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 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 low-energy weak phosphorescence in 4 and 5.