DFT and TD-DFT Investigation of Metal–Ligand Binding Mechanisms in 1H-Benzimidazole–Coumarin Hybrids: Electronic, Optical and Sensing Behaviour Analysis

Abstract

The structural, electronic and optical properties of a designed 1H-benzimidazole coumarin hybrid ligand (BC-L) were investigated here using DFT and TDDFT computations. The study will also include the biological and environmental metal ions coordination behaviour. The design optimizations of BC-L show a nearly planar structure which can promote strong ICT. The metal binding took place through N, O-chelation with Fe³⁺ and Cu²⁺ inducing the maximum structural distortion and electron redistribution. According to the bindings energy calculation values, it was found out that the ion Fe³⁺ (–47.8 Kcal mol⁻¹) has the strong interaction with protons. This assertion is supported by a significant reduction of the HOMO–LUMO gap and the maximum NBO stabilization energies. TD-DFT simulations revealed significant red-shifts for Fe³⁺ (402 nm) and Cu²⁺ (385 nm) due to enhanced ICT/LMCT transitions, while Zn²⁺ displayed a small shift characteristic of the CHEF-type fluorescence enhancement. Population studies confirmed that Fe³⁺ and Cu²⁺ withdraw a significant amount of charge. However, Zn²⁺ do not cause any change. Evidence compiled from computations establishes unambiguously a sensing hierarchy; Fe³⁺>Cu²⁺>Zn²⁺; that supports a proposition of dual-mode detection, namely LMCT which is driven fluorescence quenching (Fe³⁺/Cu²⁺), and fluorescence enhancement (Zn²⁺). The benzimidazole-coumarin hybrid has strong selectivity and tunable optical response making it a promising multifunctional chemo-sensor with potential applications in analytics and bioimaging.