Emissive or Nonemissive? A Theoretical Analysis of the Phosphorescence Efficiencies of Cyclometalated Platinum(II) Complexes

We herein report a theoretical analysis based on a density functional theory/time-dependent density functional theory (DFT/TDDFT) approach to understand the different phosphorescence efficiencies of a family of cyclometalated platinum(II) complexes: [Pt(NCN)Cl] 1; NCN = 1,3-bis(2-pyridyl)phenyl(-)), [Pt(CNN)Cl] (2; CNN=6-phenyl-2,2'-bipyridyl(-)), [Pt(CNC)(CNPh)] (3; CNC=2,6-diphenylpyridyl(2-)) [Pt(R-CNN)Cl] (4; R-CNN=3-(6'-(2 ''-naphthyl)-2'-pyridyl)isoquinolinyl(-)), and [Pt(R-CNC)(CNPh)] (5; R-CNC=2,6bis(2'-naphthyl)pyridyl(2-)). By considering both the spin-orbit coupling (SCIC) and the electronic structures of these complexes at their respective optimized singlet ground (SO) and first triplet (T-opt(1)) I) excited states, we were able to rationalize the experimental findings that 1) 1 is a strong emitter while its isomer 2 is only weakly emissive in CH2Cl2 solution at room temperature; 2) although the cyclometalated ligand of 3 has a higher ligand-field strength than that of 1, 3 is nonemissive in CH2Cl2 solution at 298 K; and 3) extension of pi conjugation at the lateral aryl rings of the cyclometalated ligands of 2 and 3 to give 4 and 5, respectively, leads to increased emission quantum yields under the same conditions. We found that Jahn-Teller and pseudo-Jahn-Teller effects are operative in complexes 2 and 3, respectively, on going from the optimized S, ground state to the optimized T-opt(1) excited state, and thus lead to large excited-state structural distortions and hence fast nonradiative decay. Furthermore, a strong-field ligand may push the two different occupied d orbitals so far apart that the SOC effect is small and the radiative decay rate is slow. This work is an example of electronic-structure-driven tuning of the phosphorescence efficiency, and the DFT/TDDFT approach is demonstrated to be a versatile tool for the design of phosphorescent materials with target characteristics.