Study of intersystem crossing mechanism in organic materials

Intersystem crossing rate from singlet excited state to triplet excited state of an organic molecule has been derived using exciton-spin-orbit-molecular vibration interaction as a perturbation operator. Incorporation of heavy metal atom enhances the spin-orbit interaction and hence the intersystem crossing rate because it depends on the square of the heaviest atomic number. We found that in the presence of heavy atom the singlet-triplet energy difference still plays an influential role in the intersystem crossing process. The derived exciton-spin-orbit-molecular vibration interaction operator flips the spin of the singlet exciton to triplet exciton after photoexcitaion from the singlet ground state with the assistance of molecular vibrational energy. From this operator an expression for the intersystem crossing rate is derived and calculated in some organic solids.