However, we found that treatment of 12-I with the small nucleophile azide resulted in both iodide displacement and reaction at the central carbon of the rhodamine scaffold, a known site of nucleophilic attack. We therefore synthesized chloropropylsilyl dyes 43 and 44 in a 2,6dimethoxy scaffold (Scheme 2), as symmetrical substitution of rhodamines at both ortho positions of the pendant phenyl sterically shields nucleophilic attack at the central carbon. Displacement of the chloride in 43 with iodide formed iodopropylsilyl dye 51, which could be further elaborated to the azide 54 (Scheme 3). Interestingly, however, the azetidine dye 44 was labile to excess iodide, which resulted in tri-iodination via displacement of the chloro group and ring-opening of both azetidines (Figure ). Si-modifications in spirolactone dyes. Si-rhodamine dyes that can spirolactonize are valuable for live cell imaging, as the spirolactone form is nonfluorescent and cell permeable, whereas the zwitterionic form is brightly fluorescent and can selectively form when bound to particular target biomolecules (Figure ). Notable examples include SiTMR (45) and JF646 (46). These dyes exist primarily in the spirolactone form even in aqueous buffer, with KL-Z lactone-zwitterion equilibrium values 35 of 0.0034 and 0.002. Estimation of the maximal extinction coefficient when ring-opened has been reported using EtOH/0.1% TFA (Table ). 9