This paper reports that fluorinated PSis with 3,3,3-trifluoropropyl and linear alkyl side chains (F-PSi)s demonstrated monovalent anion size dependent fluorescence quenching, which has potential as a novel type of chemosensor, "monovalent anion indicator". The extent of the fluorescent quenching was linearly increased by a decrease in the surface charge density of the monovalent anions. In addition, sensitivity toward monovalent anions was modulated by choosing, an adequate length of linear alkyl chains, which efficiently served as a soft but strict molecular sieve for the monovalent anions. In contrast. nonfluorinated PSi did not show such an ability to discriminate monovalent anions, suggesting that the positively charged Si main chain stimulated the electrostatic interaction with monovalent anions.