Optical triplet generation

Figure 3-24 Simplified diagram of the experimental apparatus for the optically sensitized triplet generation/TR3 studies. Requisite time delays were obtained either by optical delay or by the two-laser experimental configuration. The optical delay line is not to scale its actual length was approximately 120 ft. (Reproduced with permission from Ref. 82. Copyright 1981 American Chemical Society.)...

Radical 44 is generated by electron transfer from the parent pyrimidine to the optically generated (308 nm laser pulse) triplet states of anthroquinone-2,6-disulfonic acid. The experimental method detects transient species in the time range of 40 ns to 10 ps after the laser pulse. At pH = 1.0, the primary cation radical is the only radical found 40 ns after the pulse. The spectrum of this radical disappears at a time shorter than 1.0 ps after the pulse. A second spectrum observed 5 ps after the pulse, is attributed to radical 46, which was thought to form by net OH- addition to C6 of the cation radical. At pH = 7.0, the N3... 

In this context it is useful to remember that the concept of the possible recombination of triplet radical ion pairs is not an ad hoc assumption to rationalize certain Z - E isomerizations, although the CIDNP effects observed during an isomerization reaction played a key role in understanding this mechanism. Triplet recombination has been accepted in several donor-acceptor systems as the mechanism for the generation of fast (optically detected) triplets [169-171], and invoked for several other reaction types [172]. The CIDNP technique is a sensitive tool for the identification of this mechanism, for example, in the geometric isomerization of Z- and E-1,2-diphenylcyclopropane and in the valence isomerization of norbornadiene (vide infra). Most of these systems have in common that the triplet state can decay to more than one minimum on the potential surface of the parent molecule. 

When optically active hydrocarbons have been used as substrates, a similar pattern of insertion reactivity emerges. Phenylnitrene inserts with a maximum of 30% retention into the tertiary C—bond of optically active 2-phenylbutane implying a high degree of triplet involvement, whereas ethoxycarbonylnitrene inserts stereoselectively with 98-100% retention into the tertiary C—H of (S)-(+)-3-methyl-hexane. The result is independent of the method of nitrene generation, and of concentration, and lends support to the view that only singlet ethoxycarbonylnitrene inserts into unactivated C—bonds. 

In summary, it is well established that the species that initiates exothermic growth of a polydiacetylene chain is a diradical dimer. It can be generated thermally, the activation energy being determined by the energy of the librational motion required to temporarily shorten the C1-C4 reaction distance of a molecular pair to about 2 A, or by electronic excitation of the diacetylene moiety via UV- or y-irradiation. Upon UV-excitation the number of chains initiated is of order 10 per absorbed photon. The active precursor state is likely to be of triplet character. Even in case of optical... 

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