Structure-Emission Property Relationships in Cyclometalated Pt(II) β-Diketonate Complexes

Extending the ligand π-system of phosphorescent (C^∧C*) or (C^∧N) cyclometalated platinum(II) β-diketonate complexes can lead to large and seemingly abrupt variations of the photophysical properties such as triplet quantum yields and phosphorescence lifetimes. Quantum chemical studies using methods including elements from density functional theory (DFT) and multireference configuration interaction (MRCI) as well as spin-orbit coupling (SOC) provide a rationale for these observations. In the Franck-Condon region, the first excited singlet states (S₁) of these complexes are characterized by mixed metal-to-ligand charge-transfer (MLCT) and ligand-centered (LC) excitations. With increasing extension of the effective π-system, the lowest-lying triplet state yields more and more LC character, thus leading to a decrease of the phosphorescence rate constant. The ability to undergo efficient intersystem crossing from S₁ to T₁ is not diminished as the S₁ state largely retains its character. In the N-heterocyclic carbene (NHC) complexes investigated here, at least two triplet states are found energetically below the S₁ state. Out-of-plane distortion enhances the probability for nonradiative decay of the triplet population. In the smaller compounds emitting in the violet or blue spectral region, the phosphorescent state is separated from the lowest-lying dark metal-centered (MC) triplet state by a small barrier only, explaining their experimentally observed low photoluminescence quantum yields in liquid solution. The semiempirical DFT/MRCI-R2018 Hamiltonian employed in our studies proves well-suited for investigating the absorption and emission properties of these platinum(II) complexes. Generally, good agreement is observed between our calculated data and the experimental findings.