Ancillary Ligand Steric\nEffects and Cyclometalating\nLigand Substituents Control Excited-State Decay Kinetics in Red-Phosphorescent\nPlatinum Complexes

Achieving high-efficiency red phosphorescence remains\na significant\nchallenge, especially in cyclometalated platinum complexes where radiative\nrates are inherently slower than their iridium counterparts. In this\nwork, six red-emitting cyclometalated platinum complexes of the formula\nPt(C<sup>∧</sup>N)[(Ar)acNac] (C<sup>∧</sup>N is the\ncyclometalating ligand, and (Ar)acNac is an aryl-substituted β-ketoiminate\nancillary ligand) were synthesized and characterized. Two C<sup>∧</sup>N ligands were employed, 1-phenylisoquinoline (piq) and its cyano-substituted\nanalogue 1-phenylisoquinoline-4-carbonitrile (piqCN), which both result\nin red phosphorescence in cyclometalated platinum complexes. These\nwere paired with three (Ar)acNac ligands that are sterically differentiated\nvia the <i>N</i>-aryl group, which is phenyl in the unsubstituted\nanalogue (Ph)acNac and 2,6-dimethylphenyl or 2,6-diisopropylphenyl\nin the sterically encumbered analogues. An in-depth photophysical\nanalysis of all compounds was performed and compared to the related\ncompounds with the acetylacetonate (acac) ancillary ligand. While\nquantum yields are modest in the unsubstituted (Ph)acNac complexes,\nsteric bulk on the β-ketoiminate has a pronounced effect on\nthe excited-state dynamics and can lead to photoluminescence quantum\nyields of more than 0.50 in both solution and transparent polymer\nfilms, with the photoluminescence λ<sub>max</sub> ∼ 630\nnm. We show that both steric effects on the electron-rich β-ketoiminate\nancillary ligands and the cyano substituent on the cyclometalating\nligand play a role in achieving high-efficiency phosphorescence in\nthe red region.