Triplet diffusion

Fig. 25 Presentation of the turn over points between dispersive and non-dispersive triplet diffusion as extracted from Fig. 24 and plotted according to Fig. 3. Details can be found in [24]...

Because of the dispersive nature of the triplet diffusion, the triplet annihilation rate must also be a function of time and temperature. Alternatively, for continuous excitation the annihilation rate becomes a function of excitation dose. Figure 26 shows the saturation of the triplet density during continuous... 

Studies of the triplet state of biphenyl in a carbazole host crystal188 and of the triplet-state processes of anthracene in different environments180-183 have been reported, and triplet diffusivity in crystalline pyrene184 and the multiplet structure of spectra in the Si <-> S0, Tx -> S0, and S2 -> S0 transitions in 1,12-benzperylene (matrices at 4.2 K)185 examined. 

However, these time dependences of the phosphorescence and DF can only be expected in well-ordered molecular crystals. In fact, early experiments on triplet states used the DF decay in anthracene crystals to obtain the triplet lifetime and diffusivity [35, 36]. However, a general problem is that impurities can act as traps or quenching sites for triplets, thus reducing their lifetime and making the intrinsic lifetime of the triplets difficult to determine. The main reason is the long lifetime of the triplet state that allows for many sites to be visited during the triplet diffusion. 

ZHA 10] Zhang Y., Forrest S.R., Triplet diffusion leads to triplet-triplet annihilation in organic phosphorescent emitters . Chemical Physics Letters, vol. 590, pp. 106-110, 2013. 

Goudsmit G-H, Paul H and Shushin A I 1993 Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion J. Phys. Chem. 97 13 243-9... 

Note. Both the rearrangement In t-ButanoI) and the double bond isomerization of (114) (In Benzene) are quenched in a diffusion-controlled process by suitable triplet acceptors e.g., naphthalene or 2,5-dimethylhexa-2,4-diene). The rearrangement (114) (118) -I- (120) is also observed on irradiation in... 

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

Backstrom and Sandros<54-55) found that the phosphorescence of biacetyl in benzene solution at room temperature was quenched at a diffusion-controlled rate by aromatic hydrocarbons when the triplet energy of the hydrocarbon was sufficiently below that of biacetyl. 

When the triplet energy of the donor is 3 kcal/mole or more higher than the acceptor triplet energy, the energy transfer rate is about the diffusion-... 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

Since energy transfer from high triplet energy donors to both s-tram and s-cis conformations should be diffusion controlled, and the piperylene is more than 95% in the s-trans conformation at room temperature, the role of the c-T and c-C isomers can be neglected to a first approximation. 

Several carbonyl-containing peroxide additives have been shown to increase the initial rate of the nonoxidative photo-dehydrochlorination of PVC (54). In studies with polymeric ketones unrelated structurally to PVC, the excited singlet and triplet states of the carbonyl groups in these polymers were found to sensitize 0-0 homolysis at rates approaching diffusion control (55). Similar reactions may well occur in oxidized vinyl chloride polymers. 

The fact that dynamic 13C polarization is only possible through the indirect way via tire 1H spins suggests the mechanism of polarization transfer. Since the polarization transfer between the electrons and nuclei are driven by the dipolar interactions between them, and the fraction of the guest triplet molecules was small, it would be natural to assume that the polarization of the electron spins in the photo-excited triplet state is given to those H spins which happen to be close to the electron spins, and then the 1H polarization would be transported away over the whole volume of the sample by spin diffusion among the 1H spins. 

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