The effect of temperature on emission processes

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. 

Interesting examples are found in substituted anthracenes. The lifetimes and quantum yields of fluorescence of substituted anthracenes show different dependencies on temperature. The position of the substituent is more important than its nature. For 9- and 9,10-substituted anthracenes, fluorescence quantum yields increase steeply with decrease of temperature, while side-substituted derivatives have low yield and small temperature dependence. The variation is of the form 

Anthracene has a triplet state T2 which is about 650 cm-1 below the S, level. The T2 and S, states are affected to different extents by substitution, whereas T, is nearly unaffected. 

The temperature sensitivity arises due to disposition of T2 state with respect to S, state. If T2 is considerably above S, transfer to T, is less probable because of unfavourable Franck-Condon factor. As a consequence, fluorescence is the easiest way for deactivation and fluorescence yield is nearly unity. No dependence on temperature is expected. On the other hand, if T2 is sufficiently below S so that the density of state is high at the crossing point, fluorescence quantum yield should be less than unity as triplet transfer is fecilitated. Again no temperature dependence is observed. But if T2 is nearly at the same energy as S, a barrier to inter-system crossing is expected and fluorescence yield will show temperature dependence. 

The temperature sensitivity of phosphorescence mainly arises from fast impurity quenching processes. At low temperatures and rigid glassy medium, emission is a rule rather than exception. 

---