Naphthalimide-Rhodamine based chemosensors for colorimetric and fluorescent sensing Hg²⁺ through different signaling mechanisms in corresponding solvent systems

A couple of 1,8-naphthalimide-Rhodamine based chemosensors R6G-NA and RB-NA are successfully designed and synthesized with high selectivity and sensitivity to Hg²⁺. R6G-NA exhibits typical fluorescence resonance energy transfer (i.e. FRET) signal from 1,8-naphthalimide to Rhodamine 6G along with an obvious color change from yellow to orange when interacting with Hg²⁺ in pure CH₃CN. While RB-NA in CH₃CN/HEPES buffer (1:1, v/v) can only act a photo-induced electron transfer off (i.e. PET-OFF) process in 1,8-naphthalimide with the fluorescence intensity centered at 525 nm increasing after adding Hg²⁺. However, in pure CH₃CN system, the spirolactam ring of Rhodamine B in RB-NA can be opened by the Hg²⁺ along with the block of PET process in 1,8-naphthalimide which leads to a new emission band emerging at 575 nm and the intensity of fluorescence enhancement at 515 nm together with the color clearly changing from yellow to pink.