© Ultrafast Systems LLC /Vernier Software & Technology
31
7.2 Isomerization
of
Mercury Dithizonate
Excitation wavelength: <500 nm
Observation wavelength: 600 nm
Chemicals needed
Mercury dithizonate (HgDz)
Trifluoroacetic acid (TFA)
Ethanol, commercial grade for solvent
Background
Figure 15:
Photoinduced isomerization of mercury dithizonate
This photoisomerization event occurs within the flash lamp profile (i.e., “instantaneously” on our
time scale). The trans-to-cis back reaction occurs in the dark period following the flash, and the color
reverts thermally with a lifetime of ~650 ms. This inversion of color can be catalyzed by acids and bases.
In this experiment the solution of the HgDz complex is excited by the flash lamp and the decay of the
transient absorption at 600 nm is monitored as a function of time after the flash.
The decay is exponential in time with a rate that is first order in the concentration of an acid such
as trifluoroacetic acid (TFA). The experimental conditions are such that the TFA is at a much higher
concentration than the photo-produced trans-isomer of the complex and so the conditions correspond to
the pseudo-order situation as described in the kinetics theory section (Section 5.7). A plot of the observed
rate constant as a function of acid concentration is linear with a slope that provides the bimolecular rate
constant for the catalysis process.
h
