High efficiency OLEDs based on anthracene derivatives: The impact of electron donating and withdrawing group on the performance of OLED

New well-defined bulky anthracene derivatives with side groups having electron donating or withdrawing properties 8a-d were synthesized. The compounds contain substituted anthracene as the central core attaching 2-(4-(2-pyridinyl)- phenyl)vinyl and 4-R-phenyl [R: H (a), OMe (b) and CF3 (c), N(Ph)2 (d)] groups at 9- and 10- positions. The impact of electron donating, withdrawing and neutral groups and their influence on the molecules photophysical, charge transfer (CT), triplet transfer (TT) and triplet-triplet annihilation (TTA) properties has been investigated. Based on the photophysical studies the most promising molecule (8d) has been selected and high efficiency fluorescent OLEDs with EQE at very low current efficiency reaching 7% were obtained. The value at low current density implies a Triplet Fusion (TF) contribution of 45%, very close to the maximum theoretical value of 50% when only the singlet decay channel is open to TTA, however we believe that in this case both TTA and TADF contribute to the triplet harvesting to yield high EQE values, and this mixed triplet harvesting arises through the heterogeneity of the films. At high current density a brightness of 20000 cd/m2 was achieved and it is assigned partially to the material crystallisation. © 2015 Elsevier B.V.Devlet Planlama Örgütü Engineering and Physical Sciences Research CouncilF.T. gratefully acknowledge the (Turkish) State Planning Organization (DPT) for Financial support. G.H. also thanks Turkish DPT for financial support for her MSc. The authors are grateful to Dr. Serife Sarioglan for her initial help in electrochemistry measurements. OEM group at Physics Department/Durham University would like to thank EPSRC for partial financial support of this project. Appendix

Microstructural Analysis and the Multicolor UV/Violet/Blue/Green/Yellow PL Observed from the Synthesized ZnO Nano-leaves and Nano-rods

We report the synthesis of zinc oxide (ZnO) nano-leaves and nano-rods under high and extremely high alkaline experimental conditions, via a simple and low-temperature method. By performing transmission electron microscopy it is found that the nano-leaves and nano-rods grow along the (001) direction. Anisotropic, i.e., hkl-dependent line-shape broadening is observed in ZnO powder diffraction patterns. Rietveld analysis using Fullprof with model for handling the anisotropic size-like broadening is performed on these diffraction patterns. The refinement showed that ZnO powders belong to the hexagonal ZnS structure type with space group P63mc, and confirmed that the nano-leaves and nano-rods are oriented along the (001) direction. Results of visualization in 3D of the average crystallite shape obtained from refinement of spherical harmonics coefficients showed elongated shapes in the both samples, exhibiting a slight twisting for nano-leaves. Diffuse reflectance measurements reveal that the optical band-gap energies found for the ZnO nano-leaves and nano-rods is somewhat smaller than a wide-direct band gap of 3.37 eV. We argued that well defined and strong photoluminescence (PL) bands in the visible part that belong to the defects may influence the observed displacement of a ultraviolet (UV) near-band-edge emission, and which is related with obtained slightly lower band-gap energies than the established band gap of bulk ZnO. We discuss processes behind the multicolor UV/violet/blue/green/yellow emission band in PL spectra. (C) The Minerals, Metals and Materials Society and ASM International 201