Photoluminescence color tuning of phosphorescent bis-cyclometalated iridium(III) complexes by ancillary ligand replacement

A series of bis-cyclometalated iridium(III) complexes bearing various types of 1,3-diketonate ancillary ligands were prepared, and their photoluminescent (PL) properties were investigated, especially focusing on the emission color tuning. When the dipivaloylmethanate ancillary ligand (O^O-1a) was replaced by conjugated 1,3-diketonates such as 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (O^O-1b) and 1,3-bis(4-(dibenzo[b,d]furan-4-yl)phenyl)propane-1,3-dionate (O^O-1c), the blue-emitting bis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C2′]iridium(III) (Ir-1) and bluish green-emitting bis[2-phenylpyridinato-N,C2′]iridium(III) (Ir-2) complexes exhibited significantly red-shifted phosphorescence in solution (λPL; 474–604 nm and 521–661 nm for Ir-1a–c and Ir-2a–c, respectively: the subscripts a, b, and c corresponding to O^O-1a–c). On the other hand, the ancillary ligand replacement was less effective on tuning the emission color of the green-emitting bis[2-(2,4-bis(trifluoromethyl)phenyl)pyridinato-N,C2′]iridium(III) complex (λPL; 541–566 nm for Ir-3a–c), and no PL color change was observed for the red-emitting bis[1-(dibenzo[b,d]furan-4-yl)isoquinolinato-N,C3′]iridium(III) complex (Ir-4). The X-ray crystallographic analysis for Ir-1a–c revealed that these complexes adopt cis-C,C and trans-N,N configuration, indicating that the coordination geometry around the iridium center is not a main factor for the emission spectral differences caused by the O^O ancillary ligands. Furthermore, the ancillary ligand effect on the PL properties of the present iridium(III) complexes is independent on solvent polarity and concentrations. Taking these results into consideration, the ancillary ligand effect should be attributed to the electronic structures of the complexes, and the inter-ligand energy transfer from the triplet metal-to-ligand charge transfer level at the cyclometalated ligand (3MLCTC^N) to the ancillary ligand-related triplet level (3LCO^O or 3MLCTO^O) should play a significant role in the triplet exciton formation. The PL properties of Ir-1–Ir-4 in PMMA films were also investigated, and the remarkable blue shifts of λPLs due to rigidochromism were observed for Ir-1b,c and Ir-2b,c that have the aromatic O^O ancillary ligands. It was found that some of the complexes developed here exhibited larger ΦPLs in PMMA films than in toluene solutions.

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► Ir(III) complexes with various 1,3-diketonate ancillary ligands were prepared.
► The aromatic 1,3-diketonate ancillary ligands give rise to red-shifted PL.
► For the red-emitting complexes, the PL color is independent on the ancillary ligand.
► The ancillary ligand effect on PL is attributed to the inter-ligand energy transfer.