Selective Ca<sup>2+</sup> sensing with donor-acceptor conjugate comprising aza-crown ether and pyridinium<sup>*</sup>

<p>A pyridinium derivative appending a phenyl aza-crown ether (<b>1</b>) was prepared, wherein the pyridinium and aza-crown ether units are conjugated via the phenylene unit. Compound <b>1</b> showed an orange color associated with intramolecular charge transfer absorption, which faded upon addition of alkali and alkaline earth metal ions. This effect was particularly pronounced for the addition of Ca<sup>2+</sup>.</p>