The synthesis and spectroscopic characterisation of a new amino core-substituted naphthalene diimide is reported. The compound exhibits H+ sensing properties leading to a change in optical output through both absorption and emission. On addition of trifluoroacetic acid, an increase in the fluorescence quantum yield from 0.5 to 0.67 and a a??500-550 cm-1 blue shift of absorption and emission maxima are observed. Addition of triethylamine fully reverses these effects and the cycle is repeatable many times. The increase in fluorescence emission is attributed to protonation blocking weakly competitive photoinduced electron transfer operative in the neutral form of the molecule.