Spectroscopic and DFT-based characterization of V(V)-alizarin red S complexes: Structural, optical, and photonic...

In this study, the complexation between vanadium in its pentavalent state (V(V)) and the redox-active anthraquinone dye Alizarin Red S (ARS) was investigated to elucidate its solution speciation and coordination behavior. The V(V):ARS complexes were formed from the ARS ligand and the ammonium metavanadate salt as a source of the V(V) metal ion. Nuclear magnetic resonance (NMR) analyses indicate that the V(V):ARS complexes exist in two different structural forms in aqueous solution. NMR, attenuated total reflectance-Fourier transform infrared (ATR-FTIR), electronic absorption and fluorescence spectroscopy measurements, supported by DFT calculations, provided detailed insight into their binding modes and structural features. Twenty possible structures were modeled using quantum chemical methods, and their relative energies were calculated. Vibrational and absorbance spectra were calculated and fully assigned using the B3LYP/6-311++G(d,p)/LANL2DZ level of theory to determine the most accurate geometric structures and analyze their properties. Among the analyzed species, the dominant stoichiometric form was found to be the 2:2 complex, denoted as hh^4- ([V₂O₄(ARS)₂]^4-). The combined experimental and computational results allowed determination of the stoichiometry, electronic properties, and preferred coordination geometry of the complexes. The effect of pH on the stability of the V(V):ARS complex was also investigated. Furthermore, the interaction of the complex with the cationic surfactant DTAB was studied through absorbance and fluorescence spectroscopy. The experimental band gaps and refractive indices of the ARS ligand and the V(V):ARS complex were calculated using semi-empirical methods. Additional photonic properties - such as extinction coefficient, transmittance, refractive index, contrast, optical conductivity, electrical conductivity, and absorption cross-section - were calculated from experimental data and the influence of complex formation on these photonic properties is discussed. The V(V):ARS complex exhibits semiconducting behavior, indicating its potential application in the design of photonic materials. Additionally, these findings contribute to a fundamental understanding of V(V)-anthraquinone interactions, which is essential for designing redox-active systems for catalysis and energy-related applications.