Mechanism triplet energy transfer

The second light-requiring step ultimately leading to the tetraphenyl methane was shown to occur by energy transfer from triplet benzophenone to the fuchsone, resulting in triplet fuchsone. The mechanisms of energy transfer will be discussed in a later chapter. 

When D and A are similar molecules emission-reabsorption cannot be very important due to the usually small overlap of the emission and absorption spectra. Also, this mechanism should not be important for triplet-triplet energy transfer because of (a) low phosphorescence quantum yields in fluid solutions and (b) the low oscillator strengths for singlet-triplet absorption. 

However, the very important triplet-triplet energy transfer should not occur by this mechanism ... 

The triplet-triplet energy transfer, which is doubly forbidden by a resonance mechanism, is allowed by an exchange mechanism ... 

Thus Saltiel has concluded that the small increase in [T]S/[C], in going from benzene to acetone indicates that a mixed mechanism is operative for acetone-sensitized isomerization, that is, both triplet energy transfer and, to a minor extent, Schenck intermediates are involved. When acetophenone or benzophenone is used as a sensitizer the pss is close to the thermodynamic... 

The observed luminescence properties of the copolymer yarns can be easily explained if an energy transfer mechanism is assumed to be operating (Figure 7). Triplet-triplet energy transfer from the terephthalate units to the 4,4 -biphenyl -dicarboxyl ate units explains both the dual fluorescent/phospho-rescent emissions from the 4,4 -biphenyldicarboxyl ate units as well as the quenched phosphorescence from the terephthalate units. 

Based on luminescence studies, we postulated triplet-triplet energy transfer by electron exchange as the mechanism of photostabilization and we calculated an active quenching sphere with a radius, R0, of 19.7 A for 2,6-ND. Because the value of R0 is larger than 15 A, we postulated that energy migration was occurring. 

Understand that triplet-triplet energy transfer by the dipole mechanism is forbidden, but that the application of the Wigner spin rule shows that it can occur by the exchange mechanism. 

However, triplet-triplet energy transfer cannot occur by this mechanism as this would require both donor and acceptor to undergo a change in multiplicity ... 

The Coulombic mechanism would require that both 3D — 3D and 1A —> 3A were allowed transitions, which clearly they are not as both are spin-forbidden processes. Thus, triplet-triplet energy transfer by the long-range Coulombic mechanism is forbidden. 

Figure 6.15 Electron movements occurring in short-range triplet-triplet energy transfer by the exchange mechanism. Note that an electron initially on D moves to A and an electron initially on A moves to D ...

Because the energy rate does not imply the transition moments in the exchange mechanism, triplet-triplet energy transfer is possible ... 

In fluid solutions only the third mechanism is of importance in transfer of triplet excitation. The trivial mechanism is usually excluded because most molecules do not phosphoresce in solution, and the second mechanism seems to be excluded because the transition moments for the T - S process in the donor and the T - S transition of the acceptor should both be vanishingly small. An interesting possibility which has yet to be explored experimentally is that heavy atom-containing solvents might so enhance T <- S transition probabilities that long-range triplet energy transfer may become important. 

The Dexter mechanism can be thought of as electron tunneling, by which one electron from the donor s LUMO moves to the acceptor s LUMO at the same time as an electron from the acceptor s HOMO moves to the donor s HOMO. In this mechanism, both singlet-singlet and triplet-triplet energy transfers are possible. This contrasts with the Forster mechanism, which operates in only singlet states. 

In this context two additional aspects should be noted. First, our high-pressure experiments [64] provided detailed information about the PET dimerization of 17a in nonpolar solvents which confirm the triplex mechanism (cf. Sect. 4 and Ref. [65]). Second, analogeous experiments with triplet sensitizers indicate the existence of a Marcus Inverted Region for triplet energy transfer. Third, the mechanism described here for the PET dimerization of dienes resembles that by Mattes and Farid [66] for the dimerization of 1,1-diphenylethene. This latter example will be discussed in more detail in Sect. 3.2 of this article. 

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