A pH fluorescent molecular switch, BOPIM-dma, a boron 2-(2′-pyridyl) imidazole complex derivatives, was investigated in aqueous system. BOPIM-dma shows weak or no fluorescence in conventional solvents due to twisted intramolecular charge transfer (TICT). Upon protonation of the dimethylamino groups in BOPIM-dma, its fluorescence was turned on ascribed to the inhibition of TICT. Its emission can be switched 'on' or 'off' between pH 2.4 and 4.0, and it undergoes a reversible protonation-deprotonation reaction with a pKa of 3.12.