Room temperature tri-alkynyl benzene based discotic nematic mesophase enabling high-efficiency deep blue OLEDs

Abstract

In the hunt to develop room-temperature nematic discotic materials, we engineered a molecular structure that employs a tri-alkynyl benzene core incompatible with π-π stacking, providing flexibility to the system. Micro-segregation was prohibited due to the direct attachment of the flexible alkoxy chain to the central benzene ring, leading to a discotic nematic phase. The nematic phase was found stable down to room temperature and, on further cooling, transformed into a glassy state. Detailed analysis of differential scanning calorimetry plots, polarized optical microscopic textures, and temperature-dependent X-ray diffraction patterns confirmed the glassy nematic phase at low temperatures. The tri-alkynyl benzene moiety imparts high solid and solution-state blue emission properties to mesogens 1a and 1b. On stimulating the compounds electrically as a solid-state emitter in organic light-emitting diodes, they showed emission in the deep blue (CIEy < 0.08) region at 100 cd m−2 luminance and pure blue (CIEy < 0.11) region at maximum luminance using CBP host. Amongst all the configurations used, the device based on 0.5 wt% 1a displayed the best performance with a maximum external quantum efficiency of 4.1% with a turn-on voltage of 3.5 V and CIExy coordinates of (0.16,0.07).