Abstract
The equilibrium between different tautomers that can be colored or colorless is an important feature for rhodamine dyes. Presently, this phenomenon is mostly discussed for rhodamine B. Herein, we studied the tautomerism and acid–base dissociation (HR+ ⇄ R + H+) of a set of rhodamines in organic media. Form R is an equilibrium mixture of the colored zwitterion R± and colorless lactone R0. Absorption spectra in 90 mass% aqueous acetone reflects the correlation between the dyes structure and the equilibrium constant, KT = [R0]/[R±]. Increase in the pKa value on transferring from water to organic solvents confirms the highly polar character of the R± tautomer. To reveal the role of the solvent nature, the tautomerism of an asymmetrical rhodamine, 2-(12-(diethyliminio)-2,3,5,6,7,12-hexahydro-1H-chromeno[2,3-f]pyrido[3,2,1-ij]quinolin-9-yl)benzoate, was examined in 14 media. This chain–ring tautomerism is an intramolecular acid–base reaction; the central carbon atom acts as a Lewis acid. The interaction with other Lewis acids, Li+, Ca2+, Mg2+, and La3+, results in rupture of lactone cycle. In polar solvents, lactones undergo photocleavage resulting in formation of highly fluorescent R±, whereas the blue fluorescence and abnormally high Stokes shift in low-polar media may be explained either by another photoreaction or by spiroconjugation and charge transfer in the exited state.
Highlights
IntroductionThe number of various applications of the fluorescent rhodamine dyes in different fields of science and technique is huge
Introduction published maps and institutional affilThe number of various applications of the fluorescent rhodamine dyes in different fields of science and technique is huge
Rhodamines belong to the mostly used organic dyes. Owing to their unique fluorescent properties, they find numerous applications in various fields of chemistry and related sciences [1,2,3]. They are used for creating pH-sensitive hydrogels [4], chemosensors for metal ions [5], as molecular thermometer for nanoparticles for optical hyperthermia [6], for creating the fluorescence images of core–shell magnetic nanoparticles used in hyperthermia therapy [7], etc
Summary
The number of various applications of the fluorescent rhodamine dyes in different fields of science and technique is huge. Rhodamines belong to the mostly used organic dyes Owing to their unique fluorescent properties, they find numerous applications in various fields of chemistry and related sciences [1,2,3]. 12) and replacing of the arene by the pyridine cycle Dyes. chromophore system (compounds 8, 9), moving the amino group from 3- to 2-position stabilizes the zwitterions both in casesystem of dialkylamino‐.
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