Phosphorescence versus non-radiative de-excitation

In solution at room temperature, non-radiative de-excitation from the triplet state Ti, is predominant over radiative de-excitation called phosphorescence. In fact, the transition Ti — S0 is forbidden (but it can be observed because of spin-orbit coupling), and the radiative rate constant is thus very low. During such a slow process, the numerous collisions with solvent molecules favor intersystem crossing and vibrational relaxation in So- 

On the contrary, at low temperatures and/or in a rigid medium, phosphorescence can be observed. The lifetime of the triplet state may, under these conditions, be long enough to observe phosphorescence on a time-scale up to seconds, even minutes or more. 

Thermally activated delayed fluorescence Reverse intersystem crossing Ti — Si can 

3) The word crossing comes from the fact that the intersection between the potential energy surfaces corresponding to the Si and Tn states allows a molecule to cross from the Si state to the Tra state. The smaller the difference between the crossing point of these two 

Triplet-triplet annihilation In concentrated solutions, a collision between two molecules in the Ti state can provide enough energy to allow one of them to return to the Si state. Such a triplet-triplet annihilation thus leads to a delayed fluorescence emission (also called delayed fluorescence of P-type because it was observed for the first time with pyrene). The decay time constant of the delayed fluorescence process is half the lifetime of the triplet state in dilute solution, and the intensity has a characteristic quadratic dependence with excitation light intensity. 

Thermally activated delayed fluorescence Reverse intersystem crossing Ti — Si can occur when the energy difference between Si and Ti is small and when the lifetime of Ti is long enough. This results in emission with the same spectral distribution as normal fluorescence but with a much longer decay time constant because the molecules stay in the triplet state before emitting from Sp This fluorescence emission is thermally activated consequently, its efficiency increases with increasing tempera- 

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