Temperature dependence luminescence

The end of the present Section aims both to summarize the just mentioned peculiarities of the non-steady-state transient kinetics of the tunnelling luminescence due to step-wise temperature changes, and to develop the theoretical basis for distinguishing two alternative reasons for the tunnelling luminescence temperature dependence thermally activated defect diffusion or rotation. 

Fig. 9. Temperatures dependence of the luminescence efficiency of the system Cai-jPbiWO for three values of X (modified from JA Groenink, thesis, Utrecht, 1979)...

Temperature-dependent luminescence measurements in the range from 77 to 300 K show quenching of the peak luminescence by a factor of about 15. Similar behavior is observed in the lifetime quenching [665, 666], As the band gap of the PECVD a-Si H is about 1.6 eV, nonradiative deexcitation of Er may occur at elevated temperatures. The amount of quenching lies in between that of c-Si and LPCVD a-Si H, just like the bandgap. 

The high-spin/low-spin interconverison in a Ni11 complex of the cyclam derivative (639) bearing a luminescent naphthalene substituent has been used as a fluorescent molecular thermometer.161 The Ni11 tends to quench fluorescence of the proximate naphthalene subunit, but the two spin states exert a different influence on the emission properties. Emission is temperature dependent, since the high spin —> low spin conversion is endothermic, i.e., a temperature increase favors formation of the low-spin form. 

The first photophysical investigation performed on stereochemically pure metal-based dendrimers having a metal complex as the core is that concerning the tetranuclear species based on a [Ru(tpphz)3]2+ core (tpphz=tetrapyrido[3,2-a 2, 3 -c 3",2"-h 2",3"j]phenazine) [67]. Dendrimer 45 is an example of this family. In this compound, two different types of MLCT excited states, coupled by a medium- and temperature-dependent photoinduced electron transfer, are responsible for the luminescence behavior. However, the properties of all the optical isomers of this family of compounds are very similar. This finding is also in... 

The temperature dependence of luminescence from the sample irradiated at 1 x 1013 cm-2 with 28Si+ indicates, above —110 K, an activation energy of 90 meV for the competing nonradiative recombination process— this competing process may be the thermal dissociation of geminate pairs or bound excitons at donorlike or acceptorlike centers. The 0.09-eV value of activation energy is consistent with the results of Troxell and Watkins (1979). 

Wallace WL, Bard AJ (1979) Electrogenerated chemi-luminescence. 35. Temperature-dependence of the ECL efficiency of Ru(bpy)32+ in acetonitrile and evidence for very high excited-state yields from electron-transfer reactions. J Phys Chem 83 1350-1357... 

Even when the d-d state is at much higher energy than the emitting level, it can still be of paramount importance in the photophysics and photochemistry of the system. Indeed, a major contributor to the temperature-dependent loss of emission intensity in luminescent metal complex based sensor materials is nonradiative decay via high-energy d-d excited states.(15) The model for this is shown in Figure 4.4A. The excited state lifetime is given by... 

There are possible advantages of the temperature dependence of excited state lifetimes. The dependence can be used for luminescence-based temperature sensors, which is an area of considerable interest. Clearly, with a suitable AE, complexes that give significant lifetime changes in a wide range of possible temperatures can be designed. We discuss this issue elsewhere.(16)... 

The temperature dependence of luminescent metal complexes can be controlled by molecular design that affects the energy gap between the emitting state and the deactivating d-d or by altering the preexponential factor for thermal deactivation. The sometimes large temperature dependencies of lifetime and quantum yields for metal complexes also suggest their use as temperature sensors. 

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. 

The mechanisms of luminescence decay from an optical center are of critical importance. In particular we have to know if there are any processes internal to the center or external to it, which reduce the luminescence efficiency. It is possible to define two decay times, ir, the true radiative decay time which a transition would have in absence of all non-radiative processes, and r, the actual observed decay time, which maybe temperature dependent, as will usually occur when there are internal non-radiative channels, and which may also be specimen dependent, as when there is energy transfer to other impurities in the mineral. The quantum yield may be close to unity if the radiationless decay rate is much smaller than the radiative decay. 

Another example is the luminescence of in YPO4 with tetragonal zircon structure (Oomen et al. 1988). The emission consists of two bands, one in the UV region and one in the visible part of the spectrum. The intensity ratio of these bands is strongly temperature dependent (Fig. 5.57). 

Lindholm and Adamson144 report that photochemically, coordinated NH3 is preferentially lost from Cr(NH3)5NCS2 + while thermally NCS" is replaced. A study of the temperature dependence of Cr(NCS)e3- phosphorescence was reported.145 A general discussion of the luminescence behavior of coordination complexes has been given146 and a careful study of the Cr(C204)33 luminescence spectra has been reported.147... 

Baker and coworkers [16] reported on a self-referencing luminescent thermometer designed around the temperature-dependent intramolecular excimer formation/dissociation of the molecular probe l,3-Ws(l-pyrenyl)pro-pane (BPP) dissolved in 1-butyl-l-methylpyrrolidinium bjs(trifluoromethyl-sulfonyl)imide, [C4Cipyr][Tf2N]. Upon an increase in temperature, and hence a decrease in the IL s bulk viscosity, the excimer-to-monomer fluorescence... 

It has been possible to derive J for several salts of [(en)2Cr(OH)2Cr(en)2]4+ from low-temperature single-crystal absorption and powder luminescence spectra in the region of the 4A2 A2— aA2E transition of the paired system.444 The difference of up to 10% from the values derived from powder magnetic susceptibility data indicates a slight temperature dependence of /. 

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