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

Abstract

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 (H7···O3 and H18···O4) between cationic and anionic fragments in the pyridinium hydrogen sulfate than these (H26···O4 and H25···O12) in pyridinium bis(dihydrogen phosphate). These observations are confirmed by FT-IR and UV–vis methods, where: (i) the infrared bands due to H···O and N–C interactions in P2HP appear at lower frequencies than the same ones in the spectrum of PHS and (ii) the n → π* 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.