Probing the Local Polarity of Alkylammonium Formate Ionic Liquids and Their Mixtures with Water by Using a Carbonyl Carotenoid

Abstract

Ultrafast transient absorption experiments have been carried out to determine the local polarity of three alkylammonium formate (AAF) protic ionic liquids (PILs), methlyammonium formate (MAF), ethylammonium formate (EAF), and n-butylammonium formate (BAF), by using 12'-apo-β-carotenoic-12'-acid (12'CA) as a molecular probe. MAF is more polar than methanol; EAF and BAF have polarities similar to ethanol and n-butanol, respectively. In general, the AAF PILs follow rather closely the correlation between the S(1) /intramolecular charge-transfer (ICT) state lifetime and the polarity parameter Δf, which was previously established in organic solvents. This is in contrast to earlier results for the 12'CA probe in imidazolium-based ILs, in which the local polarity determined by the probe was much larger than that for dipolar organic solvents with the same dielectric constant. The systematic variation of the composition of EAF/water mixtures shows no indication of significant deviations from the local to the bulk composition. We also characterized the photophysical properties of the deprotonated form of the 12'CA probe. It exhibits a structured S(0)→S(2) absorption spectrum, which is blueshifted relative to neutral 12'CA. The lifetime of the S(1)/ICT state of the anion is about 170 ps, and therefore, similar to the lifetime of 12'CA in less polar solvents. The transient S(1) /ICT spectrum in methanol closely resembles that of nonpolar carotenes. Both observations suggest that the ICT character of the S(1) state of the anion is largely suppressed because the shift of electron density toward the negatively charged carboxylate group is not favorable.