Selection of an Analysis Wavelength: An Interesting Example Involving Solvatochromism and the Zwitterionic Dimroth-Reichardt's Betaine ET-30 Dye

A laboratory experiment is designed for studying the solvatochromic behavior of the zwitterionic Dimroth-Reichardt's betaine ET-30 dye. By spectrophotometric measurements, students determine the analysis wavelength for ET-30 dissolved in solvents of varying polarity. The five solvents selected for study give solutions that appear wine-red, violet, blue, green, and yellowish-green. Student observations afford an excellent opportunity for detailed discussions of how solvent polarity affects spectral transitions, which expands the presentation given in most analytical textbooks. Textbook presentations generally represent absorption and emission transitions in the form of a Jablonski diagram, with no discussion of solvatochromism and solvent-dependent wavelength shifts.