Phosphorescence lifetimes temperature-dependent

The phosphorescence lifetimes have been examined for many protein systems as a function of temperature. In the early work oxygen was not removed from the sample.(72,73) In these works the lifetimes are dominated by quenching by oxygen, and so the temperature dependencies probably represent temperature-dependent oxygen diffusion. 

Bismuto et alF compared the phosphorescence from both tuna and sperm whale apomyoglobin. The emission occurs from a tryptophan in the A helix. The temperature dependence of lifetime and the position of the 0-0 vibrational band differ as a function of temperature for the two proteins. The authors interpreted their results to indicate that the microenvironment of the tryptophan in sperm whale apomyoglobin possesses a higher degree of internal flexibility than that in the tuna protein. 

The long-lived phosphorescence of the tryptophan in alkaline phosphatase is unusual. Horie and Vanderkooi examined whether its phosphorescence could be detected in E. coli strains which are rich in alkaline phosphatase.(89) They observed phosphorescence at 20°C with a lifetime of 1.3 s, which is comparable to the lifetime of purified alkaline phosphatase (1.4 s). Long-lived luminescence was not observed from strains deficient in alkaline phosphatase. The temperature dependence of tryptophan phosphorescence in the living cells was slightly different from that for the purified enzyme, indicating an environmental effect. 

Y. Kai and K. Imakubo, Temperature dependence of the phosphorescence lifetimes of heterogeneous tryptophan residues in globular proteins between 293 and 77 K, Photochem. Photobiol. 29, 261-265 (1979). 

If rate constants are competitive, we expect the emission of prompt fluorescence, phosphorescence and delayed fluorescence of energy quanta /7v/, h jp and //vn) respectively. Although //v/ is equal to ftvED, the lifetime of delayed fluorescence will match the lifetime of triplet decay. The rate constant ArEn for E-type delayed fluorescence is temperature dependent and can be expressed as... 

In general, the natural radiative lifetimes of fluorescence and phosphorescence should be independent of temperature. But the emission intensities may vary due to other temperature dependent and competitive rate constants. 

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. 

The wavelength of the spectra scarcely changed when observed at 77 and 298 K. whereas the yield (or intensity) of the phosphorescence at 77 K (Fig. 19, spectrum a) was remarkably high compared with the yield at 298 K. This result can be attributed to the fact that the lifetime of the charge-transfer excited triplet state is markedly affected by temperature-dependent radiationless processes. Such radiationless deactivation from the excited triplet state becomes less efficient as the temperature decreases, leading to an elongation of the phosphorescence lifetime from T298 140... 

The phosphorescence lifetimes in solution are strongly dependent (about 3 orders of magnitude) on temperature from 150 to 230 K, and for deuterated samples are four times longer than that for the... 

The differences observed in the temperature dependences of luminescence for europium and terbium(III) clathrochelates can also be due to the absence of back energy transfer from encapsulated Eu cation to the clathrochelate ligand. The expression for the rate constant of metal-to-ligand back transfer contains the AE value, which is the energy difference between the triplet level of the bipyridine fragment and the metal ion phosphorescence level. The back transfer rate for the europium (III) ion is much less than that for the terbium(III) ion. This process is essential for the terbium(III) ion, and this is confirmed by differences in the temperature dependence of excited-state lifetimes for europium and terbium (III) clathrochelates in D5O the luminescence of the terbium(III) complex at 77 K is approximately an order of magnitude higher than that at 300 K, whereas for the europium(III) complex, it does not depend on temperature. 

Although the previously observed phosphorescence of ferrocene now appears to have been artlfactual, a fairly strong luminescence has been observed from the analogous ruthenium(II) compound (53,210). The emission spectrum of ruthenocene, measured at low temperatures either from the pure solid or from glassy media, appears as a rather broad but highly structured band centered around a maximum at about 17 kK. The lifetime of the emission from the solid is strongly temperature dependent. 

One main reason for our limited information comes from a fact that this molecule does not phosphoresce under any conditions and the transient absorption is very weak. By applying the TG method, a remarkably shortened triplet lifetime of about 100 ns was measured in many solvents at room temperature [85] (at a lower temperature, the lifetime is 3 jrs from the TL studies [94]). Compared with the lifetimes of about a few 10 ms for pyrazine and pyrimidine at a lower temperature, the observed lifetime is quite short. The short lifetime is neither due to the self-quenching nor to the quenching by 02, but is intrinsic. The quantum yield of the triplet formation (r/)isc) was determined from the relative intensity of the two rising components, Qs/Qtot, and the energy balance relation of Eq. (34) to be 0.1, which is much smaller than that at the lower temperature. These temperature-dependent lifetimes and isc could well be explained by a model of... 

The temperature dependence of the PL spectra of 1 is shown in Figure 3.8. There was a strong iuCTease in triplet emission intensity with decreasing temperature. At 11K, the emissive peak occured at about 508 nm for 1. The luminescence lifetime (Tp) values for 1 were determined to be about ll.Ops at 290K and 40.2 ts at 11K (versus Tp = 30 s at lOK for I), which was in line with the Jt-Jt character of the phosphorescent triplet state of the emission. The reduced conjugation in 1 shifted the phosphorescence to the blue by 0.06 and 0.38 eV, respectively, as compared with the related Pt(ll) polyynes I and 30. 

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