RELATIVE FRACTION OF EXCITED-STATE OXYGEN FORMED AS B1-SIGMA-G-PHASE PHOTOSENSITIZED REACTIONS( IN SOLUTION)

The relative fraction F(SIGMA) of excited state oxygen formed as b1SIG MA(g)+ was determined for a series of triplet state photosensitizers i n CCl4 solutions. Oxygen excited states were formed by energy transfer from aromatic ketones whose triplet-state energies E(T) ranged from 5 3 to 75 kcal/mol. F(SIGMA) was determined by monitoring the intensitie s of (a) O2(b1SIGMA(g)+) fluorescence at 1926 nm [O2(b1SIGMA(g)+) --> O2(a1DELTAg)] and (b) O2(a1DELTA(g)) phosphorescence at 1270 nm [O2(a1 DELTA(g)) --> O2(X3SIGMA(g)-)]. Ketone-dependent changes in F(SIGMA) a re not consistent with predictions made by Kearns et al. over 25 years ago. Specifically, F(SIGMA) values from high ET ketones were found to be smaller than, not equivalent to, those obtained by using a low E(T ) ketone. These data may indicate that charge-transfer-mediated indire ct coupling between states of the ketone-oxygen encounter pair can inf luence yields of O2-(b1SIGMA(g)+) and O2(a1DELTA(g)).