Fluorescence enhancement of quinolines by protonation?

RSC Advances Pub Date: 2020-08-12 DOI: 10.1039/D0RA04691D

Abstract

A study of the fluorescence enhancement of isoquinoline, acridine (benzo[b]quinoline) and benzo[h]quinoline is reported with six organic acids of different pKa values. Protonation was found to be an effective tool in the fluorescence enhancement of quinolines. A significant increase in the fluorescence intensity is observed only when strong acids are used, resulting in an over 50-fold increase in fluorescence with trifluoroacetic or benzenesulfonic acid and isoquinoline in a 1.5?:?1 ratio. The benzenesulfonic acid was found to be the most effective in the protonation of the bases despite its higher pKa value compared to trifluoro- and trichloroacetic acid. The X-ray crystal structures of 14 salts reveal the charge-assisted hydrogen bond O?N distances to vary very little, from 2.560(2)–2.714(3) ?, with the exception of the isoquinolinium benzenesulfonate where the O?N distance of 2.862(7) ? is caused by additional intermolecular interactions in the solid-state.

Graphical abstract: Fluorescence enhancement of quinolines by protonation
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