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Two bipyridinium-benzocrown ether conjugates, 1 and 2, each appending styrene and naphthalene units at the nitrogen atom of pyridine in pyridyl-pyridinium-benzocrown ether conjugates via methylene spacer, respectively, have been prepared to investigate their guest responsive color changes and sensing abilities. Non-crown ether compounds, 3 and 4, have been also prepared as the analogue of 1 and 2, respectively. All compounds showed yellow color associated with intramolecular charge transfer (CT) absorptions, where the benzocrown ether or non-crown ether unit and bipyridinium unit act as -electron donor and electron acceptor, respectively. Upon addition of metal ions as the guest, 1 and 2 showed color change to colorless with the blue shift in their CT absorptions, although 3 and 4 did not changed their color by the presence of the guest. This is due to the formation of host-guest inclusion complexes of 1 and 2 with metal ions. Such guest-induced color changes of 1 and 2 were used for alkali and alkaline earth metal ion sensing. The high sensitivities of 1 and 2 were observed for Li and Mg among alkali and alkaline earth metal ions, respectively. The sensitivity value of 1 was roughly proportional to its binding constants. Similar trend of 2 was observed for alkali metal ions but not for alkaline earth metal ions. The substituent introduced at N-position in the bipyridinium-benzocrown ether conjugate may affect the ion sensing property.

Visible Intramolecular Charge Transfer Absorption of Bipyridinium-Crown Ether Conjugates in Response to the Alkali and Alkaline Earth Metal Ions