Electric field-modulated photoluminescence (EML) measurements are presented for vacuum-evaporated films of cyclometallated Pt (II) complexes of 1,3-di(2-pyridyl) benzenes used as triplet emitters in organic light-emitting diodes (OLEDs). The excimer phosphorescence is quenched by the external electric field of 2.5 MV/cm up to 25% but the same effect on monomer phosphorescence is one order of magnitude smaller. The higher quenching effect for triplet excimers than triplet monomers in solid films of Pt complexes is rationalized assuming excimers to be populated within excimer-active domains of the films through an intermediate stage of geminate (e–h) pairs derived from dissociated monomer excitons. The EML data for excimers are successfully described in the framework of Sano–Tachiya–Noolandi–Hong (STNH) theory of geminate (e–h) pair recombination where the final recombination step (e–h capture) proceeds on a sphere of finite radius (a) with a finite speed. The conventional Onsager theory (a = 0) is sufficient to explain the EML quenching effect for monomers. The results are important for explaining the decrease of electroluminescence quantum efficiency observed in OLEDs working under high electric fields.

AbstractAppropriate functionalization of the cyclometalated ligand, L, and the choice of the ancillary ligand, X, allows the dipolar second‐order nonlinear optical response of luminescent [PtLX] complexes—in which L is an N^C^N‐coordinated 1,3‐di(2‐pyridyl)benzene ligand and X is a monodentate halide or acetylide ligand—to be controlled. The complementary use of electric‐field‐induced second‐harmonic (EFISH) generation and harmonic light scattering (HLS) measurements demonstrates how the quadratic hyperpolarizability of this appealing family of multifunctional chromophores, characterized by a good transparency throughout much of the visible region, is dominated by an octupolar contribution.

Two new cyclometallated platinum(II) complexes have been prepared that incorporate a terdentate (NCN)-C-boolean AND-N-boolean AND-coordinating ligand and a monodentate acetylide co-ligand. The complexes, namely [PtL3-C C-C6H3F2] and [PtL6-C C-C6H3F2] (where HL3 = 5-methyl-1,3-di(2-pyridyl)benzene; HL6 = 5-mesityl-1,3-di(2-pyridyl)benzene; H-C C-C6H3F2 = 3,5-difluorophenylacetylene), were prepared by ligand metathesis from the corresponding chloro complex (PtLCl)-Cl-n. Both of the new complexes are intensely luminescent in solution, displaying quantum yields superior to (PtLCl)-Cl-n. 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.

[(PtLX)-X-6] {X = Cl or NCS and L-6 = 5-mesityl-1,3-di-(2-pyridyl)benzene} display similar luminescence in solution but, in the solid state, the packing of the molecules is different, with short Pt center dot center dot center dot Pt interactions for X = NCS, leading to a red-shifted emission band. The effect has been used to generate OLEDs that emit squarely in the NIR region (855 nm).

Two cyclometallated platinum(II) complexes, N boolean AND C boolean AND N-5-fluoro-1,3-di(2-pyridyl) benzene platinum(II) chloride, FPtCl, and N boolean AND C boolean AND N-5-methyl-1,3-di(2-pyridyl) benzene platinum(II) isothiocyanate, MePtNCS, have been synthesized and characterized. Both complexes are highly efficient phosphorescent green emitters which can also display excimer emission in the red region. They have been studied as triplet emitters in solution-processed, multilayer organic light-emitting diodes (OLEDs), together with the known complex of 5-methyl-1,3-di(2-pyridyl) benzene, MePtCl, for comparison. The trend in efficiencies of the OLEDs prepared correlates with the charge-trapping properties of the complexes. The most efficiently emitting complex, FPtCl, was used as the dopant in a solution-processed white OLED, employing monomer and excimer emission.

The Pt(II) complex N boolean AND(CN)-N-2 boolean AND-1,3-di(2-pyridyl) benzene platinum chloride ((PtLCl)-Cl-1) 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 (PtLCl)-Cl-n have been prepared. The monomer and excimer emission energies in solution are compared with those of (PtLCl)-Cl-1 and (PtLCl)-Cl-22 {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 (PtLCl)-Cl-27. 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.

7-N,N-Dibutylamino-2-azaphenanthrene (L(3)), 8-N,N-dibutylamino-2-azachrysene (L(4)) and related Ir(i) complexes or alkylated salts show high second-order NLO responses, as determined by the EFISH technique and DFT calculations. L(4) is appealing as building block for NLO active materials due to its unexpected large μβ(1.907)value and its very high thermal stability.