Unexpected photo-induced oxidative cyclization and luminescence switching in molecular aggregates of two acylhydrazone-based Ir(iii) complexes

Abstract

Two iridium complexes, namely [Ir(ppy)₂(mbpymbh)]PF₆ (1) and [Ir(dfppy)₂(mbpymbh)]PF₆ (2), exhibit unexpected photoreactivity, although the incorporated ligands ppyH, dfppyH and mbpymbh have no inherent photochemical activity. The molecular aggregates of 1 and 2, formed in a CH₃CN–H₂O (v/v = 1/9) mixed solvent, show photo-induced luminescence switching, with the emission-color change from red to orange for 1, while a transition from the off state to on state with an emission band at 566 nm for 2. These changes in luminescence are assigned to photo-induced oxidative cyclization of the acylhydrazone units in 1 and 2, thus generating photoproducts 1-O and 2-O. Complexes 1-O and 2-O were isolated and identified from their crystal structures. We discussed both the effect factors and the reaction mechanism of the photo-oxidation reactions for complexes 1 and 2, as well as the luminescence properties of complexes 1, 2, 1-O and 2-O. The discovery of photo-induced oxidative cyclization in this work not only provides a new class of photo-responsive luminescent materials, but also offers a novel synthesis method for 1,3,4-oxadiazole-based Ir(III) complexes.