Cyclometalated Ir(iii) complexes containing quinoline–benzimidazole-based N^N ancillary ligands: structural and luminescence modulation by varying the substituent groups or the protonation/deprotonation state of imidazole units

Abstract

Cyclometalated Ir(III) complexes [Ir(dfppy)₂(qbiH)](PF₆) (1), [Ir(dfppy)₂(qbim)](PF₆)·H₂O (2), [Ir(dfppy)₂(qbio)](PF₆) (3) and [Ir(dfppy)₂(qbi)] (4) have been designed and prepared, in which the N^N ligands qbiH, qbim and qbio incorporate different substituent groups R on their imidazole units (H atom, CH₃ group and n-C₈H₁₇ group, respectively) in order to explore the influence of the substituent groups R and the protonation/deprotonation state of imidazole units in these Ir(III) complexes on their structures and luminescence behaviors. Crystal structures indicate that an {Ir(dfppy)₂}⁺ unit is coordinated by neutral ligands qbiH in 1, qbim in 2 and qbio in 3, while a qbi⁻ anion in 4. These Ir(III) complexes show clearly different molecular stacking modes. In compound 1, neighboring [Ir(dfppy)₂(qbiH)]⁺ cations are linked into a supramolecular chain through π⋯π stacking interactions between adjacent dfppy⁻/qbiH ligands. In 2 and 4, two neighboring iridium complex units connect each other through π⋯π stacking interactions between dfppy⁻ ligands in the former, while between qbi⁻ ligands in the latter, forming supramolecular dimers. Compared to 1, 2 and 4, compound 3 only exhibits intermolecular van der Waals interactions. At room temperature, these Ir(III) complexes in CH₂Cl₂ reveal phosphorescence with a mixing of ³MLCT and ³LC characters, emissions at 558 and 585 nm for 1, 572 (or 573) and 600 nm for 2 and 3, and 546 nm for 4. Compared to 1–3, compound 4 displays relatively weak luminescence intensity. Interestingly, upon addition of NEt₃/TFA, both 1 and 4 in CH₂Cl₂ can switch their luminescence between strong emission at 558 nm and weak emission at 546 nm, due to their acid-/base-induced structural interconversion between the protonation state and the deprotonation state of the qbiH ligand. The emissions of 1–4 in the solid state reveal different degrees of the red shift compared to their corresponding emissions in CH₂Cl₂, the broad emission bands at 542, 572 and 611 nm for 1, 553, 581 and 612 nm for 2, 544, 578 and 630 nm for 3, and 595 and 633 nm for 4. Based on the crystal structures of 1–4, this work discusses the luminescence modulation of these Ir(III) complexes by varying their substituent groups or the protonation/deprotonation state of the imidazole units.