Carbazole isomers induce ultralong organic phosphorescence

Abstract

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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Fig. 1: Paradox of the ultralong phosphorescence of carbazole.
Fig. 2: Impurity effect on carbazole derivatives.
Fig. 3: Emission characteristics with different isomer doping concentrations.
Fig. 4: Transient absorption, photoluminescence and ultralong phosphorescence mechanism.

Data availability

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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Acknowledgements

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).

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Authors

Contributions

C.C. and B.L. designed the experiments. C.C. optimized the HPLC and grew crystals. C.C., Z.C., Z.Y., Z.M. and Z.Y. contributed to the optical characterizations. C.C. and K.C.C. synthesized all compounds. A.S.B. and C.C. solved the crystal structures. C.C. and B.L. discussed the results and drafted the manuscript. B.L. supervised the project. All authors contributed to the proofreading of the manuscript.

Corresponding author

Correspondence to Bin Liu.

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The authors declare no competing interests.

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Supplementary information

Supplementary Information

Supplementary methods, Scheme 1, Figs. 1–37 and Tables 1–11.

Source data

Source Data Fig. 1

HPLC spectra of TCI-Cz.

Source Data Fig. 2

HPLC spectra monitored at the onset absorption of 346 nm for Cz and 354 nm for CPhCz.

Source Data Fig. 3

Emission characteristics with different isomer doping concentrations.

Source Data Fig. 4

Transient absorption and photoluminescence.

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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

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