Singlet Oxygen Chemistry in Water. 2. Photoexcited Sensitizer Quenching by O2 at the Water−Porous Glass Interface

Insight into the O2 quenching mechanism of a photosensitizer (static or dynamic) would be useful for the design of heterogeneous systems to control the mode of generation of 1O2 in water. Here, we describe the use of a photosensitizer, meso-tetra(N-methyl-4-pyridyl)porphine (1), which was adsorbed onto porous Vycor glass (PVG). A maximum loading of 1.1 × 10−6 mol 1 per g PVG was achieved. Less than 1% of the PVG surface was covered with photosensitizer 1, and the penetration of 1 reaches a depth of 0.32 mm along all faces of the glass. Time-resolved measurements showed that the lifetime of triplet 1*-ads was 57 μs in water. Triplet O2 quenched the transient absorption of triplet 1*-ads; for samples containing 0.9 × 10−6−0.9 × 10−8 mol 1 adsorbed per g PVG, the Stern−Volmer constant, KD, ranged from 23 700 to 32 100 M−1. The adduct formation constant, KS, ranged from 1310 to 510 M−1. The amplitude of the absorption at 470 nm decreased slightly (by about 0.1) with increased O2 concentrations. Thus, the quenching behavior of triplet 1*-ads by O2 was proposed to be strongly dependent on dynamic quenching. Only ∼10% of the quenching was attributed to the static quenching mechanism. The quenching of triplet 1*-ads was similar to that observed for photosensitizers in homogeneous solution which are often quenched dynamically by O2

Evidence for Iodine Atoms as Intermediates in the Dye Sensitized Formation of I-\u88\u92I Bonds

Visible light excitation of [Ru(bpz)2(deeb)](PF6)2, where bpz is 2,2?-bipyrazine and deeb is 4,4?-(CO2Et)2-2,2?-bipyridine, in acetonitrile solutions with iodide is shown to initiate excited-state electron transfer reactions that yield iodine atoms. The iodine atoms subsequently react with iodide to form the I?I bond in I2??. The resultant Ru(bpz?)(bpz)(deeb)+, I2?? stores ?1.64 eV of free energy and returns cleanly to ground-state products with kcr = (2.1 ± 0.3) ? 1010 M?1 s?1.QC 2011122

Singlet Oxygen Generation on Porous Superhydrophobic Surfaces: Effect of Gas Flow and Sensitizer Wetting on Trapping Efficiency

We describe physical-organic studies of singlet oxygen generation and transport into an aqueous solution supported on superhydrophobic surfaces on which silicon–phthalocyanine (Pc) particles are immobilized. Singlet oxygen (¹O₂) was trapped by a water-soluble anthracene compound and monitored <i>in situ</i> using a UV–vis spectrometer. When oxygen flows through the porous superhydrophobic surface, singlet oxygen generated in the plastron (i.e., the gas layer beneath the liquid) is transported into the solution within gas bubbles, thereby increasing the liquid–gas surface area over which singlet oxygen can be trapped. Higher photooxidation rates were achieved in flowing oxygen, as compared to when the gas in the plastron was static. Superhydrophobic surfaces were also synthesized so that the Pc particles were located in contact with, or isolated from, the aqueous solution to evaluate the relative effectiveness of singlet oxygen generated in solution and the gas phase, respectively; singlet oxygen generated on particles wetted by the solution was trapped more efficiently than singlet oxygen generated in the plastron, even in the presence of flowing oxygen gas. A mechanism is proposed that explains how Pc particle wetting, plastron gas composition and flow rate as well as gas saturation of the aqueous solution affect singlet oxygen trapping efficiency. These stable superhydrophobic surfaces, which can physically isolate the photosensitizer particles from the solution may be of practical importance for delivering singlet oxygen for water purification and medical devices