Symposium on Photochemistry 1994
Senri Life Science Centre, Toyonaka, Osaka, Japan
October 26, 1994
Oral presentation (Chika Sato) (chair: Akio Yoshimura)

New aspects on fluorescence quenching by Molecular oxygen. 2. Inhibition of long-distance electron transfer in acetonitrile
Chika Sato1, Koichi Kikuchi1, Kohji Okamura2, Yasutake Takahashi2, and Tsutomu Miyashi2
1Dept. Phys., Schl. Sci., Kitasato Univ., Japan, 2Dept. Chem., Fac. Sci, Tohoku Univ., Japan
Rate constants kq of fluorescence quenching by molecular oxygen 3O2(3Σg) and the efficiencies of enhanced intersystem crossing ΦT and free-radical generation ΦR are measured for several aromatic hydrocarbons in acetonitrile. kq is the diffusion-controlled limit, kdiff (=(3.2–3.7) × 1010 M−1 s−1), when the free energy change ΔG of actual electron transfer (ET) from the first excited singlet fluorescer 1M* to 3O2 is more negative than −0.8 eV. In the region where ΔG > −0.8 eV, kq is slightly smaller than kdiff. ΦT decreases from 1.0 to 0.4 with decreasing ΔG, whereas ΦR is null except for 2,6-dimethoxynaphthalene (DMN). In the case of DMN, the actual ET is highly exothermic (ΔG = −1.45 eV). Nevertheless, ΦR is as low as 0.003, while ΦT is as high as 0.43. As the lowest triplet energy of 2.70 eV for DMN is greater than the energy of 2.07 eV for the radical ion pair (2DMN•+ + 2O2•−), the triplet DMN cannot be produced by recombination of the radical ion pair. These facts indicate that fluorescence quenching by 3O2 is not induced by long-distance ET but rather by exciplex formation. Since the molecular size of 3O2 is considerably smaller than that of the aromatic molecule, the solvent reorganization energy of long-distance ET from an aromatic molecule to 3O2 is considered to be extremely high. For this reason, long-distance ET cannot compete with exciplex formation in fluorescence quenching by 3O2.