Exploring Charge Transfer Complexes of Fluoroquinolone Drugs and π-Acceptors (Picric Acid and 3,5-Dinitrobenzoic Acid): DFT Insights Into Electronic Interactions, Thermodynamic Stability, FMOs, and NLO Properties

This research employs density functional theory (DFT) at the B3LYP/6–311G(d,p) level of theory to delve into the intricacies of charge transfer complexes (CTCs) formed between two fluoroquinolone antibiotics, norfloxacin and ciprofloxacin, serving as electron donors and π-acceptors, namely picric acid and 3,5-dinitrobenzoic acid. The study uncovers significant charge transfer interactions manifested through alterations in bond lengths, so Mulliken charge redistribution and an analysis of thermodynamic stability, evidenced by binding energies (ΔE°_int) ranging from −8.33 to −11.21 kcal mol⁻¹, alongside spontaneous complex formation as indicated by negative Gibbs free energy changes (-ΔG°). The investigation further corroborates its findings through infrared (IR) and Ultraviolet–Visible (UV–vis) spectroscopic analyses, which strongly correlate with experimental data. This alignment not only substantiates the theoretical vibrational modes and electronic transitions computed in silico but also augments the reliability of the DFT method in evaluating CTCs. Additionally, an analysis of frontier molecular orbitals (FMOs) reveals that significant donor-to-acceptor charge transfer occurs, accompanied by a decrease in the HOMO-LUMO energy gaps (ranging from 2.85 to 3.68 eV). This reduction indicates enhanced nonlinear optical (NLO) activity of the complexes. Most notably, the computed first hyperpolarizability values (β_total) range from 8.84 × 10⁻³⁰ to 17.09 × 10⁻³⁰ esu, demonstrating that these complexes exhibit superior NLO capabilities compared to urea. This finding highlights their potential applicability as advanced optoelectronics and pharmaceutical sciences materials. In conclusion, the insights gleaned from this DFT study reinforce the utility of theoretical methods in unveiling the properties of CTCs and emphasize their significance for further exploration and practical applications in diverse scientific domains. The robust consistency between computed predictions and experimental results underscores the efficacy of DFT as a powerful tool in understanding the electronic characteristics of CTCs.