Luminescent dinuclear platinum(II) complexes featuring rigidly linked Pt(N'CNO) units: N′ = isoquinoline versus pyridine

Luminescent, square-planar platinum(II) complexes often undergo intermolecular interfacial interactions at elevated concentrations, sometimes leading to low-energy emission from excimers or aggregates and thus offering an attractive route to more efficient red- and NIR-emitting phosphors. In this study, we describe two new compounds, L^pyPt₂ and L^iqPt₂, in which two Pt(NCNO) units are appended onto a xanthene scaffold to favour the intramolecular formation of such states. The Pt(NCNO) units are based on mononuclear Pt(II) complexes known to be highly emissive, namely those of 5,5-dibutyl-2-(3-(pyridin-2-yl)-phenyl-5H-indeno[1,2-b]pyridine-9-olate in the case of L^pyPt₂, and its isoquinolin-3-yl analogue for L^iqPt₂. X-ray diffraction reveals an anti arrangement of the Pt(NCNO) units relative to one another. The emission spectrum of L^pyPt₂ in solution is dominated by a low-energy band centred at 660 nm, with weaker bands at shorter wavelengths where the corresponding mononuclear complex emits. The former is attributed to excimers that form intramolecularly, but the process is inhibited in a polystyrene host at room temperature, and in a glass at 77 K. Conversely, L^iqPt₂ displays no such excimer emission in solution, probably due to a less flexible structure impeding the attainment of the necessary geometry at room temperature. In polystyrene films at loadings >25 % by mass, both complexes begin to show low-energy emission from intermolecular excimers or aggregates. In neat films of L^iqPt₂, the low-energy band dominates the spectrum, with a remarkably high quantum yield of around 20 %, an order of magnitude higher than L^pyPt₂.