Acceptor Photolysis

A recent report by Kennedy and Diem describes the promotii effect of a glow discharge (Telsa coil) or ultraviolet irradiation iq)on the low temperature polymerisation of isobutene by titanium tetrachloride. The authors ascribed the yield increases observed to the formation of cocatalytic amounts of chlorine or hydrogen chloride arising from the decomposition of the catalyst into TiCla and a chlorine atom. The interpretation is probably valid, but we point out that HQ is not a cocatalyst for the polymerisation of i )butene by TiCl4 in hexane, one of the solvents used in that work  

We prefer to think that a combination of factors, difficult to control or separate, should be held responsible for these phenomena apart from the formation of cocatalytic substances arising from chlorine atoms, it is in fact known that both the glow discharge and ultraviolet light can effectively degas the vessel walls introducing new impurities (mois- 

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These molecules form the usual complexes with metal acceptors. Photolysis of the E-isomer of the diphosphene (163) using a mercury lamp with a pyrex filter results in the formation of a mixture of and Z- isomers in equilibrium. However, photolysis... 

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

This brings us to U(VI) as an electron acceptor in MMCT transitions. A few examples were mentioned above already. Krol et al. have shown and used the U(V)-U(VI) MMCT transition in oxygen-deficient uranates [78]. It is situated in the near-infrared. This transition plays an important role in the quenching of the luminescence of uranates. The luminescence of U(VI)02 in solution is quenched by Mn(II) and it has been shown by flash photolysis that this is due to an excited MMCT state Mn(III)U(V) [79]. 

When a A = 1849 A light acts on an ammonia molecule, the latter breaks into a hydrogen atom and an NH2 radical [13]. At the zinc oxide surface the former particle is an electron donor, whereas the latter one is an acceptor. Experiments indicate that in photolysis of ammonia in a vessel shown in Fig. 4.6. only hydrogen atoms can be detected at every level of the vessel, starting from the source. This experimental result can be accounted for by the fact that, even in presence of such acceptors as... 

The prototype o-quinone methide (o-QM) and / -quinone methide (p-QM) are reactive intermediates. In fact, they have only been detected spectroscopically at low temperatures (10 K) in an argon matrix,1 or as a transient species by laser flash photolysis.2 Such a reactivity is mainly due to their electrophilic nature, which is remarkable in comparison to that of other neutral electrophiles. In fact, QMs are excellent Michael acceptors, and nucleophiles add very fast under mild conditions at the QM exocyclic methylene group to form benzylic adducts, according to Scheme 2.1.2a 3... 

Porter and Wilkinson(56) measured the rates of quenching for a variety of triplet donors with triplet acceptors at room temperature in fluid solution by flash photolysis. The appearance of the triplet-triplet absorption spectrum of the acceptor and the simultaneous disappearance of the donor triplet-triplet absorption spectrum provided unequivocal evidence for the triplet-triplet energy transfer process. Table 6.5 provides some of the quenching rate constants reported in this classic paper. 

Another method to detect energy transfer directly is to measure the concentration or amount of acceptor that has undergone an excited state reaction by means other than detecting its fluorescence. For instance, by chemical analysis or chromatographic analysis of the product of a reaction involving excited A [117, 118]. An early application of this determined the photolyzed A molecules by absorption spectroscopic analysis. [119-121], This can be a powerful method, because it does not depend on expensive instrumentation however, it lacks real-time observation, and requires subsequent manipulation. For this reason, fluorescence is the usual method of detection of the sensitized excitation of the acceptor. If it is possible to excite the donor without exciting the acceptor, then the rate of photolysis of the acceptor (which is an excited state reaction) can be used to calculate the FRET efficiency [122],... 

Donor/acceptor association and the electron-transfer paradigm form the unifying theme for the C—C bond cleavage of various benzpinacols and diary-lethane-like donors in the presence of different electron acceptors (such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), triphenylpyrylium (TPP+), methyl viologen, nitrosonium cation, etc.). Scheme 13 reminds us how this is achieved by either CT photolysis of the D/A pair or via diffusional quenching of the excited electron acceptor A by the electron donor D. 

The problem of retention of asymmetry of the formed free radical in the fast geminate recombination of radicals was studied by photolysis of the optically active azo-compound PhMeCH—N=NCH2Ph [88,89]. The radical pair of two alkyl radicals was initiated by the photolysis of the azo-compound in benzene in the presence of 2-nitroso-2-methylpropane as a free radical acceptor. The yield of the radical pair combination product was found to be 28%. This product PhMeEtCCH2Ph was found to be composed of 31% 5,5 -(-)(double retention), 48% meso (one inversion), and 21% R.R(+) double inversion. These results were interpreted in terms of the competition between recombination (kc), diffusion (kD), and rotation (kml) of one of the optically active radicals with respect to another. The analysis of these data gave kxo[Pg.126]

In photochemical reactions, the role of DBPO will undoubtedly be that of an electron acceptor from an excited state species, as shown in reaction (51). Thus, inverted spin trapping will be feasible and an unambiguous interpretation of the appearance of PhCOO—ST will be difficult. In HFP the very strongly attenuated reactivity of benzoate ion should, however, make the homolysis mechanism predominate, as indicated by the appearance of both PhCOO—ST and Ph-ST (ST = PBN or DMPO) in the photolysis of DBPO and ST (Eberson et al., 1996a). 

The photoreduction of eight electron-acceptor- and four electron-donor-substituted nitrobenzenes has been studied and quantum yields for either starting material disappearance or product formation have been reported 7). Photolysis of 4-nitrobenzonitrile and 4-nitrotoluene in air-saturated solutions was completely quenched and thus a triplet multiplicity of the reacting excited state was derived 7). 

Carbenes and transition metal carbene complexes are among the few reagents available for the direct derivatization of simple, unactivated alkanes. Free carbenes, generated, e.g., by photolysis of diazoalkanes, are poorly selective in inter- or intramolecular C-H insertion reactions. Unlike free carbenes, acceptor-substituted carbene complexes often undergo highly regio- and stereoselective intramolecular C-H insertions into aliphatic and aromatic C-H bonds [995,1072-1074,1076,1085,1086],... 

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

It has been proposed that there may be a single-electron-transfer mechanism for the Mukaiyama reaction.62 63 For example, photolysis of benzaldehyde dimethylacetal and 1-trimethylsilyloxycyclohexene in the presence of a typical photoelectron acceptor, tri-phenylpyrylium cation, gives an excellent yield of the addition product. 

Little evidence is available to determine whether aromatic ketones can act as singlet sensitizers. If they can do so, it will be only at high acceptor concentrations because the lifetime of benzophenone singlets,154 for example, is less than 2 x 10-10 sec and that of several other aromatic ketones is estimated to be less than 10 10 sec.155 Yang231 has noticed that the photochemical addition of benzophenone to 2,3-dimethyl-2-butene is less efficient in neat olefin than in 4M olefin, which may indicate deactivation of benzophenone singlets. Golub218 has mentioned that benzophenone is a singlet sensitizer in the photolysis of 1,4-dichlorobutane. 

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