Sensitized Photooxygenation According to Type I Mechanism (Radical Mechanism) — Part Ii. Flash Photolysis Experiments

Kramer, Horst E. A.; Maute, Alfred

doi:10.1111/j.1751-1097.1972.tb06219.x

Cited by 12 publications

(2 citation statements)

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“…There are two basic sensitizing mechanisms: Electronic energy transfer or electron transfer between the dye triplet and some substrate (for a competition of both see e. g. Ref. [18]). To elucidate the redox mechanism reactions with several dye triplets and many electron donors have been investigated in this laboratory during the last years [19,20].…”

Section: The Probe Function Of Heavy Atom Substimentsmentioning

confidence: 99%

Heavy Atom Substituents as Molecular Probes for Solvent Effects on the Dynamics of Short‐lived Triplet Exciplexes

Winter

Steiner

1980

Ber Bunsenges Phys Chem

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The influence of heavy atom substituents (Br, I) in the electron donor aniline on the electron transfer reaction with thiopyronine triplet is investigated by flash spectroscopy in solvents of different viscosity and polarity. Triplet quenching constants and radical yields are determined. The results are analysed in terms of decay constants of an intermediate triplet exciplex where the heavy atom substituents significantly enhance the intersystem crossing process leading to singlet ground state formation and thus diminishing the radical yield due to exciplex dissociation. The sensitivity of the radical yield to solvent effects is strongly enhanced by heavy atom substitution though the solvent effects proper on the exciplex dynamics are substituent independent. The heavy atom effect is used to study the solvent cage effect on exciplex dissociation by means of viscosity variation. The exciplex dissociation lifetime is proportional to solvent viscosity, however, it contains a constant contribution which may be attributed to exciplex bonding. By means of the heavy atom probes it is found that increasing solvent polarity intersystem crossing in the exciplex is favored over exciplex dissociation into radicals. A tentative explanation is given in terms of solvent polarity dependence of the electronic energy gap between exciplex and corresponding Franck-Condon ground state. (1) and (2) it follows that the radical yield is determined by three factors, characterizing the behaviour of the triplet exciplex (Eq. (4» where 'Hal is the atomic spin-orbit coupling constant of the halogen and C 2 (POSHaJ is the Hiickel spin density transferred to the position of the halogen atom by the electron transfer. The latter factor readily explains the positional dependence of the substituent effect (para > ortho > meta).(1)The rate determining step in the quenching of the excited cationic dye triplet eA +) by the electron donor (D) is the formation of a triplet exciplex with radical-pair-like electronic structure e(A'D +». The triplet exciplex has two decay channels: dissociation to the free radicals (rate constant k fr ) and intersystem crossing with back transfer of the electron to the donor leading to the formation of the ground state of the reaction partners (rate constant k isc ) ' The radical yield observed is determined by the two decay constants of the exciplex:triplet exciplexes can be detected by their phosphorescence under such conditions. In a recent paper [22] we provided indirect evidence for the formation of triplet exciplexes as intermediates in electron transfer reactions. We studied the reaction of thionine triplet as an electron acceptor with aniline and its monohalogen derivatives as electron donors in methanol. In these reactions the halogen substituents have systematic influence on the radical yield (extrapolated to complete triplet quenching) which could be explained satisfactorily by the following mechanism, strongly supported by a recently determined magnetic field effect [23]:The analysis of the radical yield...

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Section: The Probe Function Of Heavy Atom Substimentsmentioning

confidence: 99%

Heavy Atom Substituents as Molecular Probes for Solvent Effects on the Dynamics of Short‐lived Triplet Exciplexes

Winter

Steiner

1980

Ber Bunsenges Phys Chem

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“…Type I sensitization of oxidation is called "Backstrom-type photosensitized oxidation" or "primary dehydrogenation photosensitized reaction with oxygen". Type I photoreactions are discussed for the photodynamic degradation of nucleic acids and nucleic acid constituents (Simon and Van Vunakis, 1962;Wacker et al, 1964;Grossweiner, 1969;Knowles, 1967;Kepka and Grossweiner, 1971;Grossweiner and Kepka, 1972;Berg and Gollmick, 1974;Kittler and Lober, 1974), of model substrates like allylthiourea (Koizumi et al, 1964;Kramer and Maute, 1972a;1972b;Zugel et al, 1972) and p-toluenediamine (Gollrnick and Berg, 1968). The oxidation half-wave potentials as measured polarographically by means of a paste electrode are shown for various sensitizer dyes, pyrimidine and purine derivatives in Table 5.…”

Section: (D) Photodynamic Actionmentioning

confidence: 99%

Selected Topics in Photochemistry of Nucleic Acids, Recent Results and Perspectives

Löber¹,

Kittler²

1977

Photochem & Photobiology

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Protonation‐State‐Driven Photophysics in Phenothiazinium Dyes: Intersystem Crossing and Singlet‐Oxygen Production

et al. 2017

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The impact of altering the solvent pH value on the photodynamic activity of thionine has been studied computationally by means of density functional theory and multi‐reference interaction methods. To this end, we have investigated the electronic structure of the ground and excited states of diprotonated (TH22+) and neutral imine (T) forms of thionine (TH+). It is well known experimentally that the T1 state of TH+ undergoes acid–base equilibrium reactions resulting in a pronounced pH effect for the efficiency of singlet‐oxygen (1O2) production. Our results show that the energy‐transfer reactions from the T1 state of TH22+ and T to 3O2 correspond to reversible equilibrium processes, whereas in TH+ this process is very exothermic in a vacuum (−0.66 eV) and in aqueous solution (−0.49 eV). These facts explain the experimental observation of a much smaller efficiency of 1O2 production for TH22+ than for TH+. Moreover, we found that the pH value significantly effected the intersystem crossing (ISC) kinetics impacting the concentration of triplet‐state species available for energy transfer. In very acidic aqueous solution (pH<2) where TH22+ is the prevailing species, the ISC proceeds with a rate constant of ≈108 s−1. In a basic medium where T is the dominant species, ISC decay occurs by means of a thermally activated channel (≈108 s−1) which competes with fluorescence (5.32×107 s−1). According to these results, maximum ISC efficiency is expected for intermediate acidic pH values (TH+, ≈109 s−1).

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Sensitized Photooxygenation According to Type I Mechanism (Radical Mechanism) — Part Ii. Flash Photolysis Experiments

Cited by 12 publications

References 3 publications

Heavy Atom Substituents as Molecular Probes for Solvent Effects on the Dynamics of Short‐lived Triplet Exciplexes

Heavy Atom Substituents as Molecular Probes for Solvent Effects on the Dynamics of Short‐lived Triplet Exciplexes

Selected Topics in Photochemistry of Nucleic Acids, Recent Results and Perspectives

Protonation‐State‐Driven Photophysics in Phenothiazinium Dyes: Intersystem Crossing and Singlet‐Oxygen Production

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