LASER FLASH-PHOTOLYSIS STUDIES OF THE CONTACT COMPLEX BETWEEN MOLECULAR-OXYGEN AND 1-METHYLNAPHTHALENE

The contact complex (3E0) between 1-methylnaphthalene (MN) and molecul ar oxygen in several solvents was excited at 355 nm, and the subsequen t processes were observed by absorption spectrometry on subnanosecond and suprananosecond time scales. Experimental evidence for a significa nt amount of charge-transfer character in the upper 3E2 is presented. The lifetime of this charge-transfer state (CT) is 20 +/- 10 ps, and t here are two competitive pathways for decay of this radical pair: (i) to the localized triplet state and (ii) to the triplet ground state. I n the nonpolar solvent cyclohexane, decay to the localized triplet sta te dominates and PHI(T) = 1; in the polar solvent acetonitrile, the co mpetition between two recombination pathways is approximately equal an d reduces the quantum yield of the triplet state to one-half. The quan tum yields of singlet oxygen (O2(1DELTA(g))) in the two solvents refle ct these triplet yields, which indicates that the precursor of singlet oxygen is the free triplet state of MN. This is supported by kinetic evidence. There is no evidence that intersystem crossing in the CT sta te results in direct entry to the singlet manifold.