Synthesis, crystal structure and photophysical properties of cyclometalated platinum(II) complexes with enhanced intramolecular donor–acceptor (D–A) interaction

Nine cyclometalated Pt(II) complexes, i.e. [(Ln)PtCl], [(Ln)Pt(CCC6H5)], [(Ln)Pt(CC-TPA)] (n = 1–3) (where HL1 = 4-phenyl-6-phenyl-2,2′-bipyridine, HL2 = 4-ethoxycarbonyl-6-phenyl-2,2′-bipyridine, HL3 = 4-ethoxycarbonyl-6-phenyl-2-(2′-pyrazinyl)pyridine and TPA = triphenylamine), have been successfully synthesized and verified by 1H NMR, MS and elemental analysis. The X-ray crystallography of [(L2)Pt(CC-TPA)] reveals that the coordinate geometry of Pt atom is a distorted square planar configuration with bond distances and angles in the normal range and the presence of the metal–metal and π–π interactions. The introduction of the electron-withdrawing unit, such as ethoxycarbonyl and/or pyrazinyl groups, into the (C^N^N) ligand with the electron-donating triphenylamine acetylide as the secondary ligand, systematically forms the strong D–A systems, which show the extended responses to the visible light resulting from the significant splitting and red-shift of the metal-to-ligand charge transfer (MLCT) and the ligand-to-ligand charge transfer (LLCT) transition bands in the absorption spectra. [(Ln)PtCl] and [(Ln)Pt(CC-Ph)] complexes display long-lived yellow to deep red phosphorescence in fluid solution at room temperature, but the emissions of [(Ln)Pt(CC-TPA)] complexes are quenched by the intramolecular photoinduced electron transfer (PET) process with the TPA group acting as an electron donor. Their electrochemical behaviors are consistent with the spectroscopic results.

D–A structured Pt(II) complexes: The neutral-charged Pt(II) complex with 4-ethoxycarbonyl-6-phenyl-2-(2′-pyrazinyl)pyridine as the cyclometalating ligand and triphenylamine acetylide as the secondary ligand shows an extensive UV–Vis light absorption band, which tailed over 750 nm.Figure optionsDownload as PowerPoint slideHighlights
► (C^N^N) ligand with ethoxycarbonyl and pyrazinyl groups as the electron-acceptor.
► Triphenylamine acetylide as the electron-donor.
► Cyclometalated Pt(II) complexes show the extended response to the visible light.
► Chemically tunable phosphorescence properties.