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.

A series of axial and equatorial diastereomers of (coumarin-4-yl)methyl-caged adenosine cyclic 3',5'-monophosphates (cAMPs), 1-6, having methoxy, dialkylamino, or no substituent in the 6- and/or 7-positions, and their corresponding 4-(hydroxymethyl)coumarin photoproducts 7-12 have been synthesized. The photochemical and UV/vis spectroscopical properties (absorption and fluorescence) of 1-6 and 7-12 have been examined in methanol/aqueous HEPES buffer solution. Donor substitution in the 6-position causes a strong bathochromic shift of the long-wavelength absorption band, whereas substitution in the 7-position leads only to a weak red shift. The photochemical cleavage of the caged cAMPs was investigated, and the photoproducts were analyzed. Photochemical quantum yields, fluorescence quantum yields, and lifetimes of the excited singlet states were determined. The highest values of photochemical quantum yields (photo-S(N)1 mechanism) were obtained with caged cAMPs having a donor substituent in the 7-position of the coumarin moiety, caused by electronic stabilization of the intermediately formed coumarinylmethyl cation. With donor substitution in the 6-position, the resulting moderate electronic stabilization of the coumarinylmethyl cation is overcompensated by the strong bathochromic shift, reducing the energy gap between the excited-state S(1) and the corresponding coumarinylmethyl cation. The rate constant for the ester cleavage and liberation of cAMP is about 10(9) s(-1), estimated for the axial isomer of 6 by analysis of the fluorescence increase of the alcohol 12 formed upon laser pulse photolysis.

Rate constants of photosensitized generation of O2(1Σg +), O2(1Δg), and O2(3Σg -) have been determined for a series of ππ* triplet sensitizers with strongly varying oxidation potential (E ox), triplet energy (E T), and molecular structure, in CCl4. We demonstrate that one common dependence on E ox and E T successfully describes these rate constants for the molecules studied here and also for all previously investigated ππ* sensitizers, independently of molecular structure or any other parameter. Photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states can be generally described by a mechanism involving the successive formation of excited noncharge transfer (nCT) encounter complexes and partial charge transfer (pCT) exciplexes of singlet and triplet multiplicity 1,3(T1 3Σ), following interaction of O2(3Σg -) with the triplet excited sensitizer. Both 1,3(T1 3Σ) nCT and pCT complexes decay by internal conversion (ic) to yield O2(1Σg +), O2(1Δg), and O2(3Σg -) and the sensitizer ground state. ic is the rate-limiting step in the nCT channel, whereas exciplex formation is rate determining in the pCT channel. Rotation of the O2 molecule within the solvent cage of 1,3(T1 3Σ) nCT complexes is fast enough to allow for a completely established intersystem crossing (isc) equilibrium, whereas significant noncovalent binding interactions slow rotation and inhibit isc between 1(T1 3Σ) and 3(T1 3Σ) pCT complexes. Upon the basis of this mechanism, we propose a semiempirical relationship that can be generally used to estimate rate constants and efficiencies of photosensitized singlet oxygen generation during O2 quenching of ππ* triplet states in CCl4. The data set includes 127 rate constants for derivatives of naphthalene, biphenyl, fluorene, several ketones, fullerenes, porphyrins and metalloporphyrins, and other homocyclic and heterocyclic aromatics of variable molecular structure and size. It is suggested that the general relationship presented here can be used for the optimization of the singlet oxygen photosensitization ability of many molecules, including those used in biological and medical applications, such as the photodynamic therapy of cancer.

The charge-transfer induced quenching processes of the lowest excited triplet state (T 1 ) of naphthalene derivatives by ground-state oxygen and of singlet oxygen O 2 ( 1 ∆ g ) by ground state naphthalene derivatives have been investigated in three solvents of different polarity. Both deactivation processes are described by one common Marcus type plot. The analysis of the data strongly indicates that exciplexes with the same degree of average partial charge transfer (pCT) and the same reorganization energy are formed in the ratedetermining step of both quenching processes. The free energies of pCT complex formation are related to the corresponding free energy of complete electron transfer by ∆G CT ) f∆G CET with a common corrective factor f, for T 1 and O 2 ( 1 ∆ g ) deactivation. The reorganization energy increases from 34 kJ mol -1 in carbon tetrachloride to 92 kJ mol -1 in acetonitrile. The charge transfer character is shown to be significantly larger than 25% and to increase with increasing solvent polarity.

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Dedicated to Professor Andre M. Braun on the occasion of his 60th birthdayWe have studied the charge-transfer-induced deactivation of np* excited triplet states of benzophenone derivatives by O 2 ( 3 S À g ), and the charge-transfer-induced deactivation of O 2 ( 1 D g ) by ground-state benzophenone derivatives in CH 2 Cl 2 and CCl 4 . The rate constants for both processes are described by Marcus electron-transfer theory, and are compared with the respective data for a series of biphenyl and naphthalene derivatives, the triplet states of which have pp* configuration. The results demonstrate that deactivation of the locally excited np* triplets occurs by local charge-transfer and non-charge-transfer interactions of the oxygen molecule with the ketone carbonyl group. Relatively large intramolecular reorganization energies show that this quenching process involves large geometry changes in the benzophenone molecule, which are related to favorable FranckCondon factors for the deactivation of ketone-oxygen complexes to the ground-state molecules. This leads to large rate constants in the triplet channel, which are responsible for the low efficiencies of O 2 ( 1 D g ) formation observed with np* excited ketones. Compared with the deactivation of pp* triplets, the non-charge-transfer process is largely enhanced, and charge-transfer interactions are less important. The deactivation of singlet oxygen by ground-state benzophenone derivatives proceeds via interactions of O 2 ( 1 D g ) with the Ph rings.Introduction. ± In the presence of molecular oxygen, the main pathway of deactivation of excited triplet states of sufficient energy E T is quenching by O 2 ( 3 S À g ), which leads to formation of the excited singlet states O 2 ( 1 S g ) and O 2 ( 1 D g ), and the ground-state O 2 ( 3 S g ), with efficiencies depending on several parameters [1 ± 9] [16 ± 19] [20]. The influence of the electronic configuration of a sensitizer on the efficiency S D of O 2 ( 1 D g ) generation has been one of the most puzzling issues in singlet-oxygen photochemistry [1 ± 9]. Although exceptions are known [8] [9], S D values are generally found to be significantly lower with np* triplet states (S D % 0.3 ± 0.5) than with pp* triplet states (S D % 0.8 ± 1.0) [1 ± 7]. Several features of the investigated np* triplets have been evoked to explain this behavior, such as their high polarizability [3], high triplet energies [7], or large Franck-Condon factors for the deactivation of ketoneoxygen complexes [4], but no clear mechanistic proof has been given. The aim of the present work is to determine the structure of the charge-transfer (CT) complexes formed with molecular oxygen and aromatic ketones having np* triplet-state configuration. Excited CT complexes play an important role in the deactivation of triplet-excited ketones by ground-state oxygen, and in the quenching of excited singlet oxygen by the ground-state ketones. These two processes will be compared with each other, and with the respective processes with naphthalene and biphenyl derivative...