TEMPERATURE-DEPENDENCE OF THE OXYGEN QUENCHING OF PI-PI-ASTERISK-SINGLET AND PI-PI-ASTERISK-TRIPLET STATES OF SINGLET OXYGEN SENSITIZERS

The rate constant for the oxygen quenching of the lowest pipi*-singlet state of chrysene (CHR) k(q)S and the rate constant for the oxygen quenching of the lowest pipi*-triplet states of CHR, 4-benzoylbiphenyl (BBP), and perinaphthenone (PN) k(q)T were determined in toluene. At room temperature k(q)S was found to be (3.2 +/- 0.3) X 10(10) M-1 s-1, indicating that oxygen quenching of the S1-state of CHR is diffusion controlled. The k(q)T values of the three sensitizers were found to be smaller than 1/9k(q)S at room temperature. With time-resolved luminescence measurements the efficiencies S(DELTA) of singlet oxygen (O-1(2)) production from the triplet states of the three sensitizers were measured. For the ketones, S(DELTA) values near unity were found at room temperature, whereas CHR exhibits a lower O-1(2) production efficiency. In addition the temperature dependence of the different quenching processes was studied in the temperature range -90-degrees-C < T < 90-degrees-C. For all compounds, deviation of triplet quenching from the diffusion-controlled reaction was observed at high temperatures. The data were interpreted with a collision complex model leading to the assumption of exciplex formation. We also determined temperature-dependent S(DELTA) values. In the case of BBP and PN, evidence for intersystem crossing between singlet and triplet exciplexes was found. The measurements were quantified by applying a kinetic scheme, which allows the determination of probabilities of exciplex intersystem crossing.