Flash studies

Mathis, P., Ikegami, I. and Setif, P. 1987. Nanosecond flash studies of the absorption spectrum of the Photosystem I primary acceptor A0 (submitted). 

In a classical flash study, Kok et al. showed that the 02-evolving complex of photosystem II is oxidized sequentially in a four-quantum, four one-electron oxidative process, the steps being named S0-S4, with S0 representing the totally reduced form (186). In the S state model (Scheme 1), S4 is only transiently stable, rapidly converting to S0 with concomitant 02 evolution. 

A laser flash study of the photoreactions of hexan-2-one and 5-methylhexan-2-one has provided evidence for the existence of the triplet 1,4-biradicals produced by the y-hydrogen abstraction typical of Norrish Type II reactivity. The photochemical behaviour of the alkanone, nonan-5-one, in urea inclusion compounds has been studied. In solution, irradiation of nonan-5-one yields hexan-2-one, propylene, and two cyclobutanols. In the clathrate, the fragmentation products were essentially the same but only one cyclobutanol was observed. The cyclization fragmentation ratio was established as 0.67, compared with 0.32 in methanol. The authors suggest that the CIS-cyclobutanol has less stringent rotational requirements and that it is this isomer (43) which is formed in the clathrate. 

The results of early flash studies were irreproducible and led to various conclusions concerning the nature of the primary photoproduct. For example, flash photolysis of Cr(CO)g in room temperature degassed cyclohexane solutions and in polystyrene film was reported to produce a transient having a maximum absorbance in the visible at 450 nm and which decayed with second-order (equal concentration) kinetics (83). The following scheme was proposed for the overall mechanism ... 

Quenching of Ru(bipy)32+ by the organic cations trans-1,2-bis(N-methYl-4-pyridyl)ethylene and l,l -dimethy 1-4,4 -bipyridine (paraquat ) and by the transition metal ion complexes Fe(H20)g and Ru(NH3) 3+ occurs at near diffusion controlled rates (241). In all four cases flash studies indicated that electron transfer is a principal component of the quenching mechanism. Selective excitation of Ru(bipy)3 in the presence of these quenchers (Qqx produced the oxidized ruthenium complex, Ru(bipy)33+, and the reduced quencher, Qj-gd quenching... 

Plots of 1/ N2 versus 1/[N20] were linear and in all cases the limiting quantum yield of N2 production was significantly less than unity. Mechanisms involving photoejection of electrons from ferrocene were shown to be unlikely by flash studies. Thus, it was concluded that an excited state of ferrocene, other than that obtained initially by photolysis at 254 nm, is quenched by electron transfer to N2O. 

Suginome and coworkers have described the photochemical (2+2)-cycload-dition of alkenes to 2-acetoxynaphtho-l,4-quinone. The resultant adducts can be converted into the corresponding cyclobutanols which react with mer-cury(II) oxide/iodine to afford a cyclobutanoxyl radical. A laser-flash study has examined the photochemical behaviour of vitamin K3. This investigation sought to provide details for the hydrogen atom abstraction reactions in this system. ... 

Decarboxylation of (175) occurs on its irradiation in an argon matrix at 10 K using 254 nm light. Spectroscopic analysis of the resulting matrix indicates the presence of a complex between carbon dioxide and the carbene (176). Tiaprofenic acid (177) undergoes facile photochemical decarboxylation, and this is reported to take place from an upper triplet excited state." A study of the transient photochemistry of 5-(p-toluyl)-l-methyl-2-pyrrolylacetic acid has been reported. Decarboxylation results in the formation of a carbanion in its triplet state. A laser-flash study using irradiation at 266 nm of the xanthene-9-carboxylate (178) has shown that the radical (179) is formed. This study used NaY zeolites and studied the oxidation of the radical within the cage structure. Calculations have indicated that decarboxylation of (180) and (181) and deprotonation of cycloheptatriene and cyclopentadiene affords the same anions (182) and (183), respectively. ... 

