Synthesis and characterization of phosphorescent isomeric iridium complexes with a rigid cyclometalating ligand

The synthesis, structures and photophysical properties of three phosphorescent isomeric heteroleptic iridium complexes are reported. The complexes are bis-cyclometalated with a rigid 9-tert-butyl-pyrazolo[1,5-f]phenanthridine (tpzp) ligand. The isomers (I1, I2 and I3) of the heteroleptic complex Ir(tpzp)2pic (pic = picolinate) were isolated from reactions and characterized by 2D NMR experiments and X-ray crystallography. The tpzp ligands in isomer I1 have a trans-N,N configuration, whereas the tpzp ligands in isomers I2 and I3 both have a cis-N,N configuration, with the three coordinated nitrogens being facial in I2 and meridinal in I3. The I1 complex was found to isomerize during the sublimation under high vacuum (0.5 × 10-6 torr) at 300 °C, giving a ratio of I1:I2:I3 = 3:1:6. Reduction potentials and DFT calculations of the Ir(tpzp)2pic isomers indicate that the LUMO for these heteroleptic complexes is localized on the picolinate moiety. The Ir(tpzp)2pic isomers all display broad, featureless emission at room temperature (?em ~ 530 nm) consistent with emission from a triplet metal-ligand to ligand (Ir-tpzp to picinolate) charge transfer (3MLLCT) state. Broad 3MLLCT emission (?em ~ 520 nm) is still observed from isomers I2 and I3 in rigid media at 77 K. However, under the same conditions structured blue phosphorescence (?em = 452 nm) is observed from isomer I1, consistent with emission from a metal perturbed 3LC state on the tpzp ligand.