The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. First-principles quantum mechanical calculations show that the exponential-decay law for any metastable state is only an approximation and predict an asymptotically algebraic contribution to the decay for sufficiently long times. In this Letter, we measure the luminescence decays of many dissolved organic materials after pulsed laser excitation over more than 20 lifetimes and obtain the first experimental proof of the turnover into the nonexponential decay regime. As theoretically expected, the strength of the nonexponential contributions scales with the energetic width of the excited state density distribution whereas the slope indicates the broadening mechanism.
Organic light-emitting diodes (OLEDs) using the red phosphorescent emitter iridium(III)bis(2methyldibenzo[f,h]quinoxaline) (acetylacetonate) [Ir(MDQ) 2 (acac)] are studied by time-resolved electroluminescence measurements. A transient overshoot after voltage turn-off is found, which is attributed to electron accumulation on Ir(MDQ) 2 (acac) molecules. The mechanism is verified via impedance spectroscopy and by application of positive and negative off-voltages. We calculate the density of accumulated electrons and find that it scales linearly with the doping concentration of the emitter. Using thin quenching layers, we locate the position of the emission zone during normal OLED operation and after voltage turn-off. In addition, the transient overshoot is also observed in three-color white-emitting OLEDs. By time-and spectrally resolved measurements using a streak camera, we directly attribute the overshoot to electron accumulation on Ir(MDQ) 2 (acac). We propose that similar processes are present in many state-of-the-art OLEDs and believe that the quantification of charge carrier storage will help to improve the efficiency of OLEDs.
The interaction between the water-soluble anionic conjugated copolymer poly{1,4-phenylene-[9,9-bis(4-phenoxy-butylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) and various surfactants has been studied in aqueous solution by UV-vis absorption spectra, fluorescence and electrical conductivity. It is suggested from the linear dependence of absorbance, fluorescence and electrical conductivity on concentration that in the absence of surfactant, moderately stable dispersions are formed. These are affected in different ways on adding cationic, anionic or neutral surfactants. With the cationic cetyltrimethylammonium bromide, quenching of fluorescence intensity and lifetime, and formation of a new emission occurs at concentrations well below the critical micelle concentration (cmc). Electrical conductivity measurements indicate a discontinuity at surfactant/polymer ratio corresponding to electroneutrality, due to complexation. With the anionic sodium dodecyl sulfate, fluorescence quenching is also observed, but is attributed to formation of some mixed polymer/surfactant aggregate. The most striking changes are observed with the non-ionic pentaethyleneglycol monododecyl ether (C 12 E 5 ), where a blue shift in fluorescence emission, dramatic increases in lifetime and quantum yield, and changes in electrical around the cmc are interpreted in terms of incorporation of single polymer chains in elongated cylindrical micelles. This is supported by 1 H NMR spectroscopic measurements.
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