Page 42 - Photoreactive Organic Thin Films

PHOTOISOMERIZATION OFAZOBENZENES 2 I

1.4). At irradiation wavelengths where both forms absorb—and there is no
wavelength region where only one absorbs—photostationary states are
reached at a sufficiently long irradiation period that should not depend on
the irradiation intensity when the thermal reaction is slow. (Figure 1.3 and
Equation 1.3). Isomerization can be effected by powerful laser irradiation at
2
98 99
1
1
wavelengths as long as 633 nm, ' where £ > 10~ 1 mol" cm"" . It is not clear
l 3
whether the (n,n*) or the (n,rc*) state is populated under these conditions.
The photoinduced and thermal isomerization reactions are nearly perfectly
reversible, and side reactions are virtually absent. In de-aerated hydrocarbon
solution, azobenzene can be irradiated for days with near UV or visible radia-
tion without any change of absorbance after the photostationary state is
established. Under air, the only side reaction is a very slow oxidation to
100
azoxybenzene. This can be checked without much effort by Mauser diag-
nostics (Section 1.2.2.3). For most azobenzenes, the application of
absorbance diagrams gives perfectly straight lines, indicating that the iso-
merization is the only reaction (Figure 1.4). This fact warrants the use of
24 25 101 102
azobenzene as a convenient actinometer. ' ' '
1.3.2.2. / Quantum Yields
Not every photon absorbed by azo compounds induces isomerization.
Table 1.1 shows a series of quantum yields of azobenzene collected from dif-
ferent authors. The spread of the values reflects the experimental problems of
a seemingly simple system.
The quantum yields for azobenzene in solution are not concentration
103
dependent.
One feature of the data in Table 1.1 is that §E->Z and §Z-*E do not add up
to unity. This is an indication that more than one potential surface is involved
in the isomerization mechanism (see Section 1.6). The most unusual fact dis-
closed in Table 1.1, however, is that the quantum yields are wavelength
dependent. (|> E_* Z is about twice as large for low-energy excitation of the
l
*(n,n*) state as it is for high-energy excitation of the (n,n*) state. Obviously,
for azobenzenes Kasha's rule—that fluorescence and photochemistry take

TABLE I. I Isomerization Quantum Yields of Azobenzene in Hydrocarbon
Solution

K -> n* n -> Jt* Reference

0.11 0.42 0.27 0.75 (32)
0.09 0.4 0.25 0.4 (113)
0.10 0.42 0.28 0.55 (71)
0.10 0.44 0.20 0.68 (103)
0.40 0.55 (104)
0.11 0.27 0.25 0.56 (112)
0.13 0.27 0.31 0.52 (105)
0.12 0.24 (106)