ABSTRACT
The discovery of triplet excitons participating in the photoluminescent processes in a growing number of pure organic emitters represents an exciting impetus for a diversity of promising opto, bio, and optoelectronic applications. In this contribution, we have studied a small-molecule dithienylbenzothiadiazole-based red-emitting dye red-1b, which shows clearly delayed fluorescence under optical and electrical excitation. The OLED device that contained red-1b as a nondoped solution-processable emitter exhibited a moderately high utilization of exciton amounting to ≈31% and slow efficiency roll-off. Magnetoelectroluminescence measurements revealed the coexistence of reverse intersystem crossing from the lowest triplet state to singlet state (RISC, E-type triplet to singlet up-conversion) and triplet-triplet annihilation (TTA, P-type triplet to singlet up-conversion). Specifically, in low current-density regime, the moderately high exciton utilization is attributed to RISC (i.e., thermally activated delayed fluorescence, TADF), whereas in high current-density regime, TTA may contribute to suppressing efficiency roll-off. Furthermore, the results showed that red-1b may represent a new kind of organic red emitters that display delayed fluorescence in a way differing from the few red emitters investigated so far.
ABSTRACT
In this work, two series of tetrathiafulvalene (TTF) and thiacalix[4]arene (TCA) conjugates (TTF-TCA) were designed by CuAAC click reactions. The results obtained from NMR and (1)H NMR NOE indicated that their conformations of thiacalix[4]arene framework may prefer to 1,3-alternate. The cyclic voltammograms of four TTF-TCA compounds containing electroactive TTF units were provided. Meanwhile, their intermolecular electron-transfer (ET) behaviors with tetrachlorobenzoquinone (Q) mediated by different metal ions, Sc(3+), Pb(2+), Ag(+), Cd(2+), and Zn(2+), in CH3CN-CH2Cl2 (V/V = 1:1) solution were studied and analyzed via UV-vis spectroscopy. It was determined that intermolecular ET between each TTF-TCA and Q ensemble was not observed without introduction of the metal ions mentioned above. The added specified metal ions most likely induced the intermolecular ET between TTF-TCA and Q ensemble, and the effects of Sc(3+) functions were the most imperative. The intermolecular ET also proved to be reliant on the structure of TTF-TCA, where TTF-TCA 7a and 7b were more effective than TTF-TCA 6a and 6b. The difference may be credited to TTF-TCA 7a or 7b possessing two independent TTF pendants and providing a more synergic coordination among the TTF radical cation and Q radical anion with a metal ion.
ABSTRACT
We demonstrate highly efficient exciplex delayed-fluorescence organic light-emitting diodes (OLEDs) in which 4,4',4â³-tris[3-methylphenyl(phenyl)aminotriphenylamine (m-MTDATA) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were selected as donor and acceptor components, respectively. Our m-MTDATA:Bphen exciplex electroluminescence (EL) mechanism is based on reverse intersystem crossing (RISC) from the triplet to singlet excited states. As a result, an external quantum efficiency (EQE) of 7.79% at 10 mA/cm(2) was observed, which increases by 3.2 and 1.5 times over that reported in Nat. Photonics 2012, 6, 253 and Appl. Phys. Lett. 2012, 101, 023306, respectively. The high EQE would be attributed to a very easy RISC process because the energy difference between the singlet and triplet excited states is almost around zero. The verdict was proven by photoluminescence (PL) rate analysis at different temperatures and time-resolved spectral analysis. Besides, the study of the transient PL process indicates that the presence of an unbalanced charge in exciplex EL devices is responsible for the low EQE and high-efficiency roll-off. When the exciplex devices were placed in a 100 mT magnetic field, the permanently positive magnetoelectroluminescence and magnetoconductivity were observed. The magnetic properties confirm that the efficient exciplex EL only originates from delayed fluorescence via RISC processes but is not related to the triplet-triplet annihilation process.
