Fine tuning of emission color of iridium(iii) complexes from yellow to red via substituent effect on 2-phenylbenzothiazoleligands: synthesis, photophysical, electrochemical and DFT study

Abstract

Four novel iridium(III) complexes bearing biphenyl (7a–7c) or fluorenyl (7d) modified benzothiazole cyclometallate ligands are synthesized. In comparison with the yellow parent complex, bis(2-phenylbenzothiozolato-N,C^2′) iridium(III) (acetylacetonate) [(pbt)₂Ir(acac)] (λ_PLmax = 557 nm, φ_PL = 0.26), 7a–7d show 20–43 nm bathochromic shifted orange or red phosphorescence in solution, with maximum photoluminescence (PL) quantum yield of 0.62, and PL lifetime of 1.8–2.0 μs. Meanwhile, the resulting complexes also exhibit intense orange or red phosphorescence of λ_PLmax = 588–611 nm in solid films. The complex 7c with two tert-butyl substituents possesses the highest phosphorescent efficiency both in dilute solution and thin solid films, therefore may be a prospective candidate for both doping and host emitting electrophosphorescent material. Furthermore, despite the observation of severe oxygen quenching for 7a–7d in solution, 7a and 7c even show efficient emission intensity quenching by oxygen in their solid state due to the existence of void channels in crystals; consequently, they are promising molecular oxygen sensor reagents. Electrochemical measurement and DFT calculation results suggest that all these chelates own declined LUMOs of 0.1 eV relative to that of (pbt)₂Ir(acac) owing to the contribution of the phenyl substituents; whereas only 7d shows a more destabilized HOMO (∼0.1 eV) compared with the parent chelate.