Heteroleptic cyclometalated iridium(III) complexes supported by triarylborylpicolinate ligand: ratiometric turn-on phosphorescence response upon fluoride binding.

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 (3)ML'CT/(3)LL'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 (3)MLCT/(3)ππ* states centered on the (C^N)2Ir moiety. Thermally induced conversion from the (3)MLCT/(3)ππ* to the (3)ML'CT/(3)LL'CT states is suggested to be responsible for the low-energy weak phosphorescence in 4 and 5.