The vinyl quinones (248) undergo different photochemistry. When these are irradiated a quantitative yield of dimers (249) is obtained. A report has been published dealing with the photocyclisation reactions of benzoyl benzo-1,4-quinones. A laser flash study at 248 nm of vitamin K3 has been reported and... 

A nanosecond flash study of the hydrogen abstraction processes of ace-naphthenequinone has been carried out. " ... 

A laser flash study of the acridinethione (129) has shown that the triplet state is populated on irradiation at 355 nm and electron transfer from... 

The irradiation of acetanilide in the cavity of zeolites (X, Y and P) results in the formation of o-aminoacetophenone as the principal product. A laser flash study of the kinetics of rearrangement of the triazine derivative (212) into the isomerised product (213) has been reported. In particular the 1,3-hydrogen migration was studied to establish the degree of quantum mechanical tunnelling involved. The results obtained indicate that tunnelling at two vibrational levels is involved. 

The involvement of a-and 7C-type dimeric radical-cations in one electron transfer to the aromatic sulfides (259) has been assessed. A laser-flash study of the behaviour of p-nitrobenzenethiol has shown that S-H fission is the dominant reaction with the formation of the corresponding thiyl radical. This occurs particularly when the irradiations are carried out in nonpolar solvents. The reactions encountered in polar solvents are different. Under these conditions the triplet state of p-nitrobenzenethiol is involved and this undergoes ready deprotonation. The amino acid derivatives (260) can be desulfurised by irradiation using triethylboron and triethylphosphite. ... 

Photo-decarbonylation of the cyclohexanone (32a) is efficient with a quantum yield of 0.9. The reaction yields the two products (33) and (34) in a ratio of 1 2. The cyclopentanone (32b) also decarbonylates photochemically but is less efilcient with a quantum yield of 0.5. A laser flash study has been carried out on these systems and has identified the biradicals produced by the Norrish type I process. The lifetime of the biradicals (35a) and (35b) are O.Ojxs and 0.5 xs respectively. In a related study the photodecarbonylation of cIs- and frans-2,6-diphenylcyclo-hexanone has been shown to yield a mixture of cis- and 2-diphenyl-cyclopentane and cIs- and... 

A laser-flash study has been used to measure the rate constants for the quenching of indane-l,2,3-trione and 5-methoxyindane-l,2,3-trione by al-kenes. ... 

The sulphonamides 226 are photochemically reactive and on irradiation in benzene or methanol afford the ring-expanded products 227. The process involves S—N bond fission to the biradical 228, which ultimately rebonds to yield 229. A 1,3-hydrogen migration completes the reaction affording the products in 50-70% yields. A laser flash study has identified the presence of long-lived transients190. 

The photochemical aspects of carbonyl photochemistry remain important subjects of research. Wagner and Thomas have used CIDNP to elucidate radical formation from a,a,a-trifluoroacetophenone. Irradiation of benzophenone and its derivatives in the presence of molecules with abstractable hydrogen atoms can give rise to intensely fluorescent compounds. This effect may interfere with the observation of nanosecond-domain kinetics.Quantum yields and kinetic isotope effects in nanosecond flash studies of the reduction of benzophenone by aliphatic amines have been measured by Inbar et Rate constant data are given in Tables 13 and 14. Winnik and Maharaj have studied the reaction of benzophenone with n-alkanes through hexane to hexatriacontane is 3.9 0.2kcal for all chain lengths.The effects of substituents on the benzophenone on these reactions have also been examined. The reactions of phenylacetophenone when used as polymerization initiator have been reviewed by Merlin and Fouassier. ... 

Evidence for orthogonally twisted triplet states of enones has come from laser flash studies, though the precise degree of twist in particular cases is dependent on the flexibility of the molecule (Gioia et al.). The rates of unimolecular decomposition of tetramethyldioxetane from various vibrational levels to ground- and excited-state acetone molecules have been measured by an elegant procedure (Cannon and Crim). 

A laser flash study of the photochemistry of acetone has examined the polarization induced by irradiation. The author suggests that this study shows the... 

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