Toward a chemiluminescent molecular device: metal ion-enhanced chemiluminescence of benzylidenacridan with 15-monoazacrown-5.

The benzylidenacridans (1a-d) with one or more methoxy groups emit light from the excited N-methylacridone (NMA) when treated with a solution of sodium hypochlorite and alkaline hydrogen peroxide, in which a Hammett relationship with a negative rho value was applied between the chemiluminescence quantum yields and the sigma values. However, the dimethylamino group drastically inhibited the chemiluminescence in spite of its low sigma value. While the compound (2) with 15-monoazacrown-5 also showed a very weak chemiluminescence upon treatment with tetrabutylammonium hydroxide and hydrogen peroxide in acetonitrile, the chemiluminescence was enhanced up to 20 times in efficiency when divalent metal cations, such as magnesium and calcium ions, were added. The chemiluminescence inhibition is proved to be due to the shortening of the fluorescence lifetime of NMA due to energy dissipation by the amino substituents, but the incorporation of calcium ion into the cavity of the azacrown significantly prolonged the lifetime. Thus, the possibility of metal ion sensing by a chemiluminescent molecular device is suggested.