ENERGY EXCHANGE IN PHOTOREACTIONS 19 



carbon-carbon bonds of benzene will have the appropriate length with 

 probability p^. This rough treatment indicates that the half-life of the 

 olefin, T = 1/p, is perhaps less by a power of 6 than that of benzene at 

 the same temperature. One may speak of a crossing region in the full 

 potential-energy space of all coordinates rather than a crossing point. 

 Crossing from excited states involving "atomic" electrons will have a 

 considerable extension in potential-energy space, because a large fraction 

 of the degrees of freedom are not sensitively involved in the crossing 

 process and many nuclear distances need not be specified. These cross- 

 ing regions will consequently be large by comparison with those for many 

 conjugated molecules. 



3. x-Electron bonds, being less stable than o--electron bonds, can be 

 deformed at lower energies. In other words, the 7r-electrons are readily 

 displaced from their eciuilibrium positions by applied electric fields. This 

 ease of polarizability has several useful features. It makes for large 

 dipole differences between states and hence large transition probabilities. 

 It makes association through van der Waals' dispersion forces, with 

 molecules whose reactions are to be sensitized, a stronger and longer- 

 lived phenomenon since these forces depend on the magnitude of the 

 polarizability. It makes possible stronger electric-field interactions with 

 neighboring molecules and thus increases the probability of energy trans- 

 fer by resonance processes, as will be discussed in Sect. 4-3. 



4. Conjugated systems generally have low ionization potentials. This 

 fact is the result of the increased number of resonance structures that 

 may be written for the ion of a conjugated molecule. According to a 

 theory of Weiss (1939a), photosensitized oxidations and reductions as 

 well as nonproductive quenching of fluorescence can be brought about 

 by electron transfers to or from the excited molecule. Stabilization of 

 ions should increase the efficiency of such a process. Further discussion 

 of Weiss's theory is given in Sect. 4-3. 



5. Because there exists a low-resistance path for 7r-electrons from one 

 end of a conjugated system to the other, electron movements within the 



CHjy /CH, CH3\ /CH, 



\/ \/ 



/ 'K HH HHH HHHH HHH HH / \ 



/\ll III III! Ill ll/\ 



HoC C — C^C — C = C — C = C — C=C — C^C — C^C — C = C — C = C — C = C — C CH, 



I II I I I I II I 



HjC C CHj CHj CH3 CHj c CH2 



C "CH3 CHj ^C 



Hg Ho 



Fig. 1-9. /:i-Carotene. 



system are very rapid. Speaking crudely, this has the effect that exci- 

 tation energy in the form of an excited electron can be made available for 

 reaction instantly at any point in the system. This state of affairs favors 

 reaction with other molecules, and it may also result in some special 



