Two heteroleptic Ir(iii)–bisthienylethene compounds: syntheses, structures and aggregation-induced luminescence

Abstract

A new bisthienylethene 2-(2-hydroxynaphthyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1H-imidazole (hnbdtiH) has been synthesized and applied for the coordination-assembly generation of two heteroleptic complexes [Ir(dfppy)₂(hnbdti)]·2CH₃OH (1) and [Ir(ppy)₂(hnbdti)]·CH₃OH (2) [dfppyH = 2-(2,4-difluorophenyl)-pyridine, ppyH = 2-phenyl-pyridine]. In the crystal structure of hnbdtiH, neighboring molecules are linked into a helical chain through N–H⋯π interactions between imidazole and naphthol rings. In 1 and 2, the {Ir(dfppy)₂}⁺ or {Ir(ppy)₂}⁺ unit is chelated by a hnbdti⁻ ligand using both N and O binding sites, and the intramolecular aromatic stacking interactions are formed between the thiophene ring and the 2,4-difluorophenyl/phenyl group. The crystal structures of 1 and 2 indicate that neighboring molecules are connected by CH₃OH molecules through hydrogen bond interactions, forming the [Ir⋯(CH₃OH)₄⋯Ir] dimer in the former, and the [Ir⋯(CH₃OH)₂⋯Ir] dimer in the latter. These dimers in 1 pack together through the intermolecular aromatic stacking interactions, while there are only van der Waals interactions in 2, resulting in their distinct luminescence behavior. At room temperature, 1 exhibits aggregation-induced phosphorescence emission. In contrast, 2 is almost non-luminescent both in solution and in the solid state. Compared with 1 and 2, free ligand hnbdtiH shows fluorescence both in CH₂Cl₂ solution and in the solid state. Moreover, no photochromism has been observed in hnbdtiH, 1 and 2.