© Ultrafast Systems LLC /Vernier Software & Technology
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Figure 12:
Ground state absorption spectrum of Congo red (trans conf.) in 20% water in ethanol
When light within the broad visible band is absorbed by the dye, some ground state molecules
are converted into an excited state in which the electronic structure of the dye is changed. This shift in
electron density causes the –N=N- bond to have significantly less double-bond character, and because of
this, the molecule becomes torsionally flexible. Thus, in its attempt to rid itself of the energy imparted by
the absorption of a photon, CR flips rapidly from a trans-excited state to a cis-ground state that is a higher
energy state than the trans-ground state. This cis-state is therefore metastable with respect to the trans-
ground state, and in fluid media at room temperature, a cis-trans isomerization will occur with the result
that CR in its initial state is regenerated. Overall then, all that happens is that light energy is converted to
heat in the solution.
However, this photoreaction provides an opportunity for the student to follow the progress of a
thermal cis-trans isomerization and measure its rate on timescales that cannot be achieved by traditional
mixing methods. Moreover, the rate is found to be catalyzed by both acids and bases, and the student is
required to find the bimolecular rate constant for catalysis by OH
-
ions.
