Ion‐Dependent Spectral Responses of Donor–π–Acceptor Chromophores Upon Alkali and Alkaline Earth Metal Coordination: A Comprehensive DFT and TD‐DFT Investigation

This study presents a theoretical investigation into the design and photophysical behavior of donor–π–acceptor (D–π–A) systems as metal‐responsive colorimetric sensors. The sensors feature an azo benzene π‐bridge linking the donor (aza–crown ethers) and acceptor groups (–H, –CHO, –NO 2 , and –CN). Density functional theory (DFT) and time‐dependent DFT (TD–DFT) calculations were employed to examine their geometric and optical properties. Among the designed structures, the –CN derivatives showed a favorable balance between absorption wavelength and oscillator strength, and their complexes with Li + , Na + , and Mg 2 + exhibited distinct metal‐dependent spectral responses within the ion series investigated here. Metal coordination modulates the D–π–A interaction and the associated intramolecular charge‐transfer (ICT) process. Atoms in Molecules (AIM), hole–electron, frontier molecular orbital (FMO), and NBO analyses collectively elucidated the coordination strength, charge transfer character, and orbital interactions underlying the observed spectral responses. The best‐performing sensors were [Mg–D1–π–CN] 2+ , showing a hypsochromic shift, and [Na–D3–π–CN] + , displaying a bathochromic shift. The computational trends align with reported D–π–A chromophore behaviors, where metal‐ion complexation typically induces blue‐shifted absorption. The results confirm preferential binding within the investigated ion set. These insights provide a coherent framework for understanding D–π–A structure–property relationships and guiding rational design of efficient colorimetric ion sensors.