Balancing charge-transfer strength and triplet states for deep-blue thermally activated delayed fluorescence with an unconventional electron rich dibenzothiophene acceptor

Manipulation of the emission properties of deep-blue emitters exhibiting thermally activated delayed fluorescence (TADF) through molecular design is challenging. We present an effective strategy to probe deeper into the role of localized (LE) and charge transfer (CT) states in the reverse intersystem crossing (RISC) mechanism. In a series of donor–acceptor–donor (D–A–D) blue emitters the dibenzothiophene functionality is used as an unconventional acceptor, while derivatives of 9,10-dihydro-9,9-dimethylacridine are used as electron-donors. tert-Butyl and methoxy substituents in the para-positions of the donor greatly enhance the donor strength, which allows exploration of different energy alignments among CT and LE triplet states. In the tert-butyl substituted compound the low energy triplet is localized on the acceptor unit, with the RISC mechanism (kRISC = 0.17 × 105 s−1) likely involving the mixture of CT and LE triplet states that are separated by less than 0.09 eV. An optimized organic light-emitting diode (OLED) based on the tBu-compound presents a maximum external quantum efficiency of 10.5% and deep-blue emission with Commission Internationale de l'Eclairage coordinates of (0.133, 0.129). However, when methoxy substituents are used, the low-energy triplet state moves away from the emissive 1CT singlet increasing the energy gap to 0.24 eV. Despite a larger ΔEST, a faster RISC rate (kRISC = 2.28 × 105 s−1) is observed due to the upper-state RISC occurring from the high-energy triplet state localized on the D (or A) units. This work shows the importance of fine-tuning the electronic interactions of the donor and acceptor units to control the TADF mechanism and achieve a deep-blue TADF OLED

Enantiospecific sp(2)-sp(3) coupling of secondary and tertiary boronic esters

The cross-coupling of boronic acids and related derivatives with sp² electrophiles (the Suzuki–Miyaura reaction) is one of the most powerful C–C bond formation reactions in synthesis, with applications that span pharmaceuticals, agrochemicals and high-tech materials. Despite the breadth of its utility, the scope of this Nobel prize-winning reaction is rather limited when applied to aliphatic boronic esters. Primary organoboron reagents work well, but secondary and tertiary boronic esters do not (apart from a few specific and isolated examples). Through an alternative strategy, which does not involve using transition metals, we have discovered that enantioenriched secondary and tertiary boronic esters can be coupled to electron-rich aromatics with essentially complete enantiospecificity. As the enantioenriched boronic esters are easily accessible, this reaction should find considerable application, particularly in the pharmaceutical industry where there is growing awareness of the importance of, and greater clinical success in, creating biomolecules with three-dimensional architectures

New electron-transporting materials for light emitting diodes: 1,3,4-oxadiazole-pyridine and 1,3,4-oxadiazole-pyrimidine hybrids

We describe the synthesis of three new isomeric 1,3,4-oxadiazole-pyridine hybrids, namely: 2,6-, 3,5- and 2,4-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl] pyridine, (PDPy-2,6, PDPy-3,5 and PDPy-2,4, respectively) and a 1,3,4-oxadiazole-pyrimidine hybrid, namely: 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl] pyrimidine (PDPmDP). The X-ray crystal structures are reported for PDPy-2,4 and the known phenylene analogue 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5-yl] benzene (OXD-7) as a 1 : 1 toluene solvate. The packing motif for molecules of both PDPy-2,4 and OXD-7 is that of discrete layers with the mean planes of all the molecules in the crystals parallel to within 6degrees. We have fabricated light-emitting diodes (LEDs) using poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) doped with rubrene as the emissive material, with and without a thermally evaporated electron conducting/hole-blocking (ECHB) layer of PDPy-2,6, PDPy-3,5 and PDPy-2,4, PDPmDP and OXD-7, in the device configuration ITO/MEH-PPV(Ru)/ECHB layer/Al. Electroluminescence spectra indicate that light is emitted only from the MEH-PPV layer. The bilayer LEDs are considerably more efficient than single layer devices, e. g. the external quantum efficiences of devices incorporating PDPy-2,6, PDPy-3,5 and OXD-7 are 0.14, 0.04 and 0.06% at 40 mA m(-2), respectively, cf. 0.007% for the reference single-layer MEH-PPV( Ru) device. There is no clear correlation between experimental EQE values and the PM3 calculated LUMO levels of the materials

A trisulfide-linked glycoprotein

The first member of a novel class of chemoselective reagents, glycosyl methanedithiosulfonates, has been synthesized, identified and employed in the first examples of chemical, site-selective construction of a trisulfide-modified protein with complete conversion. © The Royal Society of Chemistry

Elucidation of Structure and Dynamics in Solid Octafluoronaphthalene from Combined NMR, Diffraction, And Molecular Dynamics Studies

X-ray diffraction (XRD), molecular dynamics simulations (MD), and 19F NMR have been used to investigate structure and dynamics in solid octafluoronaphthalene, C10F8. Two distinct processes are observed via measurements of 19F relaxation times as a function of temperature; a faster process from T1 relaxation with a correlation time of the order of ns at ambient temperature (fitting to Arrhenius-type parameters Ea = 20.6 ± 0.4 kJ mol−1 and τ0 = 8 ± 1 × 10−14 s) and a much slower process from T1ρ relaxation with a correlation time of the order of μs (fitting to Ea = 55.1 ± 1.3 kJ mol−1 and τ0 = 4 ± 2 × 10−16 s). Atomistic molecular dynamics reveals the faster process to involve a small angle jump of 40° of the molecules, which is in perfect agreement with the X-ray diffraction study of the material at ambient temperature. The MD study reveals the existence of more extreme rotations of the molecules, which are proposed to enable the full rotation of the octafluoronaphthalene molecules. This explains both the T1ρ results and previous wide-line 19F NMR studies. The experimental measurements (NMR and XRD) and the MD computations are found to be strongly complementary and mutally essential. The reasons why a process on the time scale of microseconds, and associated with such a large activation barrier, can be accessed via classical molecular dynamics simulations are also discussed