molar extinction coefficients (emax) have been reported; the absorption curves for DD and its chloro-derivatives (b-TCDD, 1,2,3,7,8-Cl5DD and OCDD) can be found in several publications [2,3]. The absorption spectra of DD and PCDD occur in the region of 200-350 nm and consist of two bands - a weak broad long- wavelength band (A) in the region of 280-320 nm with a maximum at 300-304 nm and a short-wavelength narrow intense band (B) with a maximum at about 220-240 nm. It can be seen from the absorption curves of some dioxins that the high-frequency absorption band has one more, even shorter wavelength maximum (C); for example, in the spectra of solutions of DD, it is separated from band B by 5.5nm. As examples, Figs 3 and 4 show the absorption spectra of a solution of DD and of the b-TCDD vapour.
Luminescence
Dioxins belong to the class of luminescent compounds. Luminescence is due to the S1  S0 transition from the lower singlet electronically excited state (fluorescence) and to the T1  S0 transition from the lower triplet state (phosphorescence). Solutions of dioxins fluoresce in the region of 310-400 nm; when frozen (77 K), they display a bright long-living blue to light-blue phosphorescence. As an example, Fig. 5 presents the emission spectrum of a solution of DD in isopentane recorded at 77 K. This spectrum demonstrates an important feature characteristic of both DD and PCDD, namely, very low intensity of the fluorescence band 1 with respect to the phosphorescence band 2.
Fluorescence
The fluorescence spectra of solutions of DD and PCDD exhibit a structureless band (see Fig. 5). The use of selective laser excitation of fluorescence, which often gives rise to a fine (vibrational) structure in the spectra of solutions, still has not allowed recording of structured spectra for the molecules under study (b-TCDD, 1,3,7,8-Cl4DD and OCDD). According to calculations, the S1 state in the xanthene and DD molecules can be assigned to the πlπ* type of orbital; the change in the π-electron density on the oxygen atoms in this state is nearly the same for these two molecules. Therefore, the sharp (by two orders of magnitude) decrease in the ffl value for DD with respect to xanthene is believed to be due to the higher rate constant for the S1 Tj intersystem crossing (kisc) in the case of DD.
Phosphorescence
Phosphorescence is manifested as a relatively intense band in the spectrum shown in Fig. 5. Using specific experimental conditions, spectra with clear-cut vibrational structure of this band have been obtained for DD and PCDD. For a number of dioxins, the phosphorescence lifetimes tph at 77K have been measured. For example, in the case of DD, tph=0.54s (77 K, a solution in a 1:5:5 mixture of ethanol, isopentane and ether) 2 and 0.7s (77 K, a solution in isopentane). For chloro-substituted derivatives, the phosphorescence lifetimes are shorter than for unsubstituted DD.
It has been mentioned that dioxins are formed as trace side products in some industrial processes including chemical (production of chlorinated phenols, herbicides and soon) and pulp-and-paper (paper bleaching) processes and during the operation of garbage-incineration plants. This is accompanied by dioxin pollution of the environment. Polychlorinated dibenzop-dioxins are highly stable compounds. They do not decompose at high temperatures (up to 750oC); the half-life of PCDD in soil is about 10 years. Due to their stability, the dioxin toxicants are accumulated in soil, water, plants and animals, which creates hazard for humans and other living organisms, because dioxins are many orders of magnitude more toxic than commonly known poisons (such as potassium cyanide or curare). Realisation of the dioxin menace stimulated the governments of many countries to adopt special programmes on the protection of the environment from these toxic compounds.
One aspect of the environmental protection is the development of highly sensitive methods for determination and quantitative analysis of traces of PCDD. Since dioxins are very hazardous compounds, methods for their analysis should be highly sensitive, at the pictogram level. Yet another, equally significant requirement to the methods of dioxin analysis is high selectivity. This requirement originates due to the fact that dioxins having different structures differ markedly in toxic properties.
Conclusion
Since it is the molecular structure that accounts for the different photophysical properties of compounds, it is important to establish the geometrical structure of dioxins. X-Ray diffraction data point to a planar or nearly planar structure of DD and PCDD molecules in the crystals. No direct experimental data on the structures of these molecules in the gas phase or in solutions are available to date. Only indirect methods of determination do exist, relying on the data on the dipole moment of DD in solutions or on the results of theoretical calculations. Depending on the theoretical method of modeling employed, the equilibrium nuclear configuration of the molecule in the ground electronic state S0 is found to be either planar or non-planar. In the case where the dioxin molecules are found to be non-planar, some researchers concluded that, in particular, DD has a `butterfly' configuration.
Data on the structures of dioxin molecules are contradictory. Some investigators who give credit to X-ray diffraction data believe that the dioxin molecules are planar; other researchers who rely on calculations consider that the molecules are non-planar. Note that the intramolecular vibration mode of the `butterfly' type shows itself at a low frequency for all dioxins. Dioxin molecules might occur in different configurations depending on the experimental conditions (temperature, intermolecular interactions).

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