DFT and TDDFT studies on π-conjugated ligands for copper sensing: analyzing electronic structures and optical performance.

Context: Structural, bonding aspects and optical characteristics of a set of π-conjugated quinoline-based ligands (L1-L4) and coordinated copper (Cu²⁺) ion were investigated by using density functional theory (DFT) and time dependent DFT methods. DFT results showed that L3 exhibits the lowest HOMO-LUMO energy gap (3.05 eV) indicating high reactivity and strong charge transfer ability while its copper complex further reduces the gap to 2.52 eV. Electrostatic potential maps highlighted a negative potential region around nitrogen and carbonyl oxygen sites confirming their role in copper coordination. Natural bond orbital analysis of the L3 complex revealed the highest stabilization energy of 79.15 kcal/mol indicating substantial donor-acceptor interactions. ELF and LOL plots further supported efficient π-delocalization in L3-Cu²⁺ while NCI analysis further confirmed reduced steric repulsion around the Cu²⁺ coordination sphere compared to other complexes which support its favorable geometry and stability. QTAIM analysis indicated a mixed electrostatic covalent character of Cu-N/O bonds. TDDFT results showed strong ligand to metal charge transfer bands in the visible spectrum for L3-Cu²⁺ at 452 and 667 nm which lend credence to a mechanism of chelation-enhanced charge transfer. Non-linear optical analysis revealed enhanced first hyperpolarizability upon complexation particularly for L1-Cu²⁺ (β = 9.46 × 10^-30 esu) and L4-Cu²⁺ (β = 9.12 × 10^-30 esu). These observations provide a useful layout for generating metal ion sensors in the future with improved optical response and selectivity. These theoretical findings agree with the coordination behavior seen in experiments supporting L3-based systems in the copper ion detection applications.

Methods: Geometry optimization and frequency analyses were performed using DFT at the B3LYP/6-311G(d,p) level for non-metal atoms and LANL2DZ basis set for copper. The polarized continuum model was used for the solvation as implemented in Gaussian 16. The NBO6.0 program was utilized to investigate the bonding nature and stabilization energies of the complexes. The ORCA4.2 program was used to simulate the absorption spectrum. The Multiwfn and VMD programs were used for the topological analysis.