New fluorescent ionophores containing the 1-naphthylacetanilide moiety based on oligo-ethylene oxide (1₄ and 1₅) have been synthesized, and their photochemical behaviors have been studied. In the absence of a metal ion, the 1-naphthylacetanilide moiety showed a weak fluorescence emission (fluorescence “off state”). However, complexation with Ca²⁺ induced a large enhancement effect on the fluorescence intensity of 1₄ and 1₅ from the naphthalene ring (fluorescence “on” state). This behavior can be explained in terms of a similar twisted intramolecular charge-transfer relaxation mechanism. Such “off-on” fluorescence responses of 1₄ and 1₅ provide the digital detection of alkaline earth metal ions.