NITROXYL FREE-RADICAL ENHANCEMENT OF THE FORBIDDEN O-2(3-SIGMA-G-)[-O-2(1-DELTA-G) RADIATIVE TRANSITION IN CHLORINATED-HYDROCARBON SOLVENTS

Utilizing transient near-IR luminescence 'spectroscopy, we have shown that free radicals like TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) increase the rate of the forbidden radiative transition of singlet oxygen in chlorinated hydrocarbon solvents. The quantum yield for emission has been shown to depend on the nature of the substituent group in the 4' position. Concomitant with this effect is a slight high energy shift in the steady state singlet oxygen phosphorescence spectrum. Additional experiments with TEMP (2,2,6,6-tetramethylpiperidine) have shown that the nitroxyl group is necessary for this effect. Using the kinetic equations associated with a collision complex model, we have investigated a curve crossing (isophotic) point observed in the 1270 nm luminescence decays at various TEMPO concentrations. This information indicates that we are observing a contact complex characterized by a weak interaction energy and a short lifetime. We have also determined the conditions under which the isophotic point can be observed.