32 



RADIATION HIOI.OGY 



V 

 R 



\ 



c'iably tluriiifi the electron triinsitioii, and therefore tlie energies corre- 

 sponding to transitions N —^ R and N —* V will overlap. Transitions 

 from iV to R result in sharp bands, and from A'' to F in diffuse general 

 absorption. 



The optical and photochemical properties of cis- and trans-atiWyvAiv. 

 were carefully investigated by Lewis ct at. (1940). 7Vo/is-stilbene is 

 fluorescent (wave length 3300 to 4400 A), and its absorption spectrum 

 (wave length 2000 to 3400 A) shows distinct "oscillational" structure; 

 as-stilbene is nonfluorescent and its absorption spectrum is apparently 

 structureless. Irradiation of either pure compound with radiation of 



wave length 2537 A produces partial 

 stereoisomerization. Since cz's-stilbene 

 undergoes a photochemical side reac- 

 tion to an unknown product, quantum 

 yields had to be based on measurements 

 of the initial rates. Starting with the 

 pure fzs-compound, the quantum yield 

 of /raws-stilbene formation is 0.26 and 

 of the side reaction is 0.10. The cor- 

 responding quantum yield for the forma- 

 tion of cis- from ^rans-stilbene is 0.35. 

 The interpretation of these facts is ren- 

 dered uncertain by the lack of knowl- 

 edge of the potential-energy diagram for stilbene. The steric interference 

 between the phenyl groups, which is responsible for the relative instabil- 

 ity of the cts-form, undoubtedly renders the potential-energy curve for 

 the ground state unsymmetrical and probably has a similar effect on the 

 curves for the excited states. Conjugation between the benzene rings 

 and the ethylenic link must also affect the energy levels. For lack of 

 other information, let us assume that the potential-energy curves for 

 stilbene, although asymmetric, are otherwise essentially similar to those 

 for ethylene. A molecule excited to the state V will quickly lose its extra 

 energy of oscillation by successive impacts with solvent molecules and 

 will end up in the (approximately) 90° trough of the electronic state. 

 The sul)se(iuent transitions of the molecule are, of course, independent 

 of whether it was originally a normal cis- or /raw. s-stil bene molecule. 

 Since transitions between states A'' and V are permitted, it might be 

 expected that the excited mole('ule could emit a (luantum and return to 

 the ground state of either the cis- or trans-iorm, depending on the relative 

 asymmetries of states A'^ and V. If this were the mechanism of the 

 process, the quantum yields of fluorescence and isomerization would not 

 be complementary, and the limit of the sum of these yields would be 2 

 rather than 1. That this mechanism does not apply to this case is shown 

 by the nonfluorescence of cis-stilbene. The strong fluorescence of the 



AT 



0° 90° 180 



Fig. 1-3. Schematic potential-energy 

 diagram for ethylene. 



