Ze-Lin Zhu

Highly Stable Near-Infrared Fluorescent Organic Nanoparticles with a Large Stokes Shift for Noninvasive Long-Term Cellular Imaging

Fluorescent organic nanoparticles based on small molecules have been regarded as promising candidates for bioimaging in recent years. In this study, we report a highly stable near-infrared (NIR) fluorescent organic nanoprobes based on nanoparticles of an anthraquinone derivate with strong aggregation-induced emission (AIE) characteristics and a large Stokes shift (>175 nm). These endow the nanoprobe with high fluorescent brightness and high signal-to-noise ratio. On the other hand, the nanoprobe also shows low cytotoxicity, good stability over a wide pH range, superior resistance against photodegradation and photobleaching comparing to typical commercial fluorescent organic dyes such as fluorescein sodium. Endowed with such merits in term of optical performance, biocompatibility, and stability, the nanoprobe is demonstrated to be an ideal fluorescent probe for noninvasive long-term cellular tracing and imaging applications. As an example, it is shown that strong red fluorescence from the nanoprobe can still be clearly observed in A549 human lung cancer cells after incubation for six generations over 15 days.

High-Performance Blue OLEDs Based on Phenanthroimidazole Emitters via Substitutions at the C6- and C9-Positions for Improving Exciton Utilization.

Donor-acceptor (D-A) molecular architecture has been shown to be an effective strategy for obtaining high-performance electroluminescent materials. In this work, two D-A molecules, Ph-BPA-BPI and Py-BPA-BPI, have been synthesized by attaching highly fluorescent phenanthrene or pyrene groups to the C6- and C9-positions of a locally excited-state emitting phenylamine-phenanthroimidazole moiety. Equipped with good physical and hybridized local and charge-transfer properties, both molecules show high performances as blue emitters in nondoped organic light-emitting devices (OLEDs). An OLED using Ph-BPA-BPI as the emitting layer exhibits deep-blue emission with CIE coordinates of (0.15, 0.08), and a maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 4.56 %, 3.60 cd A(-1) , and 3.66 lm W(-1) , respectively. On the other hand, a Py-BPA-BPI-based, sky-blue OLED delivers the best results among nondoped OLEDs with CIEy values of < 0.3 reported so far, for which a very low turn-on voltage of 2.15 V, CIE coordinates of (0.17, 0.29), and maximum CE, PE, and EQE values of 10.9 cd A(-1) , 10.5 lm W(-1) , and 5.64 %, were achieved, respectively. More importantly, both devices show little or even no efficiency roll-off and high singlet exciton-utilizing efficiencies of 36.2 % for Ph-BPA-BPI and 39.2 % for Py-BPA-BPI.

Ambipolar D–A type bifunctional materials with hybridized local and charge-transfer excited state for high performance electroluminescence with EQE of 7.20% and CIEy ∼ 0.06

Construction of donor–acceptor (D–A) molecules with a highly hybridized local and charge-transfer (HLCT) excited state has been shown to be an effective strategy to achieve the maximum electroluminescence (EL) efficiency through the synchronous harvest of high photoluminescence (PL) efficiency and exciton utilization. Herein, two novel D–A-structured bifunctional (emissive and hole-transporting) materials, PPI-2TPA and PPI-2NPA, have been designed and synthesized for application in deep-blue OLEDs. As revealed by theoretical calculations and comprehensive photophysical experiments, both of them exhibit significant HLCT excited-state characteristics and ambipolar properties. Using them as emitting layers (EML) in multilayer non-doped devices presents true deep-blue Commission Internationale de l’Eclairage (CIE) coordinates of ca. (0.15, 0.06), accompanied by record-setting performance with maximum external quantum efficiencies (EQEs) of 7.20% for PPI-2TPA and 6.33% for PPI-2NPA. Remarkably, the simple bilayer devices fabricated using them as non-dopant EML and hole-transporting layers (HTLs) still deliver EQEs as high as 4.69% and 4.10% with little changes in color purity (PPI-2TPA: CIE (0.150, 0.063) and PPI-2NPA: (0.152, 0.063)). To the best of our knowledge, this performance is the highest among the reported non-doped devices in this color gamut, irrespective of whether the two newly formed molecules functioned as EML or EML and HTL simultaneously.

