Reichardt's Dye (Solvatochromic betaines)

Reichardt's dye is pleasing in structure and function:

It is related to yesterday's molecule, betaine. It is technically a "betaine," but in a much broader sense, since it is an ammonium zwitterion (has a positive and negative charge). It posesses the property of solvatochromism - that is, it actually changes color depending how polar a solvent you use.

You can get it to turn essentially any color in the visible, just by varying the solvent you use. It has found a lot of use in characterizing how polar certain microenvironments are (such as cell membranes and similar micelle-type structures). It's also been used (in at least one paper, not sure how practical this is) to characterize the water content of a solvent - water being so polar, it shows up in the form of a spectral change.

For a brief discussion (and pictures of the dye in some solvents), look here. It is named for Christian Reichardt, who has spent as much time studying solvents and solvent effects as just about anybody.