Metal chelation-controlled twisted intramolecular charge transfer and its application to fluorescent sensing of metal ions and anions

Two fluorescent ligands, N-(2-(5-cyanopyridyl))cyclen (L5) and N-(2-pyridyl)cyclen (L6) (cyclen = 1,4,7,10-tetraazacyclododecane), were designed and synthesized to control twisted intramolecular charge transfer (TICT) by metal chelation in aqueous solution. By complexation with Zn2+, L6 exhibited TICT emissions at 430 nm (excitation at 270 nm) in 10 mM HEPES (pH 7.0) with l = 0.1 (NaNO3) at 25 degreesC due to the perpendicular conformation of a pyridine ring with respect to a dialkylamino group, which was fixed by Zn2+-N(pyridine) coordination, as proven by potentiometric pH, UV, and fluorescence titrations and X-ray crystal structure analysis. We further describe that the 1:1 complexation of ZnL6 with guests such as succinimide, phosphates, thiolates, and dicarboxylates, which compete with a nitrogen in the pyridine ring for Zn2+ in ZnL6, induces considerable emission shift from TICT emissions (at 430 nm) to locally excited emissions (at ca. 350 nm) in neutral aqueous solution at 25 degreesC.