No abstract

Recently, solid-state lighting has received considerable attention in both academic and industrial research. [1,2] Of particular interest, for the replacement of the existing light sources, are organic light-emitting diodes (OLEDs) based on phosphorescent molecules. [3][4][5][6] The advantage of using these materials lies in the possibility to internally convert all the spin uncorrelated injected charges into light. Indeed, an internal quantum efficiency of nearly 100% has been achieved in devices based on the green-emitting organometallic complex Ir(ppy) 3 .[7]However, many unresolved issues are the subject of current research in order to implement efficient white light sources and expand the number of applications. In particular, the origin of the efficiency roll-off at high voltages, [8][9][10] the light outcoupling, [11,12] the long-term stability [13,14] and the generation of white light with an all-phosphor device [6,15] are subjects under intense investigation. White light generation is a key issue because of the wide range of applications involving full-color displays and lighting. [1,2] Among the different approaches, solution processed devices based on white light emitting molecules [16] have been demonstrated as well as thermally evaporated red, green and blue (RGB) blends [15] or stacks. To date, white light OLEDs (WOLEDs) with long term operational lifetimes have been obtained mainly with a combination of a blue fluorescent emitter [6] and phosphors for the other colors. Such an elegant approach relies on a well engineered harvesting of singlet and triplet excitons and requires therefore a precise doping of the RGB emitting dyes in the transporting hosts. In contrast, efficient WOLEDs based on blue phosphors can be obtained with all the emitters in one single layer, [17] simplifying the processing. Generally, however, blue phosphors have in the past turned out to be rather unstable. While a physical explanation for blue phosphor based device instability is still lacking, a shorter phosphorescence lifetime, eventually approaching the sub-microsecond time regime, would decrease the residence time of potentially unstable excited states. Moreover, processes detrimental to the efficiency, such as exciton charge-carrier quenching [8] or triplet-triplet annihilation, [9,10] could be strongly reduced with a faster exciton recombination. A shorter phosphorescence lifetime while maintaining high quantum efficiencies requires a large radiative rate. For organometallic complexes this rate is directly proportional to the spin-orbit coupling (SOC) matrix element involving the emitting triplet and the perturbing singlet state and inversely proportional to the degree of mixing between them, i.e., the singlet-triplet splitting (DE ST ). [18][19][20] Photophysical studies of the role of SOC and DE ST in tuning the radiative rate are still sparse, mainly because the large intersystem crossing (ISC) rates ($10 13 s À1 ) of such phosphors, [21] which makes detection (and therefore direct measurement of DE ST ) rather cha...

Cyclometalated (C^C*) platinum(II) N-heterocyclic carbene (NHC) complexes are emerging as a new class of phosphorescent emitters for the application in organic light-emitting devices (OLEDs). We present the synthesis of six new complexes of this class to investigate the influence of extended π systems. Therefore, six different NHC ligands with a varying number of additional phenyl substituents were used in combination with the monoanionic acetylacetonate (acac) ligand to obtain complexes of the general formula [(NHC)Pt(II)(acac)]. The complexes were fully characterized by standard techniques and advanced spectroscopic methods ((195)Pt NMR). For all complexes the solid-state structure determination revealed a square-planar coordination of the platinum atom. Absorption and emission spectra were measured in thin amorphous poly(methyl methacrylate) films at room temperature. Four compounds emit in the blue-green region of the visible spectrum with quantum yields of up to 81%.

Binuclear C^C* cyclometalated NHC platinum(II) compounds with bridging amidinate ligands were synthesized to evaluate their photophysical properties. Their three-dimensional structures were determined by a combination of 2D NMR experiments, mass spectrometry, DFT calculations, and solid-state structure analysis. The bridging amidinate ligands enforce short distances between the platinum centers of the two cyclometalated structures, which gives rise to extraordinary photophysical properties.

Two novel bipolar host materials BPTRZ and MBPTRZ were synthesized, in which the hole transporting carbazole is separated from the electron transporting triazine moiety by a fully aromatic, but nonconjugated meta-linked biphenyl unit. The additional twist at the biphenyl in MBPTRZ, which is achieved by methyl-substitution in 2- and 2′-position of the biphenyl leads to a higher triplet energy of 2.81 eV compared to 2.70 eV for BPTRZ. Both materials possess high thermal stabilities and good glass forming properties. An organic light emitting diode with MBPTRZ as host for the blue phosphorescence emitter FIrpic shows a maximum luminance of 30600 cd/m2 and a maximum external quantum efficiency of 7.0%.