Benzophenone triplet sensitizer

However, while 313 and 366 nm direct irradiation yields a cis- trans quantum yield of-0.005, the benzophenone triplet sensitized and the low energy direct irradiation yield is only —0.001. These results support the notion that two reactive excited states are involved one associated with the styrylferrocene which leads to the more efficient isomerization and a ferrocene-type excited state at lower energy which has a smaller tendency to undergo cis -trans isomerization. It is interesting to note that the 100% trans photostationary state is likely very close to the amount expected at thermal equilibrium. Such a result would be consistent with an intramolecular CT transition to yield a radical-anion ferricinium complex as in reaction (47). The radical anion could yield cis trans isomerization just as in the Ru(II)-styrylpyridine complexes. The wavelength dependence of the isomerization of the styrylferrocene is consistent with (47) in that upper excited states are required for the photooxidation of ferrocene to ferricinium in CC14.135 ... 

Figure Bl.16.2. X-band TREPR spectra obtained at 0.1 ps after 308 mn photolysis of a fliiorinated peroxide dimer in Freon 113 at room temperature. Part A is the A/E RPM spectrum obtained upon direct photolysis part B is the E/A RPM spectrum obtained upon triplet sensitization of this reaction using benzophenone.

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. 

Polymers are often side products. Photochemical addition of aldehyde to conjugated C=C units can be efficient when a triplet sensitizer (p. 316) such as benzophenone is... 

The Diels-Alder reaction of isopyrazoles 365 with MTAD gives azoalkanes 366. Direct as well as triplet-sensitized (benzophenone) photolysis of these compounds leads to the corresponding housanes (bicyclo[2.1.0]pentanes) 367. Under acidic conditions, the housanes rearrange to the corresponding bicyclic products 368 <1995JOC308,... 

Though the triplet sensitized photolysis of isoprene (159) does, as noted above, produce a complex mixture of products, one of these adducts has been used in the context of complex molecule synthesis (equation 5)71. Cyclobutane 160, which was formed in ca 20% yield by the benzophenone sensitized photolysis of 159, could be easily transformed into fragrantolol, 161, an isomer of grandisol isolated from the roots of the Artemisia fragrans, by simple hydroboration/oxidation of the less hindered double bond. 

In an attempt to sensitize the thiosulfate bond cleavage, benzophenone (10% by weight) was incorporated into the polymer film. Upon photolysis at 366 nm, the 639 cm 1 thiosulfate band was reduced (Figure 10) as in the case of direct photolysis at 254 nm and 280 nm. Since benzophenone is a known triplet sensitizer it is likely that the S-S bond cleavage in the thiosulfate group occurs from a triplet excited state in the sensitized reaction. Incidentally photolysis of a PATE film at 366 nm in the absence of benzophenone resulted in no loss of the 639 cm 1 IR peak. Unfortunately due to the film thickness, we were unable to obtain accurate quantum yields for either the direct or sensitized photolysis. Finally it should be noted that no chemical evidence has been presented to confirm disulfide formation. Results from the photolysis of a PATE-type model compound will be offered to substantiate the claim of disulfide formation as well as quantitate the primary photolysis step. But first, we consider photolysis of a PASE polymer film. 

Preparative photolysis of AETSAPPE (0.25 M aqueous solution) at 254 nm (Rayonet reactor) resulted in the formation of the disulfide product 2-amino(2-hydroxy-3-(phenyl ether) propyl) ether disulfide (AHPEPED) as the primary photoproduct Photolysis of AETSAPPE at 254 nm (isolated line of medium pressure mercury lamp) resulted in rapid initial loss of starting material accompanied by formation (analyzed by HPLC) of AHPEPED (Figure 12a and 12b) (Scheme IV). Similar results were obtained for photolysis- at 280 nm. Quantum yields for disappearance of AETSAPPE and formation of AHPEPED at 254 nm and 280 nm are given in Table I. The photolytic decomposition of AETSAPPE in water was also accomplished by sensitization ( x =366 nm) with (4-benzoylbenzyl) trimethylammonium chloride (BTC), a water soluble benzophenone type triplet sensitizer. The quantum yield for the sensitized disappearance (Table I) is comparable to the results for direct photolysis (unfortunately, due to experimental complications we did not measure the quantum yield for AHPEPED formation). These results indicate that direct photolysis of AETSAPPE probably proceeds from a triplet state. 

