A red-shift colorimetric and fluorescent sensor for Cu²⁺ in aqueous solution: Unsymmetrical 4,5-diaminonaphthalimide with N-H deprotonation induced by metal ions

4,5-Diamino-1,8-naphthalimide (DNP)-based chemosensor H1 was designed and synthesized. Probe H1 specifically recognized Cu²⁺ ions in neutral aqueous solution. The capture of Cu²⁺ by the receptor resulted in deprotonation of the secondary amine conjugated to the 1,8-naphthalimide chromophore, so that the electron-donation ability of the "N" atom would be greatly enhanced; thus probe H1 showed a 50 nm red-shift in absorption (from 464 nm to 514 nm) and a large colorimetric response, it also exhibited an on-off fluorescent response. Specifically, H1 is a pH-independent sensor in the range of 6.0 to 12.0 and can be used as a probe for Cu²⁺ in strong basic conditions. These results opened up new possibilities for construction of red-shift colorimetric and fluorescent sensors.