Negative Solvatochromism of an Anionic Thiazole-Based Dye

Abstract

The use of anionic dyes as pH-sensitive indicators has long been described. Dyes such as methyl orange, phenolphthalein, or bromothymol blue are well-investigated for this purpose. Ionic and especially anionic dyes are used in various independent fields as analytical probes, for example, to study interactions of anionic compounds and cationic surfactants, as selective chemosensors for both anions and cations, and to characterize the adsorption behavior of superabsorbent polymers (SAPs), solid agricultural waste, modified charcoal, or other compounds that are used as filter materials. Another main application of (solvatochromic) dyes is to measure the properties, that is, the polarity of solvent and solvent mixtures. One prominent example is the well-known Reichardt s dye (Betaine 30) and its derivatives, which are used widely. Usually, these chromophores are composed of a pyridinium ring that acts as the electron acceptor and a phenolic part that constitutes the electron donor. Depending on the substituents, these betaines mainly show a very pronounced negative (but also reversed or positive) solvatochromism. There are only very few examples of dyes that exhibit a pronounced negative solvatochromism that are not betaines. Recently, an extended anionic nitrophenol derivative was presented by Nandi et al. in 2012. Unfortunately, the dye was not described fully as it was generated in situ and its properties measured directly. Therefore, to fill this gap, we believe that the design of stable anionic dyes is of crucial importance for the development of new sensor molecules and methods. For this purpose, a well-known heterocyclic structure, 4-hydroxy-1,3thiazole, was investigated by our group and recognized as a surprisingly versatile chromophore. We derived this basic motif from the oxyluciferin structure, which is the lightemitting species generated during the degradation cycle of naturally occurring luciferin (Scheme 1). The ease of functionalization and good stability of this core structure have already led to several potential applications. 4-Hy-