Dinuclear platinum(ii) complexes featuring rigidly linked Pt(NCN)X units: the effect of X = SCN− in favouring low-energy, excimer-like luminescence

Abstract

Interfacial intermolecular interactions between phosphorescent, square-planar, cyclometallated platinum(II) complexes may lead to the formation of bimolecular excited states that emit at lower energy than the isolated complexes in dilute solution. We study compounds in which two Pt(NCN)Cl units are appended onto a rigid xanthene scaffold to favour the intramolecular formation of such states and thus promote low-energy emission even at high dilution {where NCN represents a cyclometallated tridentate ligand based on 2,6-di(2-pyridyl)benzene}. Here, we show how the metathesis of the monodentate Cl⁻ ligand to thiocyanate SCN⁻ has a profound effect on the emissive properties of such compounds in solution and in polymer-doped and neat films. Intramolecular Pt⋯Pt interactions are promoted by the change to SCN⁻ (as evident by a short Pt⋯Pt distance of 3.253(4) Å in the crystal, determined by X-ray diffraction). This increased propensity for the Pt(NCN) units to interact, induced by the thiocyanate, is also manifest in the emission spectra: the spectra show only the low-energy, excimer-like bands in solution, even at very low concentrations. That contrasts with the appearance of emission bands typical both of isolated Pt(NCN) units and of excimers for the chloro parent compound. Nevertheless, data at low temperature and in dilute polymer-doped films suggest that some degree of conformational change is still required to form the low-energy emitting states. Meanwhile, the change of the monodentate ligand from chloride to iodide suppresses the formation of the low-energy-emitting states and lowers the emission efficiency. Taken together, the results offer new insight into strategies for obtaining efficient NIR-emitting phosphors based on dinuclear Pt^II₂ excited states.