Near-infrared phosphorescence in a ruthenium(II) complex equipped with a pyridyl-1,2-azaborine ligand

Abstract

The 4-methyl-2-(pyridin-2-yl)-2,1-borazaronaphthalene molecule Hazab-py has been successfully used, for the first time, as a ligand in a ruthenium(II) polypyridine complex A (with the formula [Ru(dtbbpy)₂(azab-py)]⁺, where dtbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine). This compound was characterized by NMR spectroscopy and high-resolution mass spectrometry (MS), and its electrochemical and photophysical properties were fully investigated and compared to those of its homoleptic analogue [Ru(dtbbpy)₃]²⁺ (B), an archetypical mono-cationic cyclometalated complex C (with the formula [Ru(dtbbpy)₂(ppy)]⁺, where Hppy = 2-phenylpyridine), and the more structurally similar analogue [Ru(dtbbpy)₂(naft-py)]⁺ (D), where the B-N unit of the azaborine ligand is replaced by a standard CC one, resulting in the 2-(naphthalen-2-yl)pyridine ligand (Hnaft-py). The presence of the novel 1,2-azaborine ligand induces a 0.51 V decrease in the redox gap of complex A, compared to that of B, leading to electrochemical and photophysical properties that resemble those of C and D. Accordingly, the azaborine complex displays an emission band extending up to the near infrared region of the spectrum (with the maximum at 765 nm in room-temperature acetonitrile solution), arising from a triplet metal-to-ligand charge-transfer (³MLCT) state. As in the case of other mono-cationic cyclometalated ruthenium(II) complexes, A shows modest photoluminescence quantum yields (PLQYs), but higher PLQYs when compared to those of its direct CC analogue D (e.g., PLQY = 0.6 vs. 0.1% in a PMMA matrix at 298 K). Density functional theory (DFT) calculations were used to provide complete rationalization of the electronic properties of all the complexes and to identify lower-lying metal-centred triplets (³MC), responsible for the low PLQYs of such an azaborine-based ruthenium(II) complex.