In this study, a novel ratiometric chemosensor for Al³⁺ (HL) which contains coumarin and naphthalene moieties has been designed and synthesized on the basis of the mechanism of internal charge transfer (ICT). On addition of Al³⁺, the receptor exhibits a strong, increasing fluorescent emission centered at 525 nm at the expense of the fluorescent emission of HL centered at 481 nm in ethanol and water (9 : 1, v/v). This phenomenon is attributed to formation of a 1 : 1 complex (K_a = 1.03 × 10⁴), which inhibits internal charge transfer (ICT).