Water-Induced Turn-on of Lanthanide Photoluminescence\nEmission and Application in Colorimetric Sensing of Trace Water

To examine the water-induced\nphotoluminescence turn-on and its\npotential application in trace water sensing, a new series of [Ln<sup>III</sup>(dmba)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub>]·2H<sub>2</sub>O, where Ln<sup>III</sup> = La<sup>III</sup> (<b>I</b>), Pr<sup>III</sup> (<b>II</b>), Nd<sup>III</sup> (<b>III</b>), Sm<sup>III</sup> (<b>IV</b>), Eu<sup>III</sup> (<b>V</b>), Gd<sup>III</sup> (<b>VI</b>), Tb<sup>III</sup> (<b>VII</b>), Dy<sup>III</sup> (<b>VIII</b>), Ho<sup>III</sup> (<b>IX</b>), and Er<sup>III</sup> (<b>X</b>), were synthesized\nusing dimethoxybenzoic acid (Hdmba). Their single-crystal structures\nand thermal and chemical robustness were investigated, and the effects\nof lanthanide contraction and noncovalent interactions were discussed.\nThe photoluminescence and colorimetric properties of <b>I</b>–<b>X</b> were investigated. Their dependence on dehydration\nand rehydration was disclosed, from which the significant role of\nnoncovalent interactions was proposed. Based on the dehydration–rehydration-dependent\nresponses in the forms of photoluminescence emission and color, the\nturn-off (dehydration) and turn-on (rehydration) of the red emission\nof Eu<sup>III</sup> (<b>V</b>) were demonstrated. Using a mobile\nphone camera and freeware application, its use in the colorimetric\nsensing of trace water in polar organic solvents was successfully\nachieved. With respect to ethanol, acetonitrile, and acetone, linear\ncorrelations were established from 0 to 3–5% by volume of water\nwith an <i>R</i><sup>2</sup> of over 0.98. The detection\nand quantification limits were less than 0.5 and 1.5%, respectively.\nThe percentage recoveries were 92 and 110%. The underlying mechanism\nwas postulated.