A blue-red reversible optical switch has been created by combining Eu-III, pyridine, and a fluorescent organic chromophore. The initial solution exhibits the expected blue emission of the chromophore until UV irradiation leads to red emission (see Figure), a process that is ascribed to protonation of the pyridine and formation of a charge-transfer complex with the chromophore in the excited state, allowing the red Eu-III photoluminescence to be emitted.