Effect of perchlorate on the fluorescence of protonated 2,2′-bipyridine

Interaction between the monocation of an aromatic amine, 2,2′-bipyridine and perchlorate anion was studied. The protonation process by perchloric acid has been followed by the changes in the UV absorption spectra and in the fluorescence emission. Addition of lithium perchlorate with increasing amount results in a significant increase in the fluorescence of 2,2′-bipyridine (the bipyridyl cation). This increase in fluorescence indicates that the rotation in the protonated monocation is hindered in the presence of perchlorate. We assume that this hindrance is due to the formation of the BpH+·ClO4− ion-pair.