1.3- Biradicals triplet-singlet conversion

Thus, the sign of the observed polarization of the Me protons of the product 22 (absorption) is unambiguous evidence of the formation of an intermediate triplet 1,3-biradical 23T, and this fact, in turn, means that 16 enters the reaction with 21 in the excited triplet state. The end product 22 is formed after the triplet-singlet conversion of the triplet biradical 23t followed by cyclization of the singlet biradical 23s (Scheme 11), while the triplet biradical 23T also reverts to the initial reagents. Only this reverse reaction could explain the negative polarization (emission) of the Me protons of the initial thiacycloheptyne 21. It should be noted that the process described in Scheme 11 is the first example of a reaction of the excited triple state of germylene 1676. 

In the photochemical isomerization of isoxazoles, we have evidence for the presence of the azirine as the intermediate of this reaction. The azirine is stable and it is the actual first photoproduct of the reaction, as in the reaction of r-butylfuran derivatives. The fact that it is able to interconvert both photochemically and thermally into the oxazole could be an accident. In the case of 3,5-diphenylisoxazole, the cleavage of the O—N bond should be nearly concerted with N—C4 bond formation (8IBCJ1293) nevertheless, the formation of the biradical intermediate cannot be excluded. The results of calculations are in agreement with the formation of the azirine [9911(50)1115]. The excited singlet state can convert into a Dewar isomer or into the triplet state. The conversion into the triplet state is favored, allowing the formation of the biradical intermediate. The same results [99H(50)1115] were obtained using as substrate 3-phenyl-5-methylisoxazole (68ACR353) and... 

The photochemical dissociation of Me2Ge from 7,7-dimethyl-l,4,5,6-tetraphenyl-2,3-benzo-7-germanorbomadiene (14) has been studied by flash photolysis, low-temperature matrix isolation and CIDNP 3H NMR techniques30. The results suggest that a biradical (15) is formed as an intermediate species in the photoreaction. The biradical is initially formed in the singlet state, which undergoes conversion to the triplet state before irreversible decomposition to form Me2Ge and tetraphenylnaphthalene (TPN) (reaction 19). 

Nitroxides are the most common of the oxygen-centred biradicals to be reported. As a model for spin-crossover molecules, the nitronyl nitroxide (105) was prepared and by oxidation with PbC>2 afforded the triplet biradical (106) which was characterized by ESR (Scheme 15).242 The one-electron oxidation of (105) afforded the singlet cation (107) which was seen to exist in equilibrium with (106) in solution. The authors claim that pH-controlled interconversion between two species of different spin multiplicities in this way may provide die basis for novel magnetic switches or pH sensors. The N.N-dialkylamino nitronyl nitroxides (108) were prepared and afforded die diplet-state biradical cation species by one-electron oxidation with iodine.243 The authors propose that, by the similarity of die electronic structures, diese structures can be regarded as hetero-analogues of trimethylenemethane. A paper confirms die conversion of 3,3-dimethyldioxetane into die corresponding ring-opened 1,2-diol but refutes the... 

MFEs on reaction yields had only been observed in the reactions through radical pairs and biradicals for long years after the discovery of the MFEs on luminescence due to the T-T quenching and T-D one in solid and liquid phases. On the other hand, the MFEs of radical pairs and biradicals are due to the spin conversion between their singlet -triplet (S-T) states. As shown in Section 13.2, CFDEP induced by the T-D quenching were also found in many reactions in solution at room temperature. Thus, the author s group tried to find MFEs on the yields of such reactions with our ns-laser photolysis apparatus. At first, we studied the electron transfer reaction of triplet 10-methylphenothiazine ( D ) with 4-(4-cyanobenzoyloxy)TEMPO ( A-Ri") in 2-propanol at 293 K [14]. Here, Ri is the TEMPO (2,2,6,6-tetramethylpiperidin-l-oxyl) radical. The reaction scheme is shown in Fig. 13-5(a). 

The relatively low-temperature conversion of MSD to MBD can reasonably be attributed to the formation of a trimethylenemethane biradical, presumably in its singlet state, which is further stabilized by a cyclopentadienyl moiety (Scheme 9.29). Of interest is the extent to which this species might be stabilized by electron transfer to form a species reminiscent of the oxaallyl zwitterion from cyclopropa-none (see Chapter 5, Section 2). Of further interest is the fact that the dimers formed are not those from a trimethylenemethane triplet state. 

The effect of multiplicity of carbenes on their reactivity is most vividly marked in the following features rationalized by Skell et al. from experimental data [37-39]. First, the reaction of carbenes occurs in the singlet electron state at a much faster rate than in the triplet, with the absolute rates of typical reactions of addition to multiple bonds and of insertion into the C—H bonds exceeding, under normal conditions, the rate of intercombination conversion. Secondly, the singlet carbenes are characterized by one-step stereospecific addition to double bonds, as, for instance, in the cyclopropanation reaction, while the triplet carbenes react in a nonstereospecific way to form first an intermediate biradical through addition to one of the atoms of the double bond. The formation of a trimethylene radical, in the course of reaction of triplet methylene ( B ) with ethylene, has been confirmed by semiempirical [40, 41] and ab initio [42, 43] quantum chemical calculations. 

The reaction of 33a was shown, by matrix isolation photolysis at 10 K and other methods, to involve acyloxyketene 37 as an intermediate. It was shown in separate experiments that 37, synthesized by conventional treatment of the corresponding acid chloride, reacted with tetraketones to form the same adducts 35 and 36 in the same ratio as had been observed in the photochemical reaction of 33a. The photolysis then involves conversion of excited tetraketone to 37 and CO followed by thermal reaction of 37 with tetraketone presumably via 5-lactones. Limited evidence supports the singlet state as the reactive specie. If this is correct, a cyclic mechanism, as illustrated in Figure 50.9, could be involved a triplet state reaction would require zwitterionic or biradical intermediates, as illustrated. 

The biradicals formed from (21) and (23), respectively, are primarily produced in their singlet state, but intersystem crossing to the triplet diradical results, followed by carbon-carbon bond cleavage. As CO2 formation from dioxetanones is more exothermic than carbonyl compound formation from tetramethyl dioxe-tan, it is supposed that C-C bond breakage follows more rapidly, reducing the time available for singlet to triplet conversion. As a result, the yield of excited singlet states is nearly the same in both compounds. The yield of excited triplets, however, is much smaller in the dioxetanone case. 

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