Synthesis and intramolecular charge transfer of bipyridinium–crown ether conjugates in response to alkali metal ions

Two types of hosts, 1 and 2, in which benzocrown ether units were conjugated to monocationic pyridinium and dicationic bipyridinium units, respectively, have been prepared. 1 and 2 exhibited color-to-colorless changes induced by alkali metal ion, associated with blue shift of their intramolecular charge transfer (ICT) absorptions due to the formation of a 1:1 host–guest inclusion complex. 3 and 4 being the noncrown ether analogues of 1 and 2 also exhibited ICT absorptions, but did not shift upon addition of the guest because of no guest-binding abilities. The monocationic 1 and 3 showed red shift in their ICT bands upon addition of organic acid, such as p-toluenesulfonic acid due to their acceptor character changes by protonation on the terminal pyridine ring. Such guest-induced absorption variations of 1 and 2 were used for alkali metal ion sensing. The sensing abilities are roughly parallel to their binding constants.