Page 1100 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1085
1.36
1.47
1.39 1.47 SECTION 12.2
1.49
1.48 1.46 Photochemistry of
IC
1.36 b Alkenes, Dienes, and
S 2 Polyenes
1.43
1.42
1.43
1.36 1.45
1.45 1.42
1.43
ISC
S 1 a
1.47
1.40
1.47 1.39
1.40
1.41 1.39
1.47
T 2
IC
a
1.37
1.42
1.38 1.46
1.46
1.46
1.41
1.38
T 1
1.39 ISC
1.39 c
1.47 1.40
1.34
1.40 1.39
1.39
S 0
Fig. 12.7. Structures for the excited states T 1 , T 2 , S 1 , and S 2 of styrene. Singlet-singlet conical
intersections are labeled IC (internal conversion). Singlet-triplet intersections are labeled ISC (inter-
system crossing). Adapted from J. Am. Chem. Soc., 117, 6944 (1995), by permission of the American
Chemical Society.

concerning energy transfer between the chromophores. However, the fundamental
aspects of the double-bond rotation seem to be structurally similar to those for
ethene.

12.2.1.2. Photoisomerization of Stilbene Especially detailed study of the mechanism
of photochemical configurational isomerism has been done on Z- and E-stilbene. 15
The isomerization involves a twisted singlet state that can be attained from either the
Z-orthe E-isomer. Spectroscopic data have established the energies of the singlet and
triplet states of both Z- and E-stilbene and of the twisted excited states that are formed
from both isomers. Some features of the potential energy diagram are due to structural

15
J. Saltiel, J. T. D’Agostino, E. D. Megarity, L. Metts, K. R. Neuberger, M. Wrighton, and O. C. Zafiriow,
Org. Photochem., 3, 1 (1973); J. Saltiel and J. L. Charlton, in Rearrangements in Ground and Excited
States, Vol. 3, P. de Mayo, ed., Academic Press, New York, 1980, Chap. 14; D. H. Waldeck, Chem.
Rev., 91, 415 (1991); U. Mazzucato, G. A. Aloisi, G. Bartocci, F. Elisei, G. Galiazzo, and A. Spalletti,
Med. Biol. Environ., 23, 69 (1995); H. Gorner and H. J. Kuhn, Adv. Photochem., 19, 1 (1995).