2,4,6-Triphenyl-1,3,5-Triazine Functionalized fac-tris(2-phenylpyridine)Ir(III) Complexes with Polarity Sensitive T1 State Properties

Here we report the synthesis and study of three Ir(III) complexes, which are derivatives of fac‑Ir(ppy)3 (Hppy – 2-phenylpyridine). In these derivatives the third position of the pyridine ring of one of the ppy ligands is functionalized with a tolyl (Ir‑tol), or triphenyl triazine group (Ir‑meta and Ir‑para). Ir‑meta has a 3-(3,5-diphenyl-2,4,6-triazinyl)phenyl group on the ppy pyridyl, giving a meta disposition of the triazine to the ppy ligand and Ir‑para has a 4-(3,5-diphenyl-2,4,6-triazinyl)phenyl group bound to the ppy ligand putting the triazine moiety para to the ppy ligand. Complex Ir‑tol shows electrochemical properties and phosphorescence characteristics nearly identical to those of the unfunctionalized fac-Ir(ppy)3. Complexes Ir‑meta and Ir‑para show reduction potentials that are anodically shifted from those of Ir‑tol by ca. 0.6 V, consistent with reduction centered on the triazine moiety. In the non-polar media, the lowest triplet state (T1) of Ir‑meta and Ir‑para has mixed metal-to-ligand charge transfer (MLCT) and ligand centered (LC) character localized on the Ir-ppy moiety of the functionalized ligand. With an increase in solvent polarity. i.e. CH2Cl2 and dimethyl sulfoxide (DMSO), the T1 state of these complexes gains character of intraligand charge transfer (ILCT) from Ir-ppy to the triazine moiety of the functionalized ligand. This change in excited state character is accompanied by a distortion in the molecular geometry towards planarization of the acceptor and donor units. Such polarity-controlled modulation of T1 state character is traced with transient absorption (TA) spectroscopy and is found to markedly affect photoluminescent properties of the complexes.