Impurity Conundrum of Organic Room Temperature Afterglow

Commercial carbazole has been widely used to synthesize organic functional materials that entwine with the recent breakthroughs in thermally activated delayed fluorescence, organic luminescent radicals and organic laser diodes. Recently, the strategy of stabilizing triplet excited states in carbazole derivatives ignited the booming development of organic room temperature afterglow (RTA). The unusual RTA of carbazole and its derivatives was elaborated by crystal quality and packing. However, impurity hypotheses in organic RTA have been under debate for nearly a century. Here we show that an isomer of carbazole, accompanying the commercial sources with less than 0.5%, is the key to activating RTA for many carbazole derivatives. As compared to commercial carbazole, the fluorescence of lab-synthesized carbazole is blue-shifted by 54 nm and the well-known RTA disappears. The same phenomenon is also observed for a series of carbazole derivatives. Interestingly, even 0.01% isomer doping could yield the reported RTA. Our results demonstrate that the isomer doping in carbazole derivatives is responsible for their RTA. The impurity effect has also been confirmed for dibenzothiophene based RTA. We anticipate that isomer doping effect is applicable to many organic semiconductors derived from commercial carbazole, which will drive the review of organic functional materials in optoelectronics.