New “off-on-off” pH chemosensing bichromophore based on 2-hydroxyphenylhydrazone and 1,8-naphthalimide is synthesized. The probe, designed to operate by controlled photoinduced electron transfer (PET) and internal charge transfer (ICT) processes, shows a striking pH dependence of its absorption and emission properties. Due to the 1,8-naphthalimide unit the probe exhibits between pH 6 and 10 a centred at 548 nm (λ.ex = 400 nm) strong yellow-green fluorescence, however, beyond of this pH interval the emission of the probe is quenched. The “off-on-off” behaviour of the probe as a function of pH enables it to execute XNOR, IMPLICATION, XOR and INHIBIT logic gates. The INHIBIT logic gate as a borrow digit in conjunction with XOR logic gate as a difference digit executes a half-subtractor. Moreover, the parallel action of logic gates INHIBIT and XNOR executes a digital magnitude comparator.