High CT-Fluorophore Featuring a Basic Moiety into D–A Chain as a pKₐ Probe
2022
Romero, Angel H. | Cerecetto, Hugo E.
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, pKₐ. Herein, we developed a strategy for pKₐ estimation of Brønsted acids in solution through fluorometry by using a convenient pKₐ probe, N¹-aryl-7-methoxy-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one. It allowed us to obtain a linear log KSV versus pKₐ correlation derived from the selective quenching response of the probe by an interaction with different Brønsted acids. The key points of N¹-aryl-7-methoxy-2-(trifluoromethyl)benzo[b][1,8]naphthyridin-4(1H)-one as a pKₐ 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 pKₐ range (19.5pKₐ) for the log KSV versus pKₐ 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 pKₐ probes.
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