Charge Transfer Interactions in the Generation of Singlet Oxygen O2(1Δg) by Strong Electron Donors

Darmanyan, A. P.; Lee, Woo‐Jae; Jenks, William S.

doi:10.1021/jp984292q

Cited by 51 publications

(94 citation statements)

References 72 publications

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“…Interestingly, both pathways contribute to the formation of O 2 (1Δ g ). The strong effect of increasing CT interactions on k T Q and SΔ was further corroborated by Cebul et al (25), Smith (26), Grewer and Brauer (27,28), and Darmanyan et al (29).…”

Section: Discussionmentioning

confidence: 55%

Photosensitized Generation of Singlet Oxygen

Schmidt

2007

Photochemistry and Photobiology

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This work gives an overview of what is currently known about the mechanisms of the photosensitized production of singlet oxygen. Quenching of ππ* excited triplet states by O2 proceeds via internal conversion of excited encounter complexes and exciplexes of sensitizer and O2. Both deactivation channels lead with different efficiencies to singlet oxygen generation. The balance between the deactivation channels depends on the triplet‐state energy and oxidation potential of the sensitizer, and on the solvent polarity. A model has been developed that reproduces rate constants and efficiencies of the competing processes quantitatively. Sensitization by excited singlet states is much more complex and hence only qualitative rules could be elaborated, despite serious efforts of many groups. However, the most important deactivation paths of fluorescence quenching by O2 are again directed by excess energies and charge‐transfer interactions similar to triplet‐state quenching by O2. Finally, two recent developments in photo‐sensitization of singlet oxygen are reviewed: Two‐photon sensitizers with particular application potential for photodynamic therapy and fluorescence imaging of biological samples and singlet oxygen sensitization by nanocrystalline porous silicon, a material with very different photophysics compared to molecular sensitizers.

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Section: Discussionmentioning

confidence: 55%

Photosensitized Generation of Singlet Oxygen

Schmidt

2007

Photochemistry and Photobiology

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“…[51] Since the J value is expected to increase with decreasing donor-acceptor dis- tance, the J value of DPA-CM in which the donor and acceptor moiety is directly linked may be large enough that the energy of decays by bimolecular BET because of the slow intramolecular BET. Additionally, the excited state of DPA is uninvolved in this ET dynamics, because the energies of singlet and triplet excited states of DPA (E(S) = 3.78 eV [52] and E(T) = 2.90 eV [53] ) are higher than the corresponding energies of CM.…”

Section: Energy Diagrammentioning

confidence: 99%

“…Finally ET from DPA to 3 CM* occurs to produce 3 (DPAC + -CMC À ), which decays by bimolecular BET because of the slow intramolecular BET. Additionally, the excited state of DPA is uninvolved in this ET dynamics, because the energies of singlet and triplet excited states of DPA (E(S) = 3.78 eV [52] and E(T) = 2.90 eV [53] ) are higher than the corresponding energies of CM.…”

Section: Energy Diagrammentioning

confidence: 99%

Photoinduced Electron Transfer in Photorobust Coumarins Linked with Electron Donors Affording Long Lifetimes of Triplet Charge‐Separated States

et al. 2010

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Electron donor (D) substituted 3-ethoxycarbonylcoumarin (CM) derivatives [D-CM: D = 4-diphenylaminophenyl (DPA), 4-diethylaminophenyl (DEA), 4-dimethylaminophenyl (DMA), and 2-methyl-4-dimethylaminophenyl (MeDMA)] are synthesized and characterized. Photoinduced electron transfer (ET) from the D moiety to the acceptor (CM) and back electron transfer (BET) are investigated by femtosecond and nanosecond laser flash photolysis measurements. Femtosecond laser excitation at 355 nm of a deaerated acetonitrile (MeCN) solution of D-CM shows generation of the singlet charge-separated (CS) state [(1)(D(.+)-CM(.-))] by ET from D to the singlet excited state of the CM moiety ((1)CM*), and this is followed by rapid decay within 3 ns to afford the triplet excited state (D-(3)CM*). Nanosecond laser excitation of a deaerated MeCN solution of D-CM results in formation of the triplet CS state by ET from D to (3)CM*. The quantum yield of formation of the triplet CS state [(3)(DPA(.+)-CM(.-))] in the presence of iodobenzene (PhI) in deaerated MeCN increases with increasing concentration of PhI to reach 27 % at 0.5 M PhI. The triplet CS state decays by bimolecular BET because of the long CS lifetime by unimolecular BET. Formation of the long-lived triplet CS state was confirmed by electron spin resonance (ESR) measurements. The photorobust nature of DPA-CM is demonstrated by multiple laser pulse excitation (>1000 times) at 355 nm. The photoinduced ET and BET rate constants of a series of D-CM are thoroughly analyzed in light of the Marcus theory of electron transfer.

