Solvent-Specific Photolytic Behavior of Octachlorodibenzo-p-dioxin

The photolysis of octachlorodibenzo-p-dioxin (OCDD) was investigated in various organic solvents under lambda > 300 nm irradiation. The rates of OCDD photolysis were highly solvent-specific. OCDD was photodegraded rapidly in toluene, n-hexane, CCl4, and 1-octanol, whereas it underwent negligible photodegradation in acetonitrile, acetone, and methanol. Both OCDD photolysis and fluorescence emission did not take place in very polar solvents because polar solvent molecules efficiently quenched the excited OCDD (OCDD*). The addition of acetonitrile to an OCDD solution in toluene rapidly quenched both the fluorescence emission and the photolysis rate, which can be described by Stern-Volmer analysis. The efficient photolysis in nonpolar (or less polar) solvents seems to be mediated through a charge-transfer path where the solvent and OCDD* act as an electron donor and acceptor, respectively. However, OCDD photolysis in CCl4 seems to represent the opposite case in which the solvent is an electron acceptor and OCDD* is an electron donor. Hammett sigma constants that approximately represent the electron-donating power in structurally related aromatic solvents show a good correlation with the photolysis rates. We propose that the solvent specificity in OCDD photolysis is mainly ascribed to the difference in the electron donating (or accepting) tendency among various solvents. When triethylamine that easily donates an electron to form a charge-transfer exciplex with OCDD* was added, a marked enhancement in the photolysis rate was observed.