Quantification of Excited-State Brønsted–Lowry Acidity of Weak Photoacids Using Steady-State Photoluminescence Spectroscopy and a Driving-Force-Dependent Kinetic Theory

Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. pK a. Knowledge of the excited-state pK a value, pK a*, is critical for predicting excited-state proton-transfer behavior. A reasonable approximation of pK a* is pos...

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Published in:Journal of the American Chemical Society 2022-08, Vol.144 (32), p.14477-14488
Main Authors: Bhide, Rohit, Feltenberger, Cassidy N., Phun, Gabriel S., Barton, Grant, Fishman, Dmitry, Ardo, Shane
Format: Article
Language:English
Subjects:
Chemistry
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Summary:Photoacids and photobases constitute a class of molecules that upon absorption of light undergoes a reversible change in acidity, i.e. pK a. Knowledge of the excited-state pK a value, pK a*, is critical for predicting excited-state proton-transfer behavior. A reasonable approximation of pK a* is possible using the Förster cycle analysis, but only when the ground-state pK a is known. This poses a challenge for the study of weak photoacids (photobases) with less acidic (basic) excited states (pK a* (pK b*) > 7), because ground-state pK a (pK b) values are >14, making it difficult to quantify them accurately in water. Another method to determine pK a* relies on acid–base titrations with photoluminescence detection and Henderson–Hasselbalch analysis. This method requires that the acid dissociation reaction involving the thermally equilibrated electronic excited state reaches chemical quasi-equilibrium, which does not occur for weak photoacids (photobases) due to slow rates of excited-state proton transfer. Herein, we report a method to overcome these limitations. We demonstrate that liquid water and aqueous hydroxide are unique proton-accepting quenchers of excited-state photoacids. We determine that Stern–Volmer quenching analysis is appropriate to extract rate constants for excited-state proton transfer in aqueous solutions from a weak photoacid, 5-aminonaphthalene-1-sulfonate, to a series of proton-accepting quenchers. Analysis of these data by Marcus–Cohen bond-energy–bond-order theory yields an accurate value for pK a* of 5-aminonaphthalene-1-sulfonate. Our method is broadly accessible because it only requires readily available steady-state photoluminescence spectroscopy. Moreover, our results for weak photoacids are consistent with those from previous studies of strong photoacids, each showing the applicability of kinetic theories to interpret driving-force-dependent rate constants for proton-transfer reactions.
ISSN:0002-7863
1520-5126
DOI:10.1021/jacs.2c00554