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“…The question whether any or all of the intersystem crossings shown in the Scheme as dotted arrows occur between the singlet, triplet, and quartet spin channels, when MeCN at room temperature is used as solvent, is still open to debate (see, e.g., [16 ± 20] [25] [33]). It might be expected that during oxygen quenching of the 3 MLCT excited state of (substituted bipyridine)ruthenium(II) systems, the presence of the heavy ion Ru II , would enhance spin-forbidden pathways, possibly inducing intersystem crossing between singlet, triplet, and quintet channels as shown in the Scheme.…”

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confidence: 99%

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

Abdel-Shafi

Beer

Mortimer

et al. 2001

HCA

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Dedicated to Prof. AndreÂ M. Braun on the occasion of his 60th birthday Photophysical properties in dilute MeCN solution are reported for seven Ru II complexes containing two 2,2'-bipyridine (bpy) ligands and different third ligands, six of which contain a variety of 4,4'-carboxamidedisubstituted 2,2'-bipyridines, for one complex containing no 2,2'-bipyridine, but 2 of these different ligands, for three multinuclear Ru II complexes containing 2 or 4 [Ru(bpy) 2 ] moieties and also coordinated via 4,4'-carboxamide-disubstituted 2,2'-bipyridine ligands, and for the complex [(Ru(bpy) 2 (L)] 2 where L is N,N'-([2,2'-bipyridine]-4,4'-diyl)bis [3-methoxypropanamide]. Absorption maxima are red-shifted with respect to [Ru(bpy) 3 ] 2 , as are phosphorescence maxima which vary from 622 to 656 nm. The lifetimes of the lowest excited triplet metal-to-ligand charge transfer states 3 MLCT in de-aerated MeCN are equal to or longer than for [Ru(bpy) 3 ] 2 and vary considerably, i.e., from 0.86 to 1.71 ms. Rate constants k q for quenching by O 2 of the 3 MLCT states were measured and found to be well below diffusion-controlled, ranging from 1.2 to 2.0´10 9 dm 3 mol À1 s À1 . The efficiencies f T D of singlet-oxygen formation during oxygen quenching of these 3 MLCT states are relatively high, namely 0.53 ± 0.89. The product of k q and f T D gives the net rate constant k 1 q for quenching due to energy transfer to produce singlet oxygen, and k q À k 1 q equals k 3 q , the net rate constant for quenching due to energy dissipation of the excited 3 MLCT states without energy transfer. The quenching rate constants were both found to correlate with DG CT , the free-energy change for charge transfer from the excited Ru complex to oxygen, and the relative and absolute values of these rate constants are discussed.Introduction. ± Although the charge-transfer nature of the lowest excited state of tris(bipyridine)ruthenium(II) ([Ru(bpy) 3 ] 2 ) and related compounds [1] [2] is well established, it is clear that quenching by oxygen of the lowest excited state of these complexes, at least in nonaqueous solvents, results in efficient energy transfer since they have been shown to be good photosensitizers of singlet oxygen [3 ± 14]. The relatively long lifetime of the triplet, metal-to-ligand charge-transfer states 3 MLCT of many Ru II complexes results in oxygen quenching in air-saturated solutions being quite efficient [3 ± 14] since rate constants k q for oxygen quenching of these 3 MLCT states are typically in the range 10 8 ± 10 10 dm 3 mol À1 s À1 . However, there have been relatively few publications on the mechanism of quenching of electronically excited states of coordination complexes by O 2 compared with the hundreds of such studies made with organic molecules [15 ± 39]. We would like to dedicate this paper to AndreÂ M. Braun on the occasion of his 60th birthday since he is joint author of an important paper [6] in this field, on oxygen quenching of 3 MLCT states of a series of homoleptic [RuL 3 ] complexes (where L is 2,2'-bipyridine (b...

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Charge Transfer Interactions in the Generation of Singlet Oxygen O₂(¹Δ_g) by Strong Electron Donors

Cited by 51 publications

References 72 publications

Photosensitized Generation of Singlet Oxygen

Photosensitized Generation of Singlet Oxygen

Photoinduced Electron Transfer in Photorobust Coumarins Linked with Electron Donors Affording Long Lifetimes of Triplet Charge‐Separated States

Photosensitized Generation of Singlet Oxygen from (Substituted Bipyridine)ruthenium(II) Complexes

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