Modifying Emission Spectral Bandwidth of Phosphorescent Platinum(II) Complexes Through Synthetic Control

The design, synthesis, and characterization of a series of tetradentate cyclometalated Pt(II) complexes are reported. The platinum complexes have the general structure Pt(ppz-O-CbPy-R), where a tetradentate cyclometalating ligand is consisting of ppz (3,5-dimethyl-1-phenyl-pyrazole), CbPy (carbazolylpyridine) components, and an oxygen bridging group. Variations of the R group on the pyridyl ring with various electron withdrawing and donating substituents are shown to have profound effects on the photophysical properties of Pt complexes. Electrochemical analysis indicates that reduction process occurs mainly on the electron-accepting pyridyl group, and the irreversible oxidation process is primarily localized on the metal-phenyl portions. The studies of their photophysical properties indicate that the lowest excited state of the platinum complexes is a ligand-centered ³π-π* state with minor to significant ¹MLCT/³MLCT character and are strongly dependent on the nature of the electron-accepting pyridyl moiety. A systematic study of the substituent effects on the pyridyl ring demonstrates that the T₁ state properties can be tuned by altering the functionality and positions of substituents. Importantly, it is revealed that how the emission spectra of the Pt(II) complexes can be significantly narrowed and why it can be achieved by incorporating an electron-donating group on the 4-position of the pyridyl ring. Most of the Pt(II) complexes reported here are highly emissive at room temperature in dichloromethane solutions (Φ = 1.1-95%) and in doped PMMA films (Φ = 29-88%) with luminescent lifetimes in the microsecond range (τ = 0.6-13.5 μs in solution and 0.9-11.3 μs in thin film respectively) and λ_max = 442-568 nm and 440-544 nm in solution and thin film, respectively. Moreover, these complexes are neutral and thermally stable for sublimation, indicating that they can be useful for display and solid-state lighting applications.