Photochemistry of Pyrene on Unactivated and Activated Silica Surfaces

Abstract

Photolysis of pyrene at the solid/air interface of unactivated and activated silica gel proceeds slowly to give mainly oxidized pyrene products. We have identified 1-hydroxypyrene, 1,6-pyrenedione, and 1,8-pyrenedione among the main reaction products. The remaining minor products show molecular weights and spectral properties consistent with oxygenated pyrenes. Furthermore, small amounts of 1,1‘-bipyrene dimer are also formed at higher surface coverages (2 × 10-5 mol/g). When photolysis is carried out at 5 × 10-5 mol/g pyrene, photodegradation rate drops sharply and pyrene loss becomes insignificant. No significant change in the product distribution is observed when the photolysis is carried out on unactivated or activated silica. Photodegradation rate is slightly faster on activated silica compared to unactivated silica. Mechanistic studies indicate that the precursor to photoproduct formation is pyrene cation radical which is postulated to be formed by electron transfer from pyrene excited state to oxygen (type I) or by photoionization of pyrene. The cation radical reacts with physisorbed water on silica to give the observed oxidation products.

Substrate effects in the photoenhanced ozonation of pyrene

“…3a, is comparable to that previously observed by our group for solid-film pyrene under similar conditions: at an ozone concentration of ∼8×10 15 molec cm −3 , the average k obs increased from (3.2±1.1)×10 −3 s −1 in darkness to (5.3±1.5)×10 −3 s −1 in the presence of light (Styler et al, 2009). The photolysis of solid-sorbed pyrene is known to be slow (Reyes et al, 2000;Behymer and Hites, 1988). With no ozone present, we observe only a slow evaporative loss of pyrene upon illumination.…”

“…1b, we observe broad, featureless, excimer-type fluorescence at the surface of both solid pyrene films and concentrated octanol films. On solid surfaces, excimeric emission by pyrene has been largely attributed to absorption and subsequent emission by weakly bound groundstate associated pairs (Barbas et al, 1994;Reyes et al, 2000). Evidence for the existence of higher aggregates in the solid state is provided by the absorption spectrum of thin pyrene films, which exhibits a long-wavelength tail (Nautiyal and Bisht, 2010).…”

“…Given that pyrene does not react efficiently with O 2 ( 1 g ) (Reyes et al, 2000;Kubat et al, 2000), this pathway, referred to in Scheme 1 and in the following discussion as a B-type pathway, can be viewed purely as a photophysical deactivation process.…”

Electrochemical and photocatalytic reactions of polycyclic aromatic hydrocarbons investigated by raman spectroscopy

“…11,18 Several compounds, such as 1,6-pyrene-dione and 1,8-pyrene-dione, have been suggested as initial pyrene photochemical products. 18 These molecules are potential intermediate products for the reaction studied here and may give rise to the spectral pattern observed in Figure 6. …”

“…These radicals can follow a variety of reaction pathways, including the introduction of hydroxyl and carbonyl groups into the ring system. 11,18 Several compounds, such as 1,6-pyrene-dione and 1,8-pyrene-dione, have been suggested as initial pyrene photochemical products. 18 These molecules are potential intermediate products for the reaction studied here and may give rise to the spectral pattern observed in Figure 6.…”