Steric effects on controlling of photoinduced electron transfer action of anthracene modified benzo-15-crown-5 by complexation with Mg2+ and Ca2+

Benzo-crown ether based chemosensors linked by an amide bond at the 1-, 2- or 9-positions of anthracene rings were synthesized. Their complexation behavior with alkaline earth metal ions in acetonitrile was investigated using fluorescence, UV, and 1H NMR spectroscopy. In the absence of a metal ion, all compounds showed only very slight fluorescence emissions (fluorescence 'off' state) because of intramolecular charge/electron transfer process. After the complex formation with Mg2+ and Ca2+, however, only the 2-position analogue gave a fluorescence 'on' response by inhibiting the photoinduced electron transfer. Because 2-positioned anthracene was free from steric hindrance of the crown ether ring, a strongly bent complex structure was formed with Mg2+ and Ca2+, which induced a breakdown of π-conjugation between the amide moiety and the benzene ring.