X-ray Photoelectron Spectroscopy of Imidazolium-Based Zwitterions: The Intramolecular Charge-Transfer Effect

We investigate the imidazolium-based zwitterions, 1-butyl-3-methylimidazolium-2-pentafluorophosphate, together with 1-butyl-3-methylimidazolium hexafluorophosphate using X-ray photoelectron spectroscopy. The intramolecular charge-transfer from the negative headgroup to the positive headgroup is revealed in detail. In the case of the zwitterions, due to the intense charge-transfer effect, binding energies of N 1s and C² 1s are both lower; whilst P 2p is higher, compared to those of 1-butyl-3-methylimidazolium hexafluorophosphate. Such effect is found negligible for F 1s spectra. It is found that F 1s binding energies for the two samples are the same.