Commercial carbazole has been widely used to synthesize organic functional materials that have led to recent breakthroughs in ultralong organic phosphorescence1, thermally activated delayed fluorescence2,3, 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 literature1,7,8,9,10,11,12,13,14,15. 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 phosphorescence1,6,7,8,9,10,11,12,13,14,15,16,17,18.
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The data that support the findings of this study are available from C.C. and L.B. upon reasonable request. The X-ray crystallographic data for the structures reported here have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers CCDC 1953802–1953811 and 2019581–2019589. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre at www.ccdc.cam.ac.uk/data_request/cif. Source data are provided with this paper.
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This study was supported by the Singapore National Research Foundation (NRF) Competitive Research Program (R279-000-483-281), the NRF Investigatorship (R279-000-444-281) and the National University of Singapore (R279-000-482-133).
The authors declare no competing interests.
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Chen, C., Chi, Z., Chong, K.C. et al. Carbazole isomers induce ultralong organic phosphorescence. Nat. Mater. (2020). https://doi.org/10.1038/s41563-020-0797-2