Tailoring refractive index dispersion in ionic liquids: The influence of charge delocalization in cations

• Published in: Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy Molecular and biomolecular spectroscopy, 2025-01, Vol.324, p.124964, Article 124964
• Main Authors: Rodríguez-Fernández, Carlos Damián, Doval, Alejandro, Arosa, Yago, González-Núñez, Héctor, López-Lago, Elena, de la Fuente, Raúl
• Format: Article
• Language: English
• Subjects: Aromaticity; Charge delocalization; Charge transfer; Dispersion; Electronic polarizability; Ionic liquids; Refractive index
• ISSN: 1386-1425
• DOI: 10.1016/j.saa.2024.124964

[Display omitted] •Molecular polarizability density governs the magnitude of refractive index.•Aromaticity determines the refractive index dispersion.•Aliphatic cations and alkyl chains have weak influence in dispersion.•Aromatic cations produce high refractive index dispersion at short wavelengths.•Absorption bands related to π orbitals explain the dispersion in aromatic cations. In this work, we study the contributions that different molecular blocks have in the wavelength-dependence of the refractive index in ionic liquids. The ionic liquids chosen for this work are combinations of the bis(trifluoromethylsulfonyl)imide anion with cations based on four different heterocycles with different extents of charge delocalization. The analysis is performed in terms of the experimental electronic polarizability, which is obtained by combining measurements of refractive index curves and densities via the Lorentz-Lorenz equation. Exploiting the additivity of electronic polarizability in ionic liquids, the contribution of the anion and the heterocycles of the cations is separated from that of the alkyl chains. Our results show important differences in these contributions, revealing a key influence of the charge delocalization in the cationic rings on the behavior of the refractive index dispersion. The understanding of how different parts of ionic liquids affect their refractive index dependence on wavelength would allow to gain precise control of this magnitude, enabling the development of customized optical materials for diverse applications in photonics and sensing technologies.