An acidity scale, [H⁺]^hv, for proton quenching of excited states in aqueous perchloric acid

Abstract

An acidity scale for excited state protonation kinetics in aqueous perchloric acid has been developed using 1-cyanonaph-thalene as a fluorescent indicator. A comparison of the quenching rate constants obtained using this scale is made with both the more general excess acidity function, X, and the transition state activity coefficient approach. A variety of chromophores were studied including 1- and 2-cyanonaphthalenes, 1- and 2-methoxynaphthalenes, benzyl alcohols, toluenes, benzonitriles, and 2-vinylnaphthalene. Keywords: acidity scale, proton fluorescence quenching.

Photochemical Addition of 2,2,2-Trifluoroethanol to Benzonitrile and p-, m-, and o-Methylbenzonitrile

“…A reviewer has suggested that an alternate possibility for the mechanism of these addition reactions could be protonation by TFE of S 1 of the aromatic at C1 followed by cyclization of the pentadienyl cation to the bicyclic one. This suggestion has merit because we have previously shown that the fluorescence of these compounds is quenched in acidic media . However, the onset of fluorescence quenching in aqueous perchloric acid for 21 , 1 , 2 , and 3 occurs at approximately 2.4, 1.4, 0.9, and 0.4 M acid, respectively, i.e., considerably more acidic than TFE.…”

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

“…In 1973, William and Pfister demonstrated that the fluorescence of a basic N -isopropylcarbazole dyes can be selectively quenched by an interaction with Brønsted acids, and it seemed to be dependent on the acidity character of the acid . From 1987 to 1989, Pincock and Redden studied the quenching fluorescence response for the excited states of 1-cyanonaphthalene as a basic fluorescent indicator, finding a direct correlation between the acidity concentration and fluorescence intensity. − Recently, Bevilacqua and Wilcox estimated the p K a values in RNA structures by using a convenient fluorescent p K a probe, such as 2-aminopurine, giving p K a values comparable to those obtained from NMR measurements with small deviations . All these reports put in evidence that the indirect strategy using a convenient p K a probe represents a suitable solution for p K a determination, which allows to elude the typical drawbacks relative to the direct fluorescent strategy.…”