Page 58 - Photoreactive Organic Thin Films

I. PHOTO1SOMERIZAT1ONOFAZOBENZENES 37

The given error margin would exclude compatibility with the undoubtedly
wavelength-dependent isomerization yields found in steady-state experiments
(Figure 1.9). The authors seek a not very convincing explanation in the inter-
nal conversion of vibrationally excited states of S l directly to hot vibrational
states of the ground state, whose cooling time has been determined to be ca.
16 ps. To comply with the results for rotationally blocked molecules, these
vibrations should experience rotational features that do not lead to isomer-
ization. Vibrationally excited states may be important, because the lifetime of
1
the (n,n*) state is much smaller than the cooling time in a solvent. 191
61
With Fujino and Tahara's proposition, based on experimental results,
1
that no isomerization should be possible in the high energy (n,n*} state and that
l
exclusively inversion in the (n,n*} state should lead to isomerization, and with
185
Ishikawa's proposition, based on computational results, that all isomeriza-
tion should occur on a rotational pathway even in the (n,7t*) state, another
round of discussion of the isomerization mechanism in azobenzene seems to
have been opened. Further work will be necessary, but both single-route
mechanisms will need to rationalize the wavelength-dependent quantum yields.
1.6.1.5 The Triplet State
Isomerization in the triplet manifold seems to be totally separate from the
singlet route. Here also, however, isomerization quantum yields are reported
71
that are dependent on the sensitizer's triplet energy. No implications for the
isomerization mechanism have been discussed.

1.6.2 Pseudo-Stilbene-Type Molecules

1.6.2.1 The Thermal Isomerization Mechanism
Based on their isomerization experiments in polar solvents, Whitten et
149
al. stated that thermal Z —> E isomerization of donor/acceptor-substituted
194
azobenzenes proceeds by rotation. Later, Shin and Whitten modified this
point of view and saw a dual mechanism active dependent on solvent and
88 133 151 175
donor strength. Asano and coworkers, ' ' ' from their experiments
under pressure, inferred a dependence of the mechanism on solvent and even
parallel reaction paths along these mechanistic coordinates.
i .6.2.2 The Photoisomerization Mechanism
The photoisomerization mechanism in aminoazobenzene- and pseudo-
stilbene-type compounds has attracted far less attention than the mechanism
1
for azobenzenes. In pseudo-stilbenes, the (n,n*) state is buried under the
intense ic —> n* band and cannot be populated selectively. No state-specific
quantum yields are available because the yields are independent of the
160
exciting wavelength. There is only a very narrow experimental basis for a
discussion of these two mechanisms. However, this may change when pseudo-
stilbenes are subjected to ultrashort-time experiments.
135 136 l
Monoprotonation of azobenzenes is asymmetric. ' a (n,n*} state still
exists, but it is expected to shift to high energy, and the molecules are very
stilbene-like. Rotation would be the isomerization mode. This concept is sup-
ported by the strong temperature dependence of fluorescence: Azobenzene-