Emissive or nonemissive? a theoretical analysis of the phosphorescence efficiencies of cyclometalated platinum(II) complexes

Abstract

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 = l,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(RCNN)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 (SOC) and the electronic structures of these complexes at their respective optimized singlet ground (S 0) and first triplet (T opt 1 ,) 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 CH 2Cl 2 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 CH 2Cl 2 solution at 298 K; and 3) extension of π 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 pseudoJahn-Teller effects are operative in complexes 2 and 3, respectively, on going from the optimized S 0 ground state to the optimized T opt 1, excited state, and thus lead to large excitedstate structural distortions and hence fast nonradiative decay. Furthermore, a strong-field ligand may push the two different occupied d orbitais 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. © 2009 Wiley-VCH Verlag GmbH & Co. KGaA.