Syntheses, Structures, and Electronic and Optical Properties of Platinum(II) Complexes of 1,3-Bis(imino)benzene-Derived Pincer Ligands

Abstract

Six new Pt^II complexes of 5-substituted 1,3-bis­(<i>N</i>-methylimino)­benzene (bIBH)-derived ligands have been synthesized by direct reactions with K₂Pt^IICl₄ in glacial acetic acid. The ethenylene groups in the two styryl-substituted complexes undergo <i>E</i>–<i>Z</i> photoisomerization, as revealed by ¹H NMR spectroscopy. The UV–vis spectra of the styryl-/aryl-substituted complexes show intense high-energy bands due to ligand-based π → π* transitions and a number of weaker bands at lower energies in the 300–500 nm region. Time-dependent density functional theory (TD-DFT) calculations indicate that the latter absorptions have mixed intraligand π → π* and Pt­(d) → π* metal-to-ligand charge-transfer (MLCT) character. Four of the new complexes give vibronically structured emission profiles in fluid solution at room temperature. The luminescence quantum yields and lifetimes in nondegassed dichloromethane are in the ranges 0.1–0.2% and 159–170 ns, respectively. In contrast, the styryl-substituted complexes are nonemissive, due to excited-state quenching by <i>E</i> → <i>Z</i> isomerization. DFT indicates that the emissive triplet state has mixed ligand-to-ligand charge-transfer and MLCT character, with a geometry distorted with respect to the symmetric ground state. Single-crystal X-ray structures have been determined for two of the complexes, showing N–Pt–N angles of ca. 159°. The complex of an unsubstituted bIB^– ligand is deep red and forms columns with short Pt···Pt distances of ca. 3.55 Å, indicating weak intermetallic interactions