86 



RADIATION BIOLOGY 



regardless of what other species has actually absorbed the light 

 initially. 



This "sensitized fluorescence" is a subtlp source of error in lumines- 

 cence measurements. Correspondingly it may be of great importance in 

 fluorescence and quenching and in energy transfer between molecules or 

 unconjugated parts of molecules in biological systems. 



PHOTOISOMERISM 



Most excited polyene molecules therefore spend a long time, perhaps 

 10~^ sec, in the first excited singlet state, and 10~^ sec or more in the 

 triplet state if an appreciable fraction of them get into this state. These 

 times are long, i.e., by comparison with the time needed to execute 

 molecular vibrations, rearrangement, or dissociation, or rotations or 



Brownian motions and collisions 

 leading to chemical reactions. The 

 excitation energy is also ample for 

 many chemical effects, 50kcal/mole 

 or more for visible absorption, and 

 effects are easily produced for which 

 the electron density distribution in 

 the excited state is favorable. 



An especially interesting photo- 

 chemical effect is the photoisomeri- 

 zation of polyenes, as described in 

 the classic studies of Zechmeister 

 and Polgar (1943) on carotenes and 

 in those of Zechmeister and co- 

 workers on diphenyl butadienes 

 (Sandoval and Zechmeister, 1947; 

 Pinckard et al., 1948). 



Ethylene. We may begin to un- 

 derstand what happens by con- 

 sidering how the ground and excited 

 states behave in the simplest case, 

 ethylene, when the two ends of the 

 molecule are twisted with respect to each other around the double-bond 

 axis (Fig. 2-9). 



On any orbital model whatever, a 90° twist in ethylene means that the 

 orbitals we have called/ and g must become degenerate (coincide). The 

 energy of the /-orbital must have been increased, and that of the ^-orbital 

 decreased, until they are equal, as shown in Fig. 2-9. On the energy- 

 level diagram the energy of the p configuration then coincides with the 

 , center of gravity of the Jg configuration, and the lowest state of the 



90 ISO 



0. aeqrees 

 Fig. 2-9. Change of ethylene orbitals and 

 states with angle of twist. {Adapted 

 from Mulliken and Roothaan, 1947.) 



