Novel phenanthroimidazole-based blue AIEgens: reversible mechanochromism, bipolar transporting properties, and electroluminescence

Multifunctional materials are crucial and have promising applications in a wide range of organic electronics. Herein, we designed and synthesized two bipolar blue molecules named 2-(4-(4,5-diphenyl-2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-imidazol-1-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (PPI-PIM-TPE) and 1-phenyl-2-(4-(2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-phenanthro[9,10-d]imidazol-1-yl)phenyl)-1H-phenanthro[9,10-d]imidazole (2PPI-TPE) with aggregation-induced emission (AIE) and mechanochromism characteristics. They both have good thermal stability (T-d is 505 degrees C for PPI-PIM-TPE and 510 degrees C for 2PPI-TPE), strong AIE properties and reversible mechanochromism. The quantum yields in the solid state were as high as 61.9% for PPI-PIM-TPE and 73.4% for 2PPI-TPE. In addition, the two pristine solid powders are white and emit blue light. After grinding, the solid becomes yellow and emits blue-green emission. The color changes are reversible by solvent fuming. The change in emission color can be observed by the naked eye, demonstrating that they are typical mechanochromic materials. Non-doped blue OLEDs based on 2PPI-TPE exhibit an external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) of 2.48%, 6.46 cd A(-1) and 4.72 lm W-1, respectively. The doped device based on 2PPI-TPE as a dopant emitter exhibits a higher EQE, CE and PE of 3.55%, 6.67 cd A(-1) and 5.52 lm W-1. The performances of the OLEDs with these emitters are among the best of recent reports based on blue materials with AIE and mechanochromism simultaneously.