Manifold triplet

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. 

So far the roles of the three components of the triplet manifold (To,T i) have not been considered. In Fig. 5 the relationship between the energies of the three components of the T state and the S state is shown. 

Dimers (73) and (74) were formed in approximately equal amounts in all cases, although, as in the cases of 2-cyclopentenone and 2-cyclohexenone, the relative amount of (72) (either cis-syn-cis or cis-anti-cis) was found to vary substantially with solvent polarity. As in 2-cyclopentenone, this increase in the rate of head-to-head dimerization was attributed to stabilization of the increase in dipole moment in going to the transition state leading to (72) in polar solvents. It is thought that the solvent effect in this case is not associated with the state of aggregation since a plot of Stem-Volmer plot and complete quenching with 0.2 M piperylene indicate that the reaction proceeds mainly from the triplet manifold. However, the rates of formation of head-to-head and head-to-tail dimers do not show the same relationship when sensitized by benzophenone as in the direct photolysis. This effect, when combined with different intercepts for head-to-head and head-to-tail dimerizations quenched by piperylene in the Stem-Volmer plot, indicates that two distinct excited triplet states are involved with differing efficiencies of population. The nature of these two triplets has not been disclosed. 

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

Seminal studies on the dynamics of proton transfer in the triplet manifold have been performed on HBO [109]. It was found that in the triplet states of HBO, the proton transfer between the enol and keto tautomers is reversible because the two (enol and keto) triplet states are accidentally isoenergetic. In addition, the rate constant is as slow as milliseconds at 100 K. The results of much slower proton transfer dynamics in the triplet manifold are consistent with the earlier summarization of ESIPT molecules. Based on the steady-state absorption and emission spectroscopy, the changes of pKa between the ground and excited states, and hence the thermodynamics of ESIPT, can be deduced by a Forster cycle [65]. Accordingly, compared to the pKa in the ground state, the decrease of pKa in the... 

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... 

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 ... 

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...

Sensitization, which can populate the triplet manifold, was used in a number of instances. Sensitization with benzophenone was used in the photolysis of diazomethane to generate triplet methylene. The triplet methylene thus produced, however, failed to abstract much hydrogen from alkanes (cyclohexene), but... 

The dimerization of phenylnitrene to form azobenzene is a characteristic reaction of the triplet state of an arylnitrene. Thus, some intermediate formed along the reaction coordinate must undergo intersystem crossing to the triplet manifold. 

Evidence for photoassociation in the triplet manifold is at present inconclusive. Although Hoytink et al.20 have reported excimer phosphorescence from cooled ethanolic solutions of phenanthrene and naphthalene, concentration and temperature-dependent studies of the emission characteristics must be extended in order to distinguish photoassociation of the triplet state from intersystem crossing of the singlet excimer and possible triple-triplet annihilation. Certainly the decay constant of the molecular triplet state in fluid media is relatively insensitive to solute concentration21 although this... 

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