Quantum-Chemical Insights into the Phosphorescence Efficiencies of Blue-Emitting Platinum Complexes with Phenylene-Bridged Pincer Ligands

Blue phosphorescent platinum complexes with phenylene-bridged pincer ligands, [Pt­(dmib)­Cl] [1; dmib = m-bis­(methylimidazolyl)­benzene], [Pt­(mizb)­Cl] [2; mizb = bis­(N-methylimidazolium)­benzene], and [Pt­(dpzb)­Cl] [3; dpzb = m-bis­(3,5-dimethylpyrazolyl)­benzene], have been investigated theoretically to rationalize the marked differences of their phosphorescence efficiencies. On the basis of density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, the geometrical and electronic structures, absorption and emission properties, and radiative and nonradiative processes are analyzed in detail. The emission from the emissive lowest triplet state (T₁) originates from a mixture of metal-to-ligand charge-transfer (³MLCT) and intraligand charge-transfer (³ILCT) states. The calculated radiative decay rate constants of T₁ of the complexes are comparable and in the same order of magnitude with the experimental measurements. Therefore, the potential energy profiles for the deactivation processes from T₁ via temperature-independent and -dependent pathways are explored to reveal the effect of nonradiative decay on phosphorescence. The calculated results indicate that the very weak emission of 3 could be ascribed to the deactivation process via the metal-centered (³MC) state, which can be readily accessible via a spontaneous process from the T₁ state. This work provides more in-depth insight into the nature of the emissive excited state, shielding light on a better understanding of the excited-state behavior of phosphorescent platinum complexes.