Donor phosphorescence

The quenching of donor phosphorescence is a function of the acceptor concentration in rigid media and follows the expression ... 

Based on the Dexter mechanism, with a distance-dependent rate coefficient for triplet energy transfer, a non-exponential decay function for donor phosphorescence in a rigid solution was derived... 

In order to clear up the mechanism of inactivation of excited states, we examined the processes of quenching of fluorescence and phosphorescence in PCSs by the additives of the donor and acceptor type253,2S5,2S6 Within the concentration range of 1 x 1CT4 — 1 x 10"3 mol/1, a linear relationship between the efficiency of fluorescence quenching [(/0//) — 1] and the quencher concentration was found. For the determination of quenching constants, the Stem-Volmer equation was used, viz. 

Thus a plot of t //p vs. 1/[A] should yield a straight line with intercept kf + kic + klsc)IKkisc or 1 imlsc. Thus phosphorescence intensity and lifetime as a function of [A] (within the limits of our assumptions), the intersystem crossing efficiency of the donor can be obtained. 

Sensitisation by a high-energy donor, such as triplet xanthone, can populate both the Qi and the D, states by energy transfer (Figure 10.4). This results in both the photosolvation reaction and phosphorescence emission. 

Sensitisation with lower-energy donors, such as triplet Ru(bpy)3+, on the other hand, means that energy transfer is only possible to the Di state (Figure 10.4). This results in phosphorescence emission, but the photosolvation reaction does not occur. This shows that the Qi state must be responsible for the photosolvation reaction. 

D 1A —- 1D + 1 A (triplet-singlet energy transfer). This type of transfer leads to phosphorescence quenching of the donor (e.g. phenanthrene (T) + rhod-amine B (S)). 

Such transfers are detected by (i) reduction of the phosphorescence lifetime of donor in presence of acceptor, (ii) appearance of acceptor fluorescence and (iii) experimental calculation of R0 and its comparison with the theoretical value obtained from the Forster formulation. In such transfers... 

Benzophenone was found to possess the criteria for a suitable donor or sensitizer For this molecule, the intersystem crossing JD 3D occurs with unit efficiency (tfisc = 1). Under the experimental setup, no phosphorescence was observed in absence of benzophenone. After... 

Triplet—triplet energy transfer from benzophenone to phenanthrene in polymethylmethacrylate at 77 and 298 K was studied by steady-state phosphorescence depolarisation techniques [182], They were unable to see any clear evidence for the orientational dependence of the transfer probability [eqn. (92)]. This may be due to the relative magnitude of the phosphorescence lifetime of benzophenone ( 5 ms) and the much shorter rotational relaxation time of benzophenone implied by the observation by Rice and Kenney-Wallace [250] that coumarin-2 and pyrene have rotational times of < 1 ns, and rhodamine 6G of 5.7 ns in polymethyl methacrylate at room temperature. Indeed, the latter system of rhodamine 6G in polymethyl methacrylate could provide an interesting donor (to rose bengal or some such acceptor) where the rotational time is comparable with the fluorescence time and hence to the dipole—dipole energy transfer time. In this case, the definition of R0 in eqn. (77) is incorrect, since k cannot now be averaged over all orientations. 

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. 

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