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Circularly polarized lanthanide luminescence for advanced security inks

Abstract

Authenticating products and documents with security inks is vital to global commerce, security and health. Lanthanide complexes are widely used in luminescent security inks owing to their unique and robust photophysical properties. Lanthanide complexes can also be engineered to undergo circularly polarized luminescence (CPL), which encodes chiral molecular fingerprints in luminescence spectra that cannot be decoded by conventional optical measurements. However, chiral CPL signals have not yet been exploited as an extra security layer in advanced security inks. This Review introduces CPL and related concepts that are necessary to appreciate the challenges and potential of lanthanide-based, CPL-active security inks. We describe recent advances in CPL analysis and read-out technologies that have expedited CPL-active security ink applications. Further, we provide a systematic meta-analysis of strongly CPL-active Euiii, Tbiii, Smiii, Ybiii, Cmiii, Dyiii and Criii complexes, discussing the suitability of their photophysical properties and highlighting promising candidates. We conclude by providing key recommendations for the development and advancement of the field.

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Fig. 1: Security inks featuring long-lived lanthanide luminescence and rapidly emitting organic fluorophores.
Fig. 2: Circularly polarized light and enantioselective image contrast.
Fig. 3: A subset of chiral lanthanide complexes discussed as candidates for CPL-active security inks.
Fig. 4: Next-generation CPL spectrometers have time-gated detection and improved signal-to-noise ratio relative to legacy CPL spectrometers.
Fig. 5: Reported gem values for CPL-active lanthanide and Criii complexes discussed here.
Fig. 6: Relative proportion of Euiii gem values reported in the literature for each emission band.

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Acknowledgements

L.E.M. and R.P. acknowledge financial support from the Royal Society URF\R\191002, BBSRC BB/S017615/1, EPSRC EP/P025013/1 and a BBSRC Discovery Fellowship BB/T009268/1.

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L.E.M. conducted the review and meta-analysis, wrote the manuscript and designed figures. R.P. edited the manuscript, designed figures and conceived the overall direction of the manuscript.

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Correspondence to Robert Pal.

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MacKenzie, L.E., Pal, R. Circularly polarized lanthanide luminescence for advanced security inks. Nat Rev Chem 5, 109–124 (2021). https://doi.org/10.1038/s41570-020-00235-4

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