Wavelength dependency of the rate of iodine catalyzed photoisomerization of retinol and retinal.

In separate experiments, hydrocarbon solutions of the 11-cis isomers of retinol and retinal containing catalytic amounts of iodine were irradiated with monochromatic light. Changes in absorbance were followed spectroscopically for each wavelength of light and measured as a function of time. The curve obtained by plotting the change in absorbance (delta A) vs time (t) is a hyperbola, and thus the plot of 1/delta A vs 1/t forms a straight line. The half reaction time, t1/2, was extracted from this equation, giving a value for each wavelength of light. The parameter 1/t1/2 is adjusted to compensate for variation of light quanta from the xenon source, and it is plotted vs wavelength. A response spectrum is obtained that is a Gaussian. The lambda max is 510 nm for retinol and 519 nm for retinal. This shows that in the photoisomerization iodine is the absorbing species and not the carbocation, which absorbs at 589 nm.