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

Insight into the O-2 quenching mechanism of a photosensitizer (static or dynamic) would be useful for the design of heterogeneous systems to control the mode of generation of O-1(2) in water. Here, we describe the use of a photosensitizer, meso-tetra(N-niethyl-4-pyridyl)porphine (1), which was adsorbed onto porous Vycor glass (PVG). A maximum loading of 1.1 x 10(-6) mol 1 per PVG was achieved. Less than 1% of the PVG surface was covered with photosensitizer 1, and the penetration of I 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 mu s in water. Triplet O-2 quenched the transient absorption of triplet 1*-ads; for samples containing 0.9 x 10(-6)-0.9 x 10(-8) mol 1 adsorbed per g PVG, the Stern-Volmer constant, K-D, ranged from 23 700 to 32 100 M-1. The adduct formation constant, K-S, ranged from 1310 to 510 M-1. The amplitude of the absorption at 470 nm decreased slightly (by about 0.1) with increased O-2 concentrations. Thus, the quenching behavior of triplet 1*-ads by O-2 was proposed to be strongly dependent on dynamic quenching. Only similar to 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 homogencous solution which are often quenched dynamically by O-2.