The interaction between AHBH-PMO nanoparticles and Cu²⁺ ions was thoroughly investigated to uncover the molecular-level mechanisms governing fluorescence quenching and selectivity. Comprehensive spectroscopic, computational, and analytical techniques were employed to elucidate the coordination environment, binding stoichiometry, and electronic changes responsible for the observed “off-on-off” response.
Fourier-transform infrared (FTIR) spectroscopy revealed significant shifts in key vibrational modes upon Cu²⁺ addition. The O–H stretching band at 3465 cm⁻¹ shifted to 3203 cm⁻¹, while the N–H bending mode at 1556 cm⁻¹ moved to 1549 cm⁻¹. Similarly, the C=N stretch at 1624 cm⁻¹ red-shifted to 1601 cm⁻¹. These shifts indicate a change in electron density due to deprotonation and coordination, consistent with the formation of a stable complex involving the imine nitrogen and ortho-hydroxyl groups. The broadening and disappearance of certain peaks further suggest strong metal-ligand interactions and structural reorganization.
¹H NMR titration experiments in DMSO-d₆ provided direct evidence of proton loss during complexation. The signals corresponding to the imine proton (H(c), δ = 12.41 ppm), ortho-hydroxyl protons (H(e,b), δ = 11.59/11.50 ppm), and the C=N proton (H(d), δ = 8.46 ppm) vanished completely upon addition of two equivalents of Cu²⁺. This deprotonation is essential for coordination, as it generates free donor atoms capable of binding the metal ion. A broad resonance at δ = 5.94 ppm, attributed to paramagnetic Cu²⁺, confirmed the presence of unpaired electrons and supported the formation of a paramagnetic complex that quenches fluorescence via electron transfer.
High-resolution mass spectrometry detected a peak at m/z 427.9174, assigned to the species [AHBH–3H⁺ + H₂O + 2Cu²⁺], confirming a 1:2 stoichiometry between AHBH and Cu²⁺. This result aligns with Job’s plot analysis, which also indicated maximum binding at a mole fraction of 0.67, further supporting the formation of a Cu₂(AHBH) complex.
Density functional theory (DFT) calculations using the PBE0/6-31G(d) level provided atomic-level insight into the coordination geometry. The optimized structure of the AHBH–Cu²⁺ complex showed that Cu²⁺ binds in a distorted square-planar arrangement, coordinated by two nitrogen atoms from the deprotonated imine group, two oxygen atoms from the ortho-hydroxyl groups, and one additional nitrogen from the carbonyl moiety. Bond lengths ranged from 1.94 Å to 2.14 Å—within the typical range for metal-ligand bonds. The spatial configuration led to a significant reduction in the HOMO-LUMO energy gap, from 3.06 eV in free AHBH to 1.02 eV in the complex, indicating enhanced charge delocalization and increased susceptibility to non-radiative decay.Galectin-1 Protein, Mouse site
This electronic transition facilitates efficient photoinduced electron transfer (PET) from the excited fluorophore to the Cu²⁺ ion, resulting in fluorescence quenching.Vasopressin Formula The rigid silica framework ensures that the fluorophore remains in a preorganized state, maximizing the efficiency of this process.PMID:34813019 Furthermore, the stability of the complex under varying pH conditions reinforces its suitability for real-world applications.
In summary, the fluorescence quenching mechanism involves multiple synergistic effects: deprotonation of functional groups, formation of a stable 1:2 Cu²⁺–AHBH complex, significant reduction in the HOMO-LUMO gap, and efficient PET. These processes collectively enable highly selective and sensitive detection of Cu²⁺ in aqueous environments. The integration of experimental and theoretical data confirms that the AHBH-PMO system operates through a well-defined, rational design principle, offering a blueprint for next-generation chemosensors based on multi-mechanism luminescence control.MedChemExpress (MCE) offers a wide range of high-quality research chemicals and biochemicals (novel life-science reagents, reference compounds and natural compounds) for scientific use. We have professionally experienced and friendly staff to meet your needs. We are a competent and trustworthy partner for your research and scientific projects.Related websites: https://www.medchemexpress.com