Platinum(IV) Complexes with Tridentate, NNC-Coordinating Ligands: Synthesis, Structures, and Luminescence

Platinum(II) complexes of NNC-cyclometalating ligands based on 6-phenyl-2,2′-bipyridine (HL1) have been widely investigated for their luminescence properties. We describe how PtL1Cl and five analogues with differently substituted aryl rings, PtL2–6Cl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt(NNC-Ln)Cl3 (n = 1–6). The molecular structures of several of them have been determined by X-ray diffraction. These PtLnCl3 compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form [Pt(NNC)(NC)Cl]+. Elevated temperatures are required, and the reaction is accompanied by competitive reduction processes and generation of side-products; however, four examples of such complexes have been isolated and their molecular structures determined. Reaction of PtL1Cl3 with HL1 similarly generates [Pt(NNC-L1)2]2+, which we believe to be the first example of a bis-tridentate Pt(IV) complex. The lowest-energy bands in the UV–vis absorption spectra of all the PtLnCl3 compounds are displaced to higher energy relative to the Pt(II) precursors, but they red-shift with the electron richness of the aryl ring, consistent with predominantly 1[πAr → π*NN] character to the pertinent excited state. A similar trend is observed for the [Pt(NNC)(NC)Cl]+ complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for [PtL1(ppy)Cl]+ and [Pt(L1)2]2+, and 550 nm for methoxy-substituted derivatives. The lifetimes are in the microsecond range, rising to hundreds of microseconds at 77 K, consistent with triplet excited states of primarily 3[πAr → π*NN] character with relatively little participation of the metal

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

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 PtII2 excited states

Dinuclear platinum( ii ) complexes emitting through TADF: new ligand design to minimise aggregation and the S 1 –T 1 energy gap

Dinuclear platinum(ii) complexes of a new, ditopic, bis-tridentate NCN–NCN-coordinating ligand, appended with four mesityl groups, are reported. The high radiative rate constants and correspondingly efficient luminescence of the complexes involves thermally activated delayed fluorescence (TADF), thanks to a near-zero energy gap between the S1 and T1 states. The mesityl groups also serve to hinder the aggregation that was detrimental to electroluminescence efficiency in previous studies, allowing a ∼4-fold increase in OLED efficiency to be achieved (i.e. from 2.3% previously to 10% in this work). Oxidation of one of the Pt(ii) complexes led to a dinuclear Pt(iv) complex of unprecedented structure