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.