Solvatochromatic Dyes:Polarity of Solvents
Chemical Concept Demonstrated: Differences in the polarity of solvents
Demonstration:
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Reichardt's dye [2,6-diphenyl-4-(2,4,6-tripheynylpyidinio)phenolate, Aldrich
27,244-2] is placed into crystallizing dishes.
The following solvents are added to the dye.
SOLVENT
COLOR of SOLUTION
methanol
green-blue
ethanol
blue-violet
2-propanol
red-violet
acetone
orange
acetone w/ water
As the amount of water added increases, the color
of the solution changes from blue-violet to violet to
red-violet to red and then orange.
Observations:
As the polarity of the solvent increases, the colors change from green to blue to violet
to red to orange to yellow.
Explanations:
When the dye used in this demonstration absorbs light, an electron is transfered
from the negative end to the positive end of the dye, making the excited state less
polar than the ground state. Because polar solvents stabilize the ground state more
than the excited state, the change in energy increases with the polarity of the solvent.
As the solvents become more polar, the light absorbed by this dye shifts from the
low energy, long wavelength (red) to the high energy, short wavelength (violet) end
of the spectrum. Remember, however, that the color we observe is the complement
of the color of the light absorbed. As a result, we observe a change from green to
blue to violet to red to orange to yellow as the polarity of the solvent increases.
Because adding water to acetone increases the polarity of this sovent, the color of
the acetone/water/dye mixture changes from blue-violet to violet to red-violet to red
and finally to orange as the amount of water increases.