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

USTA H., Alimli D., ÖZDEMİR R., Dabak S., Zorlu Y., ALKAN F., ...More

ACS APPLIED MATERIALS & INTERFACES, vol.11, no.47, pp.44474-44486, 2019 (SCI-Expanded) identifier identifier identifier

  • Publication Type: Article / Article
  • Volume: 11 Issue: 47
  • Publication Date: 2019
  • Doi Number: 10.1021/acsami.9b12971
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.44474-44486
  • Keywords: deep-blue OLED, electroluminescence, fluorescent small molecule, oligo(p-phenyleneethynylene), hybridized local and charge transfer (HLCT) state, reverse intersystem crossing (RISC), hot-exciton, LIGHT-EMITTING-DIODES, AMBIPOLAR CHARGE-TRANSPORT, STARBURST OLIGOFLUORENES, POLYMER SEMICONDUCTORS, FLUORESCENCE, PERFORMANCE, DESIGN, DONOR, ISOMERIZATION, FLUOROPHORES
  • Abdullah Gül University Affiliated: Yes


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 (Phi(PL) = 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 pi-spacer to form an acceptor (A)-pi-donor (D)-pi-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 pi-conjugated framework packed via intermolecular "C-H center dot center dot center dot pi" and "C-H center dot center dot center dot N" hydrogen bonding interactions without any pi-pi stacking. The OLED device based on 2EHO-CNPE shows an EQE(max) of 7.06% (EQE = 6.30% at 200 cd/m(2)) and a maximum current efficiency (CEmax) of 5.91 cd/A (CE = 5.34 cd/A at 200 cd/m(2)) 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) pi-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 (T-2/T-3 -> S-1) (hot-exciton channels) that enable a high radiative exciton yield (eta(r) similar to 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.