Eu(III)-Guided Photochromism: Experiments, Understanding, and Prospects

A dye containing a Schiff base ligand (SBL) has been synthesized using rhodamine 6G and salicylaldehyde. The SBL becomes highly photosensitive in the presence of Eu³⁺ ions. The Eu³⁺ coordinated SBL exhibits well-controlled photocoloration (light greenish yellow to dark pink) through UV light-triggered photoconversion (switching of the stable enol form to the metastable keto form). However, in the absence of light, the synthesized complex (SBL-Eu^III) reverses back to its initial stable form. The reversion is temperature-sensitive (thermoreversion). The Job's plot indicates a 1:1 binding stoichiometry between the SBL and Eu³⁺ ion, while the Benesi-Hildebrand plot ensures the high ligand binding constant (2.03 × 10⁵ M^-1). NMR, FTIR, ESI-MS, XPS, SCXRD, PXRD, etc., explore the structure and bonding of the ligand and its complex. Both photoconversion and thermoreversion follow first-order kinetics with rate constants of 1.85 × 10^-3 and 1.43 × 10^-3 s^-1 at 20 °C, respectively, and involve keto-enol isomerization. The keto form is highly fluorescent (FQY = 0.554, molar extinction coefficient = 3340 M^-1 cm^-1). The developed photochromic system has been used to design a molecular logic gate and a ratiometric chemodosimeter. The comparative study highlights the superiority of Eu^III over the conventional Zn^II system and benchmark photochromic materials.