A pyridine based meta-linking deep-blue emitter with high conjugation extent and electroluminescence efficiencies

We designed and synthesized a bipolar deep-blue emitter 2,6-bis(4-(1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)pyridine (26BTPIPy) based on a meta-linking D–π–A–π–D structure. Compared to its para-linking analogue (25BTPIPy), the meta-linking in 26BTPIPy effectively shortens molecular conjugated length and restricts intramolecular charge transfer. Interestingly, unlike most other meta-linking emitters, a high fluorescence yield can be maintained in 26BTPIPy. This may be attributed to a relatively planar structure at the benzene–pyridine–benzene joint in 26BTPIPy leading to considerable overlapping of its frontier molecular orbitals. These suitable combinations of properties endow 26BTPIPy with efficient deep-blue emission and good bipolar carrier transporting characteristics. An organic light-emitting device using 26BTPIPy as an emitter shows a low turn-on voltage (2.8 V), deep-blue emission with a color index of (0.15, 0.09) and high current and external quantum efficiencies (4.16 cd A−1 and 5.15%). Besides, a bilayer device using 26BTPIPy as both an emitting and electron-transporting material also gives high performance with a current efficiency of 4.22 cd A−1 and a color purity of (0.15, 0.11).

Mechanochromic asymmetric sulfone derivatives for use in efficient blue organic light-emitting diodes

Typical π–π stacking is suppressed by the asymmetric molecular design for high fluorescence quantum yield (Φf) blue light emission, overcoming the aggregation caused quenching (ACQ) limitation. In this research, two novel blue fluorescent materials with asymmetric structure: 2-(4′-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSC) and 2-(4′-((4′-(9H-carbazol-9-yl)-[1,1′-biphenyl]-4-yl)sulfonyl)-[1,1′-biphenyl]-4-yl)-1-(4-(tert-butyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSBC), consisting of a sulfone group as the electron acceptor and two different electron donors, carbazole and phenanthroimidazole, were designed and synthesized. The two compounds have high Φf (95.3% for PSC and 81.1% for PSBC) in film because of the restricting π–π stacking, and show apparent mechanochromic properties, i.e., an emission change from deep blue to blue-green resulting from external mechanical stimuli. The emissions display 50 nm/23 nm red shifts after grinding. Organic light emitting diodes (OLEDs) using the two compounds as emitters exhibited good efficiencies: the doped PSC-based device emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.072). The PSBC-based device also emitted blue light at 444 nm with CIE co-ordinates of (0.151, 0.068). A maximal external quantum efficiency (EQE) of 5.43% was also achieved.

Bis‐Tridentate Iridium(III) Phosphors with Very High Photostability and Fabrication of Blue‐Emitting OLEDs

Abstract Sky‐blue and blue‐emitting, carbazolyl functionalized, bis‐tridentate Ir(III) phosphors Cz‐1–Cz‐3 with bright emission and short radiative lifetime are successfully synthesized in a one‐pot manner. They exhibit very high photostability against UV–vis irradiation in degassed toluene, versus both green and true‐blue‐emitting reference compounds, i.e., fac‐[Ir(ppy)3] and mer‐[Ir(pmp)3]. Organic light‐emitting diodes (OLEDs) based on Cz‐2 exhibit maximum external quantum efficiency (EQE) of 21.6%, EQE of 15.1% at 100 cd m−2, and with CIEx , y coordinates of (0.17, 0.25). This study provides a conceptual solution to the exceedingly stable and efficient blue phosphor. It is promising that long lifespan blue OLED based on these emitters can be attained with further engineering of devices suitable for commercial application.

