# Why is a spin change favourable in intersystem crossing?

All phosphorescent molecules go through the following transitions:
$$\text{excited singlet state}$$ $$\Bigg\downarrow$$
$$\text{[intersystem crossing]}$$
$$\Bigg\downarrow$$
$$\text{ excited triplet state}$$
Why does that spin change always occur, given it’s a “forbidden” transition and, thus, unfavourable and much less likely to take place?

• Well, if there's no easy path, unfavourable needs to be taken. Apr 18 '18 at 22:47
• The triplet phosphorescence is also spin forbidden as is $S_1\to T$. As it actually occurs there is an allowed spin changing mechanism that lets this happen. This is spin-orbit coupling. See this answer for an explanation of radiationless transitions in general of which intersystem crossing is an example. chemistry.stackexchange.com/questions/28883/… Apr 19 '18 at 6:23
• It doesn’t always occur,nor favorable.
– Greg
Apr 20 '18 at 8:19