The thiourea based 4-amino-1,8-naphthalimide molecules 1–5 were designed as fluorescent anion sensors and their photophysical properties investigated upon recognition of biologically relevant anions such as acetate, dihydrogen phosphate and fluoride in DMSO. Synthesised in a single step from their respective aniline precursors, 6–9, these molecules were designed on the fluorophore–spacer–receptor principle, where in the case of sensors 1–3 the thiourea anion recognition moieties were connected to the fluorophore via the 4-amino moiety, while sensors 4 and 5 had the thiourea moieties connected to the ‘imide’via a CH₂ spacer. The current study showed that 1–5 operated as photoinduced electron transfer (PET) sensors, as no significant changes were observed in their absorption spectra, while their fluorescence emissions were quenched upon recognition of ions such as AcO⁻, H₂PO₄⁻ and F⁻, which demonstrates that bidirectional PET sensing occurs in such naphthalimide based anion sensors.