Intersystem and Reverse-Intersystem Crossings in Organic Light-Emitting Diodes.

Using the first-order perturbation theory and exciton-spin-orbit-photon-molecular-vibration-interaction (ESOPMVI) operator, the rates of reverse intersystem crossing (RISC) and thermally activated delayed fluorescence (TADF) are derived. It is shown that the pre-exponential factor of the rate of TADF is not an absolute constant as is commonly assumed. Instead, it depends on the square of the atomic number and the exchange energy, but it also depends on the triplet excitonic Bohr radius as