How do ligands influence the quantum yields of cyclometalated platinum(ii) complexes, a theoretical research study

A series of cyclometalated platinum(ii) complexes have been investigated with the TDDFT method. These complexes have similar structures but distinct phosphorescence quantum yields. Theoretical calculations were carried out to explain the differences in quantum yields from the conjugation effect of the cyclometalated ligand, molecular rigidity and ligand-field strength of the monodentate ligand. The radiative decay rate constants (k_r) have been discussed with the oscillator strength (f_n), the strength of the spin-orbit coupling (SOC) interaction between the lowest energy triplet excited state (T₁) and singlet excited states (S_n), and the energy gaps between E(T₁) and E(S_n). To illustrate the nonradiative decay processes, the transition states (TS) between the triplet metal-centered state (³MC) and T₁ states have been optimized. In addition, the minimum energy crossing points (MECPs) between ³MC and the ground states (S₀) were optimized. Finally, the potential energy curves along the nonradiative decay pathways are simulated. To obtain a phosphorescent complex with a high quantum yield, the complex should retain molecular rigidity well in the S₁ and T₁ states, while showing significant structural distortion at the MECP structure.