Monocationic ionophores capable of ion-responsive intramolecular charge transfer absorption variation

Abstract

In this study, two monocationic hosts, 1 and 2, were prepared. They are structural isomers of phenyl-pyridinium compounds conjugated benzocrown ether, in which a phenyl group is substituted at the four- and 3-positions on the pyridinium unit, respectively. Both compounds showed intramolecular charge transfer (ICT) absorptions at around 340 nm, which shifted the blue side by the presence of alkali and alkaline earth metal ions due to the formation of a 1:1 host-guest inclusion complex. Compounds 3 and 4, which are analogs of 1 and 2 with a non-crown ether structure, exhibited ICT absorptions similar to those exhibited by 1 and 2, but did not show guest-induced absorption variations. The guest-induced absorption variations of 1 and 2 could be used for alkali and alkaline metal ion sensing. Both 1 and 2 detected Mg2+ with high sensitivity and showed a similar preference to smaller ions for both alkali earth metal ions and alkali metal ions. For comparison, compounds 5 and 6 were also used for ion sensing, whose structures are similar to that of 1, but different in the substituent of a hydrogen atom and the pyridyl group instead of phenyl group of 1, respectively. All monocationic hosts showed similar sensing patterns that are roughly parallel to their binding constants.