Heteroleptic Ir(iii) complexes based on 2-(2,4-difluorophenyl)-pyridine and bisthienylethene: structures, luminescence and photochromic properties

Abstract

Two bisthienylethenes 2-(2-hydroxyphenyl)-4,5-bis[2,5-dimethyl(3-thienyl)]-1H-imidazole (L1H) and 2-(2-hydroxyphenyl)-4,5-bis(2,5-dimethyl(3-thienyl))-1-phenyl-imidazole (L2H), which have a chelating N,O-donor binding site attached to the photochromic core, have been synthesized using a one-pot condensation reaction, and used to prepare the heteroleptic complexes [Ir(dfppy)₂(L1)]·2CH₃OH (1) and [Ir(dfppy)₂(L2)] (2) [dfppyH = 2-(2,4-difluorophenyl)-pyridine]. In the crystal structures of all four compounds, two thiophene groups of each bisthienylethene molecule adopt parallel conformation. Neighboring molecules in L1H and 1 are linked into supramolecular chains through hydrogen bonds. Particularly, the packing structure of 1 contains right- and left-handed 2₁ helical chains. In contrast, neighboring molecules in L2H and 2 interact only through van der Waals interactions. At room temperature, L1H and L2H in CH₂Cl₂ show fluorescence emission at 442 nm and 469 nm, respectively. Compounds 1 and 2 in CH₂Cl₂ reveal broad emission band characteristics of the Ir(III)/dfppy⁻ chromophores at 508 nm and 494 nm, respectively, with a mixing of ³MLCT and ³LC characters. At room temperature, the photochromism ability of L2H in CH₂Cl₂ is clearly weaker than that of L1H. Moreover, no photochromism has been observed in 1 and 2. It has been demonstrated that both the substituent group and {Ir(dfppy)₂}⁺ coordination could significantly influence the crystal structures, luminescence and photochromic properties of L1H, L2H, 1 and 2.