Cation-anion interaction effect on the nonlinear optical behavior of pyridinium-based ionic liquids

Influence of the cation-anion interaction (expressed as internal hydrogen bonds) on the nonlinear optical properties (dipole moment, polarizability, anisotropy of polarizability and first-order hyperpolarizability) for the ionic liquids pyridinium hydrogen sulfate (PHS) and pyridinium bis(dihydrogen phosphate) (P2HP) was investigated in this paper. It was achieved analyzing vibrational modes (FT-IR) and electronic transitions (UV-vis) in PHS and P2HP. Molecular geometry of the title compounds was optimized by means of density functional theory calculations at B3LYP/6-311++G(2d,2p) level. Structural analysis exposed stronger internal hydrogen bonds (H-7 center dot center dot center dot O-3 and H-18 center dot center dot center dot O-4) between cationic and anionic fragments in the pyridinium hydrogen sulfate than these (H-26 center dot center dot center dot O-4 and H-25 center dot center dot center dot O-12) in pyridinium bis(dihydrogen phosphate). These observations are confirmed by FT-IR and UV-vis methods, where: (i) the infrared bands due to H center dot center dot center dot O and N-C interactions in P2HP appear at lower frequencies than the same ones in the spectrum of PHS and (ii) the n -> pi* electron transitions in PHS are more pronounced in comparison with these in P2HP. Based on the weaker cation-anion interaction in P2HP with respect to PHS, the former one is characterized with a remarkably higher first-order hyperpolarizability value. Hence, the ionic liquid P2HP was referred as a better nonlinear optical material than PHS. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The influence of cation-anion interaction on the nonlinear optical properties of ionic liquids was investigated, with a focus on the internal hydrogen bonds. The structural analysis revealed differences in the strength of internal hydrogen bonds between the two compounds, leading to variations in their nonlinear optical behavior.