Oxygen‐doped Carbon Nitrides with Visible Room‐temperature Phosphorescence and Invisible Thermal‐Stimuli‐Responsive Ultraviolet Delayed Fluorescence for Security Applications.

Multi‐mode emissive materials with stimuli‐responsive producing invisible signals are very attractive for advanced security applications, but development of such materials remains highly challenging. In this work, oxygen‐doped carbon nitrides (O‐CNs) are prepared via microwave‐assisted heating of urea, which exhibit ultraviolet (UV) solid‐state fluorescence (SSFL), visible room temperature phosphorescence (RTP) and thermal‐stimuli production of invisible UV delayed fluorescence (DF) properties. Further studies confirmed that the SSFL and RTP could be attributed to the introduction of oxygen functional group (e. g., C=O) in the skeleton of O‐CNs, thus minimizing the aggregation caused quenching effect, facilitating intersystem crossing, and stabilizing the excited triplet states. The specific thermal‐stimuli production of UV DF is deemed to be the relatively large energy gap between ground and excited singlet states as well as an effective triplet‐triplet annihilation. Notably, the emission maximum of UV DF locates at ~310 nm with an ultra‐narrow full width at half maximum (FWHM) down to 19 nm, so it is completely invisible to the naked eyes, but detectable by a UV camera. To employ the unique characteristics of O‐CNs, security protection strategies with superior concealment by virtue of the thermal‐stimuli quenching visible RTP and meanwhile producing invisible UV DF are demonstrated. [ABSTRACT FROM AUTHOR]