Design and synthesis of new functionalized 8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinolines as turn-off chemosensors for selective recognition of Pd2+ ions

Abstract

We have developed two novel chemosensors 6-morpholino-8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinoline-5-carbonitrile (L1) and 6-(methylthio)-8-(thiophen-2-yl)-1,2,3,4-tetrahydroquinoline-5-carbonitrile (L2) for selective identification of highly toxic Pd²⁺ ions. The selectivity of L1 and L2 was examined with a series of metal ions, among which Pd²⁺ ions were recognized selectively with fluorescence turn-off performances in methanol with very low limits of detection (nM). Another compound L3 was synthesized and studied to understand the role of the thienyl ring during complex L1 + Pd²⁺ and L2 + Pd²⁺ formation. The fluorescence changes after addition of Pd²⁺ were also observed through the “naked-eye” (under a 365 nm UV lamp). Quantum chemical (DFT) calculations have been carried out to acquire information about the stability of the chemosensors and their complexes with Pd²⁺ (L1 + Pd²⁺ and L2 + Pd²⁺), which suggested that complex L2 + Pd²⁺ is more stable than L1, L2 and complex L1:Pd²⁺. A number of studies comprising Job plots, Stern–Volmer plots and detection limits illustrate the notable sensing abilities of the chemosensors with Pd²⁺ ions, in which probe L2 was found to be more applicable than L1.