The rate of
intersystem crossing is relatively large if the
radiationless transition involves a change of orbital type.
Note:
A typical case is the transition from the lowest
singlet state to the
tripletmanifold,
e.g.,
1π,π* →
3n, π* is faster than
1π, π* →
3π, π* and
1n, π* →
3π, π* is faster than
1n, π* →
3n, π*.
Source:
PAC, 2007, 79, 293
(Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006))
on page 331