Zece Zhu

Teaching an old acceptor new tricks: rationally employing 2,1,3-benzothiadiazole as input to design a highly efficient red thermally activated delayed fluorescence emitter

The exploitation of blue to orange emissive thermally activated delayed fluorescence (TADF) materials has been conducted comprehensively, while the equally important red TADF materials have been studied at a relatively slow pace. Three D–A–D structured fluorescent molecules, N4,N4,N7,N7-tetraphenylbenzo[c][1,2,5]thiadiazole-4,7-diamine (BTZ–DPA), 4,7-bis(9H-carbazol-9-yl)benzo[c][1,2,5]thiadiazole (BTZ–CZ) and 4,7-bis(9,9-dimethylacridin-10(9H)-yl) benzo[c][1,2,5]thiadiazole (BTZ–DMAC) were designed and synthesized by rationally employing 2,1,3-benzothiadiazole as an acceptor. The introduction of 2,1,3-benzothiadiazole not only presents an efficient input for the design of red TADF emitters, but also provides benefits for the resulting high-efficiency organic light-emitting diodes (OLEDs) which show a maximum external quantum efficiency of 8.8% at a luminance of 1.06 cd m−2 with the emission peak at 636 nm.

Rational utilization of intramolecular and intermolecular hydrogen bonds to achieve desirable electron transporting materials with high mobility and high triplet energy

Three new pyrimidine-containing star-shaped compounds, namely, 1,3,5-tri(3-(pyrimidin-5-yl)phenyl)benzene (TPM-TPB), 2,4,6-tris(3-(pyrimidin-5-yl)phenyl)-1,3,5-triazine (TPM-TAZ) and 3,5,6-tris(3-(pyrimidin-5-yl)phenyl)-1,2,4-triazine (TPM-i-TAZ), were synthesized and characterized. These new compounds exhibited favorable electronic affinity (Ea >2.81 eV) and the triplet energy levels (ET) up to ∼2.83 eV. X-ray diffraction analysis of the compounds revealed that the intramolecular and intermolecular C–H⋯N hydrogen bonds of TPM-TAZ resulted in high electron mobility up to 2.0 × 10−3 cm2 V−1 s−1. Using these compounds as the electron transporting materials, the blue phosphorescent organic light-emitting devices showed good performance, with a very low turn-on voltage of 2.4 V, a maximum current efficiency of 26.4 cd A−1, and a maximum power efficiency of 26.9 lm W−1.

Benzobisoxazole-based electron transporting materials with high Tg and ambipolar property: high efficiency deep-red phosphorescent OLEDs

Two new benzobisoxazole-based compounds, namely, 4,8-bis(3,5-di(pyridin-3-yl)phenyl)-2,6-dimethylbenzo[1,2-d:4,5-d′]bis(oxazole) (3Py-DBBO) and 4,8-bis(4-yl-triphenylphosphine oxide)-2,6-dimethylbenzo[1,2-d:4,5-d′]bis(oxazole) (TPO-DBBO), were synthesized and characterized. The steric effect of the ortho hydrogens on the phenyl ring at 4,8-positions of benzobisoxazole resulted in out-of-plane twisting, and consequently a decrease of the intermolecular π–π interaction. The two new compounds showed excellent thermal stabilities with high glass-transition temperatures (Tg) of 248 °C for 3Py-DBBO and 142 °C for TPO-DBBO. The two compounds exhibited ambipolar transport properties, with both electron and hole mobilities of 10−6–10−5 cm2 V−1 s−1. Using the compounds as electron-transport materials, the deep-red phosphorescent organic light-emitting devices achieved a maximum external quantum efficiency up to 19.3%.

Zn(2+)-cyclen-based complex enable a selective detection of single-stranded thymine-rich DNA in aqueous buffer.

It is a big challenge to develop fluorescent probes for selective detection of DNA with specific sequences in aqueous buffers. We report a new tetraphenylethene-based Zn(2+)-cyclen complex (TPECyZn), and a chemo-sensing ensemble of the Zn complex with phenol red. TPECyZn showed significant fluorescence enhancement upon binding to thymine-rich DNA in HEPES buffers. But its selectivity was not high enough to eliminate the interference from some random DNA. By constructing the chemo-sensing ensemble of TPECyZn with phenol red, the background fluorescence was eliminated due to the energy transfer from TPECyZn to phenol red. Moreover, this chemo-sensing ensemble revealed high selectivity in detecting thymine-rich single-stranded DNA over other DNA in aqueous buffer. It can detect poly deoxythymidylic acid sequence as short as 2 nt. This detection in aqueous media makes this probe feasible in real application.

Tetraphenylethene-based Zn complexes for the highly sensitive detection of single-stranded DNA

Metal–ligand coordination interactions were utilized to develop tetraphenylethene-based DNA probes. Z- and E-TPE derivatives based on Zn2+-DPA (dipicolylamine) units, namely Z-TPE2Zn and E-TPE2Zn, were tested, respectively, to oligonucleotides with different lengths and different sequences. TPE2Zn exhibited a high fluorescence enhancement when detecting ssDNA as short as 5 nt. The fluorescence intensity of Z-TPE2Zn could increase to more than 100-fold of its initial value when enough DNA was added. This fluorescent on/off ratio was much higher than that of the classical DNA probe of ethidium bromide (EB). Absorption titration experiments indicated that thymine and guanine could coordinate with the Zn2+-DPA unit more effectively than adenine and cytosine. These experiments might aid the design of metal complex-based DNA probes with high sensitivity and selectivity.

Efficient non-doped fluorescent OLEDs with nearly 6% external quantum efficiency and deep-blue emission approaching the blue standard enabled by quaterphenyl-based emitters

Simultaneously achieving excellent color purity approaching or surpassing the blue standard and outstanding device performance remains a big challenge for blue fluorescent emitters. In this study, two quaterphenyl derivatives are rationally designed and synthesized as deep-blue emitters with high photoluminescence quantum yields of nearly 80%. These emitters are applied in non-doped OLEDs yielding a peak EQE of 5.94% and CIE coordinates of (0.152, 0.085), approaching the NTSC blue standard. When applied to doped OLEDs, these derivatives emit violet-blue light with a peak EQE of 6.55% and CIE coordinates of (0.156, 0.055).

Designing Dual Emitting Cores for Highly Efficient Thermally Activated Delayed Fluorescent Emitters

Improving the efficiency and the stability of TADF-based OLED devices is of vital importance for OLED applications. In this work, two dual emitting cores (2,2′-DPXZ-PN and 3,3′-DPXZ-PN) were rationally synthesized by connecting two identical TADF emitting units and were confirmed with obvious TADF features. The thermal stability accompanied by high Td, Tg and Tm values was obtained via the design of an extension of the molecular structure. Meanwhile, the molar extinction coefficients and PLQYs of the two dual emitting cores were also dramatically improved, of which 3,3′-DPXZ-PN in 10 wt% doped CBP film resulted in a PLQY as high as 82%. Based on the high PLQY values and TADF features, maximum external quantum efficiency (EQE) values of 13.4% for a 2,2′-DPXZ-PN-based OLED device and 14.9% for a 3,3′-DPXZ-PN-based OLED device were achieved with a slow efficiency roll-off of 12% and 13.5% at a high luminance of 1000 cd m−2.