Triplet excited states relaxation

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

The second example concerns a photo-induced e.t. with this same chemical system, to which an amine (e.g. DABCO) is added as an electron donor. The fast decay to the relaxed triplet excited state of benzophenone remains unchanged, but this is now followed by two further reactions the forward e.t. step which forms the radical ions, and the back e.t. of these ions to restore the initial system. 

Galardy et al. (1973) introduced acetophenones and benzophenones for photoaffinity labeling (see also Martyr and Benisek, 1973 Katzenellenbo-gen et al., 1974). The properties of excited states generated from these and other a,P-unsaturated ketones are well understood (Turro, 1979). In most cases a triplet excited state is formed that abstracts a hydrogen atom from a donor yielding two radicals which subsequently couple. Unreacted excited species relax to the ground state and may be excited repeatedly until they react (Fig. 2.5). 

The linear correlation with slope AR/Ap = —0.36 shows that PPP bond orders can be used for excited-state relaxation. The comparable graph of AR/Ap for trans-stilbene has a linear correlation with slope —0.25. Similarly, it is seen in biphenyl[37] that the lowest triplet state has a bond order pattern corresponding to the quinonoidal structure, implying a planar geometry in the triplet state in conformity with the prediction of Lewis and Kasha[56]. 

Relaxation of the lowest triplet excited state 3 [Re(F)(CO)3(N,N)] 1 manifests itself experimentally in different ways ... 

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