Quantum Chemical Insights into Metal-Ion Enhanced NLO Response of a Fluorescent Probe for Advanced Sensing Application.

High-performance nonlinear optical (NLO) materials are essential for optoelectronic and sensing technologies, yet multifunctional systems combining fluorescence and NLO properties remain scarce. Through systematic computational analysis, we investigate 4-chloro-2-(1-phenylimidazo[1,5-a]pyridin-3-yl)phenol (IUB) complexes with strategically selected metals: alkali (Li⁺, Na⁺, K⁺) and alkaline earth (Mg²⁺, Ca²⁺) ions to probe charge/size effects, and transition metals (Ni²⁺, Zn²⁺) as contrasting d-block representatives, Ni²⁺ for its paramagnetic fluorescence quenching and Zn²⁺ for its closed-shell fluorescence preservation. This selection enables direct comparison of how electronic configuration (s/p-block vs d-block) and oxidation state (+ 1 vs + 2) governs optoelectronic properties. Kinetically stable metal-IUB complexes formation was confirmed with the interaction energy analysis. Metal coordination induces substantial electronic reorganization, reducing transition energies from 4.5 eV to 3.69 eV while causing bathochromic shifts in absorption (273-336 nm) and emission (281-381 nm) spectra. Divalent cations demonstrate superior performance, with Zn²⁺ complexes achieving exceptional first hyperpolarizability (β_tot = 5250 a.u.)-a 16-fold enhancement over the pristine ligand (324 a.u.)-while maintaining 96% fluorescence efficiency. This remarkable NLO response correlates with calculated interaction energies up to -364 kcal/mol and substantial charge transfer (NBO charges: 1.72-1.87 e for divalent ions). Detailed electronic structure analysis through FMO, NBO, NCI, TDM and QTAIM methods reveals that metal-specific effects govern property modifications: alkali/alkaline earth metals and Ni²⁺ quench emission by disrupting ESIPT pathways (CI coefficients: 91-97%), while Zn²⁺ preserves radiative decay channels through balanced orbital mixing. The observed bathochromic shifts (Δλ = 15-63 nm) and enhanced oscillator strengths (f = 0.21-0.40) demonstrate tunable light-matter interactions. These findings establish metal-doped imidazole derivatives as versatile platforms for dual-mode sensing and NLO applications, with Zn²⁺ complexes particularly promising for integrated photonic devices. The study provides fundamental insights into structure-property relationships governing fluorescence-NLO coupling in metal-organic hybrids, enabling rational design of advanced optoelectronic materials.