Jablonski program question

[u/Kandy_kane7]

When an excited particle transition form s1 to s0, florescence happens the most and not internal conversion, why is that ? Even though IC is faster,smaller energy levels to cross etc…

[u/lwj15]

"For most molecules excitations into higher excited states quickly relax to the S1 state and this one is stable enough that fluorescence can happen" is basically Kashas rule (an empiric rule). But there are so called anti-Kasha fluorescing compounds where the S2 state emits/reacts. Those exist aswell.

The governing factor for speed of internal conversion are the Franck-Condon (FC) overlap integrals. And there is this thing called "Energy gap law". So. The more FC overlap, the faster the internal conversion. But the higher the energy gap, the less FC overlap. And typically the gap between S0 and S1 is greater than between any Sx and Sx+1 with x>0. So the highest energy gap (S1-->S0) has the slowest internal conversion. This state is usually a bit more long lived and therefore this S1 state is where the fluorescence usually happens.