Page 119 - Photoreactive Organic Thin Films

APPENDIX 3A

QUANTUM YIELDS DETERMINATION

The procedure for determining QYs is summarized as follows. On the one
hand, both trans and cis isomers may be excited by the same irradiation and
interconverted, and an equilibrium of the two isomers, called the photo-
stationary state, is reached. On the other hand, thermal cis—Hrans isomeriz-
ation moves this equilibrium in favor of the trans isomer. So, the first part
of the experiment consists of eliminating the effect of thermal isomerization
on the equilibrium by determining the absorbance of hypothetical photo-
stationary states of the photoisomerization reactions only, by extrapolating
the irradiating intensity to infinity for combinations of two irradiation and
analysis wavelengths (Rau's method). In the second part of the experiment,
the obtained extrapolated values of the absorbance are used (in Fisher's
method) to determine the extinction coefficients s c, i.e., the absorbance
spectrum, of the cis isomer, and the determination of the quantum yields is
straightforward.

3A.I RAU'S METHOD
During irradiation, the concentration Q of the cis form is given by:

K
^ = 1000/o'U - lQ- '}(s' t& cc t-s' t& tc c}IA'-kc c (3A.1)
The primed quantities refer to a measurement at the irradiation wavelength;
the unprimed quantities refer to a measurement at the analysis wavelength. IQ
is the incident photon flux, A' the total absorbance of the sample, k the first-
order thermal relaxation rate of the cis isomer, e' t (respectively e' c) the