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

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 hybrid 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 performance 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 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 enables 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-phenyleneethynylene)s can be a key player in high-performance deep-blue OLEDs.