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

ACS Appl Mater Interfaces. 2019 Nov 27;11(47):44474-44486. doi: 10.1021/acsami.9b12971. Epub 2019 Oct 28.

Abstract

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.

Keywords: deep-blue OLED; electroluminescence; fluorescent small molecule; hot-exciton; hybridized local and charge transfer (HLCT) state; oligo(p-phenyleneethynylene); reverse intersystem crossing (RISC).