Abstract
New N-(pyrenylmethyl)naphtho-azacrown-5 ( 1) was synthesized as an ‘On–Off’ fluorescent chemosensor for Cu 2+. Excited at 240 nm corresponding to the absorption of naphthalene unit (energy donor) of 1, emission at 380 nm from pyrene unit (energy acceptor) is observed, indicating that intramolecular fluorescence resonance energy transfer (FRET-On) occurs in 1. When Cu 2+ is added to a solution of 1, however, the fluorescence of pyrene is strongly quenched (FRET-Off) whereas that of naphthalene group is revived. Such FRET ‘On–Off’ behavior of 1 is observed only in the case of Cu 2+ binding, but not for other metal cations. The high selectivity of 1 toward Cu 2+ can be potentially applied to a new kind of FRET-based chemosensor. The FRET On–Off behavior is supported by computational studies. The calculated molecular orbitals of HOMO and LUMOs suggest the excited-state interactions leading to FRET from naphthalene to pyrene in 1, but no electron density changes in 1·Cu 2+ complex.
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