A smart two-photon fluorescent platform based on desulfurization–cyclization: a phthalimide–rhodamine chemodosimeter for Hg2+ NIR emission at 746 nm and through-bond energy transfer

Abstract

A smart two-photon fluorescent platform based on desulfurization–cyclization was developed by attaching rhodamine to a cyanine skeleton through the Knoevenagel reaction. Based on the platform, a CyRSN probe was rationally designed and synthesized, in which morpholinyl naphthalimide and modified rhodamine thiohydrazide were linked to form a structure of monothio-bishydrazide. The probe exhibits excellent two-photon properties toward Hg²⁺ under Ti:sapphire pulsed laser 800 nm irradiation, allowing the naked-eye signal and fluorescence “turn-on” signal at 746 nm to be obviously distinguished. The addition of Hg²⁺ to the probe ensured that the naphthalimide donor and modified rhodamine acceptor were connected with electronically conjugated bonds (1,2,4-oxadiazole). Hence, a typical TBET process took place, resulting in an increase of the modified rhodamine NIR emission at 746 nm by about 34-fold as well as a decrease of the naphthalimide emission at 540 nm and producing a colorimetric change from pale yellow to green. The chemodosimeter exhibits a stable response for Hg²⁺ over other metal ions with a detection limit of 1.91 × 10⁻⁷ M, high sensitivity, a rapid response time, and pH independence under neutral conditions (pH 6.0–10.0). In addition, the CyRSN probe was successfully applied in the detection and quantification of Hg²⁺ in water samples with satisfactory recovery results.