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.