Photo triplet mechanism

Time-resolved CIDEP and optical emission studies provide further definitive characterization of the triplet and excited singlet states followed by their primary photochemical reactions producing transient radicals in individual mechanistic steps in the photolysis of a-guaiacoxylacetoveratrone. Both fluorescence and phosphorescence are observed and CIDEP measurements confirm the mainly n,n character of the lowest triplet state. The results indicate a photo triplet mechanism involving the formation of the ketyl radical prior to the P-ether cleavage to form phenacyl radicals and phenols. Indirect evidence of excited singlet photo decomposition mechanism is observed in the photolysis at 77 K. 

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. 

Arylation is possible by the addition of triplet aryl cations to neutral nucleophiles such as alkenes, alkynes, enols, and enamines (photo SN1 mechanism). For alkenes, the... 

In a time-resolved experiment, the intrinsic polarization of the system is measured with very little correction except for power saturation (98,99). To measure the initial polarization, the signal heights S and Sq (hence the enhancement factor V and the polarization factor P = /So) can be obtained directly from the time profile. For a photo-CIDEP system induced only by the triplet mechanism and the triplet reaction involves a substrate SH, a kinetic analysis yields the following relationship ... 

Triplet Excited-State Properties of Curcumin and Its Implications in Elucidating the Photo Sensitizing Mechanisms of the Pigment. 

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. 

Scheme 6 Photo-Arbuzov rearrangement of arylethylphosphites via a long-lived triplet proximate radical pair mechanism. Reprinted with permission from [22]. Copyright 2001 American Chemical Society...

The fact that dynamic 13C polarization is only possible through the indirect way via tire 1H spins suggests the mechanism of polarization transfer. Since the polarization transfer between the electrons and nuclei are driven by the dipolar interactions between them, and the fraction of the guest triplet molecules was small, it would be natural to assume that the polarization of the electron spins in the photo-excited triplet state is given to those H spins which happen to be close to the electron spins, and then the 1H polarization would be transported away over the whole volume of the sample by spin diffusion among the 1H spins. 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

Photocycloadditions are common and usually involve diradical intermediates photo-excited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and the triplet (n, it ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

Since a sensitizer (vide infra) is used in the reaction, it may also be called a photosensitized reaction. (Any photochemical reaction in which the exciting light is absorbed by some species other than the principal reactants.) Putting everything together generates the imposing descriptive name, photosensitized cycloaddition reaction. If the steps which constitute the mechanism of the reaction are discussed in detail, the prefix photo- becomes superfluous and the individual steps will be described in ordinary chemical terms. For example, if it is decided that the mechanism of the above reaction involves addition of butadiene triplets to butadiene, that step of the reaction would be simply called cycloaddition. 

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