| Abstrakt: |
Both excited singlet states, 1Sg+and 1?g, and the triplet ground state, 3Sg-, of molecular oxygen are competitively formed during the quenching by O2of triplet (T1) excited sensitizers of sufficient energy. The corresponding overall rate constants kT1S, kT1?and kT3Sas well as the T1state energies ETand the oxidation potentials Eoxhave been determined for a series of six fluorene derivatives. Graduated and in part strong charge transfer (CT) effects on kT1S, kT1S, and kT3Sare observed. These and literature data strongly indicate that quenching occurs in two different channels each capable of producing O2(1Sg+), O2(1?g), and O2(3Sg-). One proceeds viainternal conversion (IC) of excited 1,3(T1·3S) complexes with no CT character (nCT), which cannot be distinguished from encounter complexes, the other viaIC of 1,3(T1·3S) exciplexes with partial CT character (pCT). The contributions of nCT and pCT deactivation channels to the overall formation of O2(1Sg+), O2(1?g), and O2(3Sg-) depend on ETand Eox. The rate constants of the nCT channel are controlled by the excess energies of the respective IC processes by an energy gap law. The rate constants of the pCT channel depend on the change of free energy?GCETfor complete electron transfer from T1excited sensitizer to O2. Equations are presented which show the functional form of the dependence of the oxygen quenching rate constants on ETand Eox. Particular emphasis is laid on the question of whether these relations could generally be valid for pp* triplet sensitizers. |
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