Revealing the new potential of an indandione unit for constructing efficient yellow thermally activated delayed fluorescence emitters with short emissive lifetimes

Simultaneously accomplishing a high efficiency and a slow efficiency roll-off at practical luminance levels remains challenging for thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs). In this study, for the first time, we reveal the new potential of an indandione unit featuring double carbonyl moieties, which is widely used in organic solar cells, as an electron-accepting core for constructing efficient TADF emitters. As a proof of concept, two TADF emitters, 5PXZ-PIDO and 5,6PXZ-PIDO, are developed by connecting an indandione (IDO) core with electron-donating phenoxazine (PXZ) units via phenylene π-bridges. Impressively, both emitters exhibit a distinct TADF nature with short DF lifetimes of ∼2 μs, and display relatively high photoluminescence quantum yields (ΦPLs) of over 70%. More importantly, a yellow OLED based on 5,6PXZ-PIDO achieves a high external quantum efficiency of 14.2% and an ultra-slow efficiency roll-off of 16.0% at a practical luminance of 1000 cd m−2, which is outstanding among previously reported carbonyl-based TADF emitters. This finding unlocks the huge potential of indandione-based molecules as TADF emitters for high-performance OLEDs.

Blue-Emitting Bis-tridentate Ir(III) Phosphors: OLED Performances vs. Substituent Effects

A series of six Ir(III)-based blue-emitting bis-tridentate phosphors, for which a 6-pyrazoly-2-phenoxypyridine (pzyPx) chelate is functionalized with either a m- or p-CF3 substituent at the terminal phenoxy site, and a tert-butyl substituent at the 4-position of the central pyridyl group, while the substituent of a carbene pincer ancillary is varied from methyl, isopropyl and to benzyl, are strategically designed and synthesized. A majority of these blue phosphors exhibit a nearly unitary emission quantum yield (Φ) in degassed solution and enhanced stability against photo-degradation in degassed toluene solution comparable to that of green, sky-blue and true-blue emitters, i.e. fac-[Ir(ppy)3], FIrpic and mer-[Ir(pmp)3], under irradiation with a xenon lamp at a power density of 620 W m−2 and at 35 °C. Corresponding OLED devices are fabricated using these pzyPx functionalized Ir(III) phosphors co-doped in a 9-(3-(9H-carbazol-9-yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN) host material, giving a max. EQE in the range of 8.0–12.2% and stable CIEx,y coordinates between (0.16, 0.22) and (0.17, 0.27). Importantly, the max. EQE of these OLEDs is qualitatively proportional to the phosphorescence radiative lifetime recorded in either the solution state or the doped host matrix, showing a critical factor for the development of successive blue emitters.

Ternary Acceptor-Donor-Acceptor Asymmetrical Phenanthroimidazole Molecule for Highly Efficient Near-Ultraviolet Electroluminescence with External Quantum Efficiency (EQE) >4 %

A new ternary acceptor (A)-donor (D)-acceptor (A) asymmetrically twisted deep-blue emitting molecule, PPI-2BI, was synthesized by attaching two electrophilic benzimidazole (BI) units to the C2 and N1 positions of a phenanthroimidazole (PI) donor unit. Profiting from the enhanced D-A electronic coupling, the electron injecting and transporting abilities of the new triangle-shaped A-D-A molecule are considerably improved and the molecule shows high photoluminescence (PL) and electroluminescence (EL) efficiencies. By using PPI-2BI as a non-doped emitting layer (EML), the resulting organic light-emitting device exhibits emission with color coordinates of (0.158, 0.124) and a maximum external quantum efficiency (EQE), current efficiency (CE), and power efficiency (PE) of 4.63 %, 4.98 cd A-1 , and 4.82 lm W-1 , respectively. Additionally, a simple bilayer device using PPI-2BI as both the EML and the electron-transporting layer (ETL) also shows an EQE of 3.81 % with little changes to the color purity. Remarkably, a PPI-2BI-based doped device emits efficient near-ultraviolet EL with color coordinates of (0.154, 0.047) and an EQE of 4.12 %, which is comparable to that of the best reported near-UV emitting devices.