Rigidification of cyclometalating ligands for reverse saturable absorption (RSA) materials development

Abstract

Recently, cyclometalated iridium (III) complexes have been studied for use as non-linear optical (NLO) materials. Reports of both two photon absorption (2PA) and reverse saturable absorption (RSA) properties for cyclometalated iridium (III) complexes warrant their continued development for enhanced performance. Cyclometalating ligands used with iridium commonly ligate through an anionic carbon on a phenyl ring and a nitrogen on an adjacent ring, such as 2- phenylpyridine (ppy), 1-phenylpyrazole (ppz), 2-phenylbenzoxazole (pbo), and their derivatives. These ligands contain a rotational degree of freedom around the carbon-carbon bond that links the two rings. This flexibility could induce a nonradiative decay pathway in cyclometalated iridium complexes containing these ligands. Benzo[h]quinoline (bzq) was chosen as a more rigid, but structurally analogous, comparison to ppy. The complexes [Ir^III(ppy)₂(acac)]⁰ , [Ir^III(bzq)₂(acac)]⁰ , [Ir^III(ppy)₂(bpy)]PF₆, and [Ir^III(bzq)₂(bpy)]PF₆, where acac is acetylacetonate and bpy is 2,2’- bipyridine, were synthesized and characterized to assess the impact of enhanced cyclometalating rigidity in both neutral and charged iridium complexes. Through transient absorption spectroscopy, it was found that excited state localization and lifetime was drastically affected by the choice of either the acac or bpy ligand. Furthermore, the more rigid cyclometalating ligand, bzq, exhibited smaller non-radiative decay rates than the more flexible ppy analogue.