Triplet photodecomposition

Singlet as well as triplet photodecomposition has been reported. 

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

In addition to the photoxygenation/diimide (equation 6),16) silver salt (Eq. 22), 36) and triflate (Eq. 44)s6> routes, 9 has also been prepared by benzophenone-sensitized photodecomposition of the corresponding azo compound 59 and trapping of the resultant triplet diradical with oxygen (Eq. 45) 57). 

Photodecomposition accompanying directly excited delayed fluorescence will clearly be less obvious. For example, at the same rate of light absorption in a 5 X 10-SM solution of anthracene, and with the same quantum efficiency of photodecomposition, a period 1000 times as long would be required to produce the same percentage decomposition. It was in fact found that after prolonged irradiation, a deaerated solution of 5 X 10-BM anthracene was almost completely decomposed and a crystalline precipitate was formed which had the appearance of dianthracene. It seems not unlikely therefore that one or other of the excited dimers formed by triplet-triplet quenching may be partly converted into a stable dimer. If this is so, mixed triplet quenching should result in the formation of stable mixed dimers. Experiments are in hand to test these hypotheses. 

Photodecomposition of a 4-nitrophenyl moiety is a multiphoton process absorption of one photon leads to the singlet, which relaxes immediately into a long-lived triplet state. Capturing another photon transforms the singlet through a dissociative decay process into reactive intermediates, probably radicals, which react with nucleophilic sites. In the absence of a reactive species, these reactive intermediates will relax into the initial ground state 23 ... 

The potential use of the reaction was examined, but the episulfides decompose partially during the process, and the yield of cyclohexene episulfide under optimum conditions was 20%. Since episulfides may be obtained in much higher yields from the photodecomposition of COS to give triplet sulfur atoms in gas-phase reactions,26 the method hardly has synthetic importance. It was assumed that the low yield is due to deactivation of the sulfur atoms by collision with solvent molecules, causing them to polymerize to molecular sulfur instead of reacting with cyclohexene.22... 

ISC is deemed to be very efficient in aniline and even more with chloro-substituted compounds (OISc ca. 0.9, hsc > 1 x 109 s ). Thus, photodecomposition of 178 is expected to occur in the triplet manifold and to give cation 180 in the triplet state. 

Note that in all the cases considered in Table 2 the ethyl protons of the initial Et3GeCOPh demonstrate positive polarization (A). Therefore, the analysis of these effects in accordance with the existing rules11 allows us to conclude that partially reversible photodecomposition of the ketone both in the presence and in the absence of the radical traps occurs from the triplet excited state with the formation of the triplet radical pair comprised of Et3Ge and COPh radicals. The analysis employed the following -factor and hyperfine interaction values of the radicals g(Et3Ge ) = 2.0089, g( COPh) = 2.0008 and Ah(CH2) < 0.5 mT (for Et3Ge )12. 

Under these conditions photochemical decomposition from the singlet or the triplet state of azo compounds should lead to a different product ratio. This phenomenon is called the spin correlation effect. The simple scheme representing the photodecomposition of azo compounds (see above) has to be extended further if the complete mechanism has to be dealt with. 

Inter- and intramolecularly sensitized enantiodifferentiating photodecom-j positions of pyrazoline derivatives 19t and 23 were also examined [12]. Th triplet sensitized photodecomposition of frvms-3,5-diphenylpyrazoline 19t witty (— )-rotenone 21 and (4- )-testosterone 22 afforded chiral trans- and achiral cis 1,2-diphenylcyclopropanes 20t and 20c (Scheme 3, top). The enantioselectivity ... 

Kumar and Huber (129) have studied the phosphorescence, fluorescence, and photodecomposition of propynal as a function of excitation energy up to an excess vibrational energy of 6000 cm-- -. They find that the phosphorescence yield is constant until an excess energy of 2952 cm- - is reached, after which a sharp decrease in 4>p with increasing energy is noted. With the pressures used (0.6 torr propynal), any triplets formed will be... 

Photodecomposition. As reviewed by Cundall and Davies (61), both of these absorption systems have been studied, and product yields resulting from excitation from 435.8 to 238 nm have been reported (22,30,176). Since the last review by Cundall and Davies (61), most studies have dealt with the A X system, since both singlet and triplet emission have provided valuable tools in the exploration of biacetyl photochemistry (see ref. 185). Three photodecomposition processes have been proposed for biacetyl ... 

When more than one triplet state is available below the first excited singlet, it becomes more complicated to establish which of the triplet states is mainly responsible for the observed reaction. A classical example is the photodecomposition of 4-benzoylbiphenyl (109-111). Some of the interesting features known are that 4-benzoylbiphenyl has two excited triplet states below the first excited singlet and that the lowest triplet is characteristic of a it, it state. Turro and Lee (122) have shown that this triplet has a much lower reactivity than its corresponding n, it triplet abstraction reaction of benzophenone. 

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