Long-Range Interface Effects in Room Temperature Ionic Liquids: Vibrational Lifetime Studies of Thin Films

Interface effects in the room temperature ionic liquids (RTILs) 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF₄) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf₂) were investigated using ultrafast infrared polarization selective pump-probe (PSPP) spectroscopy. The CN stretch mode of SCN^- dissolved in the RTILs was used as the vibrational probe. The vibrational lifetime of the SCN^- was the experimental observable. Quite similar single SCN^- lifetimes were observed: 59.5 ± 0.4 ps in bulk BmimBF₄ and 56.4 ± 0.4 ps in bulk BmimNTf₂. Thin films of both RTILs with thicknesses in the range of 15-300 nm were prepared by spin coating on functionalized substrates. PSPP experiments were performed in a small-incidence reflection geometry. In the thin films, a second, shorter lifetime was observed in addition to the bulk lifetime, with the amplitude of the shorter lifetime increasing with decreasing film thickness. By modeling the thickness dependence of the lifetime amplitudes, the correlation length of the interface effect (constant for exponential falloff of the influence of the interface) was determined to be 44.6 ± 0.6 nm for BmimBF₄ and 48.3 ± 2.2 nm for BmimNTf₂. The values for the shorter film lifetimes were 12.6 ± 0.1 ps for BmimBF₄ and 20.2 ± 0.6 ps for BmimNTf₂; the substantial differences from the bulk lifetimes showed that some of the SCN^- anions near the interface experience an environment distinct from that of the bulk. It was also found that for the BmimNTf₂ sample only, some of the SCN^- anions reside in the surface functionalized layer with two distinct environments having distinct lifetimes.