Probing the Ni2+‐selective Response of Fluorescent Probe NiSensor‐1 with the NiCast Photocaged Complex

CTEA (N,N-bis[2-(carboxylmethyl)thioethyl]amine) is a mixed donor ligand that has been incorporated into multiple fluorescent sensors such as NiSensor-1 that was reported to be selective for Ni2+. Other metal ions such as Zn2+ do not produce an emission response in aqueous solution. To investigate the coordination chemistry and selectivity of this receptor we prepared NiCast, a photocage using a CTEA receptor. Cast photocages undergo a photoreaction that decreases electron density at the aniline nitrogen, shifting the binding equilibrium toward unbound metal ion. The unique selectivity of CTEA was examined by measuring the binding affinity of NiCast and the CTEA receptor for Ni2+, Zn2+, Cd2+, and Cu2+ under different conditions. In aqueous solution, Ni2+ binds more strongly to the aniline nitrogen atom than Cd2+; however, in CH3CN the change in affinity virtually disappears. The crystal structure of [Cu(CTEA)], which exhibits a Jahn-Teller distorted square pyramidal structure, was also analyzed to gain more insight into the underlying coordination chemistry. The studies suggest that the selectivity of NiSensor-1 in aqueous solution is due to a stronger interaction between the aniline nitrogen atom and Ni2+ compared to the other metal ions apart from Cu2+.

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