High CT-Fluorophore Featuring a Basic Moiety into D-A Chain as a pK_a Probe

Area of Science:

• Analytical Chemistry
• Physical Chemistry
• Spectroscopy

Background:

• Determining acidity (pKa) using fluorometry is challenging due to excitation-induced acidity enhancement, leading to overestimations.
• Existing fluorometric methods lack effective strategies for accurate pKa determination of Brønsted acids.

Purpose of the Study:

• To develop a reliable fluorometric strategy for estimating the ionization constant (pKa) of Brønsted acids in solution.
• To introduce a novel pKa probe, N1-aryl-7-methoxy-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one, for accurate acidity measurements.

Main Methods:

• Utilized a specifically designed pKa probe, N1-aryl-7-methoxy-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one, for fluorometric analysis.
• Established a linear correlation between the Stern-Volmer quenching constant (log KSV) and pKa by analyzing the probe's selective quenching response with various Brønsted acids.
• Focused the study on nonfluorescent acids undergoing pure proton transfer to isolate the acid-base equilibrium's effect on quenching.

Main Results:

• Achieved a linear log KSV versus pKa correlation, demonstrating the probe's effectiveness in acidity estimation.
• The probe's design, featuring a weak basic moiety and high charge-transfer (CT) character, facilitated selective quenching and a wide pKa measurement range (19.5).
• The strategy successfully restricted the quenching response primarily to the acid-base equilibrium, ensuring accurate pKa determination.

Conclusions:

• The developed fluorometric strategy offers a new perspective for accurate pKa estimation of Brønsted acids.
• The N1-aryl-7-methoxy-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one probe is a promising tool for acidity determination in solution.
• This work serves as a proof of concept for designing advanced fluorescent pKa probes.