Phosphorescence, another important type of excited state radiation transition, is the radiation emitted when excited molecules are deactivated to low energy states with different multiplicities. Phosphorescence is usually observed from the first excitation triplet (T;). The radiation emitted during the transition to the ground state (S,) (Kasha rule).
T1→So + hv(Phosphor)
General phosphorescence is much weaker than fluorescence, this is because the T state that emits phosphorescence is usually not easy to form by directly absorbing photons from the S state, and the T state is mainly formed from the S state through the system. Due to the competition of fluorescence and internal conversion process, the quantum yield of the transmutation from S1 state to T state system is greatly reduced. The quantum yield of T state formation is, for example, is
,= Lu st=kst((kst+kic+kt) (2.9)
On the other hand, unlike the fluorescence process, the phosphorescence emission process is a spin-forbidden process. The spin barrier factor is usually 10-5 to 10-8. So the phosphor emission rate constant k. Much smaller than the fluorescence rate constant k. kp ranges from 10-1 to 103s-1. The phosphor rate constant k is defined as the reciprocal of the natural phosphor radiation lifetime t (the lifetime of the excited triplet assuming only phosphor deactivation and no other deactivation process), i.e. =1/r$(2.10)
= medium st.tp/ medium p (2.11)
tp is the lifetime of the triplet, i.e. the time it takes for the triplet to be inactivated to 1/e. tp=17>kd (2.12)
It’s the inverse of the sum of the deactivation rate constants. k will be increased by the heavy atom effect or the influence of paramagnetic molecules.