Singlet manifold

Figure 9.18 shows a typical energy level diagram of a dye molecule including the lowest electronic states Sq, and S2 in the singlet manifold and and T2 in the triplet manifold. Associated with each of these states are vibrational and rotational sub-levels broadened to such an extent in the liquid that they form a continuum. As a result the absorption spectrum, such as that in Figure 9.17, is typical of a liquid phase spectrum showing almost no structure within the band system. 

In addition,<1,48,48) it was noted that whereas the azulene effect on the sensitized reaction is sensitive to changes in solvent viscosity, the azulene effect on the direct photoreaction was independent of solvent viscosity, as would be predicted for Forster-type energy transfer. The inescapable conclusion is that cis-trans isomerization upon direct irradiation of stilbene takes place in the singlet manifold. 

Thus, upon direct excitation of 4-bromostilbene, cis-trans isomerization takes place in both the singlet and triplet manifolds.<1,16 38,63)... 

The simplest scheme incorporating coupled FRET donor and acceptor manifolds in which both species can adopt the ground, singlet excited, and triplet excited states is depicted in Fig. 12.1a. Resonance energy transfer occurs between the SI states, and... 

After a very short period of time, 10-11—10-12 sec, crossing from higher to lower excited states 36> (internal conversion) and thermal equilibration 6 ) will bring the molecule to one or another of the numerous minima in its first excited state hypersurface. If the initial excitation was into the triplet manifold, this will typically be Ti if it was into the singlet manifold, it will typically be Si (Kasha s rule 31>), exceptionally S2 if the internal conversion to Si is slow (azulene 48>68>), or Ti if intersystem crossing into the triplet manifold proceeds unusually fast and is able to compete with the relaxation to Si (particularly in the presence of heavy... 

The rate constant for formation of excited-state S02 was measured by Halstead and Thrush to be 1.7 X 10 12 exp(-2100/T) cm3 molec1 s 1 [28-31], while the overall rate constant for the loss of SO is 3.6 x 10 12 exp(-1100/T) cm3 molec-1 s 1 [11], Thus, at room temperature, S02 constitutes —1.6% of the product channels. This figure can only be considered approximate, however, since mixing of states within the singlet manifold affects the interpretation of the rate constant for formation of the excited state [32],... 

Population of the triplet manifold by direct singlet-triplet absorption is a very inefficient process, being spin-forbidden. Instead, the triplet manifold is populated indirectly by excitation into the singlet manifold followed by intersystem crossing. 

The magnitude of 1kisc is governed by El-Sayed s selection rules that is, the rate of intersystem crossing from the lowest singlet state to the triplet manifold is relatively large when the transition involves a change of orbital type. For example ... 

Ordinarily, however, the population of the triplet state is achieved through excitation into the singlet manifold which is followed by intersystem crossing to the excited triplet state. Therefore, in considering the yield of phosphorescence we must consider the processes involved in formation and disappearance of both the singlet and triplet states. 

Fig. 10 Theoretical curves [equation (12)] representing the temperature dependence of the signal intensities due to a triplet species that has a singlet manifold in equilibrium. The A -values represent the energy gap of the two states.

In the case of a quintet state consisting of two weakly coupled triplets and populating with the triplet and singlet manifolds, the signal intensity of the quintet will be given by (13). 

Fig. 11 A. Spin-orbit coupling with vibronic interaction in the singlet manifold. Mechanism III Fig. 11 B. Spin-orbit coupling with vibronic interaction in the triplet manifold. Mechanism IV...

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