NEW ASPECTS ON FLUORESCENCE QUENCHING BY MOLECULAR-OXYGEN

Fluorescence quenching rate constants k(q) by molecular oxygen O-3(2)(3SIGMA(g-) were measured for four anthracenecarbonitriles (AC) in acetonitrile and benzene and for five acridinium ions in acetonitrile. The k(q)'s for acridinium ions decrease with an increase in the free enthalpy change DELTAG of full electron transfer from the first excited singlet fluorescer 1M* to O-3(2) in acetonitrile and become smaller than one-hundredth of the diffusion-controlled limit, k(diff). The k(q)'s for AC in acetonitrile decrease up to 5.3 x 10(9) M-1 s-1 with an increase in the number of cyano substituents or with an increase in DELTAG in the range -0.53-+0.25 eV. The k(q)'s for AC in benzene, 3 x 10(9) M-1 s-1, arc only one-tenth of k(diff), except for 9-cyanoanthracene (CA) with 1.0 X 1010 M-1 s-1. For both acridinium ions and AC, the energy of the second excited triplet state 3M* is much greater than that of 1M*, so that 3M* cannot be produced by quenching. The energy gap DELTAE between 1M* and the lowest excited triplet state 3M is smaller than the energy 0.98 eV of singlet oxygen O-1(2)*(1DELTA(g)) in the case of acridinium ions, but it is larger in the case of AC. On the basis of these results the fluorescence quenching mechanism by O-3(2) is discussed in detail.