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Highly Efficient Deep-Blue Electroluminescence Based on a Solution-Processable A−π–D−π–A Oligo(p‑phenyleneethynylene) Small Molecule

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posted on 28.10.2019, 13:42 by Hakan Usta, Dilek Alimli, Resul Ozdemir, Salih Dabak, Yunus Zorlu, Fahri Alkan, Emine Tekin, Ayse Can
The development of solution-processable fluorescent small molecules with highly efficient deep-blue electroluminescence is of growing interest for organic light-emitting diode (OLED) applications. However, high-performance deep-blue fluorescent emitters with external quantum efficiencies (EQEs) over 5% are still scarce in OLEDs. Herein, a novel highly soluble oligo­(p-phenyleneethynylene)-based small molecule, 1,4-bis­((2-cyanophenyl)­ethynyl)-2,5-bis­(2-ethylhexyloxy)­benzene (2EHO–CNPE), is designed, synthesized, and fully characterized as a wide band gap (2.98 eV) and highly fluorescent (ΦPL = 0.90 (solution) and 0.51 (solid-state)) deep-blue emitter. The new molecule is functionalized with cyano (-CN)/2-ethylhexyloxy (-OCH2CH­(C2H5)­C4H9) electron-withdrawing/-donating substituents, and ethynylene is used as a π-spacer to form an acceptor (A)−π–donor (D)−π–acceptor (A) molecular architecture with hybridized local and charge transfer (HLCT) excited states. Physicochemical and optoelectronic characterizations of the new emitter were performed in detail, and the single-crystal structure was determined. The new molecule adopts a nearly coplanar π-conjugated framework packed via intermolecular “C–H···π” and “C–H···N” hydrogen bonding interactions without any π–π stacking. The OLED device based on 2EHO–CNPE shows an EQEmax of 7.06% (EQE = 6.30% at 200 cd/m2) and a maximum current efficiency (CEmax) of 5.91 cd/A (CE = 5.34 cd/A at 200 cd/m2) with a deep-blue emission at CIE of (0.15, 0.09). The electroluminescence performances achieved here are among the highest reported to date for a solution-processed deep-blue fluorescent small molecule, and, to the best of our knowledge, it is the first time that a deep-blue OLED is reported based on the oligo­(p-phenyleneethynylene) π-framework. TDDFT calculations point to facile reverse intersystem crossing (RISC) processes in 2EHO–CNPE from high-lying triplet states to the first singlet excited state (T2/T3 → S1) (hot-exciton channels) that enable a high radiative exciton yield (ηr ∼ 69%) breaking the theoretical limit of 25% in conventional fluorescent OLEDs. These results demonstrate that properly designed fluorescent oligo­(p-phenyleneethynylenes) can be a key player in high-performance deep-blue OLEDs.