A new pH fluorescent molecular switch by modulation of twisted intramolecularcharge transfer with protonation

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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.