An Anthracene Derivative with a Highly Vertical Molecular Orientation

Controlling the orientation of the transition dipole moment (TDM) is very important in the field of optoelectronics. In particular, the horizontal orientation of emissive TDMs in organic materials has been extensively studied because it can improve the out-coupling efficiency of organic light-emitting diodes (OLEDs). Conversely, the vertical orientation of emissive TDMs remains virtually unexplored. Using angle-dependent photoluminescence measurements, we discovered that 9,10-bis(3,5-dimethoxyphenyl) anthracene (DMA) shows an extremely high vertical emissive TDM orientation (Θ_V) of 82% in an evaporated neat film. To the best of our knowledge, this is the highest value reported so far for organic molecules. To investigate the origin of the high Θ_V value, we conducted two-dimensional grazing-incidence wide-angle X-ray scattering measurements (2D GIWAXS) on a DMA film and observed distinct periodic peaks in the out-of-plane direction. By combination of these measurements with single-crystal X-ray diffraction analysis, the periodic peaks were identified as lamellar structures in which DMA molecules are stacked with their long molecular axes oriented vertically on a quartz substrate. Moreover, by virtue of the exceptionally high vertical TDM orientation, the DMA film exhibits highly polarized emission from the substrate edge. We also investigated the TDM orientation of other 9,10-diphenylanthracene derivatives and observed random or horizontal TDM orientations in all cases, which highlights the uniqueness of the vertical orientation of DMA.