Internal Conversion:
Once the molecules which are excited to an electronic state (say S2) higher than the S1 state reach the vibrational ground state in the electronic level, these could pass to a higher vibrational level of a lower excited state (S1) that has the similar energy. This process is referred to as internal conversion. The molecule can continue to lose energy in this state within a nonradiative way (vibrational relaxation) until it reaches the lowest vibrational level in this excited state.
Intersystem Crossing
In rare occasions, the molecule within the vibrational states of a singlet excited state might cross over to a vibrational level of a triplet state if the two have similar energy. This process is known intersystem crossing. This spin-exchange mechanism might also lead, by very rarely, to the crossing over from a triplet state to a singlet state.