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.