The organic light-emitting diode (OLED) materials, based on fluorescence or phosphorescence emitters, have been successfully applied in displays and lighting. The quantum yield of OLED devices, however, is usually low due to the limitation of spin statistics of electric current-generated excited-states and the exciton deactivations at high current density. To take advantage of all the singlet and triplet excitons and then achieve 100% internal quantum efficiency (IQE), two strategies have been proposed based on "singlet-trapping" and "triplet-trapping", respectively. The first one attempts to convert the singlet excitons to triplet excitons through efficient intersystem crossing (ISC) induced by strong spin-orbit coupling (SOC) and therefore, obtains phosphorescence materials that emit light from the lowest triplet excited-state. The second strategy tries to achieve "delayed" fluorescence by converting triplet excitons to singlet excitons through reverse ISC (RISC) process with the help of thermal activation. In this regard, the performance of OLED device is inseparable from the ISC and RISC processes. Therefore, it is of particular importance for us to study the mechanism and quantitatively predict the rate coefficients.