A Pyridyl-1,2-azaborine Ligand for Phosphorescent Neutral Iridium(III) Complexes.

A novel 1,2-azaborine (, 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene, ) has been synthesized and used for the first time as a B-N alternative to common cyclometalating ligands to obtain neutral phosphorescent iridium(III) complexes (, , , and ) of general formula [Ir(CN)(NNB)], where CN indicates three different cyclometalating ligands (Hppy = 2-phenylpyridine; Hdfppy = 2-(2,4-difluoro-phenyl)pyridine; Hpqu = 2-methyl-3-phenylquinoxaline). Moreover, the azaborine-based complex was compared to the isoelectronic C═C iridium(III) complex , obtained using the corresponding 2-(naphthalen-2-yl)pyridine ligand . Due to the dual cyclometalation mode of such C═C ligand, the isomeric complex was also obtained. All new compounds have been fully characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and the X-ray structure of was determined. The electronic properties of both ligands and complexes were investigated by electrochemical, density functional theory (DFT), and photophysical methods showing that, compared to the naphthalene analogues, the azaborine ligand induces a larger band gap in the corresponding complexes, resulting in increased redox gap (basically because of the highest occupied molecular orbital (HOMO) stabilization) and blue-shifted emission bands (, λ = 523 577 nm for , in acetonitrile solution at 298 K). On the other hand, the LC nature of the emitting state is the same in all complexes and remains centered on the pyridyl-borazaronaphthalene or its C═C pyridyl-naphthalene analogue. As a consequence, the quantum yields of such azaborine-based complexes are comparable to those of the more classical C═C counterparts (, photoluminescence quantum yield (PLQY) = 16 22% for , in acetonitrile solution at 298 K) but with enhanced excited-state energy. This proves that such type of azaborine ligands can be effectively used for the development of novel classes of photoactive transition-metal complexes for light-emitting devices or photocatalytic applications.