The synthesis of chloropropylsilyl spirolactones yielded two separable isomeric products (49a and 49b, Scheme 4). Both have a quantum yield of 0.40 in PBS, and the extinction coefficient in 95% EtOH/0.1%TFA for both is ~200,000 M -1 cm -1 (Table ), and the K L-Z lactone-zwitterion equilibrium value for 49b is 0.0045. We therefore anticipate that the open-closed equilibrium of spirolactones like 47 and 48 can be further tuned and optimized by modification of the amine donor and/or the acidity of the spirolactone leaving group. Structure and synthetic elaboration of Si-Bridge rhodamine spirolactones. Unlike asymmetric Si-substituted cationic Si-rhodamines such as 9-13 (Scheme 1), we found that asymmetric Sisubstituted spirolactones (Scheme 4) were readily separable by flash chromatography. Small molecule X-ray crystallography of 49a revealed that it is the "s,s" isomer, where the chloropropyl group and the phenyl of the spirolactone are on opposite faces of the planar dye (Scheme 4). We were unable to obtain an X-ray structure of 49b, but the HRMS and NMR are consistent with the presentation of the silyl substituents as the "r,r" isomer.