Mechanism of Quenching of Triplet States by Molecular Oxygen:  Biphenyl Derivatives in Different Solvents

Wilkinson, Frances; Abdel-Shafi, Ayman A.

doi:10.1021/jp9907995

Cited by 84 publications

(207 citation statements)

References 39 publications

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“…Aromatic hydrocarbons such as napthalenes, anthracenes, and biphenyls have also been studied for their photosensitizer ability [24]. These studies found that the competition of charge transfer interactions with the energy transfer pathway was of greater importance for biphenyls than for the napthalenes.…”

Section: Organic Dyes and Aromatic Hydrocarbonsmentioning

confidence: 99%

Photosensitized singlet oxygen and its applications

DeRosa

2002

Coordination Chemistry Reviews

2,505

2,134

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Section: Organic Dyes and Aromatic Hydrocarbonsmentioning

confidence: 99%

Photosensitized singlet oxygen and its applications

DeRosa

2002

Coordination Chemistry Reviews

2,505

2,134

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“…20 Furthermore, it was found that a CT and a non-CT (nCT) pathway compete in the quenching of triplet states by O 2 and both yield O 2 ( 1 ∆ g ) with different efficiencies. [8][9][10][11][12][13][14] However, despite the large number of data and numerous efforts, no clear relations have been found between k T Q and S ∆ and the molecular properties of the sensitizer. The major reason for this unsatisfactory situation was the missing differentiation between O 2 ( 1 Σ g + ) and O 2 ( 1 ∆ g ) formed in the photosensitization process.…”

Section: Introductionmentioning

confidence: 99%

The Effect of Solvent Polarity on the Balance between Charge Transfer and Non-Charge Transfer Pathways in the Sensitization of Singlet Oxygen by ππ* Triplet States

Schmidt

2006

J. Phys. Chem. A

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A large set of literature kinetic data on triplet (T(1)) sensitization of singlet oxygen by two series of biphenyl and naphthalene sensitizers in solvents of strongly different polarity has been analyzed. The rate constants and the efficiencies of singlet oxygen formation are quantitatively reproduced by a model that assumes the competition of a non-charge transfer (nCT) and a CT deactivation channel. nCT deactivation occurs from a fully established spin-statistical equilibrium of (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) encounter complexes by internal conversion (IC) to lower excited complexes that dissociate to yield O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)). IC of (1,3)(T(1)(3)Sigma) encounter complexes is controlled by an energy gap law that is generally valid for the transfer of electronic energy to and from O(2). (1,3)(T(1)(3)Sigma) nCT complexes form in competition to IC (1)(T(1)(3)Sigma) and (3)(T(1)(3)Sigma) exciplexes if CT interactions between T(1) and O(2) are important. The rate constants of exciplex formation depend via a Marcus type parabolic model on the corresponding free energy change DeltaG(CT), which varies with sensitizer triplet energy, oxidation potential, and solvent polarity. O(2)((1)Sigma(g)(+)), O(2)((1)Delta(g)), and O(2)((3)Sigma(g)(-)) are formed in the product ratio (1/6):(1/12):(3/4) in the CT deactivation channel. The balance between nCT and CT deactivation is described by the relative contribution p(CT) of CT induced deactivation calculated for a sensitizer of known triplet energy from its quenching rate constant. It is shown how the change of p(CT) influences the quenching rate constant and the efficiency of singlet oxygen formation in both series of sensitizers. p(CT) is sensitive to differences of solvent polarity and varies for the biphenyls and the naphthalenes as sigmoidal with DeltaG(CT). This quantitative model represents a realistic and general mechanism for the quenching of pipi triplet states by O(2), surpassing previous advanced models.

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“…We assume for both quenching processes k Àdiff gk diff /m À1 , with g 1, whereby m mol l À1 , as was already done by Wilkinson and Abdel-Shafi [19], and ourselves [12] [13] [16]. A larger value of g would lead to proportionally larger values for all nCT complex deactivation rate constants, but the relative changes of k D,T and k D,D would remain the same.…”

mentioning

confidence: 99%

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

Schweitzer

Mehrdad

Noll

et al. 2001

HCA

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Dedicated to Professor AndreÂ 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...

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Mechanism of Quenching of Triplet States by Molecular Oxygen: Biphenyl Derivatives in Different Solvents

Cited by 84 publications

References 39 publications

Photosensitized singlet oxygen and its applications

Photosensitized singlet oxygen and its applications

The Effect of Solvent Polarity on the Balance between Charge Transfer and Non-Charge Transfer Pathways in the Sensitization of Singlet Oxygen by ππ* Triplet States

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

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