A depropargylation-triggered fluorescence "turn-on" probe for the detection of Pd²⁺ based on a bispropargylamine-rhodamine conjugate

A bis-propargyl-appended rhodamine B-based receptor BPRB has been synthesised that exhibits pronounced fluorescence enhancement in the presence of Pd²⁺ ions. The addition of Pd²⁺ enhanced the fluorescence intensity of BPRB by 113-fold (Φ_f = 0.37) and BPRB was found to exhibit high selectivity towards Pd²⁺ compared to a range of other metal ions. The enhancement of fluorescence was triggered by spirolactam ring opening followed by depropargylation of BPRB in the presence of Pd²⁺, as evidenced by FTIR and NMR analyses. BPRB was able to detect Pd⁰ without the addition of a reducing agent, and the emission intensity of BPRB-Pd⁰ was almost identical to that of BPRB-Pd²⁺; however, a rapid fluorescence response was observed in the presence of PPh ₃. To explore the efficiency of the rhodamine unit, a bispropargyl derivative of cyclohexane (BPCH) was synthesised and the fluorescence response towards Pd²⁺ was examined and compared with BPRB, revealing that the rhodamine unit enhanced the fluorescence intensity by 500-fold. The fluorescence images of BPRB and BPRB-Pd²⁺ samples indicate that BPRB could be useful for imaging Pd²⁺ in living cells.