Theoretical study of complexes based on lithium tetraamine and its fluorinated derivatives in interaction with Li−, Na−, and K− anions: study of static and dynamic NLO parameters by DFT

Context

This study explores the impact of fluorine substitution on the structural, electronic, and nonlinear optical (NLO) properties of metallic complexes featuring the tetraamine lithium (TALi⁺) motif. Twelve complexes were designed by combining TALi⁺ with anions (Li⁻, Na⁻, K⁻), forming three categories (TALi-Li, TALi-Na, TALi-K), and analyzed using quantum chemical calculations. The results reveal exceptionally high static and dynamic first hyperpolarizabilities (∆β_tot × 10⁻³⁰ esu), with the largest gap (834.69 × 10⁻³⁰ esu) observed in the (TALi-K) series. Dynamic (β_tot) values reached (~ 60 million × 10⁻³⁰ esu) at 1064 nm, while second hyperpolarizabilities (γₐ_v) followed similar trends, though (TALi-K) complexes exhibited deviations in ranking. Notably, push–pull behavior was identified, with anions acting as donors and fluorinated ammonias as acceptors, bridged by (NH₃ ligands). Absorption maxima (600–900 nm) in both vacuum and solvent environments suggest tunable optoelectronic responses. Crucially, the (TALi-K) category demonstrated the highest nonlinear refractive indices (n₂), highlighting its potential for photonic applications such as optical switching. This work provides a roadmap for designing high-performance NLO materials through strategic anion selection and fluorination, advancing the development of functional materials for light-based technologies.

Method

All calculations were performed using Gaussian 16 software. Molecular geometries were optimized at the B3LYP-D3/6–31 +  + G(d,p) level in the gas phase without symmetry constraints for a series of tetraamine lithium complexes with various alkali metals (TAL-M). Nonlinear optical properties were extensively characterized through first hyperpolarizability (β_tot) calculations using seven different functionals (CAM-B3LYP, LC-wPBE, LC-BLYP, M11, wB97X, HSEh1PBE, and M06-2X) as well as MP2 theory, all with the 6–31 +  + G(d,p) basis set. Additional basis set dependence studies were conducted using CAM-B3LYP with eight different basis sets ranging from 6-31G(d,p) to augmented triple-zeta sets. Second harmonic generation properties were evaluated through (β_tot) and γ_av calculations at the CAM-B3LYP/6–31 +  + G(d,p) level. Electronic delocalization was analyzed via NBO calculations (E(2) energies) using the same theoretical approaches. Solvent effects were incorporated using the SMD model for solvation free energies (ΔG_solv) and the CPCM model for TD-DFT calculations of absorption maxima (λ_max) and oscillator strengths at the TD-CAM-B3LYP/6–31 +  + G(d,p) level. This multi-method approach ensures robust characterization of both molecular structures and NLO properties.