Light-controlled efficient photoluminescence based on an europium β-diketonate complex with single-crystal-to-single-crystal [2+2] cycloaddition.

A brand new europium(iii) β-diketonate complex undergoes a single-crystal-to-single-crystal transformation via [2+2] cycloaddition after UV irradiation, triggering strong Eu(iii) red emission turn-on, which is highly photostable even after 50 hours of irradiation. A photo-patterning process is successfully conducted for security printing application in materials science.