High CT-Fluorophore Featuring a Basic Moiety into D-A Chain as a p K a Probe.

The determination of acidity represents a significant challenge within fluorometry, and no effective strategy has been developed successfully yet. It is attributed to the fact that acidity tends to be enhanced upon excitation, giving, in general, an overestimation of the ionization constant, p K a . Herein, we developed a strategy for p K a estimation of Brønsted acids in solution through fluorometry by using a convenient p K a probe, N 1 -aryl-7-methoxy-2-(trifluoromethyl)benzo[ b ][1,8]naphthyridin-4(1 H )-one. It allowed us to obtain a linear log K SV versus p K a correlation derived from the selective quenching response of the probe by an interaction with different Brønsted acids. The key points of N 1 -aryl-7-methoxy-2-(trifluoromethyl)benzo[ b ][1,8]naphthyridin-4(1 H )-one as a p K a probe were (i) the location of a weak basic moiety in the donor-acceptor chain of the fluorophore, which favors a selective quenching of the intramolecular charge-transfer process according to the acidity of acid, and (ii) the high CT character upon excitation that promotes higher quenching magnitudes and favors a wider p K a range (19.5p K a ) for the log K SV versus p K a correlation. Other key principles were to delimit the study to pure proton transfer and nonfluorescent acids, which allowed restricting the quenching response to a process dependent mainly on the acid-base equilibrium. All these findings open a new perspective as a proof of concept to design effective fluorescent p K a probes.