Size-selective dye inclusion into the host crystals. (A) Molecular structures and approximate dimensions (the cations are not drawn). The following dyes are sodium salts of resorufin (SR), methyl orange (MO), fluorescein (SF), 5-aminofluorescein (AF), fluorescein isothiocyanate isomer I (FITC), sulforhodamine B (also known as acid red 52, AR), brilliant blue R250 (BBR). The lithium salt of 7,7,8,8-tetracyanoquinodimethane (TCNQ) was used. (B) The process of the molecular sieving by the colorless host crystals was monitored by UV-Vis spectroscopy of the dye suspension in methanol. The 3D plot shows the amount of dye uptake by the crystals with respect to the dyes' length and width (Figure ,S4; Table ). (C) Unpolarized and Polarized optical microscopy images of the colorless host crystals (top), the crystals functionalized with the sodium salts of resorufin (SR = 0.12 mmol/g, purple, middle) or with fluorescein (SF = 0.03 mmol/g, yellow, bottom). Scale bar: 30 μm.