Effect of Temperature on Photoluminescence Spectra

The effect of temperature on the PL spectra is important as regards the internal conversion between the lowest excited triplet state and the ground state, because of the longer lifetimes of triplet states (Anpo and Che, 1999). 

Therefore, the radiationless deactivation process for most emitting species in their excited triplet states is so efficient that phosphorescence spectra can be observed only when the system including the emitting species is frozen at liquid nitrogen temperature. Of course, the environmental aspects of radiationless deactivation do not determine all the whole properties. Pyridine molecules show neither fluorescence nor phosphorescence at room temperature or in the rigid glass state at liquid nitrogen temperature. 

In general, the intensity of emission decreases when the temperature increases, because of the higher probability of other radiationless deactivation processes of excited molecules (Anpo and Che, 1999 Turro, 1978). Furthermore, a much better resolution of the vibrational fine structure of the emission (fluorescence and phosphorescence) can be observed at low temperature, as shown in Section 2.1.2, for the phosphorescence spectrum of highly dispersed tetrahedral vanadium species which exhibits a well resolved vibrational fine structure related to the V=0 double bond. 

The other process, internal conversion, is strongly temperature-dependent decreasing the temperature of the system causes a sharp increase in the emission, both for fluorescence and phosphorescence spectra. 

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