Experiments performed with a medium-sized dye, sodium fluorescein, (yellow, The demonstrated size-selective sieving of a series of molecules is used to functionalize the crystals with spatially-resolved arrays of different dyes. For this purpose, we selected two differently-sized chromophores, sodium resorufin and sodium fluorescein. Such a chromophore couple can act as an energy donor-acceptor system under excitation of the donor, sodium fluorescein, by visible light the acceptor, sodium resorufin, would emit the transferred energy (30). As described above, the crystals were first suspended in a methanol solution containing the larger dye, sodium fluorescein. This sieving process was monitored by in-situ UV/Vis spectroscopy showing a gradual decrease of the concentration of sodium fluorescein in solution (Figure ). Optical images show the formation of yellow-colored crystals that are saturated with the dye, filling the larger channels, while leaving the smaller channels unoccupied. Subsequent addition of the smaller sodium resorufin to the reaction solution resulted also in the absorption of the dye and a purple coloration of the yellow crystals (Figure ). The UV/Vis spectra recorded during the latter reaction do not indicate the release of sodium fluorescein from the crystals into the solution. The amount of sodium resorufin absorbed by the crystals is much less than that observed for the sieving of this dye with "empty" crystals (~0.12 mmol/g), and is in good agreement with what one would expect by functionalization of only the smaller channels (~0.05 mmol/g). To provide support for this conclusion, we first saturated the crystals with the smaller dye, sodium resorufin, and subsequently added the larger dye, sodium fluorescein, to the reaction mixture (Figure ). As can be seen from the UV-Vis data, the latter dye cannot enter the crystals saturated with sodium resorufin as the larger channels are already filled and inaccessible.