Oxygen Quenching of nπ* Triplet Phenyl Ketones: Local Excitation and Local Deactivation

Abstract

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...

Synthesis of Benzophenone Nucleosides and Their Photocatalytic Evaluation for [2+2] Cycloaddition in Aqueous Media

“…As the electron‐donating ability of the substituent increases, the bathochromic shift of the CT band of the corresponding nucleoside also increases. This tracks well with the oxidation potentials of the benzophenones, which are 1.15 V for 10b , which has a dimethylamino substituent, 1.26 V for 10a , 2.20 V for 10c , and 2.60 V for methyl‐substituted 10d (vs. Ag/AgCl) 22. Unsubstituted benzophenone has the highest oxidation potential of 2.85 V 22…”

Photosensitized Generation of Singlet Oxygen

“…It is important to know that all mechanistic conclusions described here apply exclusively to ππ* excited triplets. Although the very vast majority of singlet oxygen sensitizers have ππ* triplet‐state configuration, it should be noted that ππ* excited ketones exhibit a significantly different behavior, which has been previously discussed in detail (36,39).…”

“…The quintet complex 5(T 1 3Σ) has no direct product channel. The overall rate constant of 1,3 (T 1 3Σ) complex deactivation k D is calculated from k T Q according to (), where k ‐diff / k diff = 1 mol dm −3 is assumed to hold true (14,20,21,32–39). …”

Elucidation of the Dexter‐Type Energy Transfer in DNA by Thymine–Thymine Dimer Formation Using Photosensitizers as Artificial Nucleosides

“…[28] Based on the similarity of the T 1 energies of BpOMe (E T = 290 kJ mol À1 ) [14] and BpMe (E T = 290 kJ mol À1 ), [14] the observed differences in our studies must be assigned to other photophysical properties.The T 1 state (np*) of Bp (and BpMe) is populated from the S 1 state by intersystem crossing via T 2 , [29] whereas the T 1 state of BpOMe (and ApOMe) is formed directly from S 1 . Apparently,s tacking of the photosensitizer is crucial for efficient energy transfer and the theoretical prediction is confirmed that well intercalated Bps will not lead to as ignificant barrier to efficient transfer from the T 1 state.…”