The quantum yield of fluorescence (QY) is dependent on the rate of the emission process divided by the sum of the rates of all other deactivation processes
QY =
kf / kf
+ ki + kx
kf is the rate of fluorescence, ki is the rate of radiationless decay and kx is the rate of intersystem crossing.
Another way to think about QY is:
QY = Number of emitted photons / Number of
absorbed photons
If the rates of these other processes are slow compared to kf then QY is high
However, if the rates of these other processes are fast
compared to kf then
QY is low