cm -1 (Table ) which may indicate that the dyes strongly favor the spirolactone form. Using anhydrous EtOH/0.1% TFA increased these values to ~124,000 and 144,000, respectively. We did not attempt to measure aqueous quantum yields as these spirolactones did not appreciably absorb in PBS alone, and the K L-Z lactone-zwitterion equilibrium value for 47 is 0.00125. Because dimethylamine-donor dyes such as SiTMR ( ) are known to adopt the open form more than azetidine-donor spirolactones like JF646 (46) (Scheme 4 and Table ), and the azetidine in dye 44 is labile to iodide, we chose to evaluate the chloropropylsilyl modification with dimethylamine donors.