Organic long persistent luminescence

Abstract

Long persistent luminescence (LPL) materials—widely commercialized as ‘glow-in-the-dark’ paints—store excitation energy in excited states that slowly release this energy as light1. At present, most LPL materials are based on an inorganic system of strontium aluminium oxide (SrAl2O4) doped with europium and dysprosium, and exhibit emission for more than ten hours2. However, this system requires rare elements and temperatures higher than 1,000 degrees Celsius during fabrication, and light scattering by SrAl2O4 powders limits the transparency of LPL paints1. Here we show that an organic LPL (OLPL) system of two simple organic molecules that is free from rare elements and easy to fabricate can generate emission that lasts for more than one hour at room temperature. Previous organic systems, which were based on two-photon ionization, required high excitation intensities and low temperatures3. By contrast, our OLPL system—which is based on emission from excited complexes (exciplexes) upon the recombination of long-lived charge-separated states—can be excited by a standard white LED light source and generate long emission even at temperatures above 100 degrees Celsius. This OLPL system is transparent, soluble, and potentially flexible and colour-tunable, opening new applications for LPL in large-area and flexible paints, biomarkers, fabrics, and windows. Moreover, the study of long-lived charge separation in this system should advance understanding of a wide variety of organic semiconductor devices4.

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Figure 1: A mechanism for organic long persistent luminescence.
Figure 2: Photophysical properties of TMB, PPT, and a TMB/PPT blend.
Figure 3: LPL decay profiles for TMB/PPT films.

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Acknowledgements

This work was supported by the Japan Science and Technology Agency (JST), the Exploratory Research for Advanced Technology (ERATO) program, and the Adachi Molecular Exciton Engineering Project, under JST ERATO grant JPMJER1305; the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT); and MEXT/Japan Society for the Promotion of Science (JSPS) KAKENHI grant JP 15K21220. We thank K. Tokumaru and M. Kotani for helpful discussions. We also thank W. J. Potscavage Jr for assistance with manuscript preparation.

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R.K. devised and performed all research. C.A. supervised the project. R.K. and C.A. discussed the results and edited the manuscript.

Corresponding authors

Correspondence to Ryota Kabe or Chihaya Adachi.

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The authors declare no competing financial interests.

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Extended data figures and tables

Extended Data Figure 1 Emission decay profiles for 10−5 M solutions of TMB or PPT in toluene at 77 K.

Extended Data Figure 2 Emission decay profiles for a 1 mol% TMB/PPT film.

a, Semi-logarithmic plot of the emission decay profile from −60 s to 120 s. The sample exhibited photoluminescence (PL) under photo-excitation (from −60 s to 0 s), and LPL after the excitation stopped. b, Semi-logarithmic plot of the emission decay profile from −60 s to 5,000 s.

Extended Data Figure 3 Dependence of PL and LPL emission on excitation power.

A 1 mol% TMB/PPT film was excited at 300 K for one second with various excitation powers. a, Excitation power versus PL intensity. The slope of one is indicative of a single-photon process. b, Excitation power versus integrated LPL intensity. Again, the slope of one indicates a single-photon process. c, Semi-logarithmic plot of emission intensity versus time. d, Logarithmic plot of emission intensity versus time.

Extended Data Figure 4 Emission spectrum of a white LED (LDA6D-E17l, Panasonic), and absorption and excitation spectra of 1 mol% TMB/PPT film.

Extended Data Figure 5 Relationship between LPL intensity and excitation power and time.

a, Relationship between LPL intensity (five seconds after the excitation stopped) and excitation power for a 1 mol% TMB/PPT film at 300 K. b, Relationship between LPL intensity (again, five seconds after excitation stopped) and excitation time for a 1 mol% TMB/PPT film at 300 K.

Extended Data Figure 6 LPL emission spectra.

a, PL spectra for a 1 mol% TMB/PPT film at the indicated temperatures. b, LPL spectra (measured five seconds after the excitation stopped) for a 1 mol% TMB/PPT film at the indicated temperatures. c, PL spectra for the indicated TMB/PPT films (where the molar percentage refers to the concentration of TMB) at 300 K. d, LPL spectra for the indicated TMB/PPT films at 300 K.

Extended Data Figure 7 Set-up for measuring LPL.

Supplementary information

LPL emission from 1 mol% TMB:PPT film at 300 K

The 1 mol% TMB:PPT film was excited by a 340 nm LED. The video is sped up by 60 times after 30 seconds. (MP4 6132 kb)

LPL emission excited by a white LED lamp

The 1 mol% TMB:PPT film was excited by a standard white LED light (LDA6D-E17, Panasonic). (MP4 380 kb)

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Kabe, R., Adachi, C. Organic long persistent luminescence. Nature 550, 384–387 (2017). https://doi.org/10.1038/nature24010

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