Coordination-controlled photophysics and antiproliferative activity of coumarin-linked schiff bases and Zn(II) complexes.

In this work, two coumarin-linked Schiff bases, Cou-NEt and Cou-Nap, together with their Zn(II)-derived products, were synthesized and investigated by spectroscopic, computational, and biological methods. Structural characterization revealed distinct Zn(II)-dependent outcomes: Cou-Nap afforded the expected Zn(II) complex, whereas reaction of Cou-NEt with ZnCl2 induced imine cleavage to yield an amine-derived Zn(II) species. All compounds showed similar visible absorption bands, while Zn(II) incorporation modified the emission response and enhanced fluorescence, most prominently for Zn-Nap. TD-DFT calculations indicated predominantly ligand-centered excited states and suggested that the weak fluorescence of Cou-NEt is associated with access to a red-shifted keto-like excited state, whereas Zn(II) coordination suppresses nonradiative deactivation through chelation-induced rigidification. DNA-binding experiments revealed moderate affinity toward ctDNA, consistent with non-covalent interaction. In vitro antiproliferative assays against MCF-7, HepG2, and A549 cells showed that Cou-NEt and Zn-Nap were the most active compounds in the present series. Molecular docking and molecular dynamics simulations using the human mTORΔN-mLST8 complex supported favorable binding profiles for these two compounds, while confocal imaging in HeLa cells demonstrated efficient cellular uptake of the Zn(II) species. Overall, the results show that Zn(II) coordination can tune the photophysical and anticancer-related behavior of coumarin-based Schiff-base systems in a ligand-dependent manner, while further structural optimization is required to improve antiproliferative potency.