To explore and demonstrate the selective sieving effects of our crystals, we suspended them in a series of eight methanol solutions with differently-sized dyes (Figure , Figure ). Follow-up optical absorption (UV-Vis) measurements of the dye solutions show the gradual decrease of their concentrations for relatively small molecules. The sieving process is completed in ~24 h. The dye loading is the highest for the three molecules that have a "width" smaller than 0.7 nm: sodium resorufin (SR), methyl orange (MO) and 7,7,8,8-lithium tetracyanoquinodimethane (TCNQ) (= group I). Significantly lower amounts of dyes are embedded into the crystals when larger dyes are used with a width between 0.7-1.2 nm: sodium fluorescein (SF), sodium 5-aminofluorescein (AF) and fluorescein isothiocyanate isomer I (FITC) (= group II). The largest molecules we used do not enter the host matrix: sodium acid red-52 (AR) and brilliant blue R250 (BBR); width >1.2 nm (= group III) (Figure ). The estimated differences between the amounts of dyes embedded into the crystals for these three groups of crystals (I: 0.11-0.14 mmol/g, II: 0.02-0.03 mmol/g and III: none) is a strong indication that selective functionalization of the different channels of the crystals is possible based on the dimensions of the guest molecules (Figure , S5, Table ). These sieving experiments show that the smallest channel is only accessible to the dyes of group I, whereas the larger channel is accessible to both small and large dyes (groups I and II).