Charged Bis-Cyclometalated Iridium(III) Complexes with Carbene-Based Ancillary Ligands
Charged cydometalated (C<^>N) iridium(III) complexes with carbene-based ancillary ligands are a promising family of deep-blue phosphorescent compounds. Their emission properties are controlled primarily by the main C<^>N ligands, in contrast to the classical design of charged complexes where N<^>N ancillary ligands with low-energy pi* orbitals, such as 2,2'-bipyridine, are generally used for this purpose. Herein we report two series of charged iridium complexes with various carbene-based ancillary ligands. In the first series the CAN ligand is 2-phenylpyridine, whereas in the second one it is 2-(2,4-difluorophenyl)-pyridine. One biscarbene (:C<^>C:) and four different pyridine carbene (N<^>C:) chelators are used as bidentate ancillary ligands in each series. Synthesis, X-ray crystal structures, and photophysical and electrochemical properties of the two series of complexes are described. At room temperature, the :C<^>C: complexes show much larger photoluminescence quantum yields (Phi(PL)) of ca. 30%, compared to the N<^>C: analogues (around 1%). On the contrary, all of the investigated complexes are bright emitters in the solid state both at room temperature (1% poly(methyl methacrylate) matrix, Phi(PL) 30-60%) and at 77 K. Density functional theory calculations are used to rationalize the differences in the photophysical behavior observed upon change of the ancillary ligands. The N<^>C:-type complexes possess a low-lying triplet metal-centered ((MC)-M-3) state mainly deactivating the excited state through nonradiative processes; in contrast, no such state is present for the :C<^>C: analogues. This finding is supported by temperature-dependent excited-state lifetime measurements made on representative N<^>C: and :C<^>C: complexes.