Materials Chemistry Frontiers Blog

Organic molecule-based crystalline dielectric materials have attracted broad attention from chemists in recent years to develop new organic electronic devices. In their designing strategy, the molecular motion induced by the external dielectric field is required to maximize the polarization effect in the materials to realize a large dielectric constant.

When people pay attention to the molecular motion in curved-π aromatics and their supramolecular complexes, it is assumed that the “curve-to-curve” contact in the curved-π aromatics will afford the smooth molecular motion in the solid state. Sumanene (1) is one of the representative buckybowls and is known to show unique properties such as bowl inversion behaviour derived from its unique bowl shape and is recognised as the potential molecular switch (Figure 1a). However, another feature of 1 to form unidirectionally arraigned π-stacking columns in the solid state makes the bowl inversion behaviour of 1 useless for the switching applications due to the large bowl-inversion energy in the 1D-column formation.

Figure 1. a) Molecular structures of 1 and 2 and schematic diagrams of the features of 1. b) Pendulum-like in-plane motion of 2 exhibiting a dielectric response in an electric field.

Recently, the group of Osaka University and collaborators of Tohoku University, Tokyo Institute of Technology and Kyoto University have demonstrated that the in-plane motion of disfluorinated sumanene 2 in the solid state is applicable as the source of stimuli responsive function instead of the bowl inversion to bring out the dielectric response (Figure 1b).

The group has focused that 2 possesses two fluorine atoms on the same benzylic carbon on pristine sumanene to possess a large dipole moment along the in-plane direction and gives the isostructural crystalline packing to 1. Thermal analyses, variable temperature X-ray diffraction and IR measurements indicated the presence of pendulum-like in-plane motion of 1 at high temperature region although no clear phase transition was involved. Indeed, the dielectric measurement using its both powder and single crystal clearly showed that both real (ε₁) and imaginary (ε₂) parts of the dielectric constant were enhanced above ~360 K at 1 MHz with a Debye-type dielectric relaxation, confirming the in-plane motion of 2 induced by the external electric field (Figure 2).

Figure 2. a) Schematic model of the relationship between the single-crystal shape of 2 and the applied electric field. b) to e) Temperature dependence of b), d) the real part (ε1) and c), e) the imaginary part (ε2) of the dielectric constant of 2 in a single-crystalline form measured at various frequencies. The direction of the electric field applied for b) and c): parallel to the c axis; for d) and e): orthogonal to the c axis.

These results, focusing on the in-plane molecular motion in the π-stacking column of a buckybowl, will help to provide a better understanding of the dynamics of solid-state curved-π systems and will accelerate their application in functional materials.

Corresponding author:

Yumi Yakiyama is an associate professor in the Division of Applied Chemistry at Osaka University (Japan). She got a PhD degree in Chemistry at Osaka University in 2010. From 2010 to 2015 she did postdoctoral studies at POSTECH (Korea). The research field of Professor Yumi Yakiyama is physical organic chemistry especially about functional organic crystals as well as metal-organic and pure organic frameworks.

https://researchmap.jp/yumiyakiyama

https://www-chem.eng.osaka-u.ac.jp/~sakurai-lab/

Deep blue electroluminescence is highly required for organic light-emitting diode (OLED) technology. However, designing fluorophores displaying adequate CIE coordinates and particularly a low CIEy is far from an easy task. We report in this work the synthesis, the physico-chemical properties and the application in OLED of deep blue emitters constructed on the dibenzosuberene (DBS) molecular fragment. Three emitters, SPA-DBS, SIA-DBS and SQPTZ-DBS, have been constructed following a similar molecular design strategy that is the spiro connection of an electron rich unit, namely N-phenylacridine (PA), indoloacridine (IA) or quinolinophenothiazine (QPTZ) to the DBS core. The PA, IA and QPTZ fragments are known to be efficient hole injecters due to their strong electron-rich character. Through a structure/properties relationship study, the group of Prof Cyril Poriel (Institut des Sciences Chimiques de Rennes- UMR 6226, Rennes) reports the electrochemical, photophysical and thermal behaviours of these three emitters.

The resulting organic materials display similar LUMO levels lying at ca -2.30 eV and different HOMO levels driven by the donor unit comprised between -5.22 and -5.48 eV. The spiro-configuration allows maintaining high T_g and T_d in accordance with OLED application. SPA-DBS displays a deep-blue emission with CIE of (0.16, 0.04), reaching an EQE of ca 1% and possessing a very low CIEy coordinate of 0.04. This CIEy coordinate fits the NSTC, ITU and EBU standards. This work not only reports a deep blue emitter for OLED but also shed light on interesting properties displayed by the DBS fragment, such as its low LUMO energy level, ca -2.3 eV, which is significantly decreased compared to its counterpart fluorene. This particularity can be advantageously used in further designs to favour the electron injection in electronic devices.

Corresponding Author:

Cyril Poriel received his PhD in 2003 from the University of Rennes 1. After a postdoctoral stay at the University of Exeter (UK), he joined the CNRS (Institut des Sciences Chimiques de Rennes) in 2005, where he is currently CNRS Research Director. His main research interest deals with the design of π-conjugated architectures for Organic Electronics. He is author/co-author of more than 120 publications, reviews and book chapters.