N-Methylpyridinium-4-phenolates: Generation of a Betaine Dye Library Bearing Different Spacer Units and Their Solvatochromism

Abstract

A library of thirteen N-methylpyridinium-4-phenolate betaine dyes was synthesized from 4-(4-methoxyphenyl)pyridines in a three step procedure involving N-methylation, a subsequent Odemethylation, and a final deprotonation. Their general structure comprised an electron-donating phenolate group and an electron-accepting 2,6-disubstituted-N-methylpyridinium moiety, connected either directly, or by a carbon-carbon double or triple bond. Their UV-Vis spectroscopic behavior exhibits a borderline inverted solvatochromism in a wide range of solvents with different polarities, changing from bathochromic to hypsochromic shifts with the increase of the solvent polarity. Inserting a double-or a triple-bond spacer between the donor and acceptor rings had the effect of increasing the solvatochromic sensitivity of the resulting dyes.

Computational Quantification of the Zwitterionic/Quinoid Ratio of Phenolate Dyes for Their Solvatochromic Prediction

“…Interestingly, the CT values obtained for dye 10 were the largest of the set studied, classifying 10 as the dye with the most zwitterionic ground state in the library. Dyes 11-13 exhibited negative solvatochromic behavior [33,34], and their solvatochromic tendency was well-predicted by our theoretical model, with CT values accompanying the increase in the polarity of the medium. Finally, the CT values obtained for dyes 7 and 8 assigned them as negative solvatochromic compounds, again, in agreement with the spectral behavior they showed experimentally [31,32].…”

“…Dye 10, known as Reichardt's betaine, is a negative solvatochromic dye widely used as a polarity sensor, whose behavior has been extensively reviewed [9,10]. Finally, dyes 11-13 are three solvatochromic dyes that show negative non-linear solvatochromic behavior [33,34]. Brooker's merocyanine 1 was the first inverted solvatochromic dye reported in the literature with a solvatochromic inversion in chloroform solution [21].…”

“…Dye 10, known as Reichardt's betaine, is a negative solvatochromic dye widely used as a polarity sensor, whose behavior has been extensively reviewed [9,10]. Finally, dyes 11−13 are three solvatochromic dyes that show negative non-linear solvatochromic behavior [33,34].…”

“…A few years ago, we called attention to this very scarce class of compounds, such as 2, 8, and 11−13, which do not display linear solvatochromic or inverted behavior, proposing a new classification of them as borderline solvatochromic dyes [34]. Therefore, we included them in our list as a challenging test for our protocol, trying to predict the spectral tendency of a group of dyes with an intermediate solvatochromic behavior.…”

“…Therefore, we included them in our list as a challenging test for our protocol, trying to predict the spectral tendency of a group of dyes with an intermediate solvatochromic behavior. However, to maintain the traditional classification of three types of solvatochromism (negative, A few years ago, we called attention to this very scarce class of compounds, such as 2, 8, and 11-13, which do not display linear solvatochromic or inverted behavior, proposing a new classification of them as borderline solvatochromic dyes [34]. Therefore, we included them in our list as a challenging test for our protocol, trying to predict the spectral tendency of a group of dyes with an intermediate solvatochromic behavior.…”