Ion-responsive Intramolecular Charge-transfer Absorption Using a Pyridinium Benzocrown Ether Conjugate

Abstract

A pyridinium benzocrown ether conjugated compound, 1, and its analogue with a non-crown ether unit, 2, have been prepared. Both compounds showed similar absorption spectra with two absorption bands at around 260 and 330 nm in acetonitrile. The bands at the longer wavelength side are associated with intramolecular charge transfer (ICT) absorption, in which the dialkoxyphenyl unit in benzocrown ether and the pyridinium unit act as the donor and acceptor, respectively. The addition of a guest, such as Li⁺ or Mg²⁺, caused a blue shift in the ICT absorption band for 1, but not for 2. This is explained by the formation of a 1:1 host-guest inclusion complex of 1 with the guest. The guest-induced absorption variation of 1 can be used for alkali and alkaline metal ion sensing. Compound 1 could detect divalent cations, especially for Mg²⁺, rather than univalent ones (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺), although Li⁺ was detected with high sensitivity among the alkali metal ions. Compound 3, which has a pyridyl unit at the para position on the pyridinium of 1, showed a similar trend to that of 1 with lower sensitivity than that of 1. The fact that the Mg²⁺/Li⁺ sensitivity ratio of 1 and 3 was estimated to be 8.63 and 5.08, respectively, suggests a higher Mg²⁺-preference of 1 rather than 3, while the Ca²⁺/Na⁺ ones were 4.98 and 4.85, respectively, when compared ions with similar ionic radii. The sensitivity values of 1 were roughly proportional to their binding constants, as shown by the binding constants with Li⁺, Na⁺, Mg²⁺, and Ca²⁺ with values of 2100, 910, 11500, and 2000 M⁻¹ for 1, respectively. The binding constants of 3 were estimated to be 1710, 650, 3000, and 1400 M⁻¹ for Li⁺, Na⁺, Mg²⁺, and Ca²⁺, respectively, but could not be obtained for alkaline metal ions. The limit concentration for the detection of 1 for Mg²⁺ was estimated to be 0.0156 mM, which was the smallest value in this system.