The Pt(II) complex N^C²^N-1,3-di(2-pyridyl)benzene platinum chloride (PtL¹Cl) is known to display efficient triplet luminescence in the green region of the spectrum, and to form an unusually emissive excimer that emits around 690 nm. In this contribution, the introduction of trifluoromethyl groups into either the 4- or 5-position of the pyridyl rings of the ligand is shown to lead to a red-shift in the excimer band, moving it into the near infra-red (NIR) region. The new ligands, synthesised by either Suzuki or Stille cross-coupling methods, are 1,3-bis(4-(trifluoromethyl)pyridin-2-yl)benzene HL²⁷, 1,3-bis(4-(trifluoromethyl)pyridin-2-yl)-4,6-difluorobenzene HL²⁸, and 1,3-bis(5-(trifluoromethyl)pyridin-2-yl)-4,6-difluorobenzene HL²⁹, from which the corresponding Pt(II) complexes PtLⁿCl have been prepared. The monomer and excimer emission energies in solution are compared with those of PtL¹Cl and PtL²²Cl {HL²² = 1,3-di(2-pyridyl)-4,6-difluorobenzene}. The order for the monomer can be rationalised in terms of the stabilising effects of the F atoms and the CF₃ groups on the HOMO and LUMO respectively. The order of excimer emission proves to be subtly different, but the most red-shifted complex in both cases is PtL²⁷Cl. The electroluminescence of neat films of the complexes as emitting layers in OLEDs displays uniquely excimer-like emission, extending well into the technologically important NIR region.

Two new cyclometallated platinum(II) complexes have been prepared that incorporate a terdentate N^C^N-coordinating ligand and a monodentate acetylide co-ligand. The complexes, namely [PtL³–CC–C₆H₃F₂] and [PtL⁶–CC–C₆H₃F₂] (where HL³ = 5-methyl-1,3-di(2-pyridyl)benzene; HL⁶ = 5-mesityl-1,3-di(2-pyridyl)benzene; H–CC–C₆H₃F₂ = 3,5-difluorophenylacetylene), were prepared by ligand metathesis from the corresponding chloro complex PtLⁿCl. Both of the new complexes are intensely luminescent in solution, displaying quantum yields superior to PtLⁿCl. OLEDs have been prepared using the new compounds as phosphorescent emitters. Although both lead to efficient devices, the best electroluminescence quantum efficiencies are obtained with the derivative of HL⁶, having the mesityl group on the cyclometallated phenyl ring. The superior performance with this complex can be rationalised in terms of the greater steric hindrance that serves to reduce aggregate-induced quenching.

Two new pincer proligands, namely 5-(p-(N,N-diphenylamino)phenylethynyl)-1,3-di(2-pyridyl)benzene (HL¹) and trans-5-(p-(N,N-diphenylamino)styryl-1,3-di(2-pyridyl)benzene (HL²) were prepared together with their N^C^N-coordinated cyclometallated platinum(II) complexes PtL¹X (X = Cl, NCS) and PtL²Cl. Both ligands are intensely luminescent in solution (quantum yields > 0.8). PtL¹X complexes display high quantum yields in solution whereas that of PtL²Cl is very low due to the ease with which trans to cis isomerisation of the diphenylaminostyryl CC bond occurs. Distinct sets of emission bands attributable to the cis and trans forms are observable in glass at 77 K, the assignments being supported by TD-DFT calculations. Organic light-emitting diodes (OLEDs) have been prepared using the new compounds as phosphorescent emitters. Remarkably, despite the inferior quantum yield of PtL²Cl in solution, the best electroluminescence quantum efficiencies are obtained with this complex, which emerges as an excellent candidate for the preparation of NIR-OLEDs.