Theoretical studies on electronic structures, spectra and charge transporting properties of a series of Pt(C^ΛN)₂ complexes

We report a quantum-chemical study of electronic, optical and charge transporting properties of four platinum (II) complexes, Pt(C^ΛN)₂ (C^ΛN = phenylpyridine or thiophenepyridine). The lowest-lying absorptions at 442, 440, 447 and 429 nm are all attributed to the mixed transition characters of metal-to-ligand charge transfer (MLCT) and ligand-centered (LC) π → π* transition. While, unexpectedly, the lowest-lying phosphorescent emissions at 663, 660, 675 and 742 nm are mainly from metal-to-ligand charge transfer (³MLCT) ligand-centered (LC) π → π* transition. Ionization potential (IP), electron affinities (EA) and reorganization energy (λ_{hole/electron}) were obtained to evaluate the charge transfer and balance properties between hole and electron. The calculated results reveal that the small structure changes of the charged states relative to the neutral ones, and the smaller IP and EA values indicate that they exhibit a relatively high structural stability versus the injection of one charge and have better holes than electron-injection abilities. The smaller λ values and the smaller differences (within 0.06 eV) between λ_hole and λ_electron indicate the better transfer balance between holes and electrons. Moreover, the attached methyl groups slightly improve the hole-injection ability and deduce the electron-injection ability. The strong phosphorescence emission and the favorable transporting properties compared with other reported complexes indicate their potential applications in as emitters OLEDs.