Page 55 - Photoreactive Organic Thin Films

34 HERMANN RAU

(B)

o 30 60 90 120 150 ISO
Z-isomer
2-isomer CNNC dihedral angle/degrees E-isomer
FKHJME 1.15 (A) The calculated singlet states relevant for isomerization by rotation according to
Cattaneo and Persico. (Adapted from reference 184, by permission.) (B) The potential energy curve sys-
tem for rotation and inversion according to Rau (From reference 34, by permission.)

Besides solving the quantum yield enigma, this concept also rationalizes
some other results. If rotation is inhibited by, say, structural design as, for
instance, in azobenzenophanes or constraint from outside as, for instance, in
114 116 117
restricted spaces as in 13-cyciodextrin or zeolites ' or in solid matrices
111
or low temperature down to 4 K, then the internal conversion from the
1 l
(7i,n*) to the (n,n*) state provides a virtually barrierless path of isomeriza-
tion. The fact that the stilbenophane analogue of Tamaoki's azobenzeno-
181 182
phane shows isomerization ' does not invalidate this reasoning—the
azobenzenes choose the easiest isomerization path.
This suggestion, based on intuitive penetration into then nonvisible
geometries of azobenzene, has met critical theoretical examination by calcula-
tions of the potential energy surfaces and critical experimental examination
that comes from ultra-short spectroscopy.

1.6.1.3 Calculations of Potential Energy Curves
l
171
Some older calculations suggest rotation in the (n,n*) state or the
172 173
activity of both mechanisms in the Si and S 2 state. ' They were of the
highest quality in their time. In a newer paper, Monti, Orfandi, and
183
Palmieri reported an ab-initio calculation of the state energies at four
special geometries for azobenzene: E and Z, planar semilinear, and 90°
twisted azobenzene configurations. These authors found that the twisted
l l 1
(n,it*) is higher in energy than the twisted (im*) state and even the (m*)
1
state of the E-isomer, whereas the semilinear (n,n*) state should be at lower
energy than the two corresponding ^ivc*) states. With these cornerstones,
the authors conjectured a system of potential energies for rotation and inver-
sion (Figure 1.16), which differs from that of Figure 1.15 for the rotation
pathway. According to the diagrams of Monti, Orlandi, and Palmieri, on