Cyclometalated Ir(iii) complexes based on 2-(2,4-difluorophenyl)-pyridine and 2,2′-(2-phenyl-1H-imidazole-4,5-diyl)dipyridine: acid/base-induced structural transformation and luminescence switching, and photocatalytic activity for hydrogen evolution

Abstract

Based on ligands dfppyH and pidpyH, cyclometalated Ir(III) complexes [Ir(dfppy)₂(pidpyH)](PF₆) (1·PF₆) and [Ir(dfppy)₂(pidpy)] (2) have been synthesized. The crystal structures indicate that each {Ir(dfppy)₂}⁺ unit is coordinated by a neutral ligand pidpyH in 1·PF₆, while by a pidpy⁻ anion in 2. The packing structure of 1·PF₆ only exhibits electrostatic interactions and van der Waals interactions among [Ir(dfppy)₂(pidpyH)]⁺ cations and PF₆⁻ ions. In contrast, the neighboring molecules in 2 are linked into a supramolecular chain structure through aromatic stacking interactions between two dfppy⁻ ligands. In solution, 1·PF₆ and 2 show acid/base-induced structural transformation due to the protonation/deprotonation of their pyridyl groups and/or imidazole units, which can be confirmed by their ¹H NMR spectra. At room temperature, compounds 1·PF₆, 2 and pidpyH in CH₂Cl₂ reveal TFA-induced luminescence switching behaviors, from a non-luminescence state to a luminescence state with an emission at 582 nm for both 1·PF₆ and 2, and emission switching from 392 nm to 502 nm for pidpyH. These switching behaviors are associated with the protonation of pyridyl groups and/or imidazole units in 1·PF₆, 2 and pidpyH. Moreover, compounds 1·PF₆ and 2 were used as photosensitizers (PS) for reduction of water to hydrogen under the same experimental conditions. It was found that the amount of evolved hydrogen and the PS turnover number are 512 μmol and 102 for 1·PF₆, and 131 μmol and 26 for 2, respectively. Thus, compound 1·PF₆ has better photocatalytic activity than 2. In this paper, we discuss the modulation of luminescence and photocatalytic activities of 1·PF₆ and 2 by varying the coordination mode and/or protonation extent of pidpyH/pidpy⁻ ligands.