Synthesis and Solvatochromic Behavior of Zwitterionic Donor–Bridge–Acceptor Systems with Oligo(p-phenylene) Spacers

Oligo(p-phenylene)s with a donor phenol group and an acceptor pyridinium moiety separated by one and two p-phenylene units were synthesized by the linear iterative Suzuki–Miyaura coupling method using aryl nonaflates as effective coupling reagents. Zwitterionic forms of these push–pull molecules were generated upon deprotonation of the phenol leading to large redshifts in absorbance maxima. UV-vis absorbance studies also revealed strong dependence of the band position on solvent polarity: a smooth bathochromic shift can be observed with the decrease of the solvent polarity. The molecule with one p-phenylene bridging unit showed the strongest solvatochromic characteristics in the series, spanning the range of 167 nm while moving from polar water to less polar N,N-dimethylformamide. The magnitude of this shift was close to Reichardt's dye — one of the most solvatochromic organic dyes known.

Determining the Degree of Deterioration of Cooking Oil by Assessing Polar Compound Content (FS14-07-19)

Objectives Degradation of oil is characterized by oxidation, polymerization and hydrolysis of the chemical compounds of the oil. This process of deterioration increases the amount of several types of compounds, including polar compounds, such as alcohols, ketones and free fatty acids. Currently, there is no universal assay for measuring the quality of frying oil, and tests that do exist to examine frying oil are often costly and time consuming. This study set out to examine the utility of Reichardt's dye, a solvatochromic dye that indicates degrees of solvent polarity, as a possible way to accurately predict the degree of degradation of cooking oils. Methods Several pH indicators were used to indicate the pH of the substances tested. Reichardt's dye was first tested against well-known indicators, including Bromophenol blue, Bromothymol blue, Bromocresol purple, and Phenolphthalein. One milliliter of each oil sample was combined with 0.5 milliliters of the Reichardt's dye solution and the color changes were observed. To determine the percentage of free fatty acids in each of the three oils, approximately 2 grams of oil with a pH indicator was dissolved in 100% ethanol and titrated with 1 M potassium hydroxide solution. The titrations assigned quantitative values to the color changes observed when Reichardt's dye was used. Results The addition of Reichardt's dye solution to all three oils indicated presence of polarized substances through varying degrees of color change whereby the fresh vegetable oil had the lowest concentration of polar compounds (dark blue) and the degraded oil had the highest concentration of polar compounds (greenish-orange). The degree of color change correlated with the results of titrations which demonstrated increasing amounts of free fatty acid (FFA) content in the fresh vegetable oil (0.17% FFA), the slightly degraded kitchen oil (2.7% FFA) and the degraded oil (15% FFA). Conclusions Our experiment showed that Reichardt's dye was effective in indicating the polarity of oil substances, a reflection of the degree of degradation in oil. Given that oxidized oil is harmful to health, it is important to measure this oxidation process. This study warrants future research into the utility of Reichardt's dye. Funding Sources National Institute of Environmental Health Sciences, NIH; University of Pennsylvania TREES Program (R25 ES021649).

Solvatomorphism of Reichardt's dye

Six different crystal structures are obtained depending on the crystallization solvent.

Reichardt’s dye: the NMR story of the solvatochromic betaine dye

Reichardt’s dye, 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)phenolate (1), has a very large negative solvatochromism in the long wavelength absorption in the UV–vis spectrum when going from nonpolar to polar solvents. This shift provides the basis of the important and widely used ET(30) scale of solvent polarity. While many papers have investigated the properties of this dye, only a few describe the 1H and 13C NMR assignments in any detail. We report herein, our detailed analysis of the proton and carbon chemical shift assignments for this molecule based on 1D and 2D NMR measurements, as well as those of the protonated and methoxy derivatives 2 and 3, respectively. Much to our surprise, some of the critical chemical shift values we observed were significantly different from those previously reported. In addition, we discovered a good correlation not only between the solvent polarity and the chemical shifts of carbons C1 and C4 of Reichardt’s dye (1), but also between the concentration of the dye and these chemical shifts.