many different heteroatom organic blocks, such as carbazole, [ 10 ] pyridine, [ 11 ] oxadiazole, [ 12 ] acridine, [ 13 ] phosphine oxide, [ 14 ] etc., are introduced to meet these requirements, because the frontier orbitals of host materials can be delicately tuned with different groups. [ 15 ] These differences in heteroatom compounds give us more options in material's design and, however, bring some unknown risks in chemistry. It is primarily resulted from the fact that organic functional groups consist of heteroatoms always serve as the activated sites by themselves or activate adjacent positions. For example, Qiao and co-workers studied the molecular stability under device operation and found that C-C bond is signifi cantly stronger than C-S, C-P, or C-N bonds. [ 16 ] Thus, there is still a quest for host materials constituted by pure hydrocarbon (PHC) backbone in academia. But in practical, these heteroatom-free hosts still lag far behind the development of heteroatom hosts. Take the sky-blue emitter FIrpic as guest for example, the best results based on PHC hosts were just around 10% in EQE, [ 17 ] while the heteroatom hosts can achieve >20% EQE, approaching the ≈100% IQE in theory. [10][11][12][13][14] There could be big room for improvement of PHC hosts; we would therefore go forward to explore which is the effective way to arrange the hydrocarbon blocks for high efficient hosts.In this communication, two new spirofl uorene-based PHC materials, SF33 and SF34, are reported by combining two 9,9′-spirofl uorene blocks in different linking ways. SF33 adopts symmetrical confi guration with meta-meta linkage, while SF34 has unsymmetrical confi guration with meta-ortho linkage. Most of previously researches about PHC hosts adopt symmetrical or repeated units as SF33, as it can increase the molecular weight for the balance between suffi cient glass-transition temperature and suitable volatility. [ 17d ] But by breaking the symmetry of SF33 in this case, SF34 presented similar thermal property but quite different electrical and optical properties. In thermogravimetric analysis (TGA) and differential scanning calorimetry measurements ( Figure S1 and S2, Supporting Information), SF33 and SF34 exhibited good thermal stability. The decomposition temperatures ( T d ), which correspond to 5% weight loss upon heating during TGA, were measured to be 433 °C and 407 °C for SF33 and SF34, respectively. In addition, SF33 and SF34 also have a high glass-transition temperature ( T g ) of 193 °C and 177 °C, respectively.The signifi cant difference between SF33 and SF34 is in the spectra measurements. The UV-vis absorption and photoluminescence (PL) spectra of SF33 and SF34 in dilute hexane solution (1 × 10 −6 mol L −1 ) were measured to investigate their photophysical properties ( Figure 1 ). SF33 and SF34 present Organic light-emitting diodes (OLEDs) have been widely touted as next-generation displays and solid-state lighting technologies because of their superior characteristics. [ 1 ] However, effi ciency and lifetime...

Two phosphorescent Ir(III) complexes Ir(ppm)2(acac) and Ir(dmppm)2(acac) were synthesized and characterized with emission ranged at 584/600 nm and high photoluminescence quantum yields (PLQYs) of 0.90/0.92, respectively. The angle-dependent PL spectra analysis reveals that the two orange iridium(III) complexes embodied horizontal orientation property. The high photoluminescence quantum yield and high horizontal dipoles ratio determine their excellent device performance. The devices based on Ir(ppm)2(acac) and Ir(dmppm)2(acac) achieved efficiencies of 26.8% and 28.2%, respectively, which can be comparable to the best orange phosphorescent devices reported in the literature. Furthermore, with the introduction of FIrpic as sky-blue emitter, phosphorescent two-element white organic light-emitting devices (OLEDs) have been realized with external quantum efficiencies (EQEs) as high as 25%, which are the highest values among the reported two-element white OLEDs.

How to maintain high power efficiency (PE) and color stability under operating brightness is critical for the white organic light-emitting diodes (WOLEDs). To this end, two novel spiro-type materials STPy3 and STPy4 were designed. These materials could act as a single host and achieve a remarkable external quantum efficiency of 27.5% at 1000 cd m; to further optimize the PEs of OLEDs, STPy3/4 and PO-T2T were used as co-host-induced exciplexes, which enhanced the PE of green OLED to over 148.0 lm W. Unfortunately, the lower triplet energy level of exciplexes than blue emitters implied it is commonly unsuitable to fabricate WOLEDs. Herein, a new allocation of gradient exciplex (AGE) strategy was developed in which the formed excitons could be rationally allocated in a consequently doped nonuniform profile. The AGE incorporated the advantages of the exciplex with an ultralow turn-on voltage of 2.3 V and efficiency stability of spiro materials. The PE at 1000 cd m was enhanced to 72.7 lm W, representing the first exciplex WOLED with the performance exceeding that of conventional fluorescent tubes.

Implementing rigorous standards for high-quality white organic light-emitting diodes (WOLEDs) demands further investigation. Herein, a novel and feasible color remedy strategy (CRS) is proposed in WOLEDs composed of binaryemitters, to arouse the green-emission, thereby complementing the spectral deficiency in white-emission. Thus, the color rendering indexes (CRIs) of binary-emissive WOLEDs can be boosted from 63 to 80 threshold, and the Commission International de I'Eclairage-(x, y) coordinates are precisely located inside the American National Standard Institute quadrangles, which can rival the WOLEDs integrating ternary or more emitters. Moreover, it is more feasible for CRS-based binary-emissive system to tune white-emission from cool white-emission (correlated color temperature (CCT) ≈ 5000 K) to eye-friendly warm white-emission (CCT ≈ 2000 K). Meanwhile, benefiting from the reduced energy loss and low driving voltage of CRS zone, all of the CRS-based WOLEDs with diverse CCTs can exceed 20% external quantum efficiency, and the highest approach 25%, as well as the highest power efficiency beyond 60 lm W −1 , which is comparable with those reported employing lightextracting techniques. In addition, it is evident that CRS-based WOLEDs also exhibit outstanding color stability within the variation of luminance in several orders of magnitude (50-12 000 cd m −2 ). Thus, this novel CRS provides an innovative pathway to fabricate high-quality WOLEDs composed of binary emitters.

A thermally activated delayed fluorescence material 2,6-bis(9,9-diphenylacridin-10(9H)-yl)pyrazine was designed and synthesized. The twisted configuration made it possesses very small singlet-triplet splitting. A red electroluminescent device based on this new host material is able to achieve ∼26% external quantum efficiency and relatively flat efficiency roll-off.