Diamine-cored tetrastilbene compounds as solution-processable hole transport materials for stable organic light emitting diodes

A series of diamine-cored tetrastilbene (DTS) derivatives bearing various aliphatic and aromatic substituents was designed and synthesized for use as solution-processed hole transport layers (HTLs) in organic light emitting diodes (OLEDs). The chemical structures of the DTS derivatives were strategically designed to increase solubility in organic solvents as well as to avoid self-crystallization, and thus ensure a stable morphology under Joule heating while maintaining efficient hole transport capabilities. The five DTS derivatives, composed of different conjugation structures, yielded reasonably good hole transport behavior with a marginal charge carrier mobility of ∼10−5 cm2V−1s−1, which is similar to that of vacuum-deposited N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB). Due to the high glass transition temperatures of the DTS derivatives, this satisfactory charge transport behavior and smooth surface morphology were conserved up to 180 °C. Green OLEDs were prepared using tris-(8-hydroxyquinoline) aluminum (Alq3):C545T as the emission layer, and the OLED performances of the solution-processed DTS HTLs and the vacuum-deposited NPB HTL were compared. A high luminance efficiency of 11.5 cd A−1 was obtained for one solution-processed DTS HTL, which exceeds that of the NPB HTL (10.01 cd A−1). Furthermore, the DTS HTLs enabled a stable OLED operation, with double the accelerated half-life of the NPB-based device. © 20171

Tree diversity reduces herbivory by forest insects

Biodiversity loss from plant communities is often acknowledged to affect primary production but little is known about effects on herbivores. We conducted a meta-analysis of a worldwide data set of 119 studies to compare herbivory in single-species and mixed forests. This showed a significant reduction of herbivory in more diverse forests but this varied with the host specificity of insects. In diverse forests, herbivory by oligophagous species was virtually always reduced, whereas the response of polyphagous species was variable. Further analyses revealed that the composition of tree mixtures may be more important than species richness per se because diversity effects on herbivory were greater when mixed forests comprised taxonomically more distant tree species, and when the proportion of non-host trees was greater than that of host trees. These findings provide new support for the role of biodiversity in ecosystem functioning across trophic level