Photolysis of the Sulfonamide Bond of Metal Complexes of N ‐Dansyl‐1,4,7,10‐Tetraazacyclododecane in Aqueous Solution: A Mechanistic Study and Application to the Photorepair of cis,syn ‐Cyclobutane Thymine Photodimer

Conditional cleavage : Photochemical SN bond cleavage of the Zn 2+ complex of N ‐dansylcyclen (ZnL 2 ) in aqueous solution was investigated. Moreover, photolysis of ZnL 2 (see scheme) facilitated photoreversion of cis , syn ‐thymine photodimer (T[ c , s ]T).Sulfonamide constitutes a ubiquitous functional group that is frequently used in organic chemistry, analytical chemistry, and medicinal chemistry. We report herein on the photolysis of a dansylamide moiety of 1‐dansyl‐1,4,7,10‐tetraazzacyclododecane ( N ‐dansylcyclen, L 2 ) in the presence of a zinc(II) ion in aqueous solution. By potentiometric pH titrations, the complexation constant for the 1:1 complex of L 2 and Zn 2+ , log  K s (ZnL 2 ), in aqueous solution at 25 °C with I =0.1 (NaNO 3 ) was determined to be 6.5±0.1. The structure of the ZnL 2 complex was confirmed by single‐crystal X‐ray diffraction analysis. During fluorescence titrations of L 2 with Zn 2+ (irradiation at 308 or 350 nm) in aqueous solution at pH 7.4 (10 m M HEPES with I =0.1 (NaNO 3 )) and 25 °C, considerable enhancement in fluorescence emission of the Zn 2+ complex of L 2 (ZnL 2 ) was observed, while metal‐free L 2 exhibited only a negligible emission change upon UV irradiation. It was revealed that this emission enhancement arose from the photoinduced cleavage of a sulfonylamide moiety in ZnL 2 , yielding the Zn 2+ ‐cyclen complex and 5‐dimethylaminonaphthalene‐1‐sulfinic acid, which has a greater quantum yield ( Φ ) for fluorescence emission than that of L 2 and ZnL 2 . For comparison, the photolysis of N ‐(1‐naphthalenesulfonyl)cyclen (L 3 ) and its Zn 2+ complex (ZnL 3 ) under the same conditions (irradiation at 313 nm) gave the corresponding sulfonate (1‐naphthylsulfonate). We also describe the results of a photoreversion reaction of cis,syn ‐cyclobutane thymine photodimer (T[ c,s ]T) utilizing the photolysis of ZnL 2 and ZnL 3 .