Sensitization triplet mechanism

Notice that this is the same as the mechanism written in Section II except that step (9), self-quenching, has been added. If we assume that the quantum yield of intersystem crossing for the sensitizer is unity and that all the sensitizer triplet is quenched either by the acceptor or by ground state sensitizer, then the following expression for 4>a, the actual quantum yield for reaction of the substrate triplet, can be derived ... 

Hutton and Stevens49 have observed that while naphthacene vapor alone produces no delayed fluorescence, pyrene vapor can sensitize the delayed fluorescence of naphthacene vapor. It is true that they explained their results in terms of a long-lived excited dirtier but in a recent private communication Stevens has agreed that delayed fluorescence in the vapor state is probably also produced by triplet-triplet quenching and hks suggested that the results with mixtures of pyrene and naphthacene vapor provide evidence for the occurrence of the mixed triplet mechanism. 

Optically active aryl sulfoxides undergo facile photoracemiza-tion.472 The photo-inversion is sensitized by naphthalene but only partially quenched by piperylene.473 The quenching action was originally interpreted as indicating a triplet mechanism for isomerization. 

Further support for a photochemical triplet mechanism in a rather different system is that the compound 17 gives a vinylcyclopentcnone (by way of intramolecular cycloaddition followed by electrocyclic ring-opening) only on triplet sensitization (equation 45) . 

Sensitized photooxidation of sulfur-containing amino acids in neutral aqueous solution occurs via electron transfer from the sulfur to the triplet state of the sensitizer (the mechanism can be adapted to other dye-sensitized photoinitiated polymerization). Electron transfer quenching in the system is followed mainly by diffusion of the sulfur-centered radical cation and benzophenone radical anion. Laser... 

Stilbene 1 undergoes isomerization in the singlet excited state on direct irradiation [13,16-23]. The mechanism was established by J. Saltiel et al. from the experiments of the azulene effect on the photostationary state isomer composition [13,24,25]. Thus, usually, quenching experiments as well as observation of the transient spectra are necessary to distinguish the singlet and triplet mechanisms of isomerization of olefin on direct irradiation. Furthermore, triplet sensitization is necessary to study the isomerization in the triplet manifold. 

On sensitized irradiation with carbonyl compounds such as benzophenone, it is expected that the triplet state of olehns will be produced. However, a radical chain mechanism is sometimes discussed when benzil or biacetyl is used as a sensitizer. Thus, some other experimental evidence such as direct observation of the triplet state by laser photolysis is needed to establish the triplet mechanism even on triplet sensitization. 

Inoue et al. have described their study of the singlet sensitized isomerization of cyclo-octene. They report that the use of methyl benzoate or other aryl esters results in the anomalously high trans-cis ratio of 0.25. This result is only anomalous if a triplet mechanism is operative but the authors have shown that in this instance a singlet state is the active species. The same sensitizer has been used in a study of the photochemical behaviour of the diene (44), which is converted into the isomer (45) and subsequently into (46). ... 

In the cis - trans one-way isomerization of anthrylethenes. Figure 2 illustrates a typical example of the concentration dependence of the quantum yields of cis - trans isomerization of 6a. increases with the increase in cis-isomer concentration [4,18-20]. The isomerization proceeds in the quantum chain mechanism described in Scheme 2, where S is the sensitizer triplet and the k s are the rate constants for the corresponding processes. 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

It consists of thioxanthone or 2-isopropylthioxanthone in combination with (2,4,6-trimethylbenzoyl)-diphenyl-phosphine oxide or bis(2,4,6-trimethyl-benzoyl)-phenylphosphine oxide. The mechanism of sensitization was reported to involve triplet-triplet energy transfer from the thioxanthones to die phosphine oxides. That is followed by formation of radicals through a-cleavage of the photoinitiators. Direct photolysis of the phosphine oxides results in an absorptive, chemically induced, d)mamic electron polarization due to the triplet mechanism of polarization of the substituted benzoyl. The phosphoras-centered radicals produced by a-cleavage of the photoinitiators are the same radicals that... 

Quinones. The method of triplet-sensitized electron transfer which produces spin-polarized radicals via the CIDEP triplet mechanism with enhanced line intensities allows their detection on the nanosecond time scale at room temperature by FT EPR. The reaction scheme shown in Figure 4 describes the generation of the radicals by the CIDEP triplet mechanism. In the laser photolysis of the acceptor A, the triplet state Aji is generated by efficient intersystem crossing (isc) from the excited singlet state Asi ( denotes spin-polarized transi-... 

The triplet-sensitized photo-decomposition of azocumene into nitrogen and cumyl radicals was investigated by TR EPR and optical spectroscopy. " The cumyl radicals observed carry an initial spin polarization and are formed with a yield which depends on both the solvent viscosity and the strength of the external magnetic field. This phenomenon is interpreted in terms of a depopulation-type triplet mechanism, i.e. competition between decay into radicals and fast triplet sub-level intersystem crossing" back to the azocumene ground state. 

Fujita and co-workers reported the photochemical reaction of allenes in acetic acid. Irradiation (254 nm) of 14a-e affords the respective enol acetates 15. In the reaction of 14b, a triplet mechanism is proposed on the basis of quenching experiments. In methanol solution, on the other hand, the photochemical reaction of tetraphenylaUene 16, using Corex or Pyrex filters, was reported by Klett and Johnson to yield the allylic methyl ether 17 as the major photoproduct along with 18 and 19 (in ratios of 10 1 1). Similar reactions of triphenylallene 20 and 1,3-diphenylallene 21 also afford aUyl ethers 22,23 (5 1), and 24, respectively. In these reactions, a mechanism involving a strongly polarized planar singlet excited state is proposed on the basis of triplet sensitization experiments. 

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