Two dibenzothiophene (DBT)-based phosphine oxide hosts, named 4-diphenylphosphoryl dibenzothiophene (DBTSPO) and 4,6-bis(diphenylphosphoryl) dibenzothiophene (DBTDPO), were prepared by short-axis substitution with the aim to selectively adjust electrical properties. The combined effects of short-axis substitution and the involvement of electron-donating S atom in conjugation effectively suppress the influence of electron-withdrawing diphenylphosphine oxide (DPPO) moieties on the frontier molecular orbitals and the optical properties. Therefore, DBTSPO and DBTDPO have the nearly same hole injection ability and the excited energy levels, while more electron-transporting DPPOs and the symmetrical configuration endow DBTDPO with enhanced electron-injecting/transporting ability. As the result, on the basis of this short-axis substitution effect, the selective adjustment of electrical properties was successfully realized. With the high first triplet energy level (T(1)) of 2.90 eV, the suitable energy levels of the highest occupied molecular orbital and the lowest unoccupied molecular orbital of -6.05 and -2.50 eV and the improved carrier-transporting ability, DBTDPO supported its blue- and white-emitting phosphorescent organic light-emitting diodes as the best low-voltage-driving devices reported so far with the lowest driving voltages of 2.4 V for onset and <3.2 V at 1000 cd m(-2) (for indoor lighting) accompanied with the high efficiencies of >30 lm W(-1) and excellent efficiency stability.

Phosphorescent organic light-emitting diodes (PHOLEDs), with 100 % theoretical internal efficiency, are being rapidly developed as a most promising approach to meet the urgent and extensive demand of energy-efficient and portable digital terminals and lighting sources.[1] Thanks to the recent breakthrough of highly efficient blue PHOLEDs [2] and outcoupling technologies, PHOLEDs in full color can already realize extremely high efficiencies that approach those of fluorescent tubes (about 70 Lm W À1 ).[3] Nevertheless, as the hosts in the emitting layers (EMLs) should have higher triplet excited energy levels (T 1 ) to confine the excitons on phosphorescent guests, [4] the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gaps in PHOLEDs are often much larger than their fluorescent counterparts, which consequently result in poor energy-level alignment and thus higher driving voltages. This drawback not only complicates the design of driving circuit, but also directly reduces power efficiency (PE). [5] Thus, the low-voltage-driving high-efficiency PHOLED remains the biggest challenge. Su, Kido, et al. have reported green PHOLEDs with extremely low operating voltages of 2.18 V for onset and 2.41 V at 100 cd m À2 through good management of the interfacial contact between electron transporting layers and anodes.[5] However, there are only a few blue PHOLEDs that achieve low driving voltages; for example, applicable luminance at a driving voltage of less than 3 V.[6] The formidable challenge is the high barriers for carrier injection and transportation deriving from the prerequisite of extremely high T 1 of the hosts, for example, 2.85 eV (0.2 eV higher than that of blue phosphor iridium-(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic; Scheme 1). This issue actually reflects the intrinsic contradiction between optical and electrical properties of the materials. As the singlet excited energy levels (S 1 ) are related to HOMO-LUMO energy gaps, the challenge has actually evolved into an original scientific problem: how to controllably adjust S 1 without changing T 1 . Unfortunately, as most of the modification approaches have the same effects on both singlet and triplet states, [7] only few hosts possess both of the energy difference between S 1 and T 1 (DE ST ) of less than 0.4 eV and T 1 of more than 2.85 eV.[6b]We recently reported several ambipolar ternary aryl phosphine oxide (APO) hosts based on indirect linkage. [6b] Their DE ST was tuned to 0.45 eV, and low driving voltages and high efficiencies were realized. However, the indirect linkage can hardly afford multiple modifications. We believe that the Scheme 1. The energy-transfer process in phosphorescent doping systems, and molecular structures of DBFxPOCzn.

Arbuscular mycorrhizal fungi (AMF), ubiquitous soil fungi that form symbiotic relationships with the majority of terrestrial plants, are known to play an important role in plant tolerance to chromium (Cr) contamination. However, the underlying mechanisms, especially the direct influences of AMF on the translocation and transformation of Cr in the soil−plant continuum, are still unresolved. In a two-compartment root-organ cultivation system, the extraradical mycelium (ERM) of mycorrhizal roots was treated with 0.05 mmol L −1 Cr(VI) for 12 days to investigate the uptake, translocation, and transformation of Cr(VI) by AMF using inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy equipped with energy-dispersive spectroscopy (SEM−EDS), transmission electron microscopy equipped with energy-dispersive spectroscopy (TEM−EDS), and X-ray-absorption fine structure (XAFS) technologies. The results indicated that AMF can immobilize quantities of Cr via reduction of Cr(VI) to Cr(III), forming Cr(III)−phosphate analogues, likely on the fungal surface. Besides this, we also confirmed that the extraradical mycelium (ERM) can actively take up Cr [either in the form of Cr(VI) or Cr(III)] and transport Cr [potentially in the form of Cr(III)-histidine analogues] to mycorrhizal roots but immobilize most of the Cr(III) in the fungal structures. Based on an X-ray absorption nearedge spectroscopy analysis of Cr(VI)-treated roots, we proposed that the intraradical fungal structures can also immobilize Cr within mycorrhizal roots. Our findings confirmed the immobilization of Cr by AMF, which plays an essential role in the Cr(VI) tolerance of AM symbioses.