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

Abstract

Heteroleptic cyclometalated iridium(III) complexes (C boolean AND N)(2)Ir(Bpic) (4-6) (C boolean AND 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 boolean AND O chelation of the Bpic ligand to the iridium center forming an (C boolean AND N)(2)Ir 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 (ML)-M-3'CT/(LL)-L-3'CT (L = C boolean AND N; L' = Bpic) states as the lowest-energy triplet excited state, while the fluoride binding to 4 and 5 induces the highly emissive (MLCT)-M-3/(3)pi pi* states centered on the (C boolean AND N)(2)Ir moiety. Thermally induced conversion from the (MLCT)-M-3/(3)pi pi* to the (ML)-M-3'CT/(LL)-L-3'CT states is suggested to be responsible for the low-energy weak phosphorescence in 4 and 5.