Acceptor Influence on Thiolate Sensing by Hemicyanine Dyes

Area of Science:

• Organic Chemistry
• Analytical Chemistry
• Materials Science

Background:

• Developing selective nucleophilic chemosensors is crucial for detecting specific analytes in complex mixtures.
• Hemicyanine dyes, featuring N-methylbenzothiazolium (Btz) or N-methyl-3,3-dimethylindolium (Ind) acceptors, are commonly used electrophilic dyes.
• These dyes possess two reactive sites (C2 and C4), with C2 being more electrophilic due to its proximity to the quaternary nitrogen.

Purpose of the Study:

• To investigate the regioselectivity of nucleophilic attack on electrophilic hemicyanine dyes.
• To compare the reactivity of Btz- and Ind-based hemicyanine dyes with thiolate nucleophiles.
• To identify optimal dye structures for selective detection of thiolates.

Main Methods:

• Synthesis of Btz- and Ind-based hemicyanine dyes.
• Reaction of dyes with sodium thiomethoxide (NaSMe) in mixed solvent systems.
• Analysis of adduct formation using Nuclear Magnetic Resonance (NMR) spectroscopy.
• Computational optimization of adduct structures to estimate stability.

Main Results:

• Thiolate nucleophiles preferentially attacked the C4 position of the hemicyanine dyes, forming enamine adducts.
• No evidence of thiolate attachment at the more electrophilic C2 position was observed.
• The Btz-containing dye (dye 1) showed higher selectivity for thiolate addition compared to the Ind-containing dye (dye 2).
• Dye 2 was more susceptible to reaction with water, forming hydroxide adducts.

Conclusions:

• The C4 site is the preferred site for thiolate addition to hemicyanine dyes, contrary to predictions based on electrophilicity.
• The N-methylbenzothiazolium (Btz) acceptor provides enhanced selectivity for direct thiolate detection.
• Diagnostic NMR signals for adducts were identified, aiding in regioselectivity determination.