IV. Photosensitized Production of Singlet Oxygen 1721 A. Oxygen Quenching of Excited Triplet States 1725 1. Parameters Influencing the Generation of Singlet Oxygen 1725 2. Mechanism of Oxygen Quenching of ππ* Triplet States 1731 3. Mechanism of Oxygen Quenching of nπ* Triplet States 1736 B. Oxygen Quenching of Excited Singlet States 1737 1. Rate Constants of S 1 -State Quenching 1737 2. Products of S 1 -State Quenching 1739 3. Mechanism of Oxygen Quenching of Excited Singlet States 1742 V. Detection of Singlet Oxygen 1745 VI. Applications 1746 A. Estimation of the a f X Radiative Rate Constant in Different Environments 1746 1. Liquid Phase 1747 2. Gas Phase 1747 3. Microheterogeneous Systems 1747 B. Estimation of the Contribution of e−v, CT, EET, and Chemical Pathways to O 2 ( 1 ∆ g ) and O 2 ( 1 Σ g + ) Deactivation 1748 C. Estimation of O 2 ( 1 ∆ g ) and O 2 ( 1 Σ g + ) Lifetimes in Different Environments 1749 1. Liquid Phase 1749 2. Polymers 1749 3. Gas Phase 1749 4. Microheterogeneous Systems 1749 5. Zeolite Systems 1750 D. Estimation of a f X Emission Quantum Yields 1750 E. e−v Deactivation of Isoelectronic Molecules 1750 F. Optimization of Singlet Oxygen Sensitizers 1750 G. Estimation of Singlet Oxygen Diffusion Lengths 1752 VII. Conclusion 1752 VIII. Acknowledgment 1752 IX. References 1752

For Abstract see ChemInform Abstract in Full Text.

Both excited singlet states 1 Σ g + and 1 ∆ g and the unexcited triplet ground state 3 Σ gof molecular oxygen are formed with varying rate constants k T 1Σ , k T 1∆ , and k T 3Σ , respectively, during the quenching by O 2 of triplet states T 1 of sufficient energy E T . The present paper reports these rate constants for a series of nine naphthalene sensitizers of very different oxidation potential, E ox but almost constant E T . These data complement data for k T 1Σ , k T 1∆ , and k T 3Σ , determined previously for 13 sensitizers of very different E T . The analysis of the whole set of rate constants reveals that the quenching of triplet states by O 2 results in the formation of O 2 ( 1 Σ g + ), O 2 ( 1 ∆ g ), and O 2 ( 3 Σ g -) with varying efficiencies by two different channels, each capable of producing all three product states. One quenching channel originates from excited 1,3 (T 1 ‚ 3 Σ) complexes without chargetransfer character (nCT), which we cannot distinguish from encounter complexes; the other originates from 1 (T 1 ‚ 3 Σ) and 3 (T 1 ‚ 3 Σ) exciplexes with partial charge-transfer character (pCT). Rate constants of formation for O 2 ( 1 Σ g + ), O 2 ( 1 ∆ g ), and O 2 ( 3 Σ g -) are controlled by the respective excess energies via an energy gap relation in the nCT channel, whereas they vary with varying free energy of complete electron transfer in the pCT channel. A fast intersystem crossing equilibrium between 1 (T 1 ‚ 3 Σ) and 3 (T 1 ‚ 3 Σ) is surprisingly observed only in the nCT but not in the pCT channel.