© Ultrafast Systems LLC /Vernier Software & Technology
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7
Flash Photolysis Experiments
7.1
Base Catalysis of the cis-trans Isomerization of
Congo Red
Excitation wavelength: <500 nm
Observation wavelength: 600 nm
Chemicals needed
Congo red (CR)
Sodium
hydroxide
Ethanol, commercial grade for solvent
Background
Congo red (CR) is a diazo dye that is a derivative of azobenzene. As can be seen from the
skeletal structure in Figure 11, CR has two identically substituted azobenzene moieties.
Figure 11:
Congo red, MW=696.67
The absorption spectrum of a solution of CR in its trans-ground state (as depicted in Figure 13) in
20% water/ethanol is shown in Figure 12. The dye absorbs strongly throughout the visible range with a
maximum near 510 nm. Note that the absorbance is very weak at 600 nm.