Efficient Persistent Luminescence from Cellulose–Halide Mixtures for Optical Encryption

Natural renewable resources with long-lived persistent luminescence have attracted wide attention since many of these luminescence materials have low toxicity and are environmentally friendly. However, many of these materials lack conjugated groups such as aromatic rings, leading to weak absorption and low luminescence quantum yields. Herein, we proposed an effective method to enhance the luminescence of cellulose, a natural polymer free of conjugated groups. Halide salts were doped in cellulose for inducing absorption transition from halide ions to polysaccharide chains and increasing intersystem crossing through the external heavy-atom effect. In addition, divalent metal ions doped in cellulose could reduce nonradiative relaxations through electrostatic interactions. The phosphorescence quantum yield of dried cellulose doped with MgBr2 could reach 5%, significantly higher than that of many other natural renewable resources. The results provided a simple and effective method for tuning the persistent luminescence of cellulose, which could be easily applied to optical encryption on cellulose-based textiles and papers. This method may be extended widely for enhancing the clustering-triggered emission of many other nonconjugated organic materials in luminescence bioimaging, anticounterfeiting, and encryption.