Impurity conundrum of ultralong organic phosphorescence

Commercial carbazole (Cz) has been widely used to synthesize organic functional materials, which are entwined with recent breakthroughs in ultralong organic phosphorescence, thermally activated delayed fluorescence, organic luminescent radicals, and organic semiconductor lasers. Recently, we discovered that different from commercial Cz, the fluorescence of lab-synthesized-Cz (Lab-Cz) is blue-shifted by 54 nm and the well-known room-temperature ultralong phosphorescence almost disappears. Detailed studies reveal the presence of a Cz isomer as the impurity, which is widespread in commercial Cz sources with <0.5 mol%. Ten representative Cz derivatives were resynthesized from the Lab-Cz and all failed to show the reported ultralong phosphorescence in the same crystal states. However, even 0.1 mol% isomer doping can recover the reported ultralong phosphorescence. The presence of the isomer in commercial carbazole triggers us to re-examine the structure-property of many optically active materials with important discoveries.