Anion–π interaction guided switchable TADF and low-temperature phosphorescence in phosphonium salts for multiplexed anti-counterfeiting

Abstract

Anion–π⁺ interactions have gained continuous attention in diverse organic aggregates, as they can effectively alter emission behavior. Herein, the anion–π⁺ interaction is introduced to phosphonium salts, which exhibit tunable thermally activated delayed fluorescence and phosphorescence emission. Intriguingly, the emission spectra evolve from deep-blue to yellow emission by regulation of the anion–π⁺ interaction strength through varying the anions, such as BF₄⁻, CF₃SO₃⁻, PF₆⁻, and NO₃, accompanied by adjustable luminescent decay times from milliseconds to several seconds. Notably, bright blue emission with a high photoluminescence quantum yield near 100% is achieved when substituting the iodide ions with larger counter anions. The phosphonium iodide with strong anion–π⁺ interaction and heavy atom effect shows a high inter-system crossing rate, which inhibits the direct and prompt fluorescence emission. The anion–π⁺ interaction and twisted structure strongly suppress π–π stacking and afford ultra-high photoluminescence yields. Furthermore, the participation of polar solvent molecules results in the solvation and bathochromic-shift phenomenon of the solid-state phosphonium iodide due to the ionic polarized host–guest structure. This work provides new insights into the anion–π⁺ interaction in luminescent phosphonium aggregates.