pH‐Controlled Luminescence Turn‐On Behaviour of a Water‐Soluble Europium‐Based Molecular Complex

We have successfully synthesized a new europium metal–organic complex (MOC) [Eu 2 (pydcH) 6 ] · 3H 2 O ( 1 , H 2 pydc = 2,6‐pyridinedicarboxylic acid) by the hydrothermal technique as well as simple solvent evaporation in water. Single‐crystal X‐ray crystallographic data of 1 confirms a molecular complex of tricapped trigonal prismatic Eu 3+ ions with a free carboxylic acid site of each 2,6‐pyridinedicarboxylic acid ligand. Complex 1 is water‐soluble and remains stable in a broad pH range of 2–11. Upon excitation at 280 nm 1 shows a characteristic ligand‐sensitized metal‐centred visible red luminescence. The luminescence of complex 1 can be further enhanced by changing the pH of the solution in the range of 2–6. Besides, the luminescence response is completely reversible in the range of pH 2–6. The mechanism of the pH‐dependent luminescence behaviour is successfully explained and supported by UV/Vis absorption and IR spectroscopic experiments.