Influence of chiral ionic liquids on stereoselective fluorescence quenching by photoinduced electron transfer in a naproxen dyad.

In a previous study of a naproxen dyad in a pair of N-methylimidazoliummethyl menthylether-NTf(2) chiral ionic liquids (J. Phys. Chem. B 2008, 112, 7555), we observed that though intramolecular electron transfer was impeded, a consistent small stereodifferentiation in the fluorescence lifetime of the dyad was obtained. We proposed that this discrimination was purely electronic in nature and did not arise from geometrical effects, which can influence nonradiative rate processes, such as intramolecular electron transfer. In our present work, we have studied the interaction of the same chiral naproxen dyad molecule in both the previously studied menthyl-based NTf(2) ionic liquids and also in bis(tertrabutylphosphonium) (TBP) d-,l-tartrate ionic liquids. Unlike in the menthyl-based IL pair, the amount of quenching is different in the bis(TBP) tartrate enantiomeric liquids and the tartrate enantiomers have a different temperature dependence on the nonradiative rate of the dyad. This chiral discrimination most likely arises from the steric effects of the different conformations of the chiral molecules. We have shown that the viscosity and polarity of the solvents can influence the rate of electron transfer. On the other hand, no such electron transfer quenching is observed in the menthyl-based NTf(2) IL solvents. To our knowledge, this is the first example of chiral ionic liquids inducing a stereoselective fluorescence quenching by photoinduced, intramolecular electron transfer.