Bisthienylethenes containing an imidazole bridge unit and their Ir(iii) complexes: influence of substituent groups on photochromism and luminescence

Abstract

Two new bisthienylethenes containing an imidazole bridge unit have been synthesized, namely tBuLH and tBuLMeH, which incorporate –C(CH₃)₃ and/or –CH₃ substituent groups. Based on tBuLH and tBuLMeH, heteroleptic complexes [Ir(dfppy)₂(tBuL)]·2CH₃OH (1) and Ir(dfppy)₂(tBuLMe)] (2) were prepared [dfppyH = 2-(2,4-difluorophenyl)-pyridine]. Crystal structures of tBuLH·HOAc and 1 were measured, indicating that two thiophene groups adopt antiparallel conformations in the former, while parallel conformations in the latter. In compound 1, a tBuL⁻ ligand uses its phenol–imidazole moiety to coordinate with an {Ir(dfppy)₂}⁺ unit, and neighboring [Ir(dfppy)₂(tBuL)] molecules interact through van der Waals interactions. Upon irradiation with 326 nm/321 nm light, both tBuLH and tBuLMeH show photochromic behavior in CH₂Cl₂. However, no photochromism has been observed in 1 and 2. At room temperature, tBuLH and tBuLMeH in CH₂Cl₂ exhibit fluorescence emission at 464 nm and 479 nm, respectively. Compounds 1 and 2 in CH₂Cl₂ reveal phosphorescence emission at 514 nm and 507 nm, respectively, with a mixed ³MLCT/³LC character. This work discusses the influence of –C(CH₃)₃ and –CH₃ substituent groups on the structures, photochromism and luminescence of tBuLH, tBuLMeH, 1 and 2.