Collisonally induced intersystem crossing
There are two factors that I recall that can enhance intersystem crossing (ISC, the conversion of an excited singlet state to an excited triplet state, or vice-versa; $\ce{S_1 &lt;=&gt; T_1}$).  One is the presence of a "heavy atom" such as bromine or iodine in the colliding molecule (or the excited molecule itself).  The other is the presence of a paramagnetic species.

Both types of molecules enhance ISC by relaxing the quantum rules which forbid  singlet-triplet conversions. When a heavy atom or paramagnetic molecule collides with a molecule in its excited state it can facilitate the electron spin flip (e.g. converting paired electrons to unpaired electrons, or vice versa) through spin-orbit coupling.  It is this spin-orbit coupling that relaxes the rules forbidding direct conversion between $\ce{S1}$ and $\ce{T1}$ states.  In essence spin-orbit coupling leads to a mixing of the singlet and triplet wavefunctions, thereby allowing singlet-triplet conversion.