Surface photochemistry: Dibenzo-p-dioxin adsorbed onto silicalite, cellulose and silica

Laser-induced luminescence of argon purged and air-equilibrated samples of dibenzo-p-dioxin adsorbed onto three different solid powdered supports, silicalite, cellulose and silica, revealed the existence of both fluorescence and phosphorescence emissions at room temperature.A remarkable difference in transient absorption spectra was found when dibenzo-p-dioxin was included within the narrow internal channels of silicalite: triplet–triplet absorption of dibenzo-p-dioxin was detected in the silicalite case as a host, simultaneous with radical cation formation immediately after pulsed laser excitation (in the hundreds of nanoseconds time scale) while in the case of cellulose and silica as adsorbents, absorption transients arising from 2,2′-biphenylquinone and possibly from the spiroketone were found. For all hosts dibenzo-p-dioxin exhibits a transient band peaking at 330–340 nm in the microsecond and millisecond time scales, which we assigned to the biradical of dibenzo-p-dioxin.Diffuse reflectance laser flash photolysis and chromatographic techniques provided complementary information, the former about transient species and the latter regarding the final products formed after laser irradiation at 266 nm. Product analysis and identification clearly show that the photodegradation products are dependent on the host, the photochemistry being much more rich and complex in the cellulose and silica cases, where the main detected photoproduts were 2,2′-dihydroxybiphenyl and 1-hydroxydibenzofuran. In the case of silicalite as host, which has a channel-like internal structure and reduced space available for the guest dioxin, photodegradation reactions are highly reduced or even inhibited and no photodegradation products were detected.