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 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.

An Efficient Hole Transporting Polymer for Quantum Dot Light‐Emitting Diodes

“…Therefore, some electron‐withdrawing CN groups are introduced into the organic framework of TFB to achieve two unreported polymers (Figure 1a). [ 34,35 ] The 2‐cyanopropan‐2‐yl (CNPr) group was chosen for its unique features, such as weak electron‐withdrawing property and no active hydrogen (α– H ) atoms. The former decreases the electrons on the polymer chain, resulting in a much more stabilized HOMO energy level.…”

“…The calculated HOMOs of two organic units in TFB, CNPr‐TFB, and CN‐TFB are −5.14, −5.33, and −5.63 eV, respectively (Table S1, Supporting Information), which agrees with the UPS results and shows the consistent tendency that electron‐withdrawing groups on the TPA moieties can stabilize the HOMOs of polymers through pulling the electrons on the polymer chain to these electron‐withdrawing groups. [ 35 ] On the other hand, the LUMO energy levels are also distributed on the fluorene and TPA moieties in these polymers. In contrast to the HOMO contributions, TPA moieties have less LUMO contributions with the percentages of 22.89%, 22.83%, and 25.68% for TFB, CNPr‐TFB, and CN‐TFB, respectively.…”

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

“…14,15 Given the fact that the HLCT concept was proposed a few years ago, to date the development of HLCT emitters still lags behind TADF emitters. [16][17][18] In particular, device performances of most HLCT-based OLEDs remain restricted as their external quantum efficiencies (EQEs) are generally <10%, 10,15,[19][20][21][22][23] leaving much room for efficiency improvement. Seen in this light, there is an urgent need for simple and effective strategies to develop high-efficiency HLCT emitters and related OLEDs.…”

Highly Efficient Blue Organic Light-Emitting Diode Based on a Pyrene[4,5- d ]Imidazole-Pyrene Molecule

“…In recent years, materials with hybrid local and CT (HLCT) excited state were also well developed, which successfully achieved the merits of high η PL and a high output of singlet excitons in a series of donor (D)-acceptor (A) materials. 36,[41][42][43][44][45][46][47][48][49][50][51][52] HLCT materials also show the potential to maintain stable η ext at high brightness. Thus, there is still room for further enhancement of the η ext of blue OLEDs and low efficiency roll-offs in pyrene-based D-A system.…”