Temperature Sensing of Thiolate Addition by Phenolate Merocyanine Dyes: Importance of the Quinone Methide Resonance Structure

Abstract

Merocyanine (MC) dyes containing an aromatic donor vinyl linked to a cationic acceptor serve as chemosensors for analyte detection. Their electrophilicity permits anion detection through addition reactions that disrupt dye conjugation. Herein, we demonstrate the temperature influence on thiolate addition to MCs containing the N-methylbenzothiazolium (Btz) acceptor. The zwitterionic phenolate dye (PhOBtz) displays impressive temperature sensitivity to thiolate addition, with the brightly colored phenolate favored upon heating and the colorless thiolate adduct favored upon cooling. In contrast, MC dyes containing neutral donors (PhOMeBtz and PhNMe2Btz) display only moderate temperature sensitivity to thiolate capture and release. Extraction of thermodynamic parameters demonstrates a strong enthalpic driving force for thiolate addition to PhOBtz that is absent for PhOMeBtz and PhNMe2Btz. Variable temperature 1H NMR studies demonstrate that PhOBtz adopts the para-quinone methide (p-QM) resonance structure. Thus, thiolate addition to PhOBtz resembles 1,6-conjugate addition to p-QMs which is accompanied by a large increase in the π-stabilization energy upon adduct formation. Manipulation of PhOBtz electrophilicity by attaching chlorine substituents to the phenolate caused the thiolate adducts to dissipate over time for p-QM regeneration. Our work provides new design ideas for the utility of phenolate MC dyes, given that they are carriers of the p-QM electrophile.

Nature‐Inspired Functional Chromophores from Biomimetic o‐Quinone Chemistry

“…[5,6] In most cases, tunable chromophoric properties can be achieved via rational combination of donor-acceptor (D-A) conjugate motifs, as typically exemplified by the aux-ochrome and antiauxochrome disposition at the termini of an even numbered polymethine chain in merocyanine dyes. [7][8][9] Interestingly, in an ever-increasing number of cases an important role in the overall optical and electronic performance of functional dyes is attributed to aggregation, which may be governed by hydrogen bonding or stacking interactions. [10,11] Accordingly, the 3D nanomolecular morphology and the supramolecular architecture of the chromophoric aggregates may represent most critical "extrinsic" factors that, combined with the "intrinsic" π-electron properties, regulate the overall behavior of the functional system.…”

“…To be defined “functional”, a chromophore must display specific properties associated with, e. g ., acidichromism, solvatochromism or intense light emission (fluorophores) upon excitation with solar or visible light [5,6] . In most cases, tunable chromophoric properties can be achieved via rational combination of donor‐acceptor (D‐A) conjugate motifs, as typically exemplified by the auxochrome and antiauxochrome disposition at the termini of an even numbered polymethine chain in merocyanine dyes [7–9] …”

Visible Fluorescent Light-up Probe for DNA Three-Way Junctions Provides Host–Guest Biosensing Applications

“…2-Methylbenzothiazole, methyl iodide, 3-fluoro-4-hydroxy-benzaldehyde, quinine hemisulfate monohydrate, sodium cholate hydrate, cocaine hydrochloride solution (1.0 mg/mL in methanol), estradiol, deoxycholic acid, and DNA samples were obtained from commercial sources and used as received. Treatment of 2-methylbenzothiazole with methyl iodide (1.5 equiv) in acetonitrile at reflux was used to prepare N -methyl-2-methylbenzothiazolium iodide, as described previously . NMR experiments were conducted on a 600 MHz Bruker Avance spectrometer equipped with an HCP triple-resonance probe.…”

“…While Btz is a classic DNA binding motif in fluorescent light-up probes, ,− negatively charged phenolate donors are not commonly employed for DNA staining. Instead, they serve as pH indicators and as solvent polarity probes due to their impressive negative solvatochromism . This latter feature stems from the dye’s preference for the zwitterionic form in water that switches to the more rigid neutral p -quinone methide ( p -QM) resonance structure in nonpolar solvents (Figure ).…”

“…The phenolate possesses spectacular solvatochromic properties and appears to be well suited for sensing the hydrophobic branch point of DNA 3WJs as p -QM formation can provide a fluorescent readout signal. Hence, to optimize dye response, the ortho-F atom was added to lower the phenolic p K a (the unsubstituted PhOHBtz has a p K a ≈ 8.0) and promote phenolate formation at pH 7.4. The 3WJ binding specificity of FPhOBtz is initially demonstrated in two host DNA 3WJ aptamers: the cocaine-binding aptamer MN4 , and the cholic acid binding aptamer (CABA, Figure ).…”