Thiourea based molecular dyad (ANTU): Fluorogenic Hg2+ selective chemodosimeter exhibiting blue–green fluorescence in aqueous-ethanol environment

A thiourea based fluorescent molecular dyad (ANTU) bearing anthracene and naphthalimide fluorophores has been designed and synthesized as a selective chemodosimeter for Hg2+ in aqueous-ethanol solution with detection limit of 4 ppb. The optoelectronic behavior displayed by ANTU at 442 and 370 nm excitation is attributed to intramolecular charge transfer (ICT) and fluorescence resonance energy transfer (FRET) processes. The molecular dyad upon interaction with different cations showed Hg2+ triggered intramolecular guanylation reaction to form a new imidazoline derivative in which the fluorescence intensity at 530 nm (at λex = 442 nm) was decreased due to restricted intramolecular charge transfer (ICT) reaction wherein at 370 nm excitation intensity of a new emission at 467 nm increased ratiometrically due to decrease in ICT and FRET processes. The naked-eye sensitive color of the solution was changed from a fluorescent green to blue. The potential application of chemodosimeter to detect Hg2+ in protein medium and on paper strip has also been described. The proposed mechanism of interaction between dyad and Hg2+ is confirmed by the change in optical behavior, FT-IR, 1H, 13C NMR and ESI-MS spectral data.

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