Absorption of light (W or visible) by the ground state of a molecule (So) generates electronically excited states, either directly (the singlet states) or after intersystem crossing from the singlet manifold (the triplet states). Alternatively, triplet states may be generated by energy transfer from another excited state (a sensitiser). In both multiplicities, very fast internal conversion leads to the lowest states (S1 and TI respectively). These states, although still quite short lived (typical lifetime 610-8 s for S1 and .
