Stereoisomeric Homo- and Hetero-Binuclear Iridium(III) Complexes with 3-Oxidopicolinate Bridging Ligand and Their Application in OLEDs

The small and simple 3-hydroxypyridine-2-carboxylic acid (Hpic-OH) is explored as asymmetric bridging ligand for the synthesis of neutral binuclear cyclometalated iridium(III) complexes. Once fully deprotonated the picO2– ligand can act as ancillary ligand toward two iridium centers adopting both the N^O– and O^O– chelation modes. To tune the energy of the excited states within such binuclear complexes, the 2-(2,4-difluorophenyl)pyridine (Hdfppy) and the 2-phenylbenzothiazole (Hpbtz) are used as cyclometalating ligands to respectively obtain both blue- or orange-emissive homo-cyclometalated complexes (BB and YY, with formula [Ir(dfppy)2]2(picO) and [Ir(pbtz)2]2(picO), respectively). Moreover, for the first time, short-bridged hetero-cyclometalated binuclear complexes are also obtained (BY and YB, with formula [Ir(dfppy)2](picO)[Ir(pbtz)2] and [Ir(pbtz)2](picO)[Ir(dfppy)2]). Depending on the reciprocal arrangement of the cyclometalating ligands on the two sides of the small picolinate bridge, two couples of diastereoisomers are obtained and efficiently separated, as proved by combined NMR and DFT studies. The reported binuclear complexes are highly emissive with photoluminescence quantum yields (PLQYs) up to 67%, which are comparable to those of their mononuclear analogues (B and Y). Due to the full reversibility of their redox processes, all the complexes are also tested in solution-processed organic light-emitting diodes, providing unique OLEDs based on hetero-binuclear cyclometalated iridium(III) complexes