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

Abstract

An acidity scale for excited state protonation kinetics in 20% ethanol:80% aqueous sulfuric has been developed using 1-cyano-naphthalene as a fluorescent indicator. The utility of this new scale is demonstrated using the proton quenching of fluorescence of a variety of chromophores. These include 1- and 2-cyanonaphthalenes, 1- and 2-methoxynaphthalenes, benzyl alcohols, toluenes, and 2-vinylnaphthalene. Keywords: acidity scale, fluorescence quenching, excited state basicity.

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

“…For the most part the k, values obtained in perchloric acid are in good agreement with the values obtained for the same compounds in 20% ethanol: 80% aqueous sulfuric acid media (2). The relative order of quenching of the naphthalene compounds is the same as before so a discussion is not required here.…”

“…rate resulting from a process requiring diffusion through the As was discussed previously (2) an acidity function is proportional to aH+, which is increasing rapidly as an acid becomes more concentrated. Static quenching is determined by the concentration, and not by the activity of the protons.…”

“…Recently, we described a procedure for establishing an acidity scale, [H+lhV, that proved to be a useful way of analyzing data for proton quenching of the fluorescence of excited states of aromatic substrates (1,2). Thus, the modified Stern-Volmer expression, eq.…”

“…[2], where a~+ is the activity of the proton relative to a standard state of infinite d i l~t i o n ,~ f &, and f 5 are the activity coefficients for the excited singlet state of the indicator and for the transition state for protonation of that excited state respectively, k:, TO', and ~k~ are the quenching rate constant, the lifetime of the excited state, and the empirical Stern-Volmer slope for the indicator 1-cyanonaphthalene, respectively. If a substrate gives a linear Stern-Volmer plot versus this function with a slope Ksv, then eq.…”

“…In our previous paper (2) we made the assumption that this might be true and, therefore, discussed the kq values as if they were correct. The reactions involved are rapid, exothermic '~u t h o r to whom correspondence may be addressed.…”