A new class of iridium(III) complexes based on fluorine substituted 2,3′-bipyridine and pyridyltetrazolate derivatives: Synthesis, crystal structures, photoluminescent and electroluminescent properties

Abstract

The new cyclometalated iridium (III) complexes, (dfpypy)2Ir(BrPyTz) (a), (dfpypy)2Ir(CF3PyTz) (b), (tfpypy)2Ir(CF3PyTz) (c), and (dfpypy)2Ir(PzTz) (d), containing fluorine-substitued 2,3′-bipyridine as the main ligands and pyridyltetrazolate or pyrazinyltetrazolate derivatives as ancillary ligand were synthesized and characterized. The investigation of the crystal structures shows that there are intermolecular interactions in the iridium (III) complexes, including π - π stacking, the weak non - classical C–H⋯N hydrogen bonds and F … π interactions. Moreover, the UV–vis spectra display concentration-dependent character. As the solution concentration increases, two weak lower-energy absorption bands appear, suggesting there are aggregates between the ground state molecules. All complexes, except d that has emission peak at 467 nm, exhibit the blue photoluminescence (PL) with emission peak at about 440 nm in dichloromethane solution, but the films doped 10 wt % the complexes in polymethyl methacrylate (PMMA) show widened and distinctly tailed PL spectra between 500 and 650 nm, indicating there are the emission originated from aggregates. The organic light-emitting diodes (OLEDs) using a as emitters were fabricated by the vacuum evaporation process. Interestingly, we found that OLEDs doped with 5 wt % a show the voltage-dependent electroluminescent (EL) characteristics. At lower driving voltage EL spectra are basically the same as PL spectra in the solutions, the devices exhibit blue emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0.23,0.32). On the other hand, at higher driving voltage in addition to blue emission from the monomer, there is the broadened red-shifted emission with a peak value of 560 nm originated from the aggregates. The emission from the monomer and the aggregates are well matched to produce white OLEDs with CIE coordinates (0.33,0.39). Furthermore, as the applied voltage is further increased, the blue emission from the monomer is suppressed efficiently and the devices display the yellow emission derived from the aggregates with CIE value of (0.44,0.45).