Heteroleptic Cyclometalated Iridium(III) Complexes Supported by Triarylborylpicolinate Ligand: Ratiometric Turn-On Phosphorescence Response upon Fluoride Binding

摘要

Heteroleptic cyclometalated iridium(III) complexes (C^N)_2Ir(Bpic) (4?6) (C^N = dfppy (4), ppy (5), btp (6)) supported by triarylborylpicolinate (Bpic) ancillary ligand were synthesized and characterized. X-ray diffraction study of 5 confirmed N^O chelation of the Bpic ligand to the iridium center forming an (C^N)_2Ir?borane conjugate. While the UV/vis absorption bands of 4?6 remained almost unchanged in the low-energy region upon fluoride addition, a ratiometric turn-on phosphorescence response was observed for 4 and 5. In contrast, the phosphorescence of 6 was little affected by fluoride binding. Experimental and theoretical studies suggest that the LUMO in neutral 4 and 5 is dominated by the Bpic ligand, which makes the weakly emissive 3ML′CT/~3LL′CT (L = C^N; L′ = Bpic) states as the lowest-energy triplet excited state, while the fluoride binding to 4 and 5 induces the highly emissive ~3MLCT/~3ππ* states centered on the (C^N)_2Ir moiety. Thermally induced conversion from the ~3MLCT/~3ππ* to the ~3ML′CT/~3LL′CT states is suggested to be responsible for the lowenergy weak phosphorescence in 4 and 5.