A series of fluorescent sensor molecules based on a phosphane sulfide derivative were designed and synthesized. The effect of the distance between the complex entities as well as the number and the length of fluorophores was investigated using both steady-state and time-resolved fluorescence methods. The complexation behavior of these sensor molecules, which have a distinct affinity for Hg²⁺, is reported. The coordination of Hg²⁺ induces a photoinduced electron transfer from fluorophore to the complexed mercury and results in a significant decrease of the fluorescence. Theses sensors exhibit very low detection limits in CH₃CN/H₂O (80/20 v:v) and excellent sensitivity to Hg²⁺ over other potentially interfering cations such as Na⁺, K⁺, Mg²⁺, Ca²⁺, Cu²⁺, Ag⁺, Zn²⁺, Cd²⁺ and Pb²⁺.