Fluorine-induced aggregate-interlocking for color-tunable organic afterglow with a simultaneously improved efficiency and lifetime

Abstract

Designing organic afterglow materials with a high efficiency and long lifetime is highly attractive but challenging because of the inherent competition between the luminescence efficiency and lifetime. Here, we propose a simple yet efficient strategy, namely fluorine-induced aggregate-interlocking (FIAI), to realize both an enhanced efficiency and elongated lifetime of afterglow materials by stimulating the synergistic effects of the introduced fluorine atoms to efficiently promote intersystem crossing (ISC) and intermolecular non-covalent interactions for facilitating both the generation of triplet excitons and suppression of non-radiative decays. Thus, the fluorine-incorporated afterglow molecules exhibit greatly enhanced ISC with a rate constant up to 5.84 × 10⁷ s^-1 and suppressed non-radiative decay down to 0.89 s^-1, resulting in efficient organic afterglow with a simultaneously improved efficiency up to 10.5% and a lifetime of 1.09 s. Moreover, accompanied by the efficient phosphorescence emission especially at cryogenic temperature, color-tunable afterglow was also observed at different temperatures. Therefore, tri-mode multiplexing encryption devices by combining lifetime, temperature and color, and visual temperature sensing were successfully established. The FIAI strategy by addressing fundamental issues of afterglow emission paves the way to develop high-performance organic afterglow materials, opening up a broad prospect of aggregated and excited state tuning of organic solids for emission lifetime-resolved applications.

Fig. 1. Design of fluorine-induced aggregate-interlocking (FIAI) for high-performance organic afterglow. (a) Jablonski diagram of the organic afterglow mechanism. The inset shows the equations of the phosphorescence quantum yield (eqn (1)) and lifetime (eqn (2)). (b) Schematic drawing of the FIAI strategy to restrict molecular motions in the crystal. (c) Molecular structures of o/m/p-FPOCz designed by FIAI. Ex., excitation; Fluo., fluorescence; Phos., phosphorescence.

Fig. 2. Photophysical properties of POCz and o/m/p-FPOCz. (a) Steady-state photoluminescence (black lines) and afterglow spectra (red lines) under ambient conditions. Insets show the molecular structures. (b) Afterglow decay profiles of the emission bands at 527 nm by 365 nm excitation under ambient conditions. (c) Key parameters for organic afterglow emission. (d) Excitation-phosphorescence mapping of the m-FPOCz crystal with a delay time of 25 ms. (e) Temperature-dependent afterglow spectra from 300 to 80 K of the m-FPOCz crystal. (f) CIE 1931 coordinates of afterglow emission of the m-FPOCz crystal from 300 to 80 K.

Fig. 3. Single crystal and theoretical analyses. (a and b) Intermolecular interactions of the selected dimer (left) and tetramer (right) in POCz (a) and m-FPOCz (b) single crystals. (c–e) The molecular packing arrangements along different directions in the m-FPOCz single crystal showing various intermolecular interactions. (f and g) TD-DFT calculated energy level diagram and the corresponding SOC constants of POCz (f) and m-FPOCz (g). (h) Proposed mechanism in boosting the organic afterglow performance via FIAI.

Fig. 4. Applications of the afterglow materials designed by the FIAI strategy. (a). Schematic construction of the lifetime, temperature and color tri-mode multiplexing encryption QR code. (b) Fluorescence spectra of red and blue dyes. (c) Steady-state (black) and afterglow emission spectra (red) of the p-FPOCz crystal. (d) Lifetime decay profiles of the short-lived luminescence of the red and blue dyes and the p-FPOCz crystal. (e) Photographs of the lifetime, temperature and color multiplexing encryption QR code under daylight and 365 nm irradiation and after ceasing the irradiation. (f) Temperature-dependent color chart with the corresponding CIE coordinate manifesting the feasibility of the p-FPOCz crystal in visual detection of temperature.