Page 30 - Photoreactive Organic Thin Films

PHOTOISOHERiZATION OFAZOBENZENES

be eliminated by choosing different irradiation intensities and extrapolating
«... rA. x „ 23
tO 10 — > °°.
For the development of the following procedures, the thermal reaction is
considered slow compared to the photoreaction. From fundamental kinetics
it is known that opposing reactions end in an equilibrium, opposing photo-
reactions in a photostationary state (pss). The rate equations of neither C E nor
c z in Equation 1.1 are of first order. However, that equation can be trans-
formed into an equivalent one describing the rate as a function of (c Zoo - c z),
i.e., the approach to the pss, which, indeed, is of first order, not in the irradia-
tion time axis but in the axis "photons absorbed":
— —
In =- = -1000 Ij (e£<j>£ -f 4<£) -~ - dt (1.2)

Remember, for A<0.01 the integral is 2.303 t. Another useful equation
therm
derived from Equation 1.1 is for the pss with k = 0:

d$%, (1.3)
c Eoo
From Equation 1.3 it is evident that the pss for different irradiation wave-
lengths has varying compositions. Figure 1.3 shows E-azobenzene that was
irradiated with 313 nm leading to a pss with 80% Z-isomer. Then the irradi-
ation wavelength was changed to 366 nm, which creates a new pss with 48%
Z-form. The same pss is reached when starting from pure E-form. 436 nm
gives ca. 15 % of Z-form in the pss. The concentration/time relation in Figure
1.3 is not purely exponential, because the irradiation time is the variable.
For the evaluation of Equation 1.2, the irradiation intensity IQ is needed.
24 25
This is determined by actinometers, ' which may be either physical in
nature or chemical reaction systems with known quantum yields. Because the
26
values of /e^(^ + e^z) for azobenzene are well documented, azobenzene
itself is used as a convenient actinometer. 27
Any physical property proportional to concentrations may be used for
the analysis according to Equation 1.2. But the concentrations, especially in
thin films, are usually determined by means of absorbance measurements at a
wavelength £ where only the Z- and/or E-forms of the azobenzene derivative
change their absorbances. Then the irradiation (A,) and analyzing (£,) wave-
lengths must be held apart:

ln 1 1000 (e + dt
TT = - '« ^ ^ A (t) (1.4)
77
K
A; 3-A§
and
s\
I . J I
Equation 1.4 can determine whether the observed photoreaction is of first
order or not, i.e., whether Scheme I is valid. Cases where Scheme I may not
be sufficient for a description of the photoisomerization include systems with