The energy-transfer properties and underlying mechanism of this donor-acceptor system were studied by analyzing their fluorescence properties. The excitation spectrum of crystals functionalized only with sodium resorufin partly overlap with the emission spectrum of crystals functionalized with sodium fluorescein. The large channels of the crystals were first saturated with sodium fluorescein (= donor). Then, the smaller channels of the yellow crystals were loaded with four different amounts (SR1-SR4) of sodium resorufin (= acceptor) by using different soaking times. This process was monitored by UV-Vis measurements of the dye solutions resulting in purple crystals.