The photo-excitation and relaxation of a new cyclometalated complex, Ir(ppy) 2 fppy, is investigated through the measurement of absorption and photoluminescence spectra. Comparison of Ir(ppy) 2 fppy with Ir(ppy) 3 reveals that the lowest energy excited state of Ir(ppy) 2 fppy is localised on the aldehyde-substituted ligand, thus giving the complex its characteristic orange emission (l max # 600 nm). OLEDs containing Ir(ppy) 2 fppy doped into poly(vinylcarbazole) (PVK) exhibit the same characteristic emission as found in solution, implying efficient energy transfer between the PVK and the iridium complex. Our results provide the potential for the synthesis of iridium complexes with different emission wavelengths through the modification of only one ligand.

Efficient white-polymeric light-emitting diodes (PLED) were fabricated as a single active layer containing blue-emitting poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl) endcapped with bis(4-methylphenyl)phenylamine; (PF2∕6am4), and yellow-orange-emitting iridium [tri-fluorenyl] pyridine complex [Ir(Fl3Py)3]. The fluorene-like ligands in the blended device prevent phase segregation and also enhance energy transfer from the polymer host to the guest due to efficient overlap of wave function (Dexter process) and host singlet emission and guest absorption bands (Förster process) which reduces the loading level required to produce white emission. The two emitted colors complement each other and doping levels of 2%–3% produce white emission. Above a certain current density, depending on the doping level, the device Commission Internationale de L’Eclairage (CIE) coordinates become bias independent and a stabilized white emission can be obtained. A white-emitting PLED (coordinates 0.348, 0.367) of peak external quantum efficiency of 2.8%, and luminance of 16000cd∕m2 at applied voltage of 5V (i.e., 4.57cd∕A) was obtained.

This work describes bipolar 2,5-diaryl-1,3,4-oxadiazole-fluorene hybrids which incorporate triphenylamine or carbazole units within the p-electron system, viz. compounds 7, 8, 14 and 16. A related bipolar bis(oxadiazolyl)pyridine system 20 is reported. The syntheses of these five new materials are discussed, along with their optoelectronic absorption and emission properties, and their solution electrochemical redox properties. Anodic electropolymerisation of 20 was observed. Calculations using DFT (density functional theory) establish that they all possess a significantly higher HOMO energy level (by 0.60-1.02 eV) than 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl]benzene (OXD-7) due to the presence of electron-rich amine moieties and increased conjugation lengths, thereby leading to more balanced charge-transport characteristics. Devices were fabricated by spin-coating techniques using the bipolar compounds as the emitters in the simple device architecture ITO:PEDOT-PSS:X:Ca/Al (X = 7, 8, 14, 16 or 20). The turn-on voltages were 2.9, 5.5, 3.6, 4.5 and 3.4 V for the devices incorporating 7, 8, 14, 16 and 20, respectively. The highest external quantum efficiency (EQE) was observed for compound 7: viz. EQE 0.36%; current efficiency 1.00 cd A 21 ; power efficiency 0.56 lm W 21 at 5.7 V. The EQE of the device fabricated from 8 was considerably lower than for devices using other materials due to low light emission. The EL emission peaked at l max 430, 487, 487 and 521 nm for 8, 14 and 16, and 7, respectively. For the 20 device l max = 521 nm and 564 nm. Thus the HOMO-LUMO gap has been modified, allowing the colour of the emitted light to vary from light blue through to green by the systematic chemical modification of the molecular subunits. The high chemical and thermal durability of these materials combined with the simplicity of the device structure and low turn-on voltages offers considerable potential for OLED applications.

Results are presented which challenge the accepted view that dinuclear transition metal-ligand complexes are unsuitable for organic light-emitting device (OLED) applications due to their low luminescence quantum efficiencies. We establish for the first time that halo-and pseudo-halobridged diiridium(III) species are viable electrophosphorescent dopants in OLEDs. New cyclometalated chloro-and isocyanate-bridged diiridium(III) complexes, viz. ) 2 NCO] 2 (complex 4) were obtained in high yield from the 9,9-dihexyl-2-(pyridin-2-yl)fluorene (flpyH) ligand 1. The X-ray crystal structures are described for 3 and the monomeric complex Ir(flpy) 2 NCO(DMSO) (5) which was obtained from 4. The solution-state photophysical properties of complexes 3 and 4 are characterised by emission from mixed 3 p-p*/ 3 MLCT states at y545-550 nm. Complex 4 displays a solution-state photoluminescence quantum yield which is five times that of the dichloro-bridged analogue 3. This is ascribed to an increase in the ligand-LUMO/metal e g gap which reduces the probability of non-radiative decay processes. Spin-coated organic light emitting devices (OLEDs) made from the host polymer poly(9,9-bis-2ethylhexylfluorene-2,7-diyl) (PF2/6) end-capped with bis-(4-methylphenyl)phenylamine (PF2/6am4) doped with 12.5 wt% of the complexes 3 and 4 show good stability: turn-on voltages are low (,4 V) with maximum EL intensities of y1300 and 13 000 cd m 22 , and peak external quantum efficiencies (EQE) of 0.1 and 0.8%, at ca. 400 and 60 mA cm 22 , respectively.

The near-infrared two-photon absorption (TPA) spectra of a series of cyclometalated iridium complexes have been measured. These complexes exhibit moderately large TPA cross-sections of approximately 20 GM at the biological relevant wavelength of 800 nm. A new complex has been designed and synthesised, and found to have an increased cross-section of 44 GM at 800 nm. Full photophysical characterisation of this complex is presented.