Reichardt's Dye

Materials

Reichardt's Dye (Aldrich # 27,244-2)

Reagent Grade Acetone (dry)

Crystalizing Dish 40 x 80 mm

DI Water

Overhead Projector

Gloves and Goggles

Procedure

1. A solution containing 10mg of the dye in 15ml of the acetone, is placed in a crystalizing dish on an overhead.
2. Water is added to the solution (small ammounts should be added at first, since a little water will dramatically increase the solvent polarity and cause a large change in the color).
3. After the addition of about 60 ml of the water (resulting mixture: 20% acetone, 80% water by vol), the solution should be orange-red. (Using less dye makes the final color too faint).

Note:This demo has been used for organic lecture and laboratory courses. It shows the dramatic effect that solvent polarity can have on absorption wavelength.  This demo has also done in lecture during a discussion of vision to illustrate how nature varies the absorption wavelength within the red, blue, and green cone cells by changing the polarity of the environment surrounding the retinal chromophore ¹.

In a relatively non-polar solvent like acetone (e = 20.7 D), Reichardt's dye is green (absorbs red); as the solvent is made more polar by the addition of water (e = 78.5 D), it becomes blue, purple, magenta, red, and finally orange (absorbs blue)². This can easily be shown with an overhead projector. The color change can be attributed to the molecule's zwitterionic ground state becoming substantially less polar upon electronic excitation. The excited state might be represented as shown below. An increase in the solvent polarity stabilizes the ground state relative to the excited state, increasing the energy of the light required for the transition. Based on the measured absorption wavelengths, the increase in trasition energy upon going from acetone Lambda-max = 662 nm) to a mixture of 20% acetone and 80% water (Lambda-max = 482 nm) is about 16 kcal/mol, most of this presumably due to the ground state stabilization.

          
                                         Ph          Ph
   Ph         Ph                           \___       \___
     \___       \___                       /---\      /---\
_    /---\    + /---\         hv          /     \    /     \
 O- /     \___ N     \__Ph    --->      O=      *---N      *--Ph
    \\   //    \\   //                    \     /    \     /
     \___/      \___/                      \===/      \===/
     /                                     /          /
   Ph                                    Ph         Ph

   GROUND STATE                          EXCITED STATE


--------------------


Solvent polarity effect on ground and excited states:



               _____   excited state - - - -  _____
                 ^                              ^
                 |                              |
              hv |  Longer                      |   Shorter 
                 |  Lambda                   hv |   Lambda 
                 | absorption                   |  Absorption
                 |                              |
               __|__                            |
                     ground state               |
                                   \            |              polar 
  non-polar                          \          |              solvent
  solvent                              \     ___|__

References:

1. L.Stryer; _Biochemistry_, 3rd ed, 1988, pp 1028 - 38 (esp p 1036)
2. D.A.Johnson, R.Shaw, E.F.Silversmith, J.Ealy; _J.Chem.Ed._, 1994, 71, 517