Page 59 - Photoreactive Organic Thin Films

HERMANN RAU

+
H , for instance, does not emit at room temperature, but does emit strongly
126
at 77 K in rigid H 2SO 4.
In the donor/acceptor-substituted azobenzenes, the n conjugation system
is predominantly affected. A decrease in energy of the (ic,ic*) states and a
176
weakening of the central double bond is previewed. The close energetic
proximity of the (njz*) and (n,jt*) states, combined with easy distortions of
the molecule from planarity, suggest vibrational coupling of the two states.
Then one can envisage fast transitions between them, and it may be the
special conditions of substitution, environment, temperature, pressure, etc.
that determine the route of isomerization. Rotation is predicted as the pre-
ferred path, but inversion may also be possible, calculations predict energetic
proximity of the transition states of the two isomerizations paths. 195
1.6.2.3 The Triplet State
As in the azobenzene type systems, the triplet pathway seems to be
decoupled from the singlet route in pseudo-stilbenes. Little is known about
the mechanism in the triplet state. The only information comes from calcula-
tions, and these show that the triplet surfaces are frequently similar in shape
to the singlet surfaces. Thus, both mechanisms may be operative in the triplet
state, too.

1.7 CONCLUDING REMARKS

The photoisomerization of all types of azobenzenes is a very fast reaction on
either the singlet or triplet excited-state surfaces according to the preparation
of the excited state, with nearly no intersystem crossing. "Bottleneck states"
have lifetimes on the order of 10 ps. The molecules either isomerize or return
to their respective ground states with high efficiency. So photoisomerization is
the predominant reactive channel, and the azobenzenes are photochernically
stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have
small quantum yields of photodegradation.
Thus, azobenzene-type compounds especially have the potential to sur-
vive many isomerization cycles. Therefore, they are preferred in photorespo-
sive devices where bistability is a goal. If substitued azobenzenes have to be
considered for such a device — usually for synthetic reasons — care should be
taken to find a compound with the characteristic n — > n* band in the absorp-
tion spectrum. Substituents meeting this demand are, cum grano salts: haloge-
48 87
nalkyl, and hydroxy or carboxy groups. ' There are many examples of such
devices in the following chapters of this monograph.
Pseudo-stilbenes are better suited for the creation of devices with a high
anisotropic molecular order and large macroscopic higher-order polarizabili-
ties. To reach this, a fast thermal Z — > E isomerization is required. For this
purpose, molecules with a long wavelength intense n — > n* band should be
selected. The selection of p-alkyamino and p'-nitro or cyano groups gives the
best donor/acceptor combinations. There are also many examples for such
devices in the following chapters.
From the knowledge of the isomerization data of Sections 1.3 through