Quenchers ferrocene

In order to determine the multiplicity of the reactive species, the photodimerization was carried out in the presence of the triplet quenchers oxygen and ferrocene. The results of these experiments are shown in Table 10.4.<41) It is obvious that the presence of oxygen exerts a large quenching effect on the production of the tram dimer and a smaller but significant effect on the formation of the cis dimer (the formation of tram dimer is decreased by oxygen by a factor of 25, while the cis dimer is decreased by a factor of 1.2). As with oxygen, the production of the tram dimer was quenched in the... 

As will be discussed later, it is possible a4> that the thermolysis involves a metal-nitrene complex whereas the photolysis involves the free nitrene. The product distribution is not affected by the presence of a photosensitizer, but since ferrocene itself is both an efficient triplet quencher as well as a sensitizer 26,27) jt is very difficult to probe the spin state of ferrocenyl nitrene at the moment of reaction. The cycli-zation appears to be a singlet reaction since the yield of 27 in benzene solution is essentially unaffected by oxygen or the presence of hydro-quinone a5>. 

It is typical that the photorearrangement 57 - 58 proceeds readily.37 This seems to rule out the triplet state of the rearranging molecule 57, reasoning that the ferrocene part of the molecule would very probably act as a triplet quencher,35 as is the case with other ferrocene derivatives structurally able to undergo triplet photoreactions.36 On the other hand, the reaction 55 - 56 cannot be sensitized with acetophenone33 (ET = 73.6 kcal mole-1 32ft). 

Ferrocene is marketed as one of the 24 compounds in the Photosensitizer and Quencher Kit wherein its efficient quenching capability is also noted.8... 

In the chromophore-quencher system (1) a ferrocenyl unit is connected to a Ru(tpy)2 + unit via a p-phenylene-acetylene spacer [141]. The weak emission of the Ru(tpy)2 chromophore is completely quenched in the dyad [142]. For this system, an EnT mechanism could be inferred by analogy with the behavior of related bimolecular systems [140], but PET followed by fast back electron transfer cannot be ruled out. This mechanistic ambiguity is common to most studies on related chro-mophore-ferrocene dyads [143-146]. 

Triplet quenchers such as Oj or ferrocene inhibit formation of the anti dimer, while the formation of the syn dimer is hardly affected. The syn-anti ratio in the triplet reaction was determined by comparing the outcome of the reaction with and without use of a quencher (Cowan and Drisco, 1970a). 

Ferrocene is a very efficient triplet quencher, its quenching constants showing practically no difference for triplets between 66.6 (triphenylene) and 42.6 kcal/mole (anthracene) 185>. Two conclusions are possible either energy is transferred to ferrocene Ti (40.5 kcal/mole) or the quenching does not involve energy-transfer at all, and weak electronic interaction on contact between ferrocene and the triplet molecule effects rapid intersystem crossing of the latter by spin-orbit coupling. 

With ferrocene as quencher and triphenylene as sensitizer the cis trans isomerization of several 4-nitrostilbenes has been examined [201], From linear plots of ([t]/[c])sens versus [Q], larger slope/intercept ratios were found as compared to direct excitation conditions (Table 8). A cis -> trans pathway partly bypassing the triplet state accounts for this difference (Section VI). Plots of ([t]/[c])scns as a function of the azulene or ferrocene concentrations are shown in Figure 10 for three stilbenes. 

Stern-Volmer constants of similar size were found for nitro- and cyanostilbenes using ferrocene as quencher [85,118]. Fluorescence quenching by oxygen in saturated solution (1 bar) is small for fram-stilbene [105]. 

Attempts to study further some mechanistic aspects of the triplet decay by using additives with low triplet energies led initially to confusion. It turned out that two groups of quenchers exist. One group of classical triplet quenchers (e.g., azulene, ferrocene, and /i-carotene) quench preferentially 3t to 11 3c is not accessible because of its very short lifetime. Molecules of the second group (e.g., oxygen and di-tert-butyl nitroxide) quench preferentially 3p to p via a spin-exchange mechanism (Section... 

Jeffares and McMurray have isolated the cyclobutane lactones (78) in low yield from the photochemical rearrangement of the 6-hydroxycyclohexenones (79). These are the first examples of products derived from cyclohexenones by an a-cleavage process. Irradiation in the presence of triplet quenchers such as ferrocene improves the yields somewhat. 

Ferrocene, (>7 -cp2)Fe is an efficient quencher of triplet excited states. " Quenching occurs by energy transfer with the lowest triplet level of ferrocene lying around 179 kJ mol The rate constants for energy transfer in benzene solution at 22 °C for a series of organic triplets are shown in Table 7.1. Donors that have triplet energies significantly above Aat of ferrocene are quenched by it at a diffusion... 

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