Pyrolysis of Al‐Based Metal–Organic Frameworks to Carbon Dot‐Porous Al2O3 Composites With Time‐Dependent Phosphorescence Colors for Advanced Information Encryption

Abstract Phosphorescent materials with time‐dependent phosphorescence colors (TDPCs) have great potential in advanced optical applications. Synthesis of such materials is attractive but challenging. Here, a series of carbon dot‐porous Al 2 O 3 composites exhibiting distinctive TDPC characteristics is prepared by high‐temperature pyrolysis of Al‐based metal–organic frameworks NH 2 ‐MIL‐101(Al). The composite synthesized at 700 °C (CDs@Al 2 O 3 ‐700) shows an obvious change in phosphorescence color from blue to green after removing the excitation light of 280 nm. Photophysical analysis reveals that two emission centers in CDs, namely carbon core and surface states, are responsible for the short‐lived blue phosphorescence (96 ms) and long‐lived green phosphorescence (911 ms), respectively. The combination of blue and green phosphorescence with different decay rates triggering the interesting TDPC phenomenon. CDs@Al 2 O 3 ‐700 has a significantly high phosphorescence quantum yield of up to 41.7% and possesses an excellent optical stability against water, organic solvents, and strong oxidants, which benefits from the multi‐confinement of CDs by the porous Al 2 O 3 matrix through rigid network, strong space constraint, and stable covalent bonding. Based on the TDPC property, multilevel coding patterns composed of CDs@Al 2 O 3 are successfully fabricated for advanced dynamic information encryption.