Commercial carbazole has been widely used to synthesize organic functional materials that have led to recent breakthroughs in ultralong organic phosphorescence¹, thermally activated delayed fluorescence^2,3, organic luminescent radicals⁴ and organic semiconductor lasers⁵. However, the impact of low-concentration isomeric impurities present within commercial batches on the properties of the synthesized molecules requires further analysis. Here, we have synthesized highly pure carbazole and observed that its fluorescence is blueshifted by 54 nm with respect to commercial samples and its room-temperature ultralong phosphorescence almost disappears⁶. We discover that such differences are due to the presence of a carbazole isomeric impurity in commercial carbazole sources, with concentrations <0.5 mol%. Ten representative carbazole derivatives synthesized from the highly pure carbazole failed to show the ultralong phosphorescence reported in the literature^{1,7,8,9,10,11,12,13,14,15}. However, the phosphorescence was recovered by adding 0.1 mol% isomers, which act as charge traps. Investigating the role of the isomers may therefore provide alternative insights into the mechanisms behind ultralong organic phosphorescence^{1,6,7,8,9,10,11,12,13,14,15,16,17,18}.

商业咔唑已被广泛用于合成有机功能材料，这些材料已导致超长有机磷光¹，热活化延迟荧光^2，3，有机发光基⁴和有机半导体激光器^5的最新突破。然而，商业批次中存在的低浓度异构杂质对合成分子性质的影响需要进一步分析。在这里，我们合成了高纯度咔唑，并观察到其相对于商业样品的荧光蓝移了54 nm，并且其室温超长磷光几乎消失了⁶。我们发现这种差异是由于在商业咔唑源中存在咔唑异构杂质，其浓度<0.5 mol％。由高纯咔唑合成的十种代表性咔唑衍生物未能显示出文献^{1、7、8、9、10、11、12、13、14、15中}报道的超长磷光。然而，通过添加充当电荷陷阱的0.1摩尔％的异构体来恢复磷光。因此，研究异构体的作用可能为超长有机磷^{1,6,7,8,9,10,11,12,13,14,15,16,17,18}背后的机理提供替代性见解。