The use of ketones with (ir, n) triplet states to sensitize photoreductions will be attended by chemical sensitization for example, with benzophenone in isopropyl alcohol and 0.5M acceptor, 1% of the sensitizer triplet will still abstract from the solvent even if the acceptor quenches at the diffusion controlled rate. Failure to determine that the quantum yield of reduction was greater than 0.01 might lead to the conclusion that triplet sensitization was occurring. 

Backstrom, Steneryr, and Perlmann18 found that in the liquid-phase benzophenone-sensitized decomposition of i-C3H7OH at 25°C, the yields were reduced by adding NO. They interpreted their results in terms of quenching of the benzophenone triplet state by NO. 

Takenaka and Lemal subsequently undertook an extensive study of the perfluorobenzene oxide (3)/perfluorooxepin (4) system.13,14 After considerable effort, benzene oxide 3 was successfully prepared by ozonization of tricyclic precursor 5 and photolytic decomposition of the resultant ozonides. Oxepin 4 was not observed directly, but was estimated by NMR computer simulations to be present in small proportion (3 %) at 55"C. The principal reaction of 3 is its rearrangement to cyclohexadienone 2, which occurs at room temperature in polar solvents, by heating in nonpolar solvents, or in the presence of Lewis acids. Photolysis of 3 with benzophenone as triplet sensitizer also produces 2. Furthermore, attempts to trap the oxepin, for example by treatment with bromine in the dark, result in cyclohexadienone 2. 

Photoisomerization of alkenes via the triplet excited state is known to be possible by triplet sensitization, usually efficient in conjugated C=C bonds that fulfill the requirement of possessing triplet excited energies below those of the typical triplet sensitizers such as acetone, acetophenone, benzophenone, etc. (Table 2). Sensitization with the opposite order of triplet excited energies is possible in cases with strong electronic or strong... 

Thophene gave a 2+2 cycloaddition reaction when irradiated in the presence of maleic anhydride (Scheme 8) (63CB498 70 JCS(CC)1474 70MI28 71MI9 73JCS(P1)2322 86JP(32)363). But performed in the presence of benzophenone as triplet sensitizer, the main product was the Paterno-Biichi adduct. 

The study of the direct irradiation and triplet sensitized isomerization of styryl-ferrocene, reaction (46), is noteworthy.134) For either direct or benzophenone sensitized photoisomerization the photostationary state is exclusively the trans isomer. 

Schemer s observations (1964) on spin inversion and bond rotation in nitrogen-containing diradicals lends support to Skell s view. Direct photolysis of the triazolines 42 and 43 gives the related aziridines, 44 and 45 with predominant retention of the geometric arrangement of the methyl and adjacent phenyl substituents. Photolysis in the presence of a triplet sensitizer, benzophenone, results in a product distribution showing much lower stereoselectivity. The inference is that...

Direct irradiation at 254 nm of 1-phenylpropene 93a in cyclohexane affords a Z/E mixture in a ratio of 65 35 [223], whilst photosensitized isomerization of 93a gives (Z/E)pss ratios of 55 45 and 88 12 upon benzophenone and chrysene sensitization, respectively (Sch. 37). Similarly, a variety of P-alkylstyrenes has been subjected to triplet sensitization [224,225]. Upon direct irradiation of ( )-P-bromo-P-nitro-styrenes, the (Z)-isomers are obtained in 40-50% yield, but the major... 

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