Polymorphism of derivatives of tert-butyl substituted acridan and perfluorobiphenyl as sky-blue OLED emitters exhibiting aggregation induced thermally activated delayed fluorescence

Abstract

Multi-functional luminophores exhibiting thermally activated delayed fluorescence (TADF), aggregation induced emission enhancement and sky-blue ↔ blue colour-changing were designed and synthesized. For the first time, perfluorobiphenyl was used as an acceptor in the design of TADF compounds, while, non-substituted and tert-butyl substituted 9,9-dimethyl-9,10-dihydro-acridine was chosen as an appropriate donor. A comparative study of donor–acceptor versus donor–acceptor–donor molecular structures of the emitters was performed. Properties of the studied perfluorobiphenyls were found to be very sensitive to the substitutions. One of the studied compounds formed two crystalline polymorphs characterized by aggregation induced delayed fluorescence (AIDF), the colour of which can be changed under external forces such as a pair of solvents, temperature and mechanical treatment. It was shown by single-crystal X-ray structural analysis that tert-butyl groups, π–π and unique C–F⋯π interactions and C–H⋯F hydrogen bonds are responsible for the crystalline polymorph formations. The synthesized compounds were tested as blue/sky-blue AIDF emitters in organic light-emitting diodes, the best of which was characterized by a low turn-on voltage of 3 V and a maximum current, power, and external quantum efficiency of 22.7 cd A⁻¹, 30.8 lm W⁻¹, and 16.3%, respectively.