A 9-cycloheptatrienylidene fluorene (9-CHF) based highly selective fluorescent chemosensor for the fluoride anion detection is synthesized and characterized. Upon gradual addition of tetrabutylammonium fluoride (TBAF) under either neutral or basic condition, the synthesized chemosensor displays ratiometric fluorescent enhancements based on the mechanism of the fluoride triggered SiO bond cleavage.