Pressure-Dependent Stability of Imidazolium-Based Ionic Liquid/DNA Materials Investigated by High-Pressure Infrared Spectroscopy

1-Butyl-3-methylimidazolium hexafluorophosphate ([C(4)MIM][PF(6)])/DNA and 1-methyl-3-propylimidazolium hexafluorophosphate ([C(3)MIM][PF(6)])/DNA mixtures were prepared and characterized by high-pressure infrared spectroscopy. Under ambient pressure, the imidazolium C(2)–H and C(4,5)–H absorption bands of [C(4)MIM][PF(6)]/DNA mixture were red-shifted in comparison with those of pure [C(4)MIM][PF(6)]. This indicates that the C(2)–H and C(4,5)–H groups may have certain interactions with DNA that assist in the formation of the ionic liquid/DNA association. With the increase of pressure from ambient to 2.5 GPa, the C(2)–H and C(4,5)–H absorption bands of pure [C(4)MIM][PF(6)] displayed significant blue shifts. On the other hand, the imidazolium C–H absorption bands of [C(4)MIM][PF(6)]/DNA showed smaller frequency shift upon compression. This indicates that the associated [C(4)MIM][PF(6)]/DNA conformation may be stable under pressures up to 2.5 GPa. Under ambient pressure, the imidazolium C(2)–H and C(4,5)–H absorption bands of [C(3)MIM][PF(6)]/DNA mixture displayed negligible shifts in frequency compared with those of pure [C(3)MIM][PF(6)]. The pressure-dependent spectra of [C(3)MIM][PF(6)]/DNA mixture revealed spectral features similar to those of pure [C(3)MIM][PF(6)]. Our results indicate that the associated structures of [C(4)MIM][PF(6)]/DNA are more stable than those of [C(3)MIM][PF(6)]/DNA under